MS391 wont start after top end rebuild

[ZinTrees]

my dad rebuilt his 391 after cooking the piston (got new hyway piston and cylinder stock replacement)
saw wont start or run
it has air, fuel, spark and compression, it "popped" once then wouldnt start again, pulled carb apart and checked it, its clean, checked spark, good spark and plug gap, coil is connected and correct gap, pulled muffler and put a lighter in the cylinder, it lit up like a Christmas tree (has fuel), and enough compression to pick the saw up with the starter cord

any help? we got stuff to do and we need this fixed asap

 

[PV Hiker]

Just out of curiosity did the reason that the "piston was cooked" get resolved?

 

[ZinTrees]

Just out of curiosity did the reason that the "piston was cooked" get resolved?

milled with a baby saw and melted the piston
"fixed it" and ate a second piston, so now its new bore and piston

 

[67L36Driver]

Glued on transfer port covers?

 

[ZinTrees]

Glued on transfer port covers?

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

 

[Bob Hedgecutter]

milled with a baby saw and melted the piston
"fixed it" and ate a second piston, so now its new bore and piston

Once bitten
Twice shy
Third time a charm?

I will suggest if it got hot enough milling to melt a piston, it got hot enough to cook the crank seals.
Would pay to pressure and vacuum test the powerhead to see if a massive air leak is not your problem.

 

[ZinTrees]

Once bitten
Twice shy
Third time a charm?

I will suggest if it got hot enough milling to melt a piston, it got hot enough to cook the crank seals.
Would pay to pressure and vacuum test the powerhead to see if a massive air leak is not your problem.

so third time a bust

I will see about a pressure test

 

[Binthewoods]

If it popped once I would double check the flywheel and make sure you didn't shear the flywheel key. If it's not cleaned and torqued down it could have sheared and now your timing would be off.

 

[HarleyT]

Popped once, means that you didn't torque down your flywheel good enough.
Usually.

 

[ZinTrees]

Popped once, means that you didn't torque down your flywheel good enough.
Usually.

If it popped once I would double check the flywheel and make sure you didn't shear the flywheel key. If it's not cleaned and torqued down it could have sheared and now your timing would be off.

its tight and timed good, only thing I can think of is possibly cylinder seal thickness making the timing off

 

[HarleyT]

Rereading your first post, it might just be flooded.

 

[ZinTrees]

Rereading your first post, it might just be flooded.

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

 

[Bob Hedgecutter]

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

Okay, you have all the answers- but you do not have the one answer you need.
If you have spark, correct air fuel mixture, it is all arriving at the correct time and compression is grand (albeit via drop rope testing), then it has to be idiot factor.
Human influence, something missed or incorrectly assembled after teardown of the top end or carb.

 

[ZinTrees]

Okay, you have all the answers- but you do not have the one answer you need.
If you have spark, correct air fuel mixture, it is all arriving at the correct time and compression is grand (albeit via drop rope testing), then it has to be idiot factor.
Human influence, something missed or incorrectly assembled after teardown of the top end or carb.

exactly

 

[ZinTrees]

we ordered new crank seals, might now be the problem, idk, but for $10 its worth the shot, will give it a good look over when we pull it apart, and see whats up with it

for now, our only running saw is a poulan (and to quote lumberjack matt "Poulan pro, the trees will go")

 

[Woodslasher]

we ordered new crank seals, might now be the problem, idk, but for $10 its worth the shot, will give it a good look over when we pull it apart, and see whats up with it

for now, our only running saw is a poulan (and to quote lumberjack matt "Poulan pro, the trees will go")

I heard from a friend that "They're called Poulans because when you go to start them you gotta poulan 'em and poulan 'em and poulan 'em...."

 

[ZinTrees]

I heard from a friend that "They're called Poulans because when you go to start them you gotta poulan 'em and poulan 'em and poulan 'em...."

this one will start 2nd or 3rd pull every time (cold, 1st pull warm)

 

[Bob Hedgecutter]

this one will start 2nd or 3rd pull every time (cold, 1st pull warm)

So I'm guessing you have never pulled the Poulan apart and put it back together again?

 

[ZinTrees]

So I'm guessing you have never pulled the Poulan apart and put it back together again?

as far as I know (not my saw) yes
although I did replace the fuel lines, filters (fuel and air), and primer bulb, runs great now, has a little bog when you first hit the throttle but if you ease on it its good

 

[Bob Hedgecutter]

as far as I know (not my saw) yes
although I did replace the fuel lines, filters (fuel and air), and primer bulb, runs great now, has a little bog when you first hit the throttle but if you ease on it its good

Because it still runs and the 391 doesn't.