This 40 used to be a great engine. It's been sitting in my garage for about 2 years and I just found a home for it, but I can't get it to run properly. No matter what I do the tuning is inconsistant. Btw I have experience tuning engines but this one is giving me hell.
So far I have:

checked fuel tank and clunk lines for leaks
added more foam padding
replaced fuel lines
replaced HS needle valve
checked carb O-ring for damage
made sure carb was tightly in place
tightened every bolt

AND STILL HAVE THE SAME PROBLEMS.

I decided to tear down the engine and noticed that the rear bearing is burnt orange. Maybe this rust is my problem but I really didn't notice any classic signs of bad bearings. No play in the crank shaft. Engine ran pretty cool to the touch. And I never noticed the prop feeling "notchy" when turning it over. But honestly this is the last thing I can think of that might be my problem. Any thoughts would be great.

AND PLEASE ONLY REPLY IF YOU HAVE EXPERIENCE WITH THESE ENGINES. NO EVOLUTION FLAMING PLEASE!!!!!!
I've never had any problems with these engines until now.

I woud guess that the bearing color isn't a problem Most engines have some baked on lube and it is usually orange to brown. Feel it with your finger, If it is rust, it will feel rough, If it is backed on oil, it will feel smooth.

If the tank isn't a new one, I would check the clunk line in the tank. I've been bit more than once with one that had a crack in it and was sucking some air after a quarter tank was used.

If you don't have the adjustment limiters removed,now is the time.We've had a couple guys with these engines at the field that couldn't get them to really peak out or idle correctly, but once we removed the limiters, they tuned just like any other engine.

Last, you may wan't to soak the carb in a jar of fuel ovenight then removeboth needlevalves and blow through the plumbing to make sureall the goo from storage is flushedout. Itake the high speedneedle out and blowinto the vent line. You should get a nice steady stream of fuel with little pressure on the vent.If it is blowing air and spittingfuel, your tank is empty or the clunk line is broke. We had one of those yesterday.After blowing through the high speed, the remvoe the idle mixneedle and blow through again. This should dislodge any crud in the spraybararea.

Last are you using a new Glow Plug? Old plughs may haveenough crud on them to keep them from doing the job, even though they glow red.

Don

Carb o rings mostly dont show any significant problem by looking but mostly they are the reason.I recommend you to expand all carb o rings and clean the carb thoroughly.

Try a different jug of fuel.

What is the engine doing besides not running properly? Does it quit? When? Send more details and we can find out why it isn't running.

It's difficult to describe exactly what it does, because it is erratic.
The glow plug is new...and the fuel is new. Also the needle limiters are off. The bearings should be here in a few days, although I'm doubtful that is the problem. I'll go ahead and put new ones in since I have the engine apart. My next move is to seal the carb. Maybe some of that liquid gasket gunk (RTV?). Maybe a small ring of that stuff will help. I don't think it would hurt anything. And I may replace the whole fuel tank set up. This tank came with the kit and maybe it has a leak. I've applied pressure to the tank and it doesn't seem to leak, but who knows. I'm going to pull it apart tonight and make sure the clunk line doesn't have a leak that I over looked. Also I don't see any air bubbles in the line so I'm not sure that is it. I haven't noticed the engine acting up at a specific fuel level either. Its the same with the tank full or almost empty. AAAAAAh!!!!

The engine runs rich for 8-10 seconds at full speed....then peaks out for another 10-15 seconds.......then revs and bogs. revs and bogs....or sometimes it just bogs down and almost dies until I lower the throttle and save it from quitting. Nothing I do with the needle seems to change this. Doesn't seem to be a tuning issue as much as an air leak issue.

What about the fuel type?
Running on castor and then moving to only synthetic for instance can give erratic behavior, try with a similar fuel that you ran in it the last time.

Otherwise it does sound like a leak or something that is loose, checked the head bolts etc?

Did you blow out the spray bar? It kind of sounds like you have the balance between the high speed and idle mix out of whack. Tighten the high speed and back it out two turns, set it to idle and tighten the idle mix and then back it out two turns.Then see how it goes. It sure sounds like crud in the spray bar though.

Don

Naturally the carb fuel inlet isn't connected to the tank vent versus the fuel outlet, right? It's happened.. with remarkably similar symptoms. Also, have you given the muffler pressure system the same attention as the fuel side? Cleaned the pressure tap, replaced the pressure line, etc.? If in doubt try tuning up the engine without the muffler pressure line attached, level on the ground/bench. If is now consistent.. aha. Not likely but just offering a possibility.

Was the glow plug replaced before you tried running it again and is still in there, or after you started having problems? Just a question in lieu of fuel delivery issues.

Engine that go lean after running for a while have been known to develop air leaks as parts heat up and fits change.

Are your head bolts tight? Plug gasket installed? i.e. everythin sealed up ship shape up top?

MJD

I did blow out the spray bar.... And every bolt was tight everywhere on the engine. But I didn't check the muffler side of things. The pressure outlet on the muffler may be gunked up a bit. Also I think the muffler gasket has seen better days. I wouldn't think a leak at the muffler gasket would do much but maybe I'm wrong. Hopefully I can get this sorted out soon because I have a tribute that is begging to be flown!

MJD...... the lines are definetly not switched. That was one of the first things I thought of when I started having troubles