Stihl 051av quits running when hot.

[Bob Rooker]

My Stihl 051av starts and runs great until it gets warmed up real good then it shuts down. After it cools off it will start and run great again until it gets warmed up. I suspect it is the ignition module (coil). What do you guys think? Thanks, Bob R

 

[Wood Doctor]

High speed carb setting could be set too lean on the top end. Open that high-speed set screw up a quarter turn and see if it does it again. I think that's worth trying first.

 

[Bob Rooker]

Thanks, I will give that a try although I had it set a little more than one turn out on the high speed jet but as you say, it's worth a try!

 

[NewToStihl]

I suspect it is the ignition module (coil). What do you guys think?

I think you hit the nail on the head Bob. Every saw I've ever seen with those symptoms (running fine, quit when it got good & warm, would run again when cool) had a bad ignition module. Run it again and when it gets warm and stops, quickly check for spark.

 

[Wood Doctor]

I think you hit the nail on the head Bob. Every saw I've ever seen with those symptoms (running fine, quit when it got good & warm, would run again when cool) had a bad ignition module. Run it again and when it gets warm and stops, quickly check for spark.

I agree. That's the next step. However, when the IM goes out, the saw usually just shuts down even at low speed with no spark or you can't even start it. His saw is vintage. So, the IM is a prime suspect, no doubt about that.

 

[Bob Rooker]

I think you hit the nail on the head Bob. Every saw I've ever seen with those symptoms (running fine, quit when it got good & warm, would run again when cool) had a bad ignition module. Run it again and when it gets warm and stops, quickly check for spark.

Thanks for the confirmation of my assessment of the situation. I will try to locate a new one, probably on e-bay since they stand behind the products that are sold on their site, unless you can suggest a better place.

 

[Bob Rooker]

I agree. That's the next step. However, when the IM goes out, the saw usually just shuts down even at low speed with no spark or you can't even start it. His saw is vintage. So, the IM is a prime suspect, no doubt about that.

And thank you, Wood Doctor, for your input on my problem. Appreciate it!

 

[NewToStihl]

Thanks for the confirmation of my assessment of the situation. I will try to locate a new one, probably on e-bay since they stand behind the products that are sold on their site, unless you can suggest a better place.

The 'bay is as good of place as any to find one. I have a decent pile of NOS parts, so I'll take a look around, but I don't think I have one.

 

[alexcagle]

Is yours a points style ignition, coil with a trigger unit, or a "one piece"?

 

[Bob Rooker]

Is yours a points style ignition, coil with a trigger unit, or a "one piece"?

It does not have points and I don't know what a "coil with a trigger unit" or a "one piece" is. Can you enlighten me? Thanks.

 

[Kenskip1]

Before you start tossing parts at it how about doing a compression check.A saw of this age more than likely has some hours on it. A compression check would be a wise way to go.As the engine get hot everything expands. This includes the cylinder ring gaps, and all associated parts.When cold the cylinder, piston and related parts contract thereby increasing compression and allowing the saw to run. Until it gets hot again. JMHO, Ken

 

[Bob Rooker]

Before you start tossing parts at it how about doing a compression check.A saw of this age more than likely has some hours on it. A compression check would be a wise way to go.As the engine get hot everything expands. This includes the cylinder ring gaps, and all associated parts.When cold the cylinder, piston and related parts contract thereby increasing compression and allowing the saw to run. Until it gets hot again. JMHO, Ken

What should the compression be? Thanks.

 

[Kenskip1]

For this saw I will take a stab and say someplace in the 135 PSI range.It varies from saw to saw, however this saw is not a high winder like some of the newer hot rods. This is what my 041 has and runs very well. I would take a reading when cold, then hot.This should tell you something. From your description I will say possibly worn rings. Ken

 

[bulletpruf]

If it starts up and runs great then then quits, I'm going with ignition module, too. No harm in doing a compression test, but I've never seen an engine with worn rings run great then suddenly quit because of worn rings.

 

[Kenskip1]

How about going to plan "B" a partially clogged gas filter?

 

[Bob Rooker]

How about going to plan "B" a partially clogged gas filter?

I recently rebuilt carburetor and installed new gas filter but thanks for the suggestion.

 

[Bob Rooker]

For this saw I will take a stab and say someplace in the 135 PSI range.It varies from saw to saw, however this saw is not a high winder like some of the newer hot rods. This is what my 041 has and runs very well. I would take a reading when cold, then hot.This should tell you something. From your description I will say possibly worn rings. Ken

I got busy today and did not have time to check compression. Will try to remember to do that first thing tomorrow. Thanks.

 

[Bob Rooker]

Kenskip1, The compression on the saw is 140 psi cold and 143 psi hot. I don't believe it is the compression. It starts good after it warms up but bogs down when I run it hard.
I'm sure that is when the IM gets hot enough to stop working properly. Unless someone has a better idea I am going to get a new IM and install it. I had a similar problem with my Husky 371xp and a new coil remedied it but this is a Stihl, not a Husky and I wanted to get some input from you guys before I made the investment in a new IM. Thanks to everyone for all the suggestions. Bob

 

[Kenskip1]

BR, for a saw of this age that is very good. However I am a little puzzled.Hot the compression usually causes the pressure to drop.However I will say that there is some unburned oil in the cylinder.How about the gas tank vent? Going back to when the saw quits, does the engine race high for a second then die? How about checking for a bad crankshaft seal.The seal gets hot expands and the engine looses vacuum and dies out.Next time it stops, pull the plug and set it on the saw and pull the rope. If you have spark then it is more than likely not the coil. I am running out of theories.Next time it quits pull the air filter of and put in the carb some mix. See if it starts, then dies. Here is a long shot,underneath the carb there is a vacuum line going from the bottom of the carb heading to a small port in the crankcase. Check for a cracked line.My 041 has this but I am not sure about the 51.Whew! Ken

 

[Bob Rooker]

You have given me a lot to consider. Thanks for your time. I appreciate it! Bob