I'm puzzled (mak 6401 carb. issue)

[Beetle-Kill]

Yesterday I was ready to start on some overlength cottonwood rounds, make 'em a little more manageable. Idle is fine, Revs. fine, get a little 4-stroking @ wot. I hit the first round, gets halfway through and farts out. It never died but it bogged something fierce. Pull it out of the cut and it was at a high idle, chain not spinning but high. Blip the trigger and it settled down and would rev. again. A few seconds in the cut and it farted out again.
I am not good at carb. tuning, never acquired the knack, but it runs like it should without a load. Air filter is good, as are the plug and gas. What could cause this?
Thanks, JB
EDIT- I will be working on this tomorrow morning, I need it up and running. Thanks again.

 

[TreePointer]

You may be running into the rev limiter on the saw's ignition coil. If I recall correctly, it's at 13,500 rpm. Maybe try backing off the high jet a little (counterclockwise for richer).

 

[Beetle-Kill]

Thanks, I'll give it a try in the morning. Exterior markings show a L, H, and S above the rubber plug/cover. Back off the H, I'll try it.
If it matters, I was cutting at about 8,700ft. and it was adjusted for 8,200ft.

 

[jeff_t]

Check the impulse line. It might have a little crack in it that opens up occasionally. Mine was cracked all the way. It would start and idle, but would bog and have absolutely nothing when I hit the throttle. Look at the fuel line while you are there. Doesn't sound like a tuning issue, especially if it started suddenly.

I don't think 500' should make that much of a difference. Maybe if you were already on the edge. If that was the case, you don't want it richer.

 

[Beetle-Kill]

Check the impulse line. It might have a little crack in it that opens up occasionally. Mine was cracked all the way. It would start and idle, but would bog and have absolutely nothing when I hit the throttle. Look at the fuel line while you are there. Doesn't sound like a tuning issue, especially if it started suddenly.

I don't think 500' should make that much of a difference. Maybe if you were already on the edge. If that was the case, you don't want it richer.

Good call. Impulse line has a split where you can't see it. It's patched and soon to be replaced. back to normal now.
Thanks, JB

 

[ScotO]

when one of my saws starts acting like that, I get looking at it REALLY good. Could be a number of things (or even a combination of things). Start with the easy things first, look at the fuel line/filter. Also, not sure if you saw has an internal impulse or an impulse line, if it's an external line check that (not just the line, but where your fuel/impulse lines attach to the carburetor. When the line degrades it sometimes becomes loose at those connections.) If those things check out, tear the carb off and break it down, could be a plugged inlet screen (seen that many times too). Could be a degraded metering diaphragm also. And there are times when the crank seals go bad, that the saw acts totally wrong. So start with the easy fixes and work your way up. I'm betting it's a carb issue from the sounds of it.. *edit* saw you found the cracked impulse line. Glad it was an easy fix....

 

[jeff_t]

Good deal.

Mine broke at the crankcase end when I put the HD air filter on. Had to stand and scratch my head for a minute and realize that it might breathe a little better, but not THAT much better.

 

[Beetle-Kill]

Jeff, what did you use for a replacement line? It looks like regular vacumn line to me.
I did have to tweak the idle a bit, and I did back off the H screw about an 1/8th of a turn, but it seemed to work.
Cottonwood never knew what hit it.

 

[jeff_t]

Yeah, it's vacuum line. Went to the local small engine shop and got a foot or so. Enough for three replacements, I think.

 

[Beetle-Kill]

Cool.
Thanks again.
JB