Fuel Delivery Diagnosis

[drf256]

I'd like some opinions.

Ported MS360. Perfect new piston and 200 psi after first runs. Laid the bar in a stump and beat on it for 4 strong runs. Saw was supposed to run fine before I got it.

New OEM fuel line and filter. Tank full. Stock Zama with compensating horn cut to fit metal filter, limiters trimmed. New impulse line and boot.

I did not check the tank vent yet, but it's the new style which rarely malfunctions. I will be checking that.

Saw starts right up. Idles fine.

I can tune high side pretty fat. Down into the 13's.

It will hold high tune for 1-2 seconds and then scream. Clocked it at 13,200 for 2-3 seconds, then takes off and hits 16k. Sounds like it's able to supply enough fuel in stock configuration for a bit, then runs dry. Like the metering side is full, gets empty, and then won't fill again.

My plan is to pressure test the new line and install a new carb. May fill tank and run with no filter just to see. Will pull current carb apart and look for any obvious issues.

Any ideas guys?

 

[Simondo]

Sounds all logical to me + as you say... pressure test to confirm all ok. A Thought....if you tune it "real" fat , will it do the same ? Long shot...fuel line pinching in a bend ? All points to a fuel starvation thing without a air leak so the usual "metering lever" hight /spring pressure deal could be going on with a saw that demands more than normal .

 

[drf256]

Tank vent is new barrel type. Tough to blow air through.

 

[5000+]

Check your crank seals, particularly the flywheel side. I bet it won't hold vacuum.

 

[rattler]

Sounds like a air leak to me AL I had one with a leaky boot that did that.

 

[drf256]

Sounds like a air leak to me AL I had one with a leaky boot that did that.

Fun

 

[Magic_Man]

Fun

If I remember right Jason fought similar issue a while back. Saw would pressure/vac test fine on the bench but after it warmed up would develop a leak.

 

[Onan18]

Fun

MS360s aren't TOO bad to put seals in Doc, worst part would be if it is the PTO side and you end up having to replace the oil line.

 

[Onan18]

If I remember right Jason fought similar issue a while back. Saw would pressure/vac test fine on the bench but after it warmed up would develop a leak.

THIS! I had the same thing happen to me on a 385 XP, wound up busting the case and putting new bearings and seals in like I should have done in the first place.

Joe

 

[Nitehawk55]

If the metering needle is set OK I also would suspect a leak .

 

[dall]

i put new oem seals in the saw before i sent the saw to al i have the stihl seal installers and didnt have trouble putting them in

history of the saw my buddy matt bought it new in 2002 and this is the first time anything has been worked on it doubt the carb was ever even touched

 

[cobey]

I had an 064 that did this with a dirty carb screen and a bad fuel line.

 

[drf256]

I'd suspect an idle issue if there was an air leak, no?

 

[rattler]

I'd suspect an idle issue if there was an air leak, no?

Mine did have a poor idle.

Sent from my SM-G935V using Tapatalk

 

[Duane(Pa)]

I'd like some opinions.

Ported MS360. Perfect new piston and 200 psi after first runs. Laid the bar in a stump and beat on it for 4 strong runs. Saw was supposed to run fine before I got it.

New OEM fuel line and filter. Tank full. Stock Zama with compensating horn cut to fit metal filter, limiters trimmed. New impulse line and boot.

I did not check the tank vent yet, but it's the new style which rarely malfunctions. I will be checking that.

Saw starts right up. Idles fine.

I can tune high side pretty fat. Down into the 13's.

It will hold high tune for 1-2 seconds and then scream. Clocked it at 13,200 for 2-3 seconds, then takes off and hits 16k. Sounds like it's able to supply enough fuel in stock configuration for a bit, then runs dry. Like the metering side is full, gets empty, and then won't fill again.

My plan is to pressure test the new line and install a new carb. May fill tank and run with no filter just to see. Will pull current carb apart and look for any obvious issues.

Any ideas guys?

Exact same thing on my MM036 with zama at one of our Pa GTG'S . Long story short....carb diaphragm. Suggested by someone way smarter than me. Loss of elasticity from E fuel most likely.... This worked for me and is no doubt one of the simplest things to try first. I don't run E fuel, but this saw spent time in the mid west. They are all high on that sh!t out there.

 

[Canadian farm boy]

When the saw acts up will it return to a normal idle?
I'm thinking dirty screen in the carb or a stiff diaphragm

 

[TJ the Chainsaw Mechanic]

Normally...and I say Normally, if a saw idles fine and takes a throttle fine you don't have an air leak. If you do it's small enough to lean out the top end, but since the low end is rich enough to override the air leak you don't notice an issue. Crank seals can be quite a POS to figure out even when they are new. I like to put a coating of grease on them for break in. Get that thing scorching hot and do a vacuum test like mentioned. Could also have a fuel puddling issue as it's rich enough to hold the tune for a short time. While idling did you try turning the saw side to side and upside down to see if there is any noticeable change in idle?

 

[Mastermind]

Zama carbs typically have too small a seat under the needle...

 

[Stihlbro]

I would swap out to a known good carb and see if the saw still has same symptoms.

If it is the carb, next I would
1.) pressure check, dunk in a container of fluid. Any leaks?
2.) disassemble and throughly clean
3.) install new kit.

 

[drf256]

It was the carb. Here it is with new Zama.