Stumped by Ovation issue

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rmz290

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So I recently picked up a used ovation that wasn't running great as a project. Sled only has 800 miles and previous owner didn't really know snowmobiles so thought it was just how 340s ran. Initially it seemed to run mostly on one cylinder and be very hard to start hot. Pulled apart carb thinking it was just dirty and it wasn't bad. Checked compression and ring gap and decided to rebuild it as the compression was part of the issue. New top end is in and compression (prior to seating rings) is a touch over 100 psi. Starts up hot fine now however still running like a dog and partially on one cylinder. Swapping plugs/wires doesn't seem to make a difference, so thinking it's a carb issue.

Carb appears to be clean, cleaned it twice, blew out small ports as best as possible. There was a mouse nest in it based on some corrosion on the air inlet side but inside looks fine.
Exhaust doesn't appear plugged.
Compression seems good.
Spark looks strong resting the plugs on the head.
Swapping plugs/wires doesn't seem to make a difference, so thinking it's a carb issue. Played with the airscrews and helped a bit but not much.
Pulling the plugs after running, the PTO side piston looks cleaner. Both plugs equally dry.

A bit stumped on this so curious if anyone had any thoughts before I started throwing money at it? Next step was going to be a new coil or finding a excel/enticer carb to swap.
 
the excel/enticer carb swap might be the easyest way to go. that doubble barrel single is a pain to get info on. they use the same carb with different jets on the enticer II.
 
Maim said:
the excel/enticer carb swap might be the easiest way to go. that doubble barrel single is a pain to get info on. they use the same carb with different jets on the enticer II.
Click to expand...

Thanks, I found a blown up excel 2 hours away for $100. Debating going that direction or just swapping to a excel intake and new knockoff VM34 (only about $30 brand new) and running premix. The only other what if in my mind was a crank case leak but we tried hitting the PTO side with carb clean and didn't notice a difference.

Going to go pick-up a inline spark checker today to rule out the ignition. Will soak the carb over night to and check everything one more time. Will also see if both cylinders will run on spray. Anything else anyone can think off before I throw money at the sled?
 
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Went through carb again, ran a needle through all the small openings, blew them out, checked to confirm they were clean with a light, checked float height. Issue remains. Checked spark with a inline spark checker and didn't see anything weird. Plugs are fresh, electrode/insulator black after about 5 mins of run-time. Seemed like some light fuel on the OD of the plug but not much.

This was running it after the carb clean.
https://youtu.be/P6SGsmropKU

Runs on 2 cylinders for a few secs, then wants to die on the other 1 and needs throttle to comeback. Pretty smokey as well.

Seems like a fuel issue to me but I've been fooled before. I've read that when the oil pumps go they fail open? Potentially still a weird electrical issue? Where would you guys look next? Thanks again for the feedback!
 
Sounds like it's loading up on fuel or pipe is plugged.

Take the exhaust pipe off and just let it run with the Y. If no change pipe is ok.

Could be fuel pump diaphragm leaking and fuel is getting sucked into the crankcase and flooding that side. Check which side the pulse line is going to and if that side seems to be wet that could be the issue. The pulse line should not have fuel in it.

Also check the rubber tip on the bottom choke/enricher plunger. I have seen them crack and then they won't seal properly. If that happens the engine is always pulling in extra fuel from the choke system and loading up.

opsled
 
Opsled, thanks for the feedback! Went and played with it some more. Pulse line was on the mag side, problem cylinder seems to mostly be the PTO side. When I rebuilt it the PTO side head and piston top were pretty much washed clean to further confirming this.

I tried running it on premix to isolate the oil pump being the issue, no change. Also pulled the exhaust and ran it and similar. I stuck a small camera in the exhaust and it appeared clean too. Decent amount of wet gas in both y-pipes.

Choke plunger looks ok to me without having one to compare against. Has to be a carb issue but unsure what else to look at in the carb. I tried blowing into the carb with it full of fuel and the float/needle was closed. Also confirmed float height against manual. The only think I can think of is the gasket between the tower the jets sit in and the body just being shot and leaking. Unfortunately yamaha does not show this on the microfiche.

Screen Shot 2021-01-03 at 12.11.58 PM.png

Plugs after running for about 1-2 mins. Both wet on base.

Screen Shot 2021-01-03 at 12.12.24 PM.jpg
Screen Shot 2021-01-03 at 12.12.14 PM.jpg
 
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Does it run properly at full throttle?
I had an ovation here one time where someone had tightened down the idle mixture screw too far, and broke the carburetor casting inside the bore of the carburetor.
Also, those plugs don't look very fresh.
Which cylinder gets hotter? Use a infared heat gun to see which side is hotter by pointing it at the exhaust manifold near the cylinder.
External coil? Have you swapped it with a known good one?
 
have not swapped the coil but spark looked good. Mag side seems to be the one that gets the warmest. Carb casting looks good. Plugs were new probably 8 mins of run time ago. Doesn't run much better at full throttle. Needs some throttle to keep running before eventually dieing on both cylinders.

I found another ovation locally in a bit rough shape for $300 that runs good. Debating buying that two swap parts then selling it or just swapping over to a Mikuni VM32/34 and running premix.
 
I've worked on many ovations over the years, never had an issue with the stock carb. Either you are over looking something in the carb or there is something else wrong.
Also, just because there is spark doesn't mean that coil is good for sure.
Have you checked to make sure none of the piston skirts are broke off?
 
74Nitro said:
I've worked on many ovations over the years, never had an issue with the stock carb. Either you are over looking something in the carb or there is something else wrong.
Also, just because there is spark doesn't mean that coil is good for sure.
Have you checked to make sure none of the piston skirts are broke off?
Click to expand...

New top end so pistons are good. The only thing I can think of in the carb is there is a seal between the body and the tower the jets sit in that was pretty compressed. Unfortunately yamaha doesn't show this on any microfiche. The carb does have some corrosion on it from an old mouse nest in the airbox and maybe that ruined some of the inner bores? From what I can see they look good but there are some small passages in that carb.

Tried switching plug leads and just pulling them on and off and no change.

When I pulled the carb there was a gas puddle in the intake as well. I didn't pay too much attention to where that seal is between the body and jet tower in regards to float level but was thinking it could be leaking there?
 
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