AA5BY

My favorite plane has a 35cc mounted inverted. All is fine except the variation of mixture between upright and inverted flight. The carb as mounted has the metering circuit plate down. It is a rear carb engine running a Vess 20A about 7500. It starts easily cold or warm, transitions fine, makes plenty of power and has never dead sticked.

During inverted flight, it runs rich. Low speed needle is as lean as it will go without compromising transition. High speed needle is adjusted until no more gain in rpm. The HS needle is not as sensitive as most gas engines.

My thinking is that the metering circuit is where to look. That gravity weight of the metering diaphragm though subtle, is just enough to cause the mixture differences. That while upright, the metering needle has a hair more closure tension than it does inverted.

Does this make any sense? Any one else experience similar? Is a fix to adjust the metering arm?

Charley

My Feedback: (2)

Get another spring; then stretch the one in the carb. See what effect that action has. If it messes things up you can replace the spring. Beats bending the lever.

CR

All Day Dan

My Feedback: (5)

Try it without the cowl on to see if it has any effect. Dan.

w8ye

My Feedback: (16)

You need a higher pop off pressure in your metering circuit so gravity has less effect on it.

AA5BY

I think there is some agreement that the metering circuit is suspect and the suggestion to use another spring stretched slightly is good advice... I'll give it a try.

If the cowl could cause a different atmospheric reference between upright and inverted, removing it for a test might be helpful but I can't imagine how it could do that. If it were not ventilated properly it could cause variations of mixture at various engine rpms, but that is a much different thing than the issue at hand.

texasporty

My Feedback: (7)

Interesting read. Let us know if the metering spring works. I have very similar problem...only it is reverse of yours, but same symptoms. By the way I flew mine with and without cowl and it didn't change either way.

WRK

My Feedback: (2)

Do you have a vent line from the carb. to the canopy or something equivalent?

texasporty

My Feedback: (7)

Yes, diaphragm cover has a fitting attached to fuel tubing that terminates inside a film canister(not sealed air tight)...this change had no effect on problem.

Tony Hallo

Some random thoughts. Can you rotate the carb 90 degrees? or is it possible you have a puddling problem?

texasporty

My Feedback: (7)

Not impossible...but would require machining or making the pulse line external. I do believe the effect would be opposite.

captinjohn

Try something like flying inverted for more testing. Fly long lines while climbing and long lines slightly diving. Try some big inverted 8,s....see what this does to engine sound of going rich. Is you fuel tank mounted real low in airplane? I could see where a high angle of attack when inverted could cause a real different airflow over cowl also.

Capt,n

av8tor1977

My Feedback: (6)

I agree with trying this...

The problem is more commonly seen on engines mounted with the cylinder upwards, and running rich in the midrange. Fuel accumulates in the crankcase while cruising around upright, and when you go inverted, the accumulated fuel enters the mix and it goes rich. However, there is still the possibility at least that there is some kind of an air flow difference in your case between upright and inverted that is causing the mixture difference for one reason or another. Flying without the cowl might address this....

AV8TOR

captinjohn

Hi AV8TOR, we are now in Aripeka Florda at a RV Park. I did bring one airplane too fly & engines to work on. I know 2 guys from near home. One flyer & one fisherman....and I have not had a chance to do either. Good too rest after a long drive & getting things in place + washing the RV ect. Best Regards, Capt,n

Tony Hallo

www.flyinggiants.com/forums/showthread.php
Hopefully you can visit this link and do some more reading.

Rudolph Hart

Most people including myself should never "tune" a gas engine that was previously running well,have you filtered your fuel well and cleaned your screens??

Tony Hallo

If you go the FG Site and seach for Kcorb in the engine forum, you will find more information regarding the weight issue and changes in tuning.

ameyam

err.. I need a technology primer here. Am fairly new to gas and only exposed to a dle20. I did notice that fuel wud pour out of the vent when the airplane was held nose down but nothing much else
ameyam

w8ye

My Feedback: (16)

You didn't run your vent line correctly. Externally it must loop to the back of the tank and return to the front.

karolh

Some very informative reading here.

Karol

captinjohn

Karolh....look for a PM....my laptop is not working. In Florida now John Capt,n that is !