Carb sync? Never again.

[Av8r3400]

This past Airventure I had the opportunity to talk with Hal Stockman (the Zipper big bore guy) and he shared with me his latest modification to the 912 engines.


He is removing the small balance tube between the intake manifolds and replacing it with a 1" diameter tube.

I just did this modification to my engine this past weekend and had the chance to test run the engine today. All I can say is WOW! What a difference this makes. The engine is now butter smooth from 1500 to 6000 rpm. A mechanical sync is all I did on reassembly and don't plan to do any more. This mod is something I've been thinking about for a long while, but didn't have the courage to do without a second opinion. Makes perfect sense to me and makes me wonder why these engines don't come this way from the start.

I forgot to snap a photo, but will do this tomorrow for the record.

[avidflyer]

Yes, we or at least I would like more info and pictures. JImChuk

[DesertFox4]

Very interesting developement. Standing by for photos.

[Rodney]

Did you have to do any machining to the intake manifold, or was this simply a bolt on addition?

Thanks
Rodney

[jabkwab]

That sounds interesting.
I am curious for the theory behind that modification.

[Av8r3400]

I did some machine work on the manifolds to position and tap 3/4 NPT holes for the fittings. I got 3/4 NPT to 1" pipe 90 degree elbows from Jegs, 1" thin wall tube from a local supplier and 1" heater noose and clamps from NAPA.

The theory behind the modification is to allow the two manifolds to equalize pressure or vacuume between them easier.



Photos later today.

[jdmcbean]

This past Airventure I had the opportunity to talk with Hal Stockman (the Zipper big bore guy) and he shared with me his latest modification to the 912 engines.


He is removing the small balance tube between the intake manifolds and replacing it with a 1" diameter tube.

I just did this modification to my engine this past weekend and had the chance to test run the engine today. All I can say is WOW! What a difference this makes. The engine is now butter smooth from 1500 to 6000 rpm. A mechanical sync is all I did on reassembly and don't plan to do any more. This mod is something I've been thinking about for a long while, but didn't have the courage to do without a second opinion. Makes perfect sense to me and makes me wonder why these engines don't come this way from the start.

I forgot to snap a photo, but will do this tomorrow for the record.

I have heard of this many years ago through some overseas sources... never tried it but you are the 3rd person I have heard glowing reports from recently..

[jiott]

This is interesting to me too. I've been noodling on the theory and believe it makes perfect sense at the higher engine rpms because the intake flow is quite high and a bad carb balance may require a high flow thru the balance tube. However, down near idle the flows are very low and an imbalance would not need a very big balance tube to correct it, so I would think that the 1" balance tube would have very little value at near idle speeds. Thru the years of doing carb syncs I have noticed that if there is an imbalance it seems to always show up as far worse down in the very low rpms, but as you run the rpm up the imbalance seems to get better. I'm not sure how to explain this except that a small motion of the throttle cable near idle has a much higher percentage change of throttle opening than it does near WOT. Also, the "starting carb" circuit does not become inactive until rpms are higher. What this noodling is leading me to believe is that even if you go with a large balance tube, you should still occasionally do a carb sync at idle. Be interested in other opinions on this.

One more thing, I don't think there is much value in installing a balance tube that has an inside diameter any larger that the smallest hole thru the fittings on each end.

[Slyfox]

Larry,
you and I talked about this a OSH and it makes total sense. Now you need to go through how you did it. I can get the parts, but I be scared to drill out holes. right now I be busy putting on tires and brakes and all that good stuff. this one will be my next thing. :)

[mr bill]

I ordered my kit from Hal (ph: 775-934-5714) today. Worst case, if it doesn't pan out, you make an adapter to go back to original design. Hal's kit consists of modified manifolds, the 3/4 pipe thread fittings, new crossover tube and all required parts for installation. When done, you send him your old manifolds. This may help with a rumbling kind of vibration when throttling back to land (3300 down to 2500 rpm) at airspeed above 90 mph. All in all, this is a fun airplane.

[Av8r3400]

Here are the promised photos. I plan to get some better fitting clamps for the connector hoses. This is the proof of concept version.

13858

13859

13860

[WWhunter]

Is that smaller hose for a manifold pressure gauge? Looks pretty simple to do, might just have to do it!

[jiott]

For those of you considering doing this, I would highly recommend looking thru the postings on this subject on Rotax-Owner.com. Click on the forums and then use the search feature, using Balance Tube as the keyword and Titles only. It will bring up about 80 postings which is interesting reading. What I got from it is:
- Results seemed to be somewhat mixed, with mostly good results, mainly at the lower rpms.
- There were several ways of hooking up the larger balance tube and fittings-some with bad results.
- Most of those with good results seemed to find that they needed to limit throttle opening to 95% or else the engine would start to miss at further opening; nobody seemed to know why this is.
- There was some concern about the intake tuning with a much larger intake system volume (can have a large effect on race cars).
- Most agreed that with perfect carb sync there is no need at all for a balance tube of any size.
- No one claimed that the larger tube would eliminate all carb sync forever, it just makes it easier and more forgiving of inaccuracies. A sync at or near idle still seemed to be a good thing to do.
- Most seemed to be happy overall; a few were removing the larger tube and going back to standard.
- There is an Italian company, I think, that was offering a kit to do this. I believe they use a spacer block in front of each carb with ports in it for the larger balance tube. It moved the carbs back an inch or two I believe.

Anyway, interesting reading; people have been experimenting with this for a number of years.

Av8r3400, curious if you have noticed missing/stumbling at 95%+ throttle opening?

[Wheels]

I have been watching this with some interest. The issues with carb sync are numerous, and mostly induced by the mechanics omission or error. Sorry, but I'm the mechanic most likely to be "That" guy. A sticky cable or poorly routed cable, a cracked or worn carb boot, a sinking float, a mis-adjusted float lever arm, a worn screw, a dirty anything etc. etc. etc. and you have a rough running Rotax. Stay clean, stay tuned, know your carb theory and fly with ease. I always say if it isn't stock, its wrong, but ... I see the Zipper kit guys are having a ball! the jury is out after about 5 years of testing. I"m not into wrenching like I was when I first started and frankly I just want to go flying. The constant tinkering with my motor and worry about its reliability just seems to increase as I add any variation to what the original engineer intentioned. And Here I am modifying the holy Cr@#p out of a motor so that I can make it fit in my plane. The only difference is, I"m not trying to improve performance, I'm just trying to make it fit and run in the parameters for which it was designed. Seems like airplanes just BEG to be worked on.

[Av8r3400]

Yes the small line is for a manifold pressure sensor.

I had the opportunity to put an hour of touch and goes on this afternoon. The results are in: The modification works wonderful. The engine is smooth through all of the rpm range. My nagging rough spot from 2500-3500 is gone.

Full throttle seemed to yield about 100 rpm more at static, with no roughness or missing. There are a lot of nervous nellies on fly-Rotax that refuse to believe anything but the party line from Rotax. I like my engine. A lot. I do believe there are areas that can be improved. This is one of them.

I had synced and synced my carbs with analog and digital manometers and could never get the midrange roughness to go away. Now it's gone. Smooth idle to 1400 with no chatter.


Very happy with the mod. No regrets.

[efwd]

Nice, good to here things are working smoothly. :)

[Slyfox]

I heard you could now go down in your idle without vibration, like 1200 with no problems.

[Norm]

I like my engine. A lot. I do believe there are areas that can be improved. This is one of them.

Very happy with the mod. No regrets.

Keep in mind this mod is with a Zipper kit. Other results may vary.
I had seen this mod talked about on other forums and had considered doing this to mine before this thread showed up. Sounds like this is more fuel to add to the should do list.
I would also love to do the Zipper mod but dollars and cents don't allow at this time. Thanks for the report.

[WWhunter]

Yes the small line is for a manifold pressure sensor.

I had synced and synced my carbs with analog and digital manometers and could never get the midrange roughness to go away. Now it's gone. Smooth idle to 1400 with no chatter.


Very happy with the mod. No regrets.

Thank you sir! This is also the RPM range that I can't seem to get the 'shake' out. The roughness in that RPM, especially when I am over all the trees around my strip, is never comforting. If I can relieve this I would be a much more ardent Rotax supporter. It just bothers the bejeesus out of me when I am landing and the motor is rough.

[jiott]

This midrange roughness at 2500-3500 rpm that many of you talk about has always puzzled me. Most of you are experienced Rotax owners with many more hours than I, so I don't doubt for a minute what you are saying. What puzzles me is I don't ever see any sign of it on my 912uls with 500 hours on it now, and there are others out there with my same experience. Being a meticulous engineer type, I carefully synced the carbs mechanically and pneumatically from day 1 and have never had a roughness problem. In fact my sync hardly ever seems to move or get out of adjustment-the last time I needed to readjust slightly was 300 hours ago; it has stayed as perfect as I can get it for 300 hours without touching it.

I'm not at all trying to imply that those with this problem are not doing the sync carefully enough; there must be something else going on that my engine fortunately does not have. The fact that your roughness shows up in a narrow range of rpms seems to eliminate things like prop balance or a partially sinking carb float. Could it be your throttle cable is not smooth thru midrange? Have you totally removed the throttle springs so that the cable does not have a consistent tension thru the stroke? I am using the lightest McFarland springs.

I believe at these rpms is where the carb idle circuit makes the switch to the main circuit; could it be some deposits are causing the switchover to be rough?

The large balance tube seems to solve whatever the underlying cause is and that is a good solution and it also seems to give you a smoother idle at lower than normal idle speeds. The lowest I can get a smooth idle is about 1600 rpm.

[Rodney]

In doing some research on the subject of a balance tube I found the following on the Rotax Owners web site: An individual there wrote::

((Hope it's ok to post this: it's a probably a better explanation than I could write. I apologize if this is not proper etiquitte.))

"Built and added a 1 1/2"D. balance tube to my 912S and it is remarkable how much difference it made in the smoothness at low RPMs. This is not new technology and has been used on intakes and exhaust systems since the forties to smooth out low end vibration. It works.

I saw it advertised recently by an italian racing equipment firm called FLYGAS but they wanted something like $1200.00 for the package. (cost me about $100.00 to build). It does not increase HP, I don't know yet about fuel consumption. Its advantage is to reduce vibration and improve manifold airflow.

Here's what happens. Firing order on the Rotax, and most other boxer engines, is such that when one cylinder is firing, the other cylinder on the same side is on a compression stroke which means that NO air is flowing into that manifold. This inrush of fuel/air mix which has suddenly stopped has nowhere to go except through the balance tube and into the other manifold which is calling for air. The Rotax balance tube is woefully small for this task, even though it is great for idling. You can see this starting and stopping of airflow in the wildly bouncing of the vacuum gauge needles during carb balancing. Because all of this is happening in fractions of seconds, most of the air in the balance tube never actually goes anywhere, but bounces back and forth in the large tube. "

If this explanation is accurate, then the it appears that this modification is more a function of the style of the engine ( ie: boxer) rather than the displacement of the engine. The zipper kits only add to the cubic inches of the engine.

Got a text from Hal and Jay this morning. Cost of their conversion is $300.00 plus shipping plus your old manifolds. I have no connection with Hal or Jay, am just posting this for the information of our group.
Well, I did buy the 114HP Zipper Kit from Hal n Jay and I can report I'm very pleased with it.

Seem like every source I read on this subject reports a major improvement in idling performance of the engine.

Hope this sheds some light on the subject.

Regards
Rodney

[Wheels]

I like that explanation! it makes sense to me and I may be more inclined to make that mod ... after I fit the current engine to my plane. Yes, I know, I said the jury is usually out on a mod after about 5 years of field data, but, it sounds like we may be closer to about 40 years of data. I would like to do some historical research. ... after I fit my current engine to my plane. Did I say I was changing engines?

[Av8r3400]

For the record my modification is to a 1" diameter balance tube.

[kitfox2009]

Hi folks
This is slightly off topic but pretty close. Today I was changing oil and decdied to check the carb balance on my 730 hour 912UL. It has been at least 4 years and 400 hours since doing this. Runs smooth but let's check.
As soon as I connect the Carbmate the engine begins running rough and both carbs start flooding.
The carbmate operates well when vacuum or pressure is applied.
I t doesn't leak.
Remove it and engine runs fine.
I use a vacuum hose as a balance tube with a tee for manifold pressure gauge.
It's been this way for many hours.
Appreciate any suggestions
Cheers
Don

[mr bill]

I would try a dual manifold pressure gauge or a different CarbMate.

[kitfox2009]

A Carbmate does not allow any air to pass back the forth like a balance tube does. That`s why after syncing the carbs the engine always runs smoother when the Carbmate is removed and the balance tube reconnected.

The issue is that my engine does not even want to run unless the balance tube is installed!! What does this tell me?

[Av8r3400]

To balance th carbs you must disconnect the balance tube and attach the two vacuume tubes to the manifolds and the carb mate.

When you disconnect the balance tube you then see the actual balance of the now totally independent carb systems. Your carbs are out of sync, this is why the engine runs rough.

The engine will alway roughen if the carbs are out of sync and no balance tube it present to equalize them. This is the essence of this modification, to allow free breathing between the manifolds.

[kitfox2009]

It`s possible I did not explain the situation well.

The engine runs great with the balance tube installed. And has for many years and hours. Smooth at all RPM`s.

When I connect the Carbmate it floods and runs really rough. In fact it won`t even keep running.

I wouldn`t think the carbs would be that far out of sync when it has been running so smoothly all along.

You may be right, though! I will start from scratch with a manual sync and adjust it from there.

Thanks for the suggestion.

Cheers
Don

[Norm]

I will start from scratch with a manual sync and adjust it from there.

Thanks for the suggestion.

Cheers
Don

Don I have been in your plane and agree it runs smooth at all rpms. I would be hesitant to start from scratch with a manual sync. Check with the vacuum gauges first but when it runs as well as it does proceed with caution.
My 2cents worth.
Wait Canadian got rid of the penny.
My nickels worth:)

[Slyfox]

hook up the carb mates to the fittings in front of the carbs. you take out the little screws and hook up there. take the balance tube and block both ends. I used a straight hose between the two, when I balance I squeeze the hose off. if blocking the two manifolds yields a rough engine then the two sides are not close, need to go full throttle and check adjustment to the carb stops. then try again. if it's from hooking up the guage then go where I suggested and try again.

[kitfox2009]

OK, here's an update for anyone interested.
Today I started from scratch.
Checked all cables for free movement.
Set throttle at minimum idle and adjusted idle stops for .004 clearance.
Then closed the idle stop screws about two thirds of a turn.
Set the mixture screws open one and half turns.
Left the balance tube connected. Started engine and warmed up to operating temperature .
Carefully adjusted both idle stop screws to give a smooth running engine at desired RPM. I set mine at about 15 or 1600 with throttle pulled completely back.
Then I shut down and connected the Carbmate directly to the balance tube nipples.
Restart but do not idle the engine. Keep it above 2500 or so and run up to 3500 or 4000 . Adjust the cables to put the GREEN in middle at the most sensitive setting through these high RPM s This may take a few restarts and fine adjustments but it is worth it.
Remove the carbmate and reconnect balance tube.
My engine seems to now run smoother than it ever has and that is saying something!

After considerable time attempting to balance with the carbmate attached through ALL
RPM ranges I have come to the conclusion that it is more effective to get a smooth idling engine by adjusting the idle screws with the balance tube connected. My thinking is if the ide is SMOOTH that is more important than where the green light is!

Anyway did a few circuits in 30C Temps tonight and it performs super smooth through all RPMs.
This a long winded explanation that may not be by the book and may not work for everyone but my old 912UL likes it.
Thanks for comments especially from Mangy that really got me thinking.
Cheers
Don

[Rodney]

Ok. I had to try this balance tube thing. Been hearing n reading about it for over a year.

I have never been able to get my engine to idle smoothly much below 1800 rpm

I made one from 1 1/2" pipe just for the heck of it.

Wheels, I don't know if this is old technology or new technology, but IT WORKS.

I installed it this afternoon and started it up. Let it warm up, but hey when the oil starts out at 90 degrees it doesn't take long. Started backing down the throttle.

Finally hit the low speed idle stops at 1400 rpm. Super smooth. I can't hardly believe it.

Am going flying this evening before the sun goes down, but for now am really pleased.

Rodney

[Slyfox]

I'm in the process of redoing my fox. I'm turning it into Badassfox. I installed the Desser 27 1/2 tires. I'm installing the T-3 spring set for the tailwheel. currently the 8inch wide tire from Matco is being built, should be here next week. I put on the balance tube last night and it is real nice. flew it this morning and yes much less vibration. the stick is nice and vibration free. I like it. coming into land there was no vibration. I generally come in real short and pull back to about 500rpm and I didn't feel anything this morning, I thought the engine quit, but it was running. now those tires. makes the look on the plane as well as ground ops is way cool. I sit up much higher. ha, watch out I'll eat you up. haha.http://www.teamkitfox.com/Forums/album.php?albumid=493

[WWhunter]

Rodney,
Did you drill out and retap the manifolds, or did you put and adapter to the factory port?
Pictures would be helpful! :)

[Rodney]

I used the existing ports. I wasn't sure that would work,but I thought I would try it and let the engine tell me if it liked it. So far it seems to work fine. Am headed to the airport shortly to fly. Will pull the cowl n take some pictures

Rodney

[Slyfox]

If interested, you can go here and look at my setup for the balance tube.
http://www.teamkitfox.com/Forums/album.php?albumid=494

[PaulSS]

http://www.rotax-owner.com/en/rotax-forum/4-general-discussion/6236-auto-synchro-carb#16469

Just chucking this out there and don't know if it's useful or not. I'll be getting the 912iS, so none of those nasty carb things for me :D

[mr bill]

What if you used a piece of 1 inch water or fuel rubber hose, with no metal tube?

[Wheels]

I don't actually know why the tube is metal except for the nipple that is on some of the metal tubes used for MAP. My old tube is metal and has no Manifold Pressure attach point. So I think If I had a rubber tube, it wouldn't matter. But I have a question.
If a person used a large diameter crossover tube and simply adapted the size for the existing manifold fittings without making them larger, As Rodney did, why wouldn't that do the same as enlarging the fittings? I understand that the air in the crossover tube is largely static, so the benefit is not in "bernoulli, or venturi, principle but rather from the larger mass of air in the tube giving a "buffer" area to the molecules in transit from acceleration to static. Have I been watching to many re-runs of Star Trek, or am I actually ... close?

P.S. My mom used to say "better to be judged a fool than to open your mouth and remove all doubt." ... I never listened to her. Maybe I should have.

[Av8r3400]

What if you used a piece of 1 inch water or fuel rubber hose, with no metal tube?

The Aluminum tube is lighter, that's why I used it.

[Rodney]

13966

13967

13968

Here are some pictures of my balance tube.

The main tube is made from Schedule 40 1.5" PVC.

Bent the tube to match the proper angle (approx 60 degrees) so the 1/2 rubber hose's would fit the existing 3/8" fittings in the intake manifold.

Then I found two 1.25 x 3/4 schedule 20 inserts. Turns out the 3/4 threaded fitting fits the 1/2" fuel tube I used.

13969

13970

Next, I turned the inserts down to fit inside the Schedule 40 pipe. I couldn't find Sch 40 fittings at my local store so had to use the Sch 20 size and make it fit.

The main pipe is 11" long - then took the pipe and fittings and hose out to the airplane for a final fitting before gluing it all up. Used regular PVC cement for the inserts and screwed the hose into the fittings with clear GE Silicone seal to secure the hose to the inserts.

I think that this could all be built out of Schedule 20 material and instead of bending it, there are 60 degree elbows available. So, maybe two short pieces of 1.5 OR even 1.25 inch PVC with a 60 degree coupling and two inserts with a threaded 3/4" female threads might work. Haven't built one, so am being a little conservative here.

After a 1 hour flight yesterday, I landed, took off the cowling and I could hold the balance tube with my hand. That tells me it isn't getting too hot.

So far, I'm really pleased with the setup. If anyone sees a serious problem, please let me know. I don't know the long term affects of heat on the plastic, but I know it took about 300 degrees of heat to bend the main tube, and it's not getting anywhere near that hot under the cowling. I didn't have to modify the manifolds, just used the existing ports.

I did go back this afternoon and used Kitfox 2009's procedure for balancing the carbs. I hooked a 1/2" fuel tube from one carb to the other, and using the idle adjustment on one carb, got the engine as smooth as I could. Then hooked up the carbmate and balanced at 3500 rpm.

I have about a six hour flight tomorrow, so will report back on how things went. Also, and I haven't really figured this out yet, I can now use my Hacman system to lean the engine. Before the balance tube mod, running anywhere from 5300 to 5600 rpm, as soon as I cracked open the leaning knob, the engine got rough. Could not get any temp rise in the EGT's or decrease fuel flow. A quarter turn was too much. Then, on my first flight after installing the balance tube, I can now lean the engine. I've been trying to get my EGT's up in the 1400 degree range to seat some new oil rings, and I can finally do that.

So far, this has been a really good improvement to the way my engine operates. Nice slow idle (1400 rpm) and I can lean the engine at cruise :D

Rodney

[Paul Z]

Here is another picture of Rodney's install. It shows the cross over tube really well.

[kitfox2009]

Hi Rodney
Think I might experiment a bit with this as well. Did you just glue in the nipple for your manifold pressure gauge?
Don

[Rodney]

Don. I found a plastic nipple that had a 1/4 pipe thread on it. Drilled a hole in the balance tube, screwed in the nipple and sealed it with silicon seal.

Think I got it at a local farm supply store, but I'm sure there must be other sources too

Thanks Paul for posting a better picture. Appreciate it

Rodney

[kitfox2009]

Update.
Well today just for fun I made up a larger balance tube for my 912Ul.
Material required is a 14 inch length of 1 inch heater hose plus two 1inch pieces of 3/4 inch and 2 approx 3 inch pieces of 1/2 inch. 4 hose clamps and 1 Mini hose nipple if you have a manifold pressure gauge.
Assemble as shown. I used the adhesive in the photo.
Have not flown yet but runs smoothly through all RPM ranges on the ground test. I realize this only increases the balance tube to 1inch diameter but might be interesting results. Ran smoothly before but maybe even better now!

Cheers
Don

[Rodney]

That looks great:D. Seems like one inch or greater does the job. Sure is nice to sit on the ramp with the idle set at 1200 to 1400 with no shake rattle n roll.

Rodney

[kitfox2009]

Hi Rodney
Just did a 2 hour low level site seeing trip around our "smoky" valley this morning. With all the fires this year our area is nick named "Smokanagan" now.
Anyway I have a feeling that the new low budget carb balance tube provides a smoother operation through all RPM ranges. Quite impressed. It`s so easy to make up and the cost is zero.
Have fun
Don

[efwd]

Hmm, why do I hear so much about trusting the engineers? Why did they not go with a one inch diameter or why have they not gone back and changed the equipment yet?

[kitfox2009]

Guess this is why USA calls our a/c type "Experimental and we Canadians call them "Homebuilt". Nice,EH!!

Have fun
Don

[Rodney]

Efwd. As Travis Tritt said in one of his songs. "It's all about the money."

Also, seriously, just because they built it, don't assume they know everything.

Too, I've been in meetings where a "junior" engineer proposed a perfectly good inexpensive solution to a problem, only to have the idea rejected by some manager who thought he knew everything.

Could be lots of reasons they didn't put one on.

Rodney

[kitfox2009]

G'morning Rodney
I believe the title to this thread is a bit misleading. I strongly believe we should all sync the carbs on a regular basis BUT unless you are able to keep "the green light in the middle" through all RPMs from idle to full throttle (It was always a compromise for me) then maybe a modification to the balance tube might be helpful.
Going to a breakfast flyin this morning. We'll see how another 2 hours air time goes.
Take care
Don

[Rodney]

Kitfox 2009

Ya I guess we kind of did the "thread creep" thing.

Sure hope you have a great flight n breakfast.

Still, a lot of valuable info in this thread n a lot of fun looking at the
different ways to build a balance tube. Like yours the best. Simple n very cost effective to build. And it works.

Maybe some day Canada n the FAA will get it worked out so those of us on a drivers license medical can visit you guys in Canada 🇨🇦. Sure would be nice

[WWhunter]

Yeah, good luck with that Rodney!!! I wrote to the RAA and AOPA about this and the answer I was given at the time wasn't very promising. I'm not sure if there are many of us on the sport pilot that care to venture into Canada. I was flying on floats for a few years and wanted to make it up there.

[jiott]

Regarding the bigger balance tube: There is such a thing as tuning the inlet manifold system volume for max performance at high power levels. Do you suppose that the Rotax engineers may have done this? Has anyone with these larger balance tubes done any kind of testing to make sure you have not degraded your high power performance? Just asking.

[kitfox2009]

HI Jim
I never have been able to have the carb mate green light in the middle through all RPMS from idle to 4000.
So this is what I did..
I installed the over size tube and adjusted both idle stop screws to give the smoothest possible idle at about 1600..I think the 80 horse engines are smoother at these low rpm.
Then I shut down and hooked up the carb mate.
Restarted and immediately ran up to 3000,4000. Adjusted the linkages to center the green dot at the most sentsive setting. It is steady at all RPMS above 3000.
Shutdown and reinstall oversize tube and go flying.
Made a breakfast trip this morning and came home direct over the top at 8500. Ran great. Pretty hard to tell is it was any better than normal. At that elevation I usually adjust a prop and throttle for about 5000 rpm and 27 inches. About 75%power according to rotax charts.The ias is only about 95 mph. We were flight training with very cute CASARA spotter so was not in a hurry!

[kitfox2009]

Sorry Jim
That probably didn't answer your question. Not sure if you would call that "testing".So take this idea and results as a very seat of your pants summary. It may not help at all.
The flight training was a lot of fun though,especially for an ole guy.
Cheers
Don

[mr bill]

The Hal Stockman cross over tube conversion is done and it works as others have said. I added 1/8 NPT hole to install a nipple for CarbMate pneumatic sync. I also used a one inch coolant hose across the manifolds. I did the mechanical sync as carefully as I could and when the CarbMate was hooked up, the carbs were in the green at 3500 rpm and at idle with no further adjustment needed. The rough spot between 2500 and 3500 rpm is gone.

[Wheels]

I'm pretty impressed with what you guys are seeing. I imagine a cracked tube or broken seal would act like a carb boot failure. Am I right? Just wondering what the failure chain looks like before we go there. I don't see any extra links in it from the old aluminum and rubber hose assembly that came with the engine.

[Pilotharry]

Fellow Flyers,
I finally did the enlarged balance tube mod on my 912UL over the last couple weeks and have flown for about 2 hours now and can firmly confirm that:
-It runs very smooth over the full throttle range
-No change in my performance numbers except that it feels lot smoother overall. (it feels more powerful)
-Cold start is like a car with no shudder or vibration.

Not enough time to say if it changed my fuel consumption but I would assume it will run more efficient just because of all that smoothness and both banks running hand in hand.

I will remember to take some pictures next time but all I did is that I drilled and taped the existing balance tube holes to 3/8"NPT and screwed in the 3/8" 45 degree male to female fittings in them. I drilled out the fittings ID to remove any casting imperfections and increase the ID a bit.Then I took a 3" long 3/8" NPT nipple and cut it in 2 halves. Screwed each half into the fittings added. Connected both sides with a 9/16" Vacuum hose from PartSource. All fittings were thread locked on final assembly.
Feel free to contact me with any questions and I will consider doing all the machining and supply parts as kit for anyone who would like to give it a try for something around $300.:)

Happy Skies,
Harry

[DesertFox4]

Thanks Harry. I have recently talked to a couple Rotax powered Kitfox owners looking into this modification.

[Pilotharry]

Here are some photos of the setup before mounting on engine. I will get more of the complete setup when I get a chance to visit hangar and open the cowl (its -19 C outside)

[jiott]

I would really not want to put those iron fittings into the aluminum manifold. Lots of ways to use aluminum fittings.

[Pilotharry]

Original fittings in the manifold are not aluminium. So, whats your reasoning for not using these ones? I can easily replace them with other 'recommended' material.:D

[jiott]

I'm pretty sure my original stock fittings are aluminum. Galvanic corrosion with dissimilar metals would be an issue here. Cad plated steel fittings may be better, but your unplated water pipe fittings would be a worst case in my opinion. Aluminum fittings are readily available, light weight, and look much better also. Just throwing my opinion out there.

[Av8r3400]

The small OEM fittings are definitely aluminum.

Aluminum 90º elbows are readily available through Jegs or Summit Racing. That's where I got mine, 3/4" NPT to 1" Tube.

[Wheels]

Sorry if I"m bringing my "North Idaho Hick fix" to the party but I didn't change the fittings.
I clamped a hose to the fitting and then added a rubber hose cross tube with a larger Inner Diameter so that it fit snuggly over the outer diameter of the short piece on my elbow. No drilling, no dissimilar metal, and the result is the same. A larger space in the crossover tube, period. Anyone see where I have gone wrong?
I'm 60 hours into it and its perfect all through the range on my 912UL

Cant resize the pix on my mac, i'm working on it. If you want pix send me your email.
Jayandpamela@yahoo.com

[rv9ralph]

Off Topic Warning..

Wheels, check the Mac App Store for Multiple Image Resizer. Quick easy, free and works.

Ralph

[Wheels]

Thanks. Here's a pic.

[Pilotharry]

I'm pretty sure my original stock fittings are aluminum. Galvanic corrosion with dissimilar metals would be an issue here. Cad plated steel fittings may be better, but your unplated water pipe fittings would be a worst case in my opinion. Aluminum fittings are readily available, light weight, and look much better also. Just throwing my opinion out there.

Just for healthy discussion please -
Checked and the original fittings are not aluminium..at least on mine. If the difference of material is a big concern here with galvanic corrosion in this environment/application, then how about the two bolts on each manifold threaded straight into the manifold that hold the carb sockets on? Second would be the main 4 bolts that hold the manifold on cylinders ..that are not aluminium and are screwed into aluminium cylinder heads.

I am still on the look for aluminium fittings but all I could source for now have smaller ID on the tube side. I will keep looking.

[jrevens]

That's a good question and good points. I believe that there would be very little potential for corrosion with galvanized steel fittings and the cast aluminum manifolds. First of all, it is a generally very dry location and additionally the threads will probably be sealed from moisture by some kind of thread sealing compound. Secondly, the 2 metals are very close together on a galvanic series chart. Aluminum and plated (cadmium or zinc) steel are in contact with each other all over the place on most aircraft engines, including fasteners, fittings, etc. Stainless steel however would not be as good a choice from a potential corrosion standpoint.

[Pilotharry]

Thank you John for your valuable input. Yes, the joint is completely sealed with thread lock.

Happy skies!
Harry