jdlmodelt

the compression new on these is 181-210psi. I think you indicated you have from 60-80psi? 

The numbers you posted for compression are pretty low for hot or cold. The carburetor isn't making your compression go down when the engine is hot. There is something mechanical that causes your compression to drop when hot. It's either as someone else indicated, decompression mechanism holding the exhaust valve(s) open, or scoring on your cylinder wall and piston, or some other mechanical issue with your valves. At this point, you would spend less time if you just pulled the top end off unless you are able to determine the decompression mechanism is the culprit.

On 2/3/2021 at 7:20 PM, freebo86 said:

Thats the issue. So many that I don’t even know where to begin, lol. Quite overwhelmed and frightened considering seems like the possibility is endless along with the money train that I can throw at it. 

Does this model have the fuel pump that ticks when its pumping? My fuel pump is fairly audible and It was a semi continuous to semi random ticking rythm.

13 minutes ago, Goober said:

Ok thanks for catching me up. very good. So ignition is normal, it starts up easily with the choke, runs well, even after warmup. then while riding it dies suddenly and without getting hot or other symptoms?

Valve train noise since you fixed the muffler?
 

plug doesn’t look terrible sooty.

 

Not overheating. Fan kicks on? Were you riding it and fan kicked on or stationary?

 

does your choke cable stick at all? choke valve seems to be working. Lube the cable to be sure it’s free.

 

you have low compression while it’s warm and higher compression when cold?
 

I like Retro’s suggestion. 

The decompression interfering while warm/hot makes sense with regards to the compression. The only other thing I can think of is do a top end overhaul. Sounds like you'll see things in there that you can't see right now. Mine had cupped rockers where they ride on the cam. Cam was fine. My top (intake) side of piston and cylinder were scored due to overheating or lack of lubrication. Now, new rockers, bored out 0.5mm and new Wiseco 10:1 piston and she runs.

it makes what I would describe as a chirping sound.

Haha! Agreed. I've never been much of a Polaris fan.

3 hours ago, Goober said:

Yup there’s a better explanation for the primary adjustment in the manual than what I could do. And it usually takes me a few trials to tune the carb—engine gets too hot for me to get the pilot screw the way i like it.

 

also—did you change the choke valve? Does the engine choke right away when you activate it? If so it’s probably ok

The choke works great. This engine starts in single digit F temperatures great. I did notice I've lost a slight bit of my power after that adjustment I did on the metering rod to butterfly valve, so, I will play with that some more. I had to put my own project aside to work on a guy's 2002 polaris 500 ATV. It acted like it had jumped a tooth on the cam chain. It was lacking some compression and backfiring when attempting to start. It ran fine when last parked. Turns out the spark plug threads in the head were down to 2 or 3 threads and the spark plug was just wobbling around in the socket area. I'll need to do a helicoil repair on that one next week. Then I have to rebuild the top end on Dad's Hisun 500. That UTV has been a total POS. But we keep it limping along. 

On 2/3/2021 at 7:20 PM, freebo86 said:

Thats the issue. So many that I don’t even know where to begin, lol. Quite overwhelmed and frightened considering seems like the possibility is endless along with the money train that I can throw at it. 

Does this model have the fuel pump that ticks when its pumping? My fuel pump is fairly audible and It was a semi continuous to semi random ticking rythm.

On 1/18/2021 at 9:16 PM, retro said:

My .02 cents on the valve train noise (I fixed a 450 with ALL of these conditions about a year ago):

 

I am guessing that the valve lash was run excessively loose for so long that the top of the valve stems are hammered out & cupped. The lash adjuster screws are mushroomed bad too, so no way ya gonna be able to properly adjust the lash on either valve using a flat feeler gauge.

 

Also the camshaft and lifters may have become worn & hammered out on the base of the cam lobe ramps, so much that valve lash may be a lot looser at the bottom of the lobe ramps compared to the centers of the base circle (where lash adjustment is done). The cam chain & tensioner block is probably shot too, due to all the hammering shock over time.

 

So.... cam, lifters, chain, tensioner, both valves and both lash adjuster screws are probably shot. Hammering shocks may have loosened up a valve guide in the head too. A guide might be moving up & down with it's valve while its running.

 

Inspect the lash screws and valve stem tips (look for a loose guide too) to learn whether ya might have serious damages or not?

This was the case with my 88 TRX350D Foreman. Rocker surfaces that ride on the cam were so worn they were cupped. I was fortunate that everything else was still within spec. I replaced rockers and installed a new piston 0.5mm oversized and it runs like a champ now. And it was super noisy no matter what I did to adjust the rocker lash. Now its super quiet.

So I picked up a 135 as that was what the shop had in their assortment of jets. I get home. pull mine out and observe with a magnifying glass and wouldn't you know. Mine already has a 132! Lol! I did readjust the jet metering tube synchronization with the butterfly valve so the metering tube is resting on the metering jet at idle. When I adjusted the synchronization with the idle screw all the way off while overhauling the carbureator, I had the two opening at the exact same time, however, after installing, the idle screw has to be advanced a few turns to get to engine idle, and consequently, lifts the jet meter tube allowing more fuel. This may have been the contributor to the plug running rich and mostly at idle which should only be getting fuel through the air/idle adjustment screw? It's my understanding the meter jet and rod are the acceleration and high idle fuel supply? Hope I'm making sense.

Hello, I recently had my 88 Foreman 350D bored out 0.5mm and installed a Wiseco 10.25:1 piston. I'm running 91 octane fuel as that's the best I can get here in Colorado. I live at 6400 fee elevation. There is what I would describe as a chirpping sound at low rpm loads. I'm wondering if this is engine pinging and if anyone has determined a way to adjust the spark timing as this is my machinist's recommendation?

Other than the chirpping sound, the engine is super torquey and strong.

Thanks,

James