[Cperkins]

Step father bought the sled a couple of days ago. Seller stated they didn't know what was wrong, they had needle and seat replaced.
Not trusting people I checked plug on mag side soaked but has strong spark. At this time I did full rebuilds on both carbs, drained all fuel from case ( there was alot ) and put fresh plugs into the sled. Started the sled and it is only running on pro cylinder. I found that I turning the fuel off gets the mag cylinder to fire, turn fuel on and it stops firing. Swap fuel lines same issue, swap carbs same issue, swap choke and throttle plungers, same issue. Tested compression 110 on mag side and 120 on pto side... I would not think this would cause the issue but I have now rebuilt the top end on the sled pistons rings, gaskets honed cylinders the whole nine... now testing the compression pre brake in both are at 125, now I expect aftet brake in this will stay close to the sames (at least balance out ) but I am being preemptive and looking for other ideas.

 

[LTSIII]

I have the same problem with my 91 trail. Next thing I'm going to do is fill the bowl full of fuel on the bench and check and see if the floats are sinking. Also going to see if the float check valve will hold pressure.

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[Cperkins]

Swap carbs if issue stays its not carbs if it follows then bench test your carb, my issue did not move when swapping

 

[LTSIII]

Swap carbs followed Carbs. Took apart the carb that was sailing for the millionth time. Floats did not think they are OK. Put it back on same problem swapped. Float Needle vowels did not change. Now the thing will start at all. Hoping for fire.

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[LTSIII]

Pardon the spelling on a few words did not proofread it good enough using text to talk moshe time at cat figure out what I'm saying but I can't either so it's a push.

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[Cperkins]

Well found the problem, issue was the fuel pump still don't know what's wrong with old one or why swapping lines did not show the problem but the pump swap fixed this issue

 

[Coolhand]

22 years old usually what happens is the diaphragm goes bad. Not uncommon for a pump that old on a 2-stroke. I change mine every 8-10 years regardless and then throw the one that was on there in the back of the snowmobile for a spare.

 

[Cperkins]

It had been recently replaced looked like a shiny new makuni pump

 

[Coolhand]

Usually they can still look new after a few years. But sadly even a new pump can be bad out of the box. It doesn't happen very often with one made by Mikuni though. But I have seen it happen in the past. Had one 30 some years ago for a single carburetor on a old Arctic Cat new pump was bad out of the box.