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Discussion Starter · #1 ·
Hopefully someone can point me in the right direction here.

It's a 90 indy 650 carbed. Carbs are clean, and brand new jets are installed.

I'm trying to tune her up for the season and run across a problem while trying to sync the carbs (uni-sync air flow meter).

I can get the sled to idle great, right around 2000 rpms, and she'll idle just fine until I give it some throttle. At that point the engine takes off up to around 4 or 5k for a few moments (even though I'm off the gas), and after maybe 5-10 seconds will drop back down to idle.
This happens even if I give it the tinyist amount of throttle.

What's going on here?
 

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Discussion Starter · #3 ·
Just replaced the throttle and choke cables. :)
 

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You need to sync your carbs. It's difficult with three carbs but it will make a world of difference. If you have a 3 cyl vacuum tool it will be easy.
 

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Play with the air screws, when the idle is hanging adjust them and see if it brings the rpm's back down. Also make sure that you didn't put to much tension on the throttle cable when you synced them, make sure the slide is closing all the way. You should have some play in the throttle, you should be able to slide a credit card between the throttle and stop before the slides begin to move. Also make sure that the airbox isn't pulling on the center throttle cable.
 

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I have 92 Indy 650 and mine did the same thing. What I found was one of the cyl base gaskets was blown out ever so slightly thus causeing a air leak. DON"T run it too long like that you may melt down a piston. A easy way to find out which cyl it is , is spray starter fluid around the cyl base area. As the fluid gets sucked in the RPM's will go up. Carefull starter fluid burns real well. PS: Question what kind of pistons are you running. I have been using Wiseco lites however I'm planning on going back to stock due the fact the wiseco's keep burning hole's threw the center. You might say the wiseco lites have me lite in my pocket book.
 

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dm, your piston is not causeing the hole in itself! Going to OEM's will just mean your burning down OEM pistons. Fix the problem, not the symtom. Burning the middle is a clear lean situation, or possibly preignition/detonation.
 

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Jet the middle up, try kimpex or oem pistons, also every rebuild with this motor requires a over bore, if you just hone and replace piston/rings you'll keep eating pistons due to excessive piston/ cylinder clearence, hole in the piston is a lean condition though.
 

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ripperd2,No kidding I thought they just melted down threw osmosis. Well enough of that. I ended up reinstalling the wisco lites. Side wall clearance was found to be within spec's,ring end gap checked also found to be within no fileing required. Timing found to be 1/2 deg advanced. Flywheel removed and stater ajusted. Engine timing checked bang on with timing light. Also found the check valves on the atmospheric sense lines to be faulty. Thus causeing under hood PSI to be greater which in turn was pushing the fuel out of the carb bowl. This explaned why it was a differnt piston each time as it was a differnt check valve each time. I have removed the check valve altogether as they are only there in the event the float bowl starts to flood out. Worst case here would be you would flood the engine. So all said and done the old girl is running great I,m getting a 100MPH out of her. Got big lakes up here.
 
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