Single vacuum gauge to sync 912 carbs?

[avidflyer]

Hi all, I got the used 912 running on the Kitfox 4 rebuild I'm doing, and didn't have a carb mate. Got to thinking, and remembered I had this Penski vacuum gauge, plus some 1/4" shut off valves. I hooked separate hoses to each intake manifold, put a cheap snowmobile fuel valve in each line, teed them together with the other leg of the T going to the vacuum gauge. I would open one valve, with the other one shut, get a vacuum reading, and then do the same from the other side and get a reading. Fairly soon, I was within 1/2 a pound of vacuum or less on each carb at the same time. My question is, would I have been able to do a better job with a carbmate or for that mater and electronic carb balancer? I do know it's running pretty smooth. JImChuk

[Dave S]

Jim,

Your method is solid.

Any instrument system where more than one gauge is used introduces the potential for analytical variation between the gauges. A single gauge rules out that factor.

Throw in electronics and you have additional variables to control (temperature/resistance/calibration/etc)

Famous engineering quote...."The price of reliability is simplicity"

I'd do it by your method in a heartbeat.

[jiott]

I agree, its better than the two gauge method. I assume you blocked the crossover tube even though you didn't mention it.

[109JB]

Never balance the carbs on a 912 but for snowmobiles, motorcycles, etc. I have always used the 2 bottle method. This is probably because the first time I had to balance carbs was on a motorcycle when I was 16 yrs old and dirt poor.

https://www.youtube.com/watch?v=7D_f05_a74Y

[Dave S]

Hey John,

That's perfect! Hard to get anything more accurate than that. Nothing wrong with being frugal so a person can spend more on avgas.

Probably one of the best uses of an old corona bottle I have seen yet.

Do you happen to recall what size stopper fit the neck of the bottle?

[109JB]

Hey John,

That's perfect! Hard to get anything more accurate than that. Nothing wrong with being frugal so a person can spend more on avgas.

Probably one of the best uses of an old corona bottle I have seen yet.

Do you happen to recall what size stopper fit the neck of the bottle?

Wasn't my video. Just one showing the method. I normally use 20 ounce plastic Coke/Pepsi/Dr.Pepper.... bottles and then drill holes in the cap to tightly fit the hose I'm using. Hot melt glue them to a scrap of wood and away you go.. When done I usually keep them around for the next time, but wind up throwing them away before needing them again.

The important thing to note is that the tubes that go to the engine end right inside the bottle and there isn't enough fluid combined to fill a single bottle. That way once all the fluid goes to one side it can't get sucked in the engine. Air just starts bubbling up through it.

[Dave S]

John,

OK - I hear that - do you happen to know how much vacuum a snowmobile engine pulls vs a 912?

I am wondering if a plastic bottle could hold up to the idle vacuum ( or negative manifold pressure) on a 912?

I'd be reluctant to use anything other than a glass bottle on the 912 :confused: Could be a bit of a mess if the plastic collapses and sucks the liquid into the engine.

[avidflyer]

I agree, its better than the two gauge method. I assume you blocked the crossover tube even though you didn't mention it.

Actually, I removed the cross over tube and connected both hoses to the intake manifolds where the crossover tube would go. JImChuk

[109JB]

John,

OK - I hear that - do you happen to know how much vacuum a snowmobile engine pulls vs a 912?

I am wondering if a plastic bottle could hold up to the idle vacuum ( or negative manifold pressure) on a 912?

I'd be reluctant to use anything other than a glass bottle on the 912 :confused: Could be a bit of a mess if the plastic collapses and sucks the liquid into the engine.

No idea really on the vacuum. Never "measured" it. All I can say is I used this method with the plastic bottles on twin cylinder 2 stroke snowmobiles and a variation but still plastic bottles on 4 carb 4 cylinder 4- stroke motorcycles and didn't have any issue. I will say that something like the really thin water bottles seem like they would collapse.

[avidflyer]

Just got a chance to look at the video, and I think I may have seen it before but forgot about it. I think I will build one, would be hard to get more accurate then that. The vacuum gauge will get me pretty close, but this should get it right on the money. Thanks for the video link! JImChuk