Carb sync? Never again.

[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.

Crossover.jpg

Crossover2.jpg

Crossover3.jpg

[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?