I am running a Zenoah 41cc that wa taken out of a back pack blower. The compression seems good, it takes about 15 seconds to bleed off pressure of a TDC piston in a fuel mix wetted cylinder. The best carb is a walbro HDA. I have tried several other carbs. The HDA, and an HDB have had rebiild kits installed. There is no air leak around the carbs. I have tried all possible permutations of high and low needle valve settings. The ignition is CH set at 31 BTDC. It seems to like a smaller gap on the plug, say 0.020".

The blanka-de-blank thing won't idle a less than about 2500 rpm, on a good day. Unless I install brakes on the airplane this is a problem. It rolls along at about 5 mph at idle. As I drop down to about 25-2600 rpm it just stops dead. I can fiddle with the low speed ajustment and find a best spot whers it neither loads up or leans out but it still wont go slower. I can set it up to run properly from high speed, about 7500 with a 20x8 and all the way down to 2500 without getting rich or lean. If I chop the throttle suddenly from high speed it does do the hitchie, but that is pretty normal for a carb wihtout a check valve.

Would it be a good idea to mess with the timing? That is a pain because the magnent in on a disk on the PTO. I have to disassemble a lot of stuff to get at it.

Got any ideas??

As long as it runs OK on the top end, your timing is probably OK.

I'd try a different carb?

What happens if you idle it down to about 1500? does it shut off? If so try opening the low speed needle or you may have to clode the low speed needle a turn or two. If the low speed needle has no effect on the engine at all then the passage in the carb is blocked. using compressed air you may be able to blow out the blockage. If it won't idle lower then 2500 then you have a air leak.

If you had the throttle plate out and assembled it backwards, you may get a reaction like this

Check all your gaskets and the crankcase for a vacuum leak

All the carbs I tried did the same. Some did not run at all. I tried a ZAMA that once ran good on another engine, no good. I have another smaller walbro that idles as poorly and cuts the top end by about 800 RPM. On a whim I tried some tiny walbros, like the ones tht come on Ryobis and featherlites, did not run much at all. Throttle plate equals butterfly?? I have read about filing a littlle notch in the butterfly to enhance idle performance, what 's the deal with that?

The jug is glued down with high temp silicone gasket goop as are the crank case parts. The timing disk on the pto is a bear to get off. The mount stand off is a convaluted thing. Is there a way to actually check/find leaks vs taking it all apart and re-sealing everyting? Could I put soapy water on the case and jug seams and put air pressure down the spark plug hole?? Make a plate to cover the exhaust or carb port and put the piston in the right sport then pressurize the case?

Your leak is likely through a shaft seal on one end or the other

Being as you have tried all these carbs and the results are the same, that pretty much eleminates the carb?

I wouldn't bother with the notch for now.

FWIW
Those back pack blower engines are known to get clogged exhaust ports and /or mufflers and won't idle right.
I've had a few of these with this problem.

That will work just fine, blocking the exhaust and intake, it wont matter where the piston is.

It does not leak. I made up some plates to cover the intake and exhaust ports. I fitted one with a shrader valve. I pressurized the case and jug to 50 psi. I have zero bubbles when I submerge the whole thing in a bucket of water.

The muffler is the bottom of an aluminum gas cylinder. It was machined up all pretty. It has a round disk with a step on it that bolts to the engine. One bolt holds the disk to the can. The can has a hole in it to let the exhaust out. I stole the idea from somone who makes such a "puck" muffler for Ryobis. It is not blocked. I increased the size of the hole until I got full RPM.

I am thinking of maching an intake manifold out of aluminum bar stock. At least I will know everything is tidy and right. The old plastic unit is a bit grungy.


Any more ideas??

I do not think I would use over 2-3 lbs of air pressure to check for leaks...you could blow a seal. Use plenty of soapy water to find leak. It will show up as bubbles will form at leak. You are lucky if you did use 50 psi not to blow seals. Maybe instead of trying various carbs...you should try a easy to mount and adjust RCecell ignition. A different advance curve and with the ability to change timing more easy...you will end up with a very good engine. Capt,n

The carb must be insulated from the cylinder heat

Not in all cases....look at photo. Capt,n http://www.toni-clark.com/english/mo...untanaS140.htm

I made an aluminum manifold for a Kiortz fourty something cc and it worked great. Maybe the gaskets proxide enough heat insulation? That engine was a fine performer until it hit the ground at 100+ mph. The crank is bent and I can't figure how to straighten it. I have beat on it and pried on it to no avail. Anyway, back to the Zenoah.

Could it be that the oil seals on the ends of the crank are old and hard? I was thinking that if tht were so when it is running they might not snap back fast enough. If the bearings were a little worn, they seem tight, but, if they were; a little imbalance, would open a gap in the seal that I don't see in the static test.

http://www.toni-clark.com/english/mo...untanaS140.htm

That is a beautiful airplane. Two things strike me as odd. The exhaust mainifold is right next to the ignition box and is touching the spark plug wire, you would thing that would casue the obviuous problem. The throttle serovo is in the engine compartment. I was under the impression that was begging for RF interference. That guy obviously knows what he is about, and it works so, who am I to critique it?

The double ended cranks usually twist where the crankpin is pressed in

The static test should work and prove the crankcase is sealed or not.

I saw the idea to make a variable timing gadget. That is not a viable option here but, I have done it in the past. I have this engine out of the airplane so I can play a bit on the test stand. The timing disk came off more easily than I expected, I used a heat gun to expand it. If I were to fuss with the timeing which way should I go from the standard 30-deg BTDC to smooth out the idle?

28 degrees is the usual sweet spot

I've played with moving the timing around and there was not much difference between 25 and 32

Start it up and spray carb cleaner at the seals at idle, if the rpm changes you have a leaking seal, if not then look elsewhere.
Put the stock ignition back on and see if that solves the problem.I don't trust aftermarket ignitions on zenoahs.

It seems it was a carb issue. I filed the butterfly such that it closed to cover the mid range jet. The fit around the barrel of the carb is very tight but, still moves freely. I also closed the little punch hole in the butterfly. That did the trick. I can get it to chug along at 12-1300 rpm. I now have an issue with it wanting to flood. That is probably because I messed with the pop off valve arm. It is more difficult to start but once it is going it is good. It may just that I have to adjust my procedure to overcome that.

In walbro repair manuals they refer to the "pop off arm" and "pop off pressure" etc

The "Pop Off Arm" referred to is part of the zero pressure demand regulator.

Another way to think of the pop off valve operation for those of us who do not have Walbro pressure testers . . . .

It lets no gasoline pass by the needle and seat unless there is a slight vacuum on the diaphragm coming from the throttle area of the engine.

This allows only gasoline that is needed for the engine to pass the needle and seat

I suppose if you put enough fuel pressure behind the needle, it could eventually release and pop off but this is not part of the normal "in operation" of the carburetor.