G1A exhaust/intake leak

[jimd]

Juswt bought a G1A (I'm pretty sure that's what it is) and runs pretty well forward, but pretty noisy. I just assumed it was an exhaust leak. Reverse will often not even spin up enough to move. After getting it home and inspecting it I thinkit has a leak right under the exhaust port. In forward, I can feel the exhuast pulse , and in reverse, it pulses fuel/air mixture.
I haven't had time to pull the cylinder to inspect fully, but does this sound like something common or have I cracked the cylinder?
JImD

[Michael Eddie]

Hi, Most likely there is an exhaust leak, these Yamaha's are pretty touchy when there is a intake leak and usually do not run. It will not run if the cylinder is cracked as well.

[jimd]

That's what I'm banking on. I went out this evening and tried and it lit right up, and ran both directions fine. In fact, it would dit there an d idle forward, with nothing touching the throttle. So I'm thinking after it warms up, some metal it warping a bit, and opening up. normal glof course type use would probably never show it up. It was running it around for 30 minutes the other night that got it warm enough to leak
JImD

[jimd]

OK, I tore it down tonight, and the cylinder had a couple of scores, I honed them out. Piston had a crack, and as I expected, the base of the cylinder gasket was blown out. Who is the best to get the parts from?
JImD

[Michael Eddie]

I'm surprised it still ran with a cracked piston, guess that motor is "unbreakable".

[jimd]

The crack was in the lower skirt. Not really a significant thing either. The major issue was the lower gasket leaking. Local shop had the gaskets in stock, but not the piston, so it's going back together with the cracked piston. If it fails I will replace the cylinder and piston which really should be replaced anyway.
JImD

[Michael Eddie]

Ok, thanks for clearing that up

[jimd]

OK update.
Bored engine .020 and installed all new gaskets/piston and rings. Did the muffler BBQ. No real improvement. Idle is a little better and midrange too, top end is the same (as I expected the top end to be)

So... I can go out and run the wheels off of it, but when it warms up, it loses low end, and will eventually die. It will not restart until I let it set a while.
Any thoughts?

[Michael Eddie]

Did you replace the crank seals too?

[jimd]

apparently not, What am I looking for?

[Michael Eddie]

Just throwing some things out there. The seals should be replaced when you change the rings, since they wear out and once their worn the engine will never run right. Make sure the whole intake setup is sealed off, all gaskets in place and the air box is sealed.

Another thought is, once the engine dies when warm try running a compression test just to see.

[jimd]

I looked at the parts diagram for the engine, and see the 2 seals. I guess I don't see how those will affect running.

Is there a procedure online for seting the carb? I used to fly RC airplanes and we ran 2 stroke engines (some larger than this one) and the engine sounds very rich to me.

[Michael Eddie]

Maybe I am misinterpreting your problem. After running for a few minutes, what does it do. Simply die and never start up? Start running very choppy? Start doing intake backfires?

Does choking it help at all?

Has the carburetor been cleaned yet? (sorry if you answered this already)

Per post #12 the carburetor is set to 1 1/2 turns out on the mixture screw. (one on top)

[jimd]

Cold engine starts and runs fine. When warm, runs fine as long as 1/2 or more throttle. Lower 1/2 throttle yields sluggish response (as if flooding) and unless I get back on throttle above half, it will stop going forward, and I can hear the starter running, with no exhaust noise.
It will not re-start after this for about 20 minutes.

I have not disassembled the carb.
Top screw looks like a jet. It is slotted, and if moved any beyond fully seated will vibrate out while running. I removed the screw completely, and I see a small passage sideways through it. Doesn't really look like a mixture screw to me.
On the side there is what looks like a normal mixture screw, but it has no effect on the running or idle unless I screw it all the way in, and it will kill the engine.

Choking while running kills the engine, and no choke is needed to start.
JimD

[Michael Eddie]

I had to go and refresh my memory. Yes it is the lower screw not the upper screw. 1 1/2 turns out. When it dies, spray some starter fluid down the carburetor and see if it will run. If it has no effect, let it sit until it will run again and then spray some starter fluid on the visible crank seal behind the clutch while it is running and see if the engine run quality improves.

[jimd]

today the rubber carb to cylinder adaptor broke. Now it goes to full throttle at startup. But when I looked at a parts diagram, I realized Im missing a vac container. Would this contribute to the problem?