Seized or not? Old Stihl fs80av

[champagnecharly]

I have an old fs80av. Was working great up until last week. For some reason coil had come loose and rubbed against the flywheel. Anyway.. since, it feels like it's seized. When I pull there is considerable resistance (i think at tdc) . looking at piston and pot there is nothing untoward.. and if I pull with the sparkplug out it pulls fine with no resistance. Any ideas?

[spudulike]

If seized, the saw will feel notch but not really at TDC, pull the muffler off and look at the piston, if seized, it will have scores on the skirt and may have the rings welded in to the grooves.

 

It sounds more likely that the change in ignition timing may have done something to the piston crown through pre ignition - probably worth taking off the cylinder to see what is occuring inside the cylinder.

 

If the saw turns over without the plug and without resistance but is harder with the plug in, it sounds like just decent compression as that is the only difference.

 

You can get puddled fuel in the lower crankcase that can cause this issue - worth turning the engine upside down with the plug out and turn it over to see what comes out.

 

The last saw I had you type of problem with was a 390XP and it didn't turn over when I removed the plug - it has so much carbon in the head, taking the plug out dislodged a big bit that caught the squish band and stopped the saw at TDC.

 

Are you sure the coil is free and there is no play in the crank bearings?

[champagnecharly]

I've taken the pot off. All clear. no grooves. I tried to test compression but couldn't pull enough and at that stage compression was just 60psi so low..

 

Could the play in bearings be due to the knock against the coil? given there is no sign of anything and no fuel top or bottom... would i be guessing right that the bearings have taken the shock?

 

FYI ran tests with flywheel off and away from ignition. Frustrating cause I'd finally just got this running well..

[spudulike]

I've taken the pot off. All clear. no grooves. I tried to test compression but couldn't pull enough and at that stage compression was just 60psi so low..

 

Could the play in bearings be due to the knock against the coil? given there is no sign of anything and no fuel top or bottom... would i be guessing right that the bearings have taken the shock?

 

FYI ran tests with flywheel off and away from ignition. Frustrating cause I'd finally just got this running well..

 

If you hold the flywheel centre and wobble it up and down then side to side, you should feel if there is wear in the crak bearings.

 

60psi is non existant - have you pulled it over 5-6 times hard or is hat just one pull and do you usually get 150 -170psi on a fit engine?

 

If your compression reading is right, you may have a lump of your piston ring missing, damage or cracked piston crown or a small end/big end failure - even with a badly seized engine I would expect 80-100 psi:confused1:

 

It is worth checking the ends, I usually rock the crank back and forth with the engine assembled and feel for excesive play before the piston moves up and down - there should be little play. If the engine is in bits, hold the piston on TDC and lift and drop it up and down and feel for play, it should be close to zero!

 

I have seen excessive play in the gudgeon pin piston seat before!

 

It is also possible you have a bent con rod - the impact with the coil may have done some bad damage! If the engine isn't a clam engine - loosen the cylinder bolts and with the plug out, rotate the engine slow whilst very lightly pushing on the top of the cylinder - any rocking from side to side and the rod is bent. You will also see heavy scuffing on the piston sides and cylinder wall.

 

Have you checked the crank isnt bent:thumbdown:

[champagnecharly]

If you hold the flywheel centre and wobble it up and down then side to side, you should feel if there is wear in the crak bearings.

 

Flywheel not on however lots of movement in all directions from shaft! This must be it.. It also creates a similar noise as when trying to pull.

 

 

60psi is non existant - have you pulled it over 5-6 times hard or is hat just one pull and do you usually get 150 -170psi on a fit engine?

 

If your compression reading is right, you may have a lump of your piston ring missing, damage or cracked piston crown or a small end/big end failure - even with a badly seized engine I would expect 80-100 psi:confused1:

 

I can't pull that much without fear of damaging. That was 2 half pulls.

 

 

It is worth checking the ends, I usually rock the crank back and forth with the engine assembled and feel for excesive play before the piston moves up and down - there should be little play. If the engine is in bits, hold the piston on TDC and lift and drop it up and down and feel for play, it should be close to zero!

 

I have seen excessive play in the gudgeon pin piston seat before!

 

From holdin git in TDC and moving (or trying to move ) piston up and down

results in 0 movement.

 

It is also possible you have a bent con rod - the impact with the coil may have done some bad damage! If the engine isn't a clam engine - loosen the cylinder bolts and with the plug out, rotate the engine slow whilst very lightly pushing on the top of the cylinder - any rocking from side to side and the rod is bent. You will also see heavy scuffing on the piston sides and cylinder wall.

 

From holdin git in TDC and moving (or trying to move ) piston up and down

results in 0 movement.

Also no markings other than what i'd call normal wear. No scores or shardes.

 

Have you checked the crank isnt bent:thumbdown:

 

If it were crank, would i not have difficulty turning even with plug out?

 

I've never changed bearings.. how do I get in that deep? I presume oil seals get pulled out but then?

[Ray1]

Could it be that the flywheel side bearing is shot, when pulled over with the plug in the compression at tdc is causing the crank in the bearing to move and the magnets on the flywheel are coming into contact with the coil, when the plug isnt in there is no compression causing the crank in the bearing to move so the flywheel spins as it should.

[champagnecharly]

It would stand to reason that that was why the problem arose in the first instance. However at the moment with flywheel & coil off, with the plug in there is still an issue.

 

The bearings are easy to come by and at varied prices.. LS engineering sell the "part" at 14 ex each however they can be found from bearing specialists for around £3 a pop down to 1.50.... big difference! Any advice on replacing bearings would be appreciated.

[spudulike]

It would stand to reason that that was why the problem arose in the first instance. However at the moment with flywheel & coil off, with the plug in there is still an issue.

 

The bearings are easy to come by and at varied prices.. LS engineering sell the "part" at 14 ex each however they can be found from bearing specialists for around £3 a pop down to 1.50.... big difference! Any advice on replacing bearings would be appreciated.

 

From your aearlier answer about there being lots of play in the crankshaft, it sounds like the bearing has worn and this has allowed the flywheel to contact the coil doing further damage.

 

Fitting new bearings is ok if you have some engineering experience, splitting the crankcase is the first difficult part, a selection of pullers, a big G clamp and bench vice are useful - if you need to clump the end of the crankshaft, use a mallet and NOT a hammer.

 

Pushing the old bearings out - do this with a suitable sized socket located on the outer race and use a big bench vice or G clamp. Inserting the new bearings - some heat the casing and freeze the bearing to gt an easier fit - I usually just clean the seats and use a socket on the OUTER part of the bearing and use a big G clamp to squeeze them in - make sure they go in the correct way if they are not symetrical and get them going in straight.

 

Never push the bearing in from the inner race!

 

Good luck - it will make a man of you:thumbup:

[champagnecharly]

Any thoughts on bearings and sourcing them?

 

The bearings are easy to come by and at varied prices.. LS engineering sell the "part" at 14 ex each however they can be found from bearing specialists for around £3 a pop down to 1.50.... big difference!

Would the £3 bearing suffice or are there tolerances etc I need to take into account?

[spudulike]

Any thoughts on bearings and sourcing them?

 

The bearings are easy to come by and at varied prices.. LS engineering sell the "part" at 14 ex each however they can be found from bearing specialists for around £3 a pop down to 1.50.... big difference!

Would the £3 bearing suffice or are there tolerances etc I need to take into account?

 

The OEM suppliers will buy their bearings from the big suppliers like SKF, and add their mark up. As long as the bearing is specifiied to the same spec - rpm, OD, ID etc, it should be OK but the manuactures do have a habit of having some wierd sizes!