Saw dies soon after starting - dealer says scored.

[esean]

Trying to help a friend with a lightly used Stihl MS192T. Saw will start, but dies after 10-60 seconds of idling. Will start to rev just a little if you ease very gently into the throttle, but too much kills it immediately. Idle speed seems fine. Adjusted L but no difference. Dealer told him it looks like it was scored from straight gas, which owner denies, but he did probably use E10 that sat too long. I pulled the muffler and could see some light scratches on the piston, and possibly a slight bit of transfer on the cylinder, but nothing like the pics I've seen here of saws that are obviously toasted. I don't currently have the saw to get pics.

Owner said the saw was running fine the last time used, which was a couple of months ago. I'm new to saw repair and diagnosis, but this sounds like a fuel system problem to me, especially given the history of using old ethanol gas. Are these symptoms consistent with scoring? Would a good compression test rule out scoring?

Thanks!

 

[buzz sawyer]

Scoring might result in a low compression reading but a low reading might also be due to an air leak, which could cause the scoring problem. I would also do a vac/pressure test to look for a leak. Can we see a pic of the scoring you described? btw, how old was the fuel?

 

[esean]

The owner has the saw so I don't have quick access for pics. Don't know exact fuel age, just know he used E10, never paid attention to how old it was, ran the saw infrequently over 7 years, and never drained the saw - so it's a safe bet the fuel got pretty stale at times.

 

[lone wolf]

The owner has the saw so I don't have quick access for pics. Don't know exact fuel age, just know he used E10, never paid attention to how old it was, ran the saw infrequently over 7 years, and never drained the saw - so it's a safe bet the fuel got pretty stale at times.

Pics are key to online diagnose!

 

[Saw Fixr]

Sometimes, just adding a liquid carb cleaner into the tank and let it run/sit a few times will do the trick. Otherwise take the carb off, clean it and reinstall. Tune accordingly.
This of course, is assuming that the scoring on the cylinder isn’t too great. Worth a try since you said that the saw does run....

 

[cuinrearview]

You admitted yourself to seeing aluminum transfer on the cylinder wall, yet you're questioning the same diagnosis by a dealer? Tell me what I'm missing? If the little window into the jug through the exhaust shows some scoring, imagine what you can't see. Time to pull this saw down.

 

[Ronie]

My money is on an air leak. When it's idling, spray some carb cleaner around the seals, base of the cylinder and the carb boot. If you get a change in rpm, you have a leak.

 

[esean]

I could see some light streaks that MAY transfer, but I don't know exactly what to look for. Not like pics I've seen here of cylinders that most say just need cleaned up, and not much worse than the cylinder in one of my good saws. I just figured since it starts normally and runs (for a bit) that compression may still be adequate and the dealer was just defaulting to the "straight gas" diagnosis to try and sell a new saw. Thanks for the ideas - I'll see if the owner wants to investigate further. Agreed that I need pics before I can expect you to give me a reasonable diagnosis.

 

[m21bmr]

If you are diagnosing the saw further...the MS-192 effectively has the carburetor mounted in the rear of the top handle and it is connected to the cylinder with a rubber boot. This boot is an area to check closely for air leaks.
More often though, when I have been chasing air leaks, the engines will rev higher than expected, not refuse to rev with throttle application. In my experience, failure to rev when throttle is applied is more likely a fuel delivery or low compression issue.
In your first post, you mentioned adjusting the L screw. Try opening up the H screw some to see if it will rev up better then. As a test, I would screw the H screw clockwise to gently seat the screw, and note the number of turns it takes to seat. Then back it out about 1/4 turn more than it was initially set at and give that a try. It is possible (probably likely) that if the carb limiters may not allow you to fully seat, or open the H screw any further than it already is. If you can perform a compression test, that would also be a good piece of information to have. If compression looks good (I'd hope for 120 psi or more), a new carburetor might solve the problem.

 

[MacAttack]

In my personal experience, stale premix runs like s--t and doesn't want to start, but doesn't cause damage to the piston/cylinder. A lean condition, or straight gas causes damage.

 

[stihl86]

In my personal experience, stale premix runs like s--t and doesn't want to start, but doesn't cause damage to the piston/cylinder. A lean condition, or straight gas causes damage.

It's my understanding old stale fuel has reduced octane levels.
That can wipe out a cylinder and piston.

 

[MacAttack]

It's my understanding old stale fuel has reduced octane levels.
That can wipe out a cylinder and piston.

If you are talking about detonation/ pre-ignition due to low octane i would agree, that will destroy an engine if it has enough compression or heat in the cylinder.
You could be correct, I've just not personally experienced engine damage in 2 strokes with stale premix, they just start and run badly.

 

[SteveSr]

In my personal experience, stale premix runs like s--t and doesn't want to start, but doesn't cause damage to the piston/cylinder. A lean condition, or straight gas causes damage.

Considering that 2-strokes are significantly fuel cooled engines it is the high volatiles in the fuel that do the cooling. With old, stale fuel these volatiles have already "left the building" and the fuel that remains can't provide sufficient cooling leading to overheating and scoring. The leaner that you are currently running (as in newer limiter cap saws) the more likely you are to have overheating problems with old fuel.

 

[RED-85-Z51]

Stale gas usually shows up as a black piston for me..alot of heat and the oil breaks down

Sent from my LM-G820 using Tapatalk

 

[MacAttack]

Stale gas usually shows up as a black piston for me..alot of heat and the oil breaks down

Sent from my LM-G820 using Tapatalk

Honestly I can usually tell if gas is ok from a smell test... if it has any hint of staleness, i just dump it. Has not failed me yet.

 

[Okie]

esean; the OP

Couple thoughts:
Do you have a reliable compression tester for small cc engines? (by reliable I mean a comp tester that you know reads chainsaw compression accurately)

If so I would suggest a compression tests and it should be at least 120 PSI.
If the compression is good I would suspect fuel related issues. If it will start easily, sit and idle but the throttle has to be lightly feathered to get the rpms up is symptoms of fuel related. Air leaks most generally cause no idle or a rough idle. You can try the carb jet adjustments mainly because you do not know where the other guys left the jet adjustments.

Most of them saw shops want to sell new saws instead of do any non-warranty repairs. (therefore I take any info that they provide with this thought)
After the saw shop tells you your saw is no good they will usually gladly sell you parts and pieces so as you can try to fix the saw yourself. (just setting on a limb like a buzzard waiting for you to return for a new saw)

Constantly using E10 in small engines is also just asking for troubles also, so pay them now or pay later and You can Thank Al Gore. and the other swamp dwellers for the E10 grain alcohol for getting the idea of making fuel from food to keep prices low and NOT pollute the enviro. Heck Ray Charles could see that was not a good idea.

 

[RED-85-Z51]

Honestly I can usually tell if gas is ok from a smell test... if it has any hint of staleness, i just dump it. Has not failed me yet.

Yes, but doing a failure analysis after a tank of garbage fuel and a half tank of fresh went thru and it has no power...i usually find a blacl piston, stuck rings...signs things got very hot

Sent from my LM-G820 using Tapatalk

 

[esean]

Okie - I don't have a tester and don't plan on doing a teardown myself, I'm just helping the owner decide what to do next. I'm thinking along the same lines as you: if compression checks out OK, then time to start on the carb and fuel system. If compression is bad, then the shop might have been right. I need to see if the shop already checked compression, the owner is clueless as to how they made their diagnosis. Thanks!

 

[Mad Professor]

Stale E10 mix will pick up water and phase separate. The water/ethanol phase has no lubricant in it.

 

[SteveSr]

Okie - I don't have a tester and don't plan on doing a teardown myself, I'm just helping the owner decide what to do next. I'm thinking along the same lines as you: if compression checks out OK, then time to start on the carb and fuel system. If compression is bad, then the shop might have been right. I need to see if the shop already checked compression, the owner is clueless as to how they made their diagnosis. Thanks!

Forget the compression test. I have seen 2-strokes with relatively good compression still have scored pistons.

The foolproof scoring test is to just pull the muffler and have a look at the piston. Since this is a top-handle saw I don't know how involved this might be. @lone wolf or someone else might be able to help. For a regular saw it is usually just a couple of bolts.