MS391 wont start after top end rebuild

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can you explain what that would be? I dont know much about these

e512d6b356e1e961c0a7e7973279d420.jpg


Covers glued on to the flywheel and clutch side of the cylinder.

Likely made to simplify the die casting process.

They will develop an air leak.

I think that thermal expansion & contraction breaks the glue bond because the covers stay some cooler than the cylinder.

I discovered the leak in the above saw by spraying carb cleaner along the sides while I kept it running.
 
e512d6b356e1e961c0a7e7973279d420.jpg


Covers glued on to the flywheel and clutch side of the cylinder.

Likely made to simplify the die casting process.

They will develop an air leak.

I think that thermal expansion & contraction breaks the glue bond because the covers stay some cooler than the cylinder.

I discovered the leak in the above saw by spraying carb cleaner along the sides while I kept it running.
the ones on this new cylinder are one part as far as I can see, and never ran so there should be zero chance of it having a leak
the old cylinder felt the same as this when trying to start it, would pop and nothing, but it the issue was a destroyed cylinder (this one is amazingly perfect)
 
we burned the fuel out with a lighter, and have had it sitting in the shop (90*) for a few hours, I dont think its flooded

WOT, and try to start it. Best way to un-flood a saw. Also open the throttle and with compressed air, blow the intake while slowly turning over the engine. I have had to deal with a neighbors flooded saw, that I thought for sure wasn't flooded. Took a lot, I mean lots and lots of pulls and suddenly it started.
 
WOT, and try to start it.
we have tried, lots, and lots, and lots of pulling (im actually sore from it), nothing, we had the carb/intake apart and its all good, not flooded, cylinder isnt flooded either

even with an air leak at the crank (we have new seals on the way, gonna put them on tomorrow) it should still run (I think), it has everything it needs to run (compression, air, fuel, and spark) im just completely stumped

if it doesnt work after we put the new crank seals in, im gonna be a little upset, since its so simple, and I should be able to fix it, it has everything it needs to run, but it just wont run
 
we have tried, lots, and lots, and lots of pulling (im actually sore from it), nothing, we had the carb/intake apart and its all good, not flooded, cylinder isnt flooded either

even with an air leak at the crank (we have new seals on the way, gonna put them on tomorrow) it should still run (I think), it has everything it needs to run (compression, air, fuel, and spark) im just completely stumped

if it doesnt work after we put the new crank seals in, im gonna be a little upset, since its so simple, and I should be able to fix it, it has everything it needs to run, but it just wont run

If you are pulling like a schoolboy and it is not flooding, nor firing, nor running- then it does NOT have everything it needs and crank seals (although new will help the saw in general) are not likely to help your current situation.
 
I would recheck the key first. Same thing happened on my MS460 when I attempted to start it the first time after rebuild. Thought I had it tight enough. Replaced key and torqued flywheel nut to spec and she fired right up. Also could be flooded. Even if you burned out the cylinder the crankcase is still full of gas. Set lever to run, full throttle and pull.

Good luck!
 
I'm going to go back to this-
pulled carb apart and checked it, its clean

IF the saw is indeed not flooding, I am reverting back to the little incidence of the carb being "pulled apart" and maybe something was not put back together correctly, or in the correct placement, or correct order- thus preventing fuel getting from the tank to the cylinder as a combustible mix.
Direct cylinder prime should prove or disprove the theory- but my guess is "we have already done that."
 
fuel is a yes, and it is in the bottom of my heart tank, checked fuel lines, all good, poured a little gas into the cylinder, nothing, only option after that is spark and air, we checked spark was good, and the gap is correct, checked coil gap, its good, quite strong spark
I highly doubt it, but maybe its not getting air? if ive force fed it gas, spark is good, compression is good, then it can only be air (someone please say im wrong, I dont want it to be the most likely impossible problem)
 
If you are pulling like a schoolboy and it is not flooding, nor firing, nor running- then it does NOT have everything it needs
1625191077980.png

it has compression, fuel, and spark, air shouldnt be an issue (no air filter, shouldnt have any restriction)
maybe the carb is just THAT far out of tune? (We pulled the limiter caps, which now makes the stihl manual useless as it is instructing a stock tune with limiters)
anyone got a tune that works on a 391 without limiter caps?
 
fuel is a yes, and it is in the bottom of my heart tank, checked fuel lines, all good, poured a little gas into the cylinder, nothing, only option after that is spark and air, we checked spark was good, and the gap is correct, checked coil gap, its good, quite strong spark
I highly doubt it, but maybe its not getting air? if ive force fed it gas, spark is good, compression is good, then it can only be air (someone please say im wrong, I dont want it to be the most likely impossible problem)

If the combustion chamber was in total vacuum, maybe there would be no air in there- but at the moment there is air, trust me.
 
Could be- at an outside chance, the plug sparks just fine outside the cylinder, but inside and under compression- it is not.
I cannot be bothered reading back through- but you have tried a new plug? I mean that is like the first and foremost you try when a saw doesn't run.
 
I hope you counted the number of turns it took to gently seat each needle after you pulled the limiter caps. If you didn't now you don't have a baseline. I assume you didn't so I would guess 1.25 turns out on each should get it to run.

My advice for the next one. Just work your way back through the parts you touched and verify whether they work. Don't start poking around all willy nilly changing things and expect to diagnose it.

I just had a thought, did you change/check your coil to flywheel gap? If you're too far out it may have spark but not have enough voltage to fire under compression.

This is about all I have to offer now.

Otherwise :dumb2:
 
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