MS440 running problem

[sakman]

This MS440 is driving me nuts so I'm reaching out. It will run for 5 seconds but won't draw fuel unless the choke is on half way. Here's what I did to it.
New Wolf Creek Saw shop cyl and piston.
New fuel line
New carb kit. Walbro

Crank seals are good and hold 10 psi pressure and 10" vacuum. That shows the base gasket and pulse line are good.
Anyone have an idea where I should look next?

 

[Wood Doctor]

This MS440 is driving me nuts so I'm reaching out. It will run for 5 seconds but won't draw fuel unless the choke is on half way. Here's what I did to it.
New Wolf Creek Saw shop cyl and piston.
New fuel line
New carb kit. Walbro

Crank seals are good and hold 10 psi pressure and 10" vacuum. That shows the base gasket and pulse line are good.
Anyone have an idea where I should look next?

You are running lean and she's starved for fuel. Open the carb's hi speed setscrew at least a quarter turn or so.

 

[J D]

Are you sure the impulse isn't restricted or blocked

 

[Old2stroke]

If you can't cure the problem by adjusting the carb mixture screws, it sounds like the fuel pump isn't working. Faulty impulse signal to the carb or something wrong with the fuel pump diaphragm. How was it running before you did the top end rebuild?

 

[sakman]

Impulse line is fine and pressure/vacuum tested.
New walbro check valves and diaphragm. Needle is set level with the body.
It wasn't running before the repairs. The owner stripped out the plug threads.

 

[Old2stroke]

Does it respond at all to carb mixture adjusting? Recently some one had a saw with the same sort of "run for 5 sec" issues and after chasing down anything that could be carb/fuel/air leaks, it turned out to be a bad coil.

 

[Wood Doctor]

Does it respond at all to carb mixture adjusting? Recently some one had a saw with the same sort of "run for 5 sec" issues and after chasing down anything that could be carb/fuel/air leaks, it turned out to be a bad coil.

Or a bad connection to the coil. I ran into one that had so much goop on it that it shorted out. However, Sakman's problem still seems more like a fuel issue than electrical. New carb may be on the agenda.

 

[sakman]

It's definitely a fuel problem. It'll throttle up and down but after a while it will just run out of fuel. If I just stick a finger in the carb to act as a choke it will keep running.This is easier than fumbling with the choke lever!

Oh I can see it now. Yup I fixed your saw with a new cylinder. Just keep your finger here and it'll run fine.

 

[Wood Doctor]

It's definitely a fuel problem. It'll throttle up and down but after a while it will just run out of fuel. If I just stick a finger in the carb to act as a choke it will keep running.This is easier than fumbling with the choke lever!

Oh I can see it now. Yup I fixed your saw with a new cylinder. Just keep your finger here and it'll run fine.

I understand your frustration. You may have red and white plastic setscrew limiters on that carb. I hate those, especially whenever they block an adjustment that might allow the saw to run. Some of them are almost impossible to remove.

You might want to try installing an aftermarket carb just to see if that solves the problem. If it fails, you haven't lost much. Just an idea and likely worth pursuing.

 

[J D]

I'd be going over the carb again checking metering level, gaskets & surfaces around all the little valve flaps, pressure testing fuel inlet, etc.
When you pressure/vac tested the crank case did you connect to the impulse line? If so did you rotate the engine & see the readings fluctuate?
Some after market cylinders don't have the impulse port properly line up with the pattern of the case & get blocked by the gasket or sealant. If you can blow through the impulse line into the crankcase then it'll be fine

 

[lone wolf]

Sound's like it could need a new OEM ,carb don't but China crap.

 

[Old2stroke]

Are you SURE the gasket is on the right side of the fuel pump diaphragm? Goes between the cover and the dia., not between the carb body and the dia. Also did the new gasket/dia. match the old ones exactly?

 

[sakman]

Impulse line goes into the crankcase nowhere near the cylinder base.
Pump diaphragm is against the carb body.

I walked by it this morning. Just gave it the finger and kept walking..

 

[Captain Bruce]

I feel your pain. 440 sitting here for a month.......rebuilt carb, fuel lines, muffier screen, and it still doesn't want to stay running beyond initial fire-up. Runs on Hi-idle after 1/2 choke. Turned up the idle and got it running steady and strong, on 1/2 choke. Blip the throttle, and it begins to go away....trigger doesn't matter. It stumbles, and dies.

Carb screws = H & L = : 1, 1 1/2, 1 3/4, and 2 turns out.

Chain IDLE screw = backed out to no contact, and than set back at 2 turns. Engine wouldn't idle or rev.

Went up in 2 turn increments, until it was running strong, but on 1/2 choke.

Blip the throttle down to a normal idle, and the 440 says good-bye.

 

[stihltech]

Inline spark tester to see if ignition shuts down first. If not, recheck fuel hose that it is open under pressure and vacuum. Got caught on this a couple times with aftermarket hoses. Does pulse hose have a good open pulse? Not just hold pressure.. Aftermarket carb? Failure rate over 50%. 6 in the last year. All cured by original equipment. The saw in good shape is well worth it.

 

[capetrees]

This MS440 is driving me nuts so I'm reaching out. It will run for 5 seconds but won't draw fuel unless the choke is on half way. Here's what I did to it.
New Wolf Creek Saw shop cyl and piston.
New fuel line
New carb kit. Walbro

Crank seals are good and hold 10 psi pressure and 10" vacuum. That shows the base gasket and pulse line are good.
Anyone have an idea where I should look next?

Real basic question

Is the fuel filter in the tank cleared?

 

[Captain Bruce]

It's definitely a fuel problem. It'll throttle up and down but after a while it will just run out of fuel. If I just stick a finger in the carb to act as a choke it will keep running.This is easier than fumbling with the choke lever!

Oh I can see it now. Yup I fixed your saw with a new cylinder. Just keep your finger here and it'll run fine.

Just had a real head scratcher....focused on the complete fuel system, twice because chinesium carbs are a coin-toss.
Pulled the muffler, and it barked on 1/2 a pull, and wanting to run.....the muffler was packed with cheap oil tar. Soaked it over-nite in thinners. Blew it out. Let it sit. Cleaned it. Painted it. Re-installed it. The saw runs properly now.

 

[Wood Doctor]

It could have been running rich for quite some time to build up that much oil tar. I just repaired an old saw that had almost the same problem. I had never seen that much packed tar in a muffler before. Most of the time the heat from a chainsaw burns it off. This saw had crud all over it and an air filter that was in gobs. How it even ran at all, I do not know.

 

[sakman]

Fixed!

And yes I'm an idiot. As I mentioned above I was sure I had the pump diaphragm mounted correctly and laying against the carb body.

Well by writing this, guess what.. Mr. I've had 100 carbs apart in the past did it wrong. The gasket was against the carb body. Corrected it and a little squirt of gas down it's throat and it runs just fine now.

Thanks to all for responding.