Dolmar PS5100 engine racing after a few cuts.

I was just out cutting with my 5100 and after a few cuts it would not idle down. The engine just wanted to keep racing.
Even set the switch to stop and it kept racing till i put the chain against a log to stop it. Any ideas ?

Sounds lean or hot.
How old is the saw ? How long have you had it ? Is the air filter cover set to winter or summer ? Does it have the metal ring in the carb. boot ? Is the fuel fresh ? Does it have the correct spark plug in it ? Is there sawdust packed in behind the starter and in the fins of the cylinder ? (The air-cooling system isn't the best to start with on these saws. They improved it on the 5105.) Was the carb. tuned before the engine was fully warmed up ? Is the muffer on tight ?
Just a few ideas to start with.

The first part makes me think air leak/carburetion, the second part would suggest ignition. I can't believe the kill switch didn't kill it, a bad kill switch wouldn't make it rev up if that were the only problem. I'm not sure how they are wired, but that would make me lean toward ignition also. I just had the ignition go out on a 5105, but it wouldn't rev up, it would "porpoise" in mid range after it got fully warmed up. Dolmar replaced it at once with no hassles.

Sorry I can't be of more help.

Red Elm said:

The first part makes me think air leak/carburetion, the second part would suggest ignition. I can't believe the kill switch didn't kill it, a bad kill switch wouldn't make it rev up if that were the only problem. I'm not sure how they are wired, but that would make me lean toward ignition also. I just had the ignition go out on a 5105, but it wouldn't rev up, it would "porpoise" in mid range after it got fully warmed up. Dolmar replaced it at once with no hassles.

Sorry I can't be of more help.

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Sometimes it happens when they rev out of control. Sounds like an air leak or carb problem as was said before. Check your carb and if need to know how to pressure test for an air leak there is a thread in my sig that is very good.

Bump!

This is common issue with the 5100, I had a few that would do that and there was no fix. I believe it an overheating issue, and one reason Dolmar redesigned the 5100. One thing that did seem to help was running 98 octane without ethanol. With that said, do a leak down test to make sure it's just not a simple air leak.

Andyshine77 said:

This is common issue with the 5100, I had a few that would do that and there was no fix. I believe it an overheating issue, and one reason Dolmar redesigned the 5100. One thing that did seem to help was running 98 octane without ethanol. With that said, do a leak down test to make sure it's just not a simple air leak.

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Thanks Andy
You put me on the right path. Turns out my Dolmar did not like the same gas that all my other saws run fine on, which is regular gas with ethanol mixed at 40/1.
I just mixed a batch of 93 octane E-Free gas at 50/1 and it runs just fine : )
Thanks for the ideas everyone.

It is toast for sure,

I will take it off your hands for a small fee.

Caz said:

Thanks Andy
You put me on the right path. Turns out my Dolmar did not like the same gas that all my other saws run fine on, which is regular gas with ethanol mixed at 40/1.
I just mixed a batch of 93 octane E-Free gas at 50/1 and it runs just fine : )
Thanks for the ideas everyone.

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Not very likely that your problem is fuel if you are mixing as you stated. If the carb has never been removed, check the screws. There was a problem for several years from the factory, with the carb screws not torqued tight enough.

sugarbush said:

Not very likely that your problem is fuel if you are mixing as you stated. If the carb has never been removed, check the screws. There was a problem for several years from the factory, with the carb screws not torqued tight enough.

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The bad fuel was overheating the saw. The 5100's had a design flaw that caused overheating issues, I had and worked on more than one 5100 that overheated. Anytime a saw that is in good working order over revs after making an extended cut, it's overheating. This is a well known fact.