18’ 800 power loss at wot under heavy load

[Pine Rob]

With these winter storms coming in my sled is finally unhappy after 2700 hard miles with 0 issues

2018 pro 800 155 was a deep day and kept getting a intermittent bog/loss of power at wot. Would go from 8200 to about 4k and sound like it would drop a cylinder. After letting off throttle it would take a few seconds for power to kick back in but if I held throttle it would stay at that 4k for 10-15 seconds. Air vents got continually cleaned and would have the issue even with vents clean. Would run 7500 rpm or less with 0 issue and could run wot for 30 seconds occasionally but most the time when I would wot

Replaced plugs
Changed fuel tank and fuel pump assembly from a known good sled
Compression 112pto 120 mag

Stock power plant stock exhaust (good exhaust donuts so don’t think it’s leaking exhaust in the hood)

Thanks all for the input

 

[Pine Rob]

Update.

took the throttle body off and inspected the reeds (usually reeds fail and cause low rpm issues not high rpm) and they look like new and are still seating properly.

While I had the reeds out I inspected the motor through the intake and the piston skirts and cylinder walls look like new.

Running out of ideas at this point. Wondering if a coil might be failing under the load and dropping a cylinder. Injector failing under a load? Don’t think injector would act like this but possible I suppose

Going it clean the exhaust valves tomorrow (don’t think that’s my issue) and test it Tuesday. Will be bringing an extra coil with me to replace on the mtn as I can’t simulate the symptoms

Until then the wife’s 600 will have to do circles around the boosted boys in this deep snow.

 

[Pine Rob]

Final update

Pulled the power valve assembly and the mag side power valve bolt was missing not allowing the actuator to open the power valve properly. The valves are a little gummy and will be cleaned while the assembly is removed. Going to reassemble with a generous amount of red loc tite and keep sending this old 800.

While all the exhaust and power valves were removed I inspected the exhaust side of the of the pistons and cylinder walls. And for 2700 hard ish miles everything looks like new still.

 

[Spiderman]

Make sure that power valve cable is all good and not broke also. They have a tendency to brake.

 

[Pine Rob]

Make sure that power valve cable is all good and not broke also. They have a tendency to brake.

I gave everything a thorough once over in the whole engine compartment while I had it all apart. Ill keep that in my end thanks for the knowledge.

 

[HECKS]

I just pulled my new top end/cylinders down again after 12 days of riding to find this mess.
According to Indy Specialties, they recommended to remove 1/16 - 1/8" off the exhaust valve tips to remedy this, and they replace valves at 4000 miles. When I replaced the cylinders and pistons I should have put new valves in, but they looked mint still. Sled has 4800 miles. Food for thought.

 

[gtwitch]

This problem as far as I know goes back to at least 2007 700 Dragon that I still have and it is my loaner sled, This grinding of the exhaust valves I did taking .030 to .040 off the exhaust valves, That sled now has over 9000 miles on it is still running strong. I have done that to all of my engines for myself and several for my riding group over the years and have had no problems!!

 

[HECKS]

This problem as far as I know goes back to at least 2007 700 Dragon that I still have and it is my loaner sled, This grinding of the exhaust valves I did taking .030 to .040 off the exhaust valves, That sled now has over 9000 miles on it is still running strong. I have done that to all of my engines for myself and several for my riding group over the years and have had no problems!!

Good to hear. I've been on Polaris' since 1999, and had never heard of this issue....until now. But I have never ran a chassis this long either.
Sure wish I came across this info last season when I replaced the crank and top end with HG7 jugs lol.

 

[DKP]

So you just take a little off the tips?

 

[J270wsm]

Final update

Pulled the power valve assembly and the mag side power valve bolt was missing not allowing the actuator to open the power valve properly. The valves are a little gummy and will be cleaned while the assembly is removed. Going to reassemble with a generous amount of red loc tite and keep sending this old 800.

While all the exhaust and power valves were removed I inspected the exhaust side of the of the pistons and cylinder walls. And for 2700 hard ish miles everything looks like new still.

Had a similar issue last yr with my 17axys. Wife was riding it and got stuck, when I jumped on to get it out, it would start to rev up then fall on its face right when the exhaust valves should be opening. Upon inspection it was a broken exhaust valve.