Gas in transmission in 250R ATC

[rmcollector]

Hi all, been awhile since I been on, hope all is well.

I picked up a 86 250R ATC and gave it a fresh bore .030 with a new piston due to cylinder scoring. Rings are in spec, newer carb is cleaned and float height is correct. New reeds. Compression is tight. The bike keeps flooding the crankcase with gas and now when I drained the oil in the transmission case, it is full of gas. I thought possibly the carb, and switched it out with another carb...and same thing. It will start after some time, but it's extremely hard to get it started and running pretty rich from the load of gas in it.

Any ideas on this issue? Any help would be appreciated!

[Tri-Z 250]

When you say compression is tight...by kick or leak down test? I believe it’s a new top on a motor that may have sat or the seals on the crank have met the end of their life cycle. Your pushing gas into the trans, other than inner case crack my $ is on the seals.

[shortline10]

Very simple , your carburetor float valve isn’t working properly on either carburetor and if your getting fuel in the tranny cavity then your crank seals or your center crank case gasket has failed .
Time for a rebuild !

[rmcollector]

When you say compression is tight...by kick or leak down test? I believe it’s a new top on a motor that may have sat or the seals on the crank have met the end of their life cycle. Your pushing gas into the trans, other than inner case crack my $ is on the seals.

Yes, right now it's at 167 psi. Wouldn't compression be low with a bad crank seal?

[rmcollector]

Very simple , your carburetor float valve isn’t working properly on either carburetor and if your getting fuel in the tranny cavity then your crank seals or your center crank case gasket has failed .
Time for a rebuild !

Ok, I know the crank seals were replaced about 2 months ago. Seems strange the float on both carbs would be messed up, but anythings possible I guess.

[El Camexican]

It needs to be disabled and put back together properly. A pressure test before will till you what went wrong and another after the rebuild will ensure it won’t happen again.

[yaegerb]

Yes, right now it's at 167 psi. Wouldn't compression be low with a bad crank seal?

167 is low for a brand new rebuild unless the rings haven’t seated yet. You should be running 180+. A compression test isn’t going to tell you if your crank seals are bad. A crank case pressure test will. I second all comments so far. It’s time to break the motor down and do new seals/gaskets and while you are in there I would do bearings and check your crank for wear.

[ps2fixer]

I'm not sure if the OP knows what a leak down test is, but you pressurize the cylinder and leave it and check for a leak over time. It's very critical to not have any air leaks with a 2 stroke else it could run too lean and melt the piston. Anything from the crank case compartment getting into the transmission says there's a bad gasket somewhere, either the gasket failed, or it didn't get installed correctly like the gasket folded over on you etc. Could be the crank seals too. I'm not sure how the transmission breather tube is setup on the 250r, but generally it ultimately ends up at the carb one way or another, could be possible fuel get in the transmission through that hose too if the layout/design makes sense for that to be even possible.

https://mobiloil.com/en/article/car-...-leakdown-test

With a chain saw normally there's block off plates put on the intake and exhaust to test crank seals and crank case gaskets, cylinder base gasket, etc.

I would post up a screen shot from the service manual for the compression specs, but sadly there isn't any for any year of atc250r. I checked the TRX250R manual too, and I don't see a spec there either.

[rmcollector]

167 is low for a brand new rebuild unless the rings haven’t seated yet. You should be running 180+. A compression test isn’t going to tell you if your crank seals are bad. A crank case pressure test will. I second all comments so far. It’s time to break the motor down and do new seals/gaskets and while you are in there I would do bearings and check your crank for wear.

Rebuild only has about 15-20 min of run time, I would think psi would go up slightly after rings seat. That was going to be my next step was to do a pressure test. I would think the only way gas getting into the transmission would be a bad crank seal? Not sure how else it would enter. Do you know off hand the size I need for the expansion plug? Going to go to the hardware store and put something together so I can test the engine.

[ps2fixer]

Here's a quick edited image I did to show where the center crank case seal could leak between crank to transmission.




Have you had any problems with tuning? Another sign of a leak is the engine tends to run lean.

[rmcollector]

Here's a quick edited image I did to show where the center crank case seal could leak between crank to transmission.




Have you had any problems with tuning? Another sign of a leak is the engine tends to run lean.

Ok thanks for that. As far as tuning, when I am able to get it started it runs pretty rich, which makes sense with all the fuel build up. It wants to start, but then dies after a few seconds. If crank is cleared of buildup with a fresh plug it will fire up and run, but won't idle. Ive done this several times.

[ps2fixer]

Might sound odd, but pretty sure when a 2 stroke loads up, it's the oil in the crank case that causes the problems. Just guessing, but if there's gas getting into the transmission, then oil from the transmission is probably getting into the crank case. That oil will still burn and probably make a fair bit of carbon to make the appearance of a rich condition. Further support of this idea is the fact you swapped out carbs, so it's not like it's the carb is just dumping fuel because of a bad needle & seat. The amount of oil mixed in the fuel could be a factor too, but my experience mainly revolves around chain saws.

From my understanding for oil mixing, if you ride light, or ride average (short bursts of WOT), then you want less oil in your fuel mix and tune the carb for the new fuel ratio (less oil in the fuel = more fuel so either need to lean the carb or add more air). Same goes for the opposite, if you're drag racing where all you do is WOT, then you want very heavy mix of oil, and the engine probably won't run great unless it's floored. Also the oils have advanced a lot since these machines were made. I can't really suggest a good ratio for a given application, but this is just a rule of thumb.

I'm real interested in what you find to be the cause of the running issues. Sometimes the weirdest problems are the ones we can learn the most from.

[ironchop]

See that red spot?

If the rod had ever let go in those cases and it banged into that wall there in the pic, that might be your issue.... I've seen holes broke thru in that area or even cracks. That lets the piston draw oil from the trans thru that hole/crack or gas leak into the trans as well. It might smoke and seem rich because your actually burning some trans fluid too

Or as mentioned above, your float stuck and the cases filled up with fuel and then leaked into the trans side of the cases thru a broken or cracked area

I'm just throwing out another possibility to look for





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[rmcollector]

That's interesting, I hope it isn't cracked or has a hole in it. I will find out more when I run the leak test, going to get the stuff today.

I took the stator cover off last night and tried to take the flywheel off with a flywheel remover tool, well, I didn't get anywhere because the damn threading inside on the flywheel is striped and it will not hold to where it will pop off. Does anyone know how to take it off without the tool and not cause any damage?

[rmcollector]

Ok, so the leak down test was done several times @ 8 psi for 10-15 minutes and it hasn't leaked even a tiny bit.

I noticed air coming out of the nipple on the case underneath the boot and reed while I was trying to pump up pressure, so this was plugged up temporarily with a hose, this may sound pretty silly...but what is this used for, and does it need to be sealed off?