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Zenoah G23 stopped sucking fuel

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Old 01-12-2018, 10:19 AM
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karolh
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Default Zenoah G23 stopped sucking fuel

I ran this my engine in my CG Extra a couple weeks ago and behaved as usual, perfectly. Decided to give it another run before going to the field and lo and behold it refused to start but would run quite readily on a prime through the carb throat. On removing the cowl I saw at the feed line was empty so decided to check the tank and found nothing out of place but redid the plumbing. I also took the opportunity to give the carb a through clean and gave all the diaphragms and check valve a proper inspection and everything seemed okay but after doing all of that the problem still exists. The engine will happily run for a few seconds on a carb throat prime but refuses to pull fuel from the tank to run on its own. All carb bolts are tight and the pulse hole is clear so what am I missing here ???
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Old 01-12-2018, 11:56 AM
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Sounds like you covered the bases. Maybe a hole to small to see in the diaphragm? Is there a fuel filter in the system? Is something stuck in the clunk?

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Old 01-12-2018, 12:31 PM
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You sound like you know what you are doing. But you do know a dry carb sometimes doesn't want to pump right? When I re-assemble a carb, I always flood it with WD-40 first. Other thoughts...

Sometimes the little filter screen in the carb can be clogged but not look like it.

A cracked or otherwise leaking carb insulator?

About the only other thing would be an air leak somewhere in the supply. You might try hooking a new, separate fuel supply directly to the carb and see if it will pump with the onboard system by-passed.

If all indeed is in good shape, the only other thing would be if the engine developed a secondary compression leak. Possibly one of the crank seals.

AV8TOR

Last edited by av8tor1977; 01-12-2018 at 12:44 PM.
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Old 01-12-2018, 02:12 PM
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karolh
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Originally Posted by av8tor1977 View Post
You sound like you know what you are doing. But you do know a dry carb sometimes doesn't want to pump right? When I re-assemble a carb, I always flood it with WD-40 first. Other thoughts...

Sometimes the little filter screen in the carb can be clogged but not look like it.

A cracked or otherwise leaking carb insulator?

About the only other thing would be an air leak somewhere in the supply. You might try hooking a new, separate fuel supply directly to the carb and see if it will pump with the onboard system by-passed.

If all indeed is in good shape, the only other thing would be if the engine developed a secondary compression leak. Possibly one of the crank seals.
AV8TOR
In answer to your and other questions :
New carb screen installed
Using an in perfect condition Ralph Cunninghan G10 insulator block
Tank has been completely re-plumbed with new felt clunk and leak tested.
The fuel system has no in-line fuel filter
Engine has excellent compression, typical of most Zen engines.
To ensure that the carb was not dry I flooded it with fuel during the re-assembly.

I plan on acting on Daveopam's suggestion and replace all diaphragms just in case. I made a similar post on that other forum hoping that Ralph Cunningham (Antique) the Zen guru will chime in as he does not visit here anymore.

Last edited by karolh; 01-12-2018 at 02:15 PM.
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Old 01-13-2018, 12:45 AM
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I wasn't speaking of combustion chamber compression, I was speaking of "secondary", or crankcase compression. If an engine has a crankcase leak, it will run when primed, but not pump fuel. This is most often caused by failed/leaking crankshaft bearing seals. Being that this is a G-23, it is an older engine and the seals might be suspect.

The only other things not totally covered would be those diaphragms, or a sticking inlet needle. I once had a needle tip deteriorate, and it would stick in its seat. That one drove me nuts in finding the problem.... Luckily though, I always have a bunch of carbs lying around and can always throw a different carb on an engine to help narrow the possibilities of a vexing troulbleshoot.

And Ralph is a great all around engine guy. It will be interesting to see what he says.

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Last edited by av8tor1977; 01-13-2018 at 12:57 AM.
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Old 01-13-2018, 09:03 AM
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karolh
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Latest update, problem solved. Acting on Dave's suggestion I decided to replace all diaphragms starting with the one on the metering side of the carb. After removing both the diaphragm and it's gasket and separating them I noticed a small amount of a sticky oily like substance on the very edge of the rubber diaphragm and thought it quite strange.

So as to equalize the pressure within the Extra's cowl during flight I had soldered a small brass nipple in the centre of the cover plate and attached a piece of tubing from it into the fuse as a fix which worked great. Just on a whim I tried blowing through the tube which was still attached to the nipple and found I could not. Removing the tube from the nipple exposed the problem as it seems that over the 15 years of the model and engine association some of the same sticky substance noticed on the diaphragm, most likely an accumulation of fuel fumes had completely blocked the hole in the nipple. Cleaned it up and with new tubing all is well once more.

Thanks to all who contributed to my post which certainly assisted in helping to find the problem which for awhile had me pulling at my few remaining hairs.
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Old 01-13-2018, 10:02 AM
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Great news!!
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Old 01-13-2018, 11:56 AM
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Good deal!! (What an odd and rare problem. Good catch!)

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Old 01-13-2018, 02:14 PM
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Originally Posted by av8tor1977 View Post
Good deal!! (What an odd and rare problem. Good catch!)
AV8TOR
Odd and rare indeed which just goes to show that in problem solving nothing should be taken for granted as not in my wildest dreams would I have thought of that nipple being blocked. Part of the good news is now I do have a much better understanding of just how these carbs work.
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Old 01-13-2018, 07:22 PM
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Always a good thing! I have always felt that it is extremely important to fully understand whatever you are trying to repair. You started out with clearly above average understanding and analytic ability.

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Old 01-15-2018, 08:05 AM
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karolh, I followed this thread but never responded. From reading your posts here over the years I didn't think I had much to offer that you didn't already know. Good to hear you found the problem and I'll put your cure on my list of things to check when engines start acting bad and everything I check looks good.
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