I have a Thunder Tiger .42 that belongs to our club. It is old and has been through a lot - when I got it the carb mounting lugs were broken off and it was heavily varnished. I took it apart and cleaned it using the24 hours in antifreeze and a crock pot trick. Looks like new! I tried to get a new carb from Ace Hobby through my LHS - no joy. I finally got one direct from Ace after a lot of negotiating, but it too was used. I replaced all gaskets.
Now to the problem. I ran the engine up today and found it has a great idle. It starts very easily. However, there is virtually no needle valve control - I can back it out all the way with very little effect. If screwed in beyond less than four turns out, it quits at half throttle or more. I can feel a fine spray of raw fuel on my hand when it is in back of the engine. I am running muffler pressure, and Byron's 15%.
When it does run at full throttle, it cannot four cycle - it just two cycles with a lot of power and little smoke. After a short period it clearly slows and then stops - I think this is because it is so lean that the engine is getting too hot to run.
Everything points to a lean condition at full throttle. Air is getting in somewhere besides the thrttle barrel I guess. Is it possible that the crankshaft seal is shot, and thus is sucking air in there? Why is the needle valve useless? Any thoughts would be appreciated - it looks so good, I really do not want to abandon it.

Sounds like the front bearing seal has given up the
ghost....fuel mist coming out of the engine. If you
replace it....get a bearing with a rubber seal on it.

Dave.

it could be leaking air around the needle. try some fuel line around the needle and see if it doesn't help. the o rings in the needle are probably shot also.

I don't know your carb but if it's got leaky O rings on the needle as pikebishop mentioned then your muffler pressure will be forcing fuel past them which could be the spray you're feeling. In other words, fuel isn't getting into the carb which makes it run lean. Also check to see if the idle needle is clear of the main jet at full throttle. Make sure there's no blockages inside the carb.

Don't worry about front bearing seals on your engine, THERE AIN"T NONE on a GP .42, which is a bushing engine.

I suspect your low end is too lean and is affecting your entire setup (assuming there are no airleaks anywhere). Try opening up your low end needle valve at least one complete turn and see if that improves fuel flow.

Uh, Dave,

Sorry, but this is a bushed engine. The only thing in front of the bronze bushing in the case is a thrust washer, drive washer, prop washer and prop nut.

Lightning Fan,

You have a leak somewhere. The needle valve should have an o-ring to seal it; if it is messed up simply use a short piece of silicone fuel tubing on the needle to make the seal. Make sure the needle valve assembly is securely attached to the carb, you might have a leak on the fuel inlet barb that swivels until it is tight. The carb should have a gasket between it and the case, if not then use some silicone gasket maker to seal it after it is throughly cleaned. Use the silicone sparingly! Further, the rear cover has a gasket, too...if it is missing silicone works there, too...sparingly! In either case, let the silicone dry overnight before running it! After that, check all screws for uniform torque/tighness. There's not much else that could be wrong with it after that IMO.

Good luck!

....is this fun, or what ? You've got a half dozen guys
all trouble shooting your motor....right here on the
Universe....

Shame on me for not knowing that your motor was
bushed.... Here's an idea....determine where
the fuel mist is coming from....and resolve the problem.

See this picture....a typical bushed motor. See the thin shim
that goes between the prop hub, and the case ? (some refer
to the prop hub as a drive washer) That thin washer is there
for a reason....is yours missing ?

That thin washer on a bushed motor acts as the front seal.
Some refer to it as the thrust washer.....however, it has
nothing to do with the thrust of the crankshaft. Keep in
mind that the propeller pulls the crankshaft forwards....not backwards. There is no washer on the front of the engine....
that has anything to do with "thrust".

...all the "thrust" in a bushed motoer is addressed at the
rear of the motor....between the crankshaft flange, and the
rear bushing. In ball bearing motors the thrust load is
seperated, and borne by both the rear and front bearings....assuming the the crankshaft flanges are correct.

All the thrust of a bushed motor is absorbed right
here....on the crank flange....and the rear bushing.

This does not change....

....this is the "seal" on a bushed motor. Without it,
massive air leak and fuel misting will be noted.
Excuse the grounge.....this motor has been un-op for
20 years. It is a close fit, but it is not touching the case.

I do not discount the opinions of my fellow modelers.
However, I've never seen a fuel leak so bad, at the
needle....that fuel mist could be felt on the hand at the rear
of the motor. And....I might say that I've never seen a tank pressurized to the point....that it was blowing fuel out the
connections of the fuel lines.

However, I have seen engines where fuel was leaking out between the cylinder liner, and the cylinder, due to extreme
miss-torqueing of the screws....

Once again....run the motor on the bench if necessary....and
find out where the fuel is leaking from....therein lies the problem.

Dave.

Flyboy Dave
The thrust washer has nothing to do with a seal it is used to keep the aluminim prop washer from contacting the aluminum case and gauling it when using an electric starter. If no one used a starter where they pushed back on the shaft the steel washer would not be needed for separation.
Some of the GP 42 carbs tend to leak around the plastic fuel nipple and that is why they changed to an aluminum one with a soft aluminum washer (gasket) on each side. Maybe the carb was a warranty return and faulty that they sent. It was mentioned that it wasn't new.

Thanks guys!!
Some more data based upon your comments.
The washer on the front of the engine referenced a number of times has a gap btwen the front of the crankcase and the prop drive washer of about 1/32". The nipple is aluminum. I just rebuilt the engine and made all new gaskets, tightened screws, etc ... that ain't it. The gap seems normal for other bushed engines I've seen.
Don't you think a big clue is the lack of needle valve control, and that it will never run rich at ANY throttle setting? It barely runs with the needle almost backed out all the way.
In thinking about this more, I think the reason the guy at ACE happened to have this used carb for me is because .... it was defective in someone else's engine. The jets have GOT to be blocked. I'll tear the carb down to parade rest and let you know.

....thanks for the help....If I was at the site where the
offending engine was....perhaps I would be more able
to accurately trouble shoot the darned thing....

I was pretty well convinced that the rearward play of
the engines were taken up....by the clearance of the
rod/crankpin (big end) against the rear back-plate of
the motor. Not one to jump to conclusions....I checked
again.... Yep....sure enough (see pic) after
a few hundred starts....and completely wearing out this
poor old O.S. .25,....I can't see any "galling" of the prop
hub against the crankcase. See, no shiny surface.....save
for one scratch....if ya' look real close....probably from
a crash, or a piece of dirt getting in there.

In fact....the prop hub, and washer do not contact the
front of the crankcase....even when pushed rearward as
far as it will go.

I do know one thing about two stroke engines....a sealed
crankcase....will offer better running qualities....than one
with air-leaks....

Flyboy Dave.

Try adding more tank pressure. My Tiger Shark 52 was doing the same thing. I closed the muffler opening a tad at a time until it worked and of course re-adjusted the needle valves. It worked perfectly. I also tached before and after and no change at all.

m_b

....I'm thinkin' the same as you....that darned carb
must be half plugged.....If you have a "banjo" fuel
connector like this, and it was leaking that bad....
you would have spotted it by now

First thing I would do is take apart and re-assemble the carb with a piece of fuel tubing over the needle valve where it meets the barrel.
I have an even older TT GP 40. Yep, fuel and oil (mostly just oil) will spray out of the front bushing. Always has and always will. It can be a little messy but part of the reason it comes out is for lubricating the front bushing.
Opening or closing the air-bleed hole will only affect idle. The hole gets closed off by the throttle barrel at about 1/4 throttle.
If you decide to open the needle jet hole a little be very careful. It won't take much to muck it all up.
As others have said, sealing all possible airleaks around the carb and needle valve is the first step to success.
During re-assembly are you sure the cylinder sleeve was put in it's correct orientation? Just a thought.

Good Luck

Well guys, no good.
Took the entire engine apart again -- the sleeve is pinned so you can only put it in one way. I screwed around with that carb for two days.
It idles fine, but at full throttle, she just does not get enough fuel and quits. Never four strokes.
I pulled the carb, connected the pump from my fuel bottle to it and pumped direct - fuel pours out of the jet just fine.
Even with the needle out of the carb, it does the same thing.
I drilled the carb at the needle valve, hoping to open it - stiff wires revealed no blocks. Drilling did nothing.
There's an OS .46 in the plane now.
DAMN!!
This is a crusade now, so I'll order another carb and see what happens.
See ya - good flying.