Ms390 Potential Air Leak?

[Teewhy]

Hi Lads,

I’m putting an Ms390 together to give to a buddy and it’s behaving strangely when I start it up.
It’ll cough on choke and then start as normal, but it’ll then race without any throttle input.
It doesn’t respond to any carb adjustments either as far as I can tell, it’s currently at 1 turn out on each. The throttle adjustment isn’t set too high either.
Eventually it just dies on its own and needs to be choked again to start.

Here’s a video, what does this read like to you guys? Diagnosing air leaks isn’t something I have much experience in.
Hope this link works

Ms390, Suspected Air leak?

(In my defence the dogs didn’t look that big when I ordered them )

Thanks Lads

 

[FederalQ]

Those look like the coveted Dolmar/makita Spike bars! Pressure/vacuum test In order to rule out an air leak. Is that an OEM carburetor? I would start checking the fuel line and filters first. Others here with far more experience will chime in

 

[Teewhy]

Those look like the coveted Dolmar/makita Spike bars! Pressure/vacuum test In order to rule out an air leak. Is that an OEM carburetor? I would start checking the fuel line and filters first. Others here with far more experience will chime in

Ahh I had no idea, I just liked the look of them, that’s pretty neat.
It’s an aftermarket carb, although I’ve had good luck with the manufacturer in the past.
Thanks @FederalQ

 

[hacskaroly]

I’m putting an Ms390 together to give to a buddy and it’s behaving strangely when I start it up.

What all have you done under "I'm putting an Ms390 together"? Are you just cleaning up the saw or have you done anything to it (pulled cylinder...etc)?

It was kind of hard to tell from the video, but when the saw started racing, did you kill it or did it die on its own? Like @FederalQ stated, check your fuel filter and fuel lines make sure fuel is flowing. Have you recently taken a look into the carb? Sometimes they get enough gunk around them that it blocks any vent holes it has and crud can get in. Might be worth a shot of giving it a good clean inside and out. Make sure the metering lever is set at the correct height. Pressure testing the saw and carb will help identify/rule out problem areas. If you have some non-flammable break cleaner, while the saw is running, you can give it a few squirts around the intake boot, clutch and flywheel side. If you have an air leak in those areas, the saw will immediately shut down (stops the combustion cycle - doesn't hurt the saw as opposed to ether which is all fire and no lube). Sometimes while the saw is running if you tilt it to one side or another it may also race indicating potential air leak on that side. Also, what is the condition of the air filter?

 

[FederalQ]

I’m betting it’s the aftermarket carburetor!

 

[ZeroJunk]

Does not strike me as an air leak, but it could be.
Make sure the impulse line is intact and not stopped up with some excess Dirko or whatever you used to seal the pan.
Usually with an air leak they will run, but just not idle down.
IOW, if it is a fully adjustable carb , with an air leak you should be able to add enough fuel that it would run at some RPM in the idle position. Just not idle down as low as it needs to because of too much air.

Likely a bad carb, or needs some tweaking.

 

[Teewhy]

Well lads I had chance to look at her this afternoon.
Monumental air leak in the crank seal on the clutch side, see picture of the remains.
The carb passed pressure and vacuum no problem, I adjusted the metering lever a little and we’ll see how that get on when she’s back together.

@hacskaroly That break cleaner trick is one for the back pocket, thanks for that.

Air filter, impulse line and flue line are all solid, with the lines being pressure checked too.
The decomp valve was leaky too so I’ll replace that.
I’m hoping that the air leak is the only source of my issues but we’ll see.
It was hilarious finding it, I couldn’t even get the slightest pressure on the hand pump so I turned my compressor down to 7psi and hooked it up, could hear the air flying out

Blatant air leak

Thanks for all the help men, it’s muchly appreciated

 

[Teewhy]

Also, forgive my ignorance, is this little spring part of the exploded crank seal?

 

[hacskaroly]

Also, forgive my ignorance, is this little spring part of the exploded crank seal?

Yes, it sits around the inner lip of the seal and keeps it tight against the crankshaft.

 

[Gearshifter]

I'm replacing a seal on a 039 for a friend. I love it how you have to strip the entire saw to replace a seal ....

 

[Wood Doctor]

I'm replacing a seal on a 039 for a friend. I love it how you have to strip the entire saw to replace a seal ....

I have run into the same thing. Poor design to say the least.