Stihl 261CM struggles to start warm

[mattbred]

I bought my 261 mtronic new 3 years ago. It worked really well for about 2 years but has now developed an issue.

I always use ethanol free premium fuel with Stihl oil/stabiliser mix. I've replaced the spark plug, fuel solenoid (with white one), and fuel filter and cleaned the air filter.

Here is the situation. The saw starts and runs great cold. I let it warm up for a minute and can cut a bunch of rounds. I can shut it off and start it within a minute no problem.

If I let it sit for about 5-10 minutes and try to start it again, it will not start in any position. It will ignite here and there for a couple RPM and then die. If I hold it WOT and give it some pulls, then choke it, I might get lucky and get it running. If I give it ANY throttle it immediately bogs down and dies. I have to go through this routine for about 15 minutes to get it working properly again. Meanwhile, I have to be very delicate with the throttle or it bogs and dies.

I did notice one thing the last time it happened. I opened the fuel cap and gas sprayed out, like it was under a ton of pressure. There were also lots of little bubbles rising up. It would not start with the gas cap off.

Any ideas?

 

[RI Chevy]

Maybe bad tank vent?
Do you let saw idle for 10 to 15 seconds before shutting down?
If not, then try it.

Maybe a carb kit for the carb? Even though its mtronic, the carb still has diaphragms and screens inside.

Could also be a bad coil...[emoji848]

 

[rogue60]

Sounds like vaper lock all pump fuel boils at a lower temp now days not just fuel with ethanol in it.

 

[drf256]

I’d check the popoff pressure on the carb. Check exhaust screen.

Tank vent should allow pressure to build in the tank which it seems to be doing.

Take a look at the piston through the exhaust port. Make sure it looks ok.

If the popoff pressure is too low, the pressure in the tank will allow the fuel to push out into the carb bore and flood that saw while off. While it’s running, the MT will try to compensate. While the saw is off, it can’t. Just hook up your pressure with a gauge to your carb fuel inlet. It should hold at least 15psi. They are usually set to over 25.

Vapor lock still a possibility. Happened to me when saw was set in the sun on a hot day while off.

Could be many other things, but do the simple stuff first. In my limited experience with MT, problems are usually not the MT system itself. The problem with MT is that it changes your diagnostic process. It compensates for so many issues, it makes standard diagnosis from your prior thought process nearly impossible.

 

[mattbred]

I've looked into vapour lock and that does fit the bill. I'll try letting the saw idle for a bit longer after running it before shutting it down.

The flywheel fins are clear, the side cover is clear, and the spark arrestor is clear.

Is there anything else I can check without pulling the carb? I may do so if letting it cool down longer doesn't help.

I'll also try new fuel, but considering it's done it before on different gas, I doubt that's the issue.

 

[Creaky limb tree care]

Perhaps the compression is reduced when the saw is warm due to worn rings?

 

[mattbred]

I wouldn't think so. It's got barely any hours on it and it runs great up to when I shut it off. It also feels like it has good compression when trying to start.