MS 261 won't 4 stroke after muffler mod and timing advance

[dhhusqihl3]

Seeking just a little advice if anyone has experienced this before. I have a MS 261 that I muffler modded and it tuned fine, then I advanced the timing 15 thousandths off the key and it won't 4 stroke until it gets to a point where the rpm comes down and it basically sounds like it's drowning. When I lean it back just past that though it runs like wild but won't 4 stroke. Seems as if there is no happy medium. Anyone ever seen this before? Thinking I will go back and pull some of the advance out or maybe go back to factory just to make sure it will tune correctly again. Thoughts?

 

[CR888]

I look at timing changes differently to most. Many will advise a 3° or 5° advance but they don't really know exactly where the timing is on your saw stock. There is easily a few degrees wiggle room in a stock key. So it's something you need to play with to get right. Usually I make small changes test and repeat 5+ times to get it right. Tip: just screw 2 of your starter essembly screws down while testing so it's quicker. But yes your probably right in thinking you need to retard your timing some. Nobody want to 'retard' their saw. But I've found on the very odd saw that gains in performance and running conditions can be improved by retarding the timing a few degrees. It all depends where the timing in said saw was set to begin with.

 

[dhhusqihl3]

So @CR888 you do believe that it not 4 stroking can be a result of the timing being a little to far advanced?

 

[huskihl]

It’s possible that the carburetor diaphragms need replaced to get some adjustability and consistency from tuning. If that doesn’t do it, I would try lowering the metering lever a little bit to soften the adjustment curve

 

[huskihl]

Try it without the air filter in place

 

[CR888]

Good advice above. While timeing can effect your max rpm, if you can't get the saw to 4 stroke at all I'd look at the induction system ie carb, filter. Also for good measure just remove spark arrestor and burn off carbon with a blow torch.

 

[drf256]

I’m with Kev. Any restriction in the fuel system can do that. It’s counterintuitive.

When you tune it fat, it will stay fat because the airflow isn’t pulling a lot of fuel vs air ratio, even though there is a restriction.

When you lean it out, it races away when it starts to pull more air than fuel and the fuel can’t keep up with it.

I know it makes no sense, but I’ve had it happen a few times, mostly with hot 036’s. There is 1/16 turn between 12k and suddenly 16k. It’s generally the carb diaphragms or needle/seat.

 

[hseII]

I’m with Kev. Any restriction in the fuel system can do that. It’s counterintuitive.

When you tune it fat, it will stay fat because the airflow isn’t pulling a lot of fuel vs air ratio, even though there is a restriction.

When you lean it out, it races away when it starts to pull more air than fuel and the fuel can’t keep up with it.

I know it makes no sense, but I’ve had it happen a few times, mostly with hot 036’s. There is 1/16 turn between 12k and suddenly 16k. It’s generally the carb diaphragms or needle/seat.

If Kev & Doc day this,

This is what it is.


Sent from my iPhone using Tapatalk Pro

 

[dhhusqihl3]

So I was finally able to get it to 4 stroke but it was 3 and a half rounds out. Seemed excessive and when it didn't stroke the high end rpm was low. So low I didn't even worry about putting the tach on it. The highest rpm I got with what seemed lean though was 14,600 so I only wanted to bump it back a touch. I pulled the plug and it looked pretty good, not chocolate colored but more of a shade lighter but not white or even tan. (don't have a great shade name for it). Anyway I did adjust the metering level to allow a little more fuel in but same result, adjustment and all. Air filter is actually new and very clean. Considering this saw has a muffler mod, the spark arrestor/screen is out of play. So I went back and put the timing back and low and behold the problem disappeared. Of course the saw was crazy rich but I was able to lean in back and it tuned exactly how it was suppose to. Apparently my saw just doesn't like a timing advance is all I've come up with.

 

[CR888]

Well its good you figured it out and now have a respect for changes in timing.