MS 361 dies out after creeping on the throttle a bit… need some help with tuning

[2-StrokeDude]

First off, Merry Christmas to all of you guys!!

So I’ve got my MS 361 all rebuilt and running well, chopped a couple of small pines down with it and cut up some logs and it runs really good! However, I keep having this issue with it where if I’m cutting just little twigs on a log before chopping it up, I just use the throttle slightly to get through it, after letting off the gas, it just dies out. Then when trying to restart it, it will either take one pull to start it back up or I’ll have to choke it and lock the throttle to get it started up again, even if it is warm. I’ve tried adjusting the L screw on the carb a bit and it’s not seem to do a whole lot. I’ve got the L screw set a little rich. Any ideas what could be causing this?

L Screw is set at 1 1/4 turns and the H screw is set at 1 1/2.

Hoping this isn’t an air leak since I vacuum/ pressure tested this chainsaw after the rebuild which I posted about and it passed both of them. It idles great and will idle for minutes on end if it doesn’t die out after letting off the throttle.
View attachment IMG_0963.mov

 

[pioneerguy600]

What kind of condition is the internal diaphragms inside the carb/ A stiff metering diaphragm can cause erratic spool up and transition down to idle.

 

[ZeroJunk]

Does sound like a carb issue. Tune the carb on idle too max RPM and then see if it will transition. If it will not richen the idle mixture maybe 1/8 and see if it will, and so on incrementally. You may need to turn the idle speed screw up to offset the richer mix. If it keeps cutting off rebuild the carb

 

[Vintage Engine Repairs]

Follow the above advise first, then give this a go

 

[stihltech]

STOP adjusting. Not the issue. Lack of fuel passing through is. Recheck fuel system. I hope no AM fuel hose or filter.
Take a breath. Go back through carburetor. Check the pulse passage at the intake while it is off.
I have had some success by probing the of idle/transfer ports in the throttle bore with a tag wire. A little corrosion will restrict flow. Very important to smooth acceleration. Also the accelerator pump.
As long as you are SURE there are no vacuum or pressure leaks.

 

[Vintage Engine Repairs]

Even at idle you’re actually lean. The cause could be varied. If richening the L a 1/8 of a turn and the idle up to compensate doesn’t solve your issue, then start from the beginning.

Check the piston skirt (scoring)
Fuel filter (restricted?)
Tank vent (blocked)
Fuel line (damaged, kinked?h
Carburettor (cleanliness, air tight, diaphragms, drillings, check valves, metering height etc?)
Fittment between carb and manifold
Impulse (hard, cracked, loose?)
Intake manifold (cracked, hard, loose?)

Re do the pressure and vacuum test - turn the crank during vacuum.

It actually doesn’t sound like it goes lean after it comes off throttle, but that’s likely because of excess fuel in the crank case. It sounds more like spark, but I don’t think it is spark related, It’s just the way it dies out so quickly like you’re hitting the kill switch.

 

[2-StrokeDude]

What kind of condition is the internal diaphragms inside the carb/ A stiff metering diaphragm can cause erratic spool up and transition down to idle.

Carb is brand new

 

[lone wolf]

Carb is brand new

Knock off or OEM?

 

[2-StrokeDude]

STOP adjusting. Not the issue. Lack of fuel passing through is. Recheck fuel system. I hope no AM fuel hose or filter.
Take a breath. Go back through carburetor. Check the pulse passage at the intake while it is off.
I have had some success by probing the of idle/transfer ports in the throttle bore with a tag wire. A little corrosion will restrict flow. Very important to smooth acceleration. Also the accelerator pump.
As long as you are SURE there are no vacuum or pressure leaks.

Thanks for the advice, yes, unfortunately I have some aftermarket fuel lines and filter but they seem like they’ve been OK. I’ll look into getting some OEM ones. I need to replace the impulse hose line as it’s getting hard anyways.

 

[2-StrokeDude]

Knock off or OEM?

Knock off unfortunately but is from a good saw parts supplier

 

[2-StrokeDude]

Knock off unfortunately but is from a good saw parts supplier

Just can’t justify buying $100 Walbro carb for a saw that I’m not gonna use a whole lot, I don’t use chainsaws a huge amount. When I got the saw the L screw adjustment was broken off so the old carb is junk otherwise I would 100% be using the OEM one. I’ve got an ultrasonic cleaner so I’d be able to get it like brand new. It’s just a shame that the screws busted off on the old one.

 

[lone wolf]

Knock off unfortunately but is from a good saw parts supplier

Well I have to question that carb,did you try opening up the L jet a tad yet?

 

[ZeroJunk]

We get $116 for a carb for a BR700. I can but one on eBay for $12.. That's what is on my own blowers.

They work OK most of the time.

 

[2-StrokeDude]

Even at idle you’re actually lean. The cause could be varied. If richening the L a 1/8 of a turn and the idle up to compensate doesn’t solve your issue, then start from the beginning.

Check the piston skirt (scoring)
Fuel filter (restricted?)
Tank vent (blocked)
Fuel line (damaged, kinked?h
Carburettor (cleanliness, air tight, diaphragms, drillings, check valves, metering height etc?)
Fittment between carb and manifold
Impulse (hard, cracked, loose?)
Intake manifold (cracked, hard, loose?)

Re do the pressure and vacuum test - turn the crank during vacuum.

It actually doesn’t sound like it goes lean after it comes off throttle, but that’s likely because of excess fuel in the crank case. It sounds more like spark, but I don’t think it is spark related, It’s just the way it dies out so quickly like you’re hitting the kill switch.

I was at first thinking that maybe something was grounding out the spark, because it definitely sounds like if you were to just hit the kill switch. Seems like it is flooding out to me.

To give you some background on the saw, I have a new crank in it, crank bearings, meteor cylinder and piston kit, as well as a new carb and fuel lines/filter. Everything is new pretty much. Did a vacuum/pressure test on it before starting it and breaking it in and turned the piston up and down when it was under vacuum and it held fine. Pressure test held fine as well

Idles really good and I had it idling for 10 minutes or so at one point when I was moving some logs around. Doesn’t seem like there’s any air leaks since I don’t think it would idle if it was leaking anywhere. I can have it upside down and on its side and it will not die out. Just dies when it’s coming off the throttle at low imput like in the video I had of it.

 

[2-StrokeDude]

We get $116 for a carb for a BR700. I can but one on eBay for $12.. That's what is on my own blowers.

They work OK most of the time.

That’s been my experience with the aftermarket carbs, I repair some small engines on the side a lot of blowers, trimmers and lawnmowers, most of the time the aftermarket stuff works, but sometimes you get a crappy one

 

[2-StrokeDude]

Well I have to question that carb,did you try opening up the L jet a tad yet?

Yes, I have tried opening the L jet, doesn’t seem like it’s helped a whole lot. I’ll try and start the saw today and mess with the carb tuning a little bit more, hopefully I can get it figured out. Just really hoping it’s not any air leaks since I don’t wanna have to pressure/vac test again, that was a pain.

According to the manual, the initial settings without the limiter caps on is one turn for both the L and H screw. Maybe I’ll just set it to that and try and adjust from there.

The aftermarket carb did not have those limiter caps on there so hopefully adjustments aren’t coming loose or anything.

 

[Vintage Engine Repairs]

Yes, I have tried opening the L jet, doesn’t seem like it’s helped a whole lot. I’ll try and start the saw today and mess with the carb tuning a little bit more, hopefully I can get it figured out. Just really hoping it’s not any air leaks since I don’t wanna have to pressure/vac test again, that was a pain.

According to the manual, the initial settings without the limiter caps on is one turn for both the L and H screw. Maybe I’ll just set it to that and try and adjust from there.

The aftermarket carb did not have those limiter caps on there so hopefully adjustments aren’t coming loose or anything.

Pressure and vac test is literally a 5 minute job, shouldn’t be difficult?

Edit: you said “aftermarket carb” you should have said that originally.. all bets are off for that thing working as it should… bin it and rebuild your oem one

 

[Mad Professor]

^^^^What he said. And use kit from OEM carb source. I'd trust the Tilotson kits for other carbs too.

 

[ZeroJunk]

Yes, I have tried opening the L jet, doesn’t seem like it’s helped a whole lot. I’ll try and start the saw today and mess with the carb tuning a little bit more, hopefully I can get it figured out. Just really hoping it’s not any air leaks since I don’t wanna have to pressure/vac test again, that was a pain.

According to the manual, the initial settings without the limiter caps on is one turn for both the L and H screw. Maybe I’ll just set it to that and try and adjust from there.

The aftermarket carb did not have those limiter caps on there so hopefully adjustments aren’t coming loose or anything.

I put an aftermarket carb on an 028 Super for a friend last week. It had been in the shop for a while since I usually buy three at a time.

Idled fine. But, it was too lean on high side. So, I turned a 1/4 turn richer. Nothing. Another 1/4 turn . Nothing. 1/2 turn, still 14, 000 or better. I was about to throw it in the trash, and opened it up like another full turn. Pig rich.

So, the high needle actually did it's function about 3 turns, maybe more, out and worked fine.

It's a little rich to bash Chinese made carbs when they are all made there.

QC not as good I'm sure but, I seem to have good luck with them.

Buy about 5 at a time for BR500,550,600,700. Five at a time for FS90's FS85's FS91's .

Doesn't mean you didn't get a bad. Doesn't mean rebuilding the old one is a bad idea .

 

[2-StrokeDude]

Pressure and vac test is literally a 5 minute job, shouldn’t be difficult?

Edit: you said “aftermarket carb” you should have said that originally.. all bets are off for that thing working as it should… bin it and rebuild your oem one

I know a pressure/vac test is not something that should take that long, the problem is sealing everything off like the intake and the exhaust especially when you don’t have the right tools for it. Took me a while to get that figured out. Also I’ve never done it before and this is the first full top and bottom end rebuild I’ve done on a saw so I’m still learning a bit.