900 Cat running on 1 Cyl

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Hey Guys,
Trying to fix a young friends sled for him, 03 MtnCat 900, & have a few questions..
He put in a new Mag side piston & cyl last season, ran it a couple times & noticed his oil was never going down so he ran premix. The first mech that rebuilt this thing had 150PSI on both jugs written down.

I just changed his oil pump & tested compression & only get 125PTO, 110Mag. When sled is cold she fired on both cyl & idled good. Once warm she drops the Mag side. I've switched plugs & plug wires side to side, still dead on Mag side.

Is this too low compression to cause a no fire? What else could I test, check?

What is the best set of pistons & rings to go with on these big motors?

 

[stanredrider]

Carb or EFI? If efi try swapping injectors to see if problem stays on same cylinder or switches. I had an 03 800 that would run great till warmed up and then problems burnt down couple times (same cylinder) and after rebuilding the last time still had an issue so switched injectors and it leaned out the other cylinder. New injector and sled never ran better, so worth a shot anyway.

 

[Readymedic]

Compression and fire are two different items. Coils, stator and CDI for spark;; piston, cylinders and rings for compression. Compression has nothing to do with spark.
Are your coils functioning?? How about your caps? Unthread your plug caps and cut a small piece off of your wires, then thread the caps back on. Check for spark. That can solve a lot of problems.
Are you puking fuel out of the exhaust? Is it flooding??

 

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This is a carbed sled. I pulled plugs & checked for spark. Both cyl have spark. Plus I metered the coil & wires & they check out good.
When sled is cold comp is 140PTO, 130Mag. It fires up & idles fine.
I ran the sled up & down my range road & when she gets hot it drops the cyl. I pulled back into the yard & pulled plugs. Both have spark but comp when hot is 125PTO, 110Mag. One other thing I noticed is the sled hangs up at 3000RPM once hot too. Takes a long time to idle back down & that's when I notice its only firing on PTO side.

There is no extra fuel dumping out the pipes or in the air box. I don't want to tear the motor down if I don't have to... but I don't know what else could be causing the loss of a cyl once hot???

 

[m8magicandmystery]

a plug can fail in a heated/loaded condition and spark ok on test..

when it fails hot can you spray a touch of ether in that mag side to see if she kicks..

 

[Readymedic]

I agree. Try a few different plugs, sometimes brand new means nothing.
It is normal for the compression to drop when the engine is hot, so don't worry there.
My 900 also hangs around 3000 rpm and takes a while to decrease, that's pretty common on these big bores and the big carbs. Quite a few times, I have to choke it to get the rpm to drop down.
If you have no fuel leaking from the exhaust flanges, your not flooding.
I would definitely guess, a spark issue. Check your 2 grounds on the magside bolt, wiggle them, or better yet, just put new ends on them, make sure you solder them and heat shrink them.(stator ground and ECU ground)

 

[Arctic Thunder]

If the rpms are hanging it could be a crank seal? giver a shot of starting fluid behind the clutch and see if the rpm's pickup.

As for compression. It shouldn't be falling off that much. My old 900's ran 150-153 at 1800' elevation, warm, cold didn't matter.

Sounds like you might need to do a little top end work.

Thunder

 

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Thanks guys,
I have tired 3-4 new plugs in the Mag side & it still stops running on that cyl once warmed up.
All grnd wires look good & solid.
What about carb settings? I know that Mag side plug is wet when I pull it out so it is getting fuel but does one side need bigger Main jet, airscrew setting?

Also another problem with this sled is when riding it cuts out... I found that the throttle lever pin has lots of play. I can hold the pin forward & ride & not have it cut out. The switch is all cleaned & working. I lubed the pin & throttle lever but there is tons of slop. Is there a replacement for this or any adjustment.

So many issues on a used TURD!! I don't even know which way to go with this thing for the kid!!

 

[diggerdown]

Your pin slop is there by design. That is the throttle safety switch. If you grip the throttle wrong it will try to shut the engine down. There are little O-rings that wear out and are replaceable. A lot of guys just unhook the switch and add a tether to prevent a frozen throttle cable run away!
RPM hang is usually a slight lean condition but not unusual on these engines. They need a good sized carb throat to feed it at high rpm and it is hard to block enough air off. Could be a crank seal leaking, base gasket, poor boot seal, or throttle cable to tight. May need different pilots. Top end is getting there, mine ran close to the same warm or cold, 145 range. Coil or stator could be getting weak and you loose the spark on the one side. Plugs may fire while checking them but will not fire under compression if the electrical power is too low.

 

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Thanks for that info. I will diconnect the stop switch & take it for a run to see what does.
I'll also try to srpay the crank seals to see what happens.

 

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Sprayed the crank seals with WD40 & no RPM change. So seals are good.
Called up the owner & asked a few questions on how they did the new piston install last time. Got to the Ring Gap & he never heard of that before. Explained what it was & the reply was.... Nope just took rings out of box & put them on piston & slapped it all together.
So tore the topend down & waiting for parts. You can see the ring grooves around the entire cyl, so it was in there to tight & ended up not seating or sealing properly.

Thanks for the help so far on this.

 

[Arcticatdan]

Sounds more like the stator. Had a Jag special that ran great cold the ran bad when warmed up. Acted like bad plug. Had stator rewound and ran better than when newer.

 

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Well Im stumped again. Took this long just for my dealer to get 1 piston, ring & gasket set in.
Put her all together tonight. New topend, new oil pump, carbs pulled & cleaned.

145 psi both holes. Fired her up & still runs on 1 cyl..
Swapped plugs & wires from side to side & it only runs on PTO side.

What the HeLLLLLL am I not finding??

I can see elec issue if it switched sides when I changed plugs or wire from side to side, but to keep failing on Mag side what gives??

 

[Slednadz]

Have you checked your reeds?

 

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Yes, pulled both sets & they are good.
Would the APV system cause any of this? Just been searching the cat forum & seen mention of the APV not in sinc causing weird issues.

I will be checking that tomorrow.

 

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Just wanted to add. That since the rebuild it doesnt even run on the mag side now. Where before the build it would run on both cyl untill hot then drop to PTO only.

 

[Sledbrain2]

I really think you have a bad stator. Also the voltage regulator can cause some issues similar. If you could swap out a good stator before sending it out to be rewound might not be a bad idea. I had a 93ext,99 600efi,02 800efi,06 M7efi all had there stators rewound near the 1500 mile to 2500 mark. Heat takes them out I wonder if he has an aftermarket can?
I believe there is a tps on that carb model, I know on a efi it tells the cpu how much fuel depending on throttle position. I think on a carb the only input would be spark or spark advance. I think you can just unplug and see if that is the issue, not 100 percent sure however.

 

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The sled does have a can. But I dont see how it can be electrical.'
It only has 1 coil. 2 plug wires & 2 plugs. I switch plugs still only runs on PTO, switch plug wires still only PTO fires. So that tells me it has spark from both plugs & wires.

Its getting fuel as the plug is wet when I pull it. Maybe its getting to much but it isnt leaking or running out of the pipe.

Im going to check carbs again today & APV after work. I hope its something simple Ive missed.

 

[diggerdown]

Hope you find something simple with the carbs. Be sure to check your carb vent tube and float level. Another possibility would be the crank is out of sync. I would think it would have to be quite a bit off not to fire at all.

 

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Well found the problem for now. It is a bad float.
I pulled carbs down & cleaned again. Everything went back in the same carb except for the floats. I switched sides.

It ran for a few secs & I thought all was good & then she started to run like crap & fuel was coming out the upper vent hose on the carb & this time it was the PTO side that wasn't running.

So now to see how long it will take for the dealer to find this part. Only took a month for piston, rings, & gasket.

I see no way that you can adjust these floats as they are all plastic. Is there a way? And can you just change the float needle & not the entire assembly?

Thanks for all the help everyone.