338xpt issue ideas

[mweba]

Back ground: Frankensaw built from a box of 338 parts. 338 engine, flywheel, coil, and carb. Saw ran a solid day with out issue. Now at random it runs with a "miss". Can cut with it for an hour sometimes, five minutes the next.

Changed coils and plugs same results. Checked flywheel but do not have a spare to throw at it. Different carb...... same results. Need fresh eyes for this one guys. May be something simple I'm overlooking.


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[SkippyKtm]

That's a toughie, I know you already changed the coil but here goes anyway...I had a 372 with a blue coil that ran like that, it would only idle no matter what ya did. I found out it was a bad coil. I changed the coil to the black, unlimited variety and the issue was gone. Another thing you might want to check is if the muffler is plugged, pull the screen out and see if that makes a difference. I usually clean the screens with a torch.

 

[mweba]

That was my train of thought as well. I have another coil at the shop all be it very unlikely, there is a reason that coil was pulled from another saw. Also I have not pulled the kill lead off of the coil. There may be an odd short there. It seams the timing advances considerably and it fights itself to run.

Earlier I was suspecting the auto decomp but not sure how that would cause this issue.

 

[SkippyKtm]

That was my train of thought as well. I have another coil at the shop all be it very unlikely, there is a reason that coil was pulled from another saw. Also I have not pulled the kill lead off of the coil. There may be an odd short there. It seams the timing advances considerably and it fights itself to run.

Earlier I was suspecting the auto decomp but not sure how that would cause this issue.

Let me know what you find! It sure seems to be acting like a bad coil, is it blue by chance?

 

[TK]

I'm not sure how much the auto-decomp may drop compression, but most standard decomp valves only drop it approx. 10psi and they will still run. You can hear the difference as it runs like an air leak fluttering if the valve sticks open.

 

[mweba]

Let me know what you find! It sure seems to be acting like a bad coil, is it blue by chance?

Both coils are black. I put a tach to it and it shows 2800 at idle when it is messing up. So either the timing or the issue is elsewhere. Should get back to it later tonight.

 

[CJH1]

Is there any slop in the flywheel keyway? That might throw timing off.

 

[pgg]

Sounds like the timing is shagged for whatever reason. That idle is lumpy as hell. Dodgy flywheel? Dodgy coil would have to be No1 suspect, they DO crap out on those XPT's, I've been thru a few on my 335's, usually bloody hard to start though with that problem, they're the same coil as on all the 3-series huskys right thru to 395's and their like, don't seem to be super-compatible with the tiny flywheel on the compact top handle models. Having spare parts that are also unknowingly dodgy really screws with logic when trying to pinpoint problems. Pinhole in HT lead arcing to the cylinder head is another thing to check

 

[blsnelling]

Is this a test, lol? Sounds like your chain brake is on. It doesn't rev up at all when you pull the throttle.

 

[mweba]

Is this a test, lol? Sounds like your chain brake is on. It doesn't rev up at all when you pull the throttle.

Ha ya a test of sorts. Tell me whats wrong and you pass lol. Chainbrake isn't on but it will not rev. Grabed another coil.

 

[blsnelling]

Ha ya a test of sorts. Tell me whats wrong and you pass lol. Chainbrake isn't on but it will not rev. Grabed another coil.

You dog, lol

 

[mweba]

Got this saw back on the bench last week. Threw a new coil on it and same issue. Pulled the flywheel to check key, no issue. Ended up hanging a screw driver on the magnets. One would hold it and the other would not so I swapped with another saw. Seems one magnet lost most of its magnetism. Saw is running good so far. Will keep you undated if things change.

 

[SkippyKtm]

Sounds like we were on the right track, eh?:tongue2:

 

[CJ1]

Got this saw back on the bench last week. Threw a new coil on it and same issue. Pulled the flywheel to check key, no issue. Ended up hanging a screw driver on the magnets. One would hold it and the other would not so I swapped with another saw. Seems one magnet lost most of its magnetism. Saw is running good so far. Will keep you undated if things change.

Thats one I have never thought of, good catch. CJ