Rat motor not running right

[Tether]

I thought I had my Super Rat project done but the motor is just not running right.
Symptoms:
Starts right up first kick usually.
Seems to run fine at first then as it warms up it starts to idle faster and when reved up and the throttle released the rpm drops slowly like a lean condition/air leak.

About the engine:
Fresh rebuilt Ace 100 lower by a known good builder. New Super Rat standard bore cylinder and piston. Used Super Rat head. Rebuilt Super Rat carb, 230 main, 30 pilot, not sure what needle is in it but looks to be in good condition. Motor has about 15 minutes of run time on it, mostly just heat cycles and up and down the driveway a couple times. Running 20:1 Yamalube 2R with 92 octane non-ethanol. B10HS plug. Timing checked good using a buzz box. Carburetor slide and cable working freely. Stock Rat pipe & silencer cleaned.

Pressure test failed I think, dropped from 6 to 3 pounds in one minute. Using leak detection fluid I could not find any leaks around the intake, cylinder base, head, center case, or my homemade testing apparatus.

Compression test (cold engine) 200 pounds. This seems high to me.

Any ideas? Advice?

Thanks, Bob

[socalhodaka]

Leak maybe at the left main seal.?

[Tether]

Any easy way to check the left main seal? Or do I need to pull the flywheel and stator?
This engine was rebuilt with all new bearings and seals. I would hope that is not the problem.

[Al Harpster]

Only way I know to check seal on flywheel side is to remove flywheel & stator.
I have had the same problem you report with my Suzuki. Drove me crazy. Had to disassemble to find it. I was real unhappy.
If you can't find the leak where you've looked & you've looked a couple times it's probably tear down time.
My GUESS: crankcase sealant didn't seal. But a guess is all it is.

[Bullfrog]

Regardless of what might have to be removed to find the leak, you MUST find the leak. A leak of that size MUST be found and fixed. There are no shortcuts. No. It matters not who assembled the engine - the leak must be found. Take a deep breath and dive in.

NOTE: I'd advise continuing the search with spray-on soapy water until you actually find the leak. You'll have a much better idea of what must be done to fix the leak IF you know where it is. Starting a full tear down before finding the leak will have you shooting in the dark on trying to fix the leak.

Ed

[Tether]

Thanks guys,
Ok I’m off to tear into it so I can check the left seal. I figured this is where this would go but was hoping for something simple.

What about the 200 psi compression?

[Tether]

Checked the left main seal, no leaking.
It’s time to pull the engine out of the bike and put it on the bench so I can get to everything better with the bubble juice.

[Bullfrog]

200 is too high. Anything over 160 and you will have to use expensive racing fuel . . . and I think, mayhaps, 200 is too high for even the highest zoot racing fuel.
Ed

[Tether]

Ed, That’s what I thought about the compression. I don’t know the history of the head I’m using, possible that it has been shaved. I have access to another head that I can borrow to see if that brings the compression down to a reasonable place.
Multiple issues are always fun.

[Tether]

Update,
Put the other head on and that got the compression down to a more reasonable 170.
It did not change the pressure test though, so the engine is coming out.
My friend will need the head back for his project so I am looking for a good Super Rat head.
I had a Wombat that had the same head issue. Some people out there ruining these things trying to get more power I guess.

[MTrat]

The leakdown air has to be escaping somewhere. Careful and diligent use of soapy water should reveal the source, without a teardown...

[Tether]

Found the leak, at the head on the exhaust side. Of course this is an area I could not see well with the engine in the bike. Barely leaking but leaking.

Is this an area that will cause the symptoms I described?

Anyone have a good 93 Rat head you are willing to part with?

Bob

[Bullfrog]

"barely leaking" - previous test was 3psi lost in one minute - that's not barely leaking. (of course that was with a different head)

I'd recommend getting the head leak sealed up right away . . . and re-checking. A leak directly into the transmission cavity or the clutch cover volume will show up as bubbles at the transmission vent (if the transmission dip stick cap is in place).
Ed

[Tether]

Maybe I shouldn’t have said barely leaking. I could definitely see bubbles forming in that area. I checked the transmission vent and no bubbles there.

[Al Harpster]

The good news: a leak at the head is one of the easiest to fix. I've never been that lucky.

Highest compression I ever got on my Harbor Freight compression Gage was around 160-165 & I was afraid to run it on 93 octane.

That was when I was set up with an new Ace piston & a new Super Rat head.

This piston & head combination is considered "bad", "Don't do it"

Could this be a factor in your situation?

Or could you try another Gage just to be sure you're calibrated?

[Tether]

Al,
I had a Harbor Freight compression tester also. The first time I used it was on a freshly rebuilt Kawasaki KL250
It registered about 80psi, scared the crap out of me. I have a good one now.
I hope the piston is correct, I bought the piston and cylinder together from Terry, it is the single ring piston. The heads have the correct part No. 93....

[Al Harpster]

I believe that's the right piston.

I'm curious how you got to 200 psi.

That's a big number. Must have been good exercise to kick it over.

My opinion: 160 psi ought to be plenty.

Other guys may think differently.

I'd be afraid of knocking.

[Tether]

I’m pretty sure the head has been modified/ shaved. I had the same issue on one of my Wombats.