MS362 Carb and Compression Issues

[PogoInTheWoods]

I have a MS362 on the bench that starts and runs from high idle on up but won't idle..., period. Has a Walbro WTE-8 carb with accelerator pump and the diaphragms still look decent. It's also only blowing 128psi of compression though the piston and rings look new through the exhaust port.

Anyone experience accelerator pump issues with this carb (similar to the 200T Zamas)?

I'm also thinking maybe the low secondary compression at less than 130psi could result in the primary compression being too low to create enough impulse to drive the pump diaphragm at idle. FWIW, the carb has the amber (teflon?) pump diaphragm which I believe is the most flexible of the ones I'm familiar with (blue, black, clear, amber).

I have no idea what the compression should be on a MS362 but I'm guessing 160 ~170 (and the decomp is there for a reason).

Thanks in advance for any input or enlightenment.

 

[tree monkey]

does it just die? and at what rpm? does it sound rich/lean? do you need to choke it to restart it?

 

[PogoInTheWoods]

Thanks for the reply.

Sounds relatively normal at high idle and WOT. A hair lean if anything. Limiter cap in place on the H. Can't get the L dialed in at all to even hear rich or lean there.

It just comes off WOT after releasing the throttle (or high idle after a blip) and dies. And it does need choked to restart. Holds vac/pressure all day long. Fuel line and filter are fine. Tank vent works as it should (holds pressure/allows vac). I didn't check popoff. Did do a few cycles in the USC before putting the carb back on. Same results. Just got the WTE kit this afternoon for new diaphragms. Has the pump 'o' ring included along with the other normal goodies..., except the metering lever spring, of course. Has the amber pump diaphragm.

Also got new rings for it just because. 128psi seems awfully low for that saw. Curious to see what the end gap is before and after.

 

[drf256]

Sure your comp gauge is correct?

Sounds like the carb is bad.

As dumb as this may sound, did you toss a new plug in it?

 

[PogoInTheWoods]

Gauge reads 2psi low compared to a Snap-On.

I agree the carb isn't working right. Not 100% convinced it's actually bad. Could be the accelerator pump 'o' ring is shot and sucking air when the throttle plate is in the idle position. Could be weak impulse due to the low compression creating a similar (but markedly worse) condition than the infamous HDA-199 problem..., or both. Could be the metering diaphragm or spring...
Gets the accelerator pump inspection and new kit today. Not sure I want to put it back together without the new rings just to test the carb with a new kit, tho. Then again...., if I do both and it then runs right I'll always wonder what fixed it.

Also, Used the plug that was in it. Looked fine. Has spark you could weld with.

 

[Chainsaw Jim]

The symptoms point to a bad carb diaphragm.

 

[PogoInTheWoods]

Got sidetracked today on a 200T with the C1Q accelerator pump problem (fixed). May still get to the 362 carb this evening.

 

[PogoInTheWoods]

Back to the 362 carb today with the fresh WTE kit. Pulled the accelerator pump plug out after removing the throttle rod and the piston just stayed in the bottom of the chamber. Had to give it a nudge with a pic for it to spring up like it should have. 'O' ring looked ok but was definitely larger than the one in the kit and my have been hanging up? Pump piston moves freely now but the new 'o' ring seems to allow for too much slop to seal in the chamber like it should. Sheesh. Putting it back together with the new 'o' ring anyway to see what happens. Saving the old one just in case...

Anyone else replace the 'o' ring with the one in the kit with good results? Or has anyone bypassed the pump circuit with success like is done with the 200T C1Q's.

Or has anyone had good results using one of the aftermarket replacement carbs as a last resort?

 

[Lightning Performance]

You can bypass the pump circut but I doubt that is what's up. I have coverd the lean gap with a rich low till the carb fix. The Oring gets swollen from E gas. Dry it out and it should shrink. If the surface bleeds black it's done for.

Case in point. The 200T in my box has the dead pump now.

 

[PogoInTheWoods]

So the replacement 'o' ring is probably the correct size and this could be as simple as the old one swelling to the point of jamming the pump circuit open? Hmmm. Guess I'll find out..., sorta. Still have the other variable of the low compression possibly affecting the impulse to the pump diaphragm at idle.

 

[Lightning Performance]

I'd be searching for that low compression issue. TM is usually right. Even he would find that issue first.

My 362 blows 170 with gasket delete no port job. Golf piston with cabers. Work saw. Piston was a test. OEM was toasty. It needed work to clear the crank. OEM clips, pin and bearing. Same as stock piston specs. Gained weight. 518 goo o.o14 quench.

 

[PogoInTheWoods]

The low compression pretty much has to be the rings since the saw will hold vac/and pressure all day long. That said, there's definitely a carb question now, too.

I'll throw it back on with the new diaphragms and pump 'o' ring to see what happens. A little searching seems to indicate there isn't an aftermarket knock-off available for the WTE-8 either.

 

[PogoInTheWoods]

We have idle. Yay. Carb rebuild including accelerator pump 'o' ring got it back in business.

Had to wind out the low quite a bit to find any respectable spool up response but at least it settles right in at 2800 and is a one pull hot start like it should be, now. Left the rings alone and obviously still have the compression issue as a result..., and question what kind of power it's gonna have in a cut. Sure as hell don't need the decomp to start it. Hell, that would just flood it anyway.

Soooo...

I'm chalkin' this one up to a stuck accelerator pump due to a swollen 'o' ring. The owner of this saw is 85 years old and is still cuttin' trees down for a living. I'm guessing he'll actually appreciate the lower compression..., at least when it comes to starting it. Never used the decomp in the first place and still uses the choke to shut a saw down. "Been doin' it that way for 70 years and ain't about to change now." LOL

Thanks for the input, fellas.