Saw surging under load.

[Exophysical]

My 365xt has started surging a bit. It does a little bit of surging at idle right after a cold start which goes away once it warms up, after that it idles and runs good unloaded at any RPM. In the wood however, at/ near full throttle the RPM's jump up every few seconds, then back dow etc. I did play with the carb tuning a bit, looking at the plug I could see a difference, but the saw continued to run pretty much the same whether I went 1/4 turn up or down (1/2 turn total variance). I checked it with a tach and WOT rpm is a little under 13000, so not running against the rev limiter.

Wondering where I should start looking? Thanks.

 

[djg james]

My 365xt has started surging a bit. It does a little bit of surging at idle right after a cold start which goes away once it warms up, after that it idles and runs good unloaded at any RPM. In the wood however, at or near full throttle, it the RPM's jump up every few seconds, then back dow etc. I did play with the carb tuning a bit, looking at the plug I could see a difference, but the saw continued to run pretty much the same whether I went 1/4 turn up or down (1/2 turn total).

Wondering where I should start looking? Thanks.

Something similar (maybe?) has been posted here recently, several times. As well with my 038. I believe the best place to start (as in my case) was a new fuel line/filter, impulse line, gas tank vent, air filter and spark plug. I don't know for sure which of those fixed my problem, but knock on wood, my saw is running strong again. I'm definitely not an expert, just passing along what I've gleaned from the previous threads.

 

[Exophysical]

Something similar (maybe?) has been posted here recently, several times. As well with my 038. I believe the best place to start (as in my case) was a new fuel line/filter, impulse line, gas tank vent, air filter and spark plug. I don't know for sure which of those fixed my problem, but knock on wood, my saw is running strong again. I'm definitely not an expert, just passing along what I've gleaned from the previous threads.

Alright, thanks. I did the plug and filter recently but will go through the rest of the system. I had done some searching, but most of the stuff I could find about surging was not about saws surging only under heavy load.

 

[Bob Hedgecutter]

Pressure and vacuum test.
I know, it is everyones answer to a guy that does not own the tools to do it- but if you can be pretty sure the fuel delivery system is good- it can answer a lot of pesky questions straight off the bat.
If it tests out good, then you might readdress the fuel delivery system.
In the wood and under load- you might be flexing things enough to expose a small air leak by letting the chain bite in and lifting the rear handle- pushing down on the front.
If you have a crack/small tear in manifolds, small leak in PTO side seal, it will lean out the immediate combustion charge and rev up. Lessen the load/flex and it will calm back down.

 

[Exophysical]

Pressure and vacuum test.
I know, it is everyones answer to a guy that does not own the tools to do it- but if you can be pretty sure the fuel delivery system is good- it can answer a lot of pesky questions straight off the bat.
If it tests out good, then you might readdress the fuel delivery system.
In the wood and under load- you might be flexing things enough to expose a small air leak by letting the chain bite in and lifting the rear handle- pushing down on the front.
If you have a crack/small tear in manifolds, small leak in PTO side seal, it will lean out the immediate combustion charge and rev up. Lessen the load/flex and it will calm back down.

Oddly enough it does not surge if I lean on it, surging is worse if I just let the weight of the saw do the work.

I'll try and find someone to do the leak down tests, was just wanting to try a few things before I did that.

 

[Bob Hedgecutter]

Oddly enough it does not surge if I lean on it, surging is worse if I just let the weight of the saw do the work.

I'll try and find someone to do the leak down tests, was just wanting to try a few things before I did that.

Okay, maybe put up some good clear photos of your chain and bar- not likely, but possible the state of that could have something to do with your issues.

 

[Exophysical]

Alright, thanks for everything, you guys have been great. Short version is that it was a tuning issue and the saw was intermittently four stroking in the cut.

Long version, I recently did the transfer mod/ 372 conversion on this saw, opened up the muffler a bit, and finally popped those restrictor caps off the carb so I could tune it properly. Having heard horror stories about people mistaking the rev limiter for 4 stroking and burning up their saw, I picked up the only Tach I could find, which was a real cheap affair that cost about $15. I did notice that I ended going way richer on the high idle than the suggested initial tuning, and I'm thinking the cheap tach steered me horribly wrong.

So I rechecked all my work, took the jug off, transfer caps, carb etc... no sign of leaks and still surging once reassembled. When I had it apart I did notice that everything was sopping wet inside. Doing some test cuts I also noticed that leaning out the carb 1/4 turn didnt fix the surging, but it did take less pressure on the saw to get it to settle down. I kept leaning it out and eventually the saw stopped surging in the cut, at that point I just tuned it by ear, now she's running like a top.

 

[Gord404]

I don't very often hear about checking the spark plug color, trying out a recently put together 390xp the other day and ran it long enough to give me a reading through some big fir and it was a little too much on the light side so opened up the high a bit and after a cool down it was just the color I like to see in that paticular non strato saw.

 

[Exophysical]

I don't very often hear about checking the spark plug color, trying out a recently put together 390xp the other day and ran it long enough to give me a reading through some big fir and it was a little too much on the light side so opened up the high a bit and after a cool down it was just the color I like to see in that paticular non strato saw.

I'm almost wondering if the strato saws need to run a bit lean, at least by my standards. To me, when the saw was still four stroking the plug looked ok, and according to the tach I shouldnt have been too far out of spec. In my earlyer post I stated that I suspected the tach of not working right, but since then I've used it on a couple non-strato saws with good results. Not sure what to think, now?

 

[thompsoncustom]

Strato saws are harder to tune and if you have done any mods than a tach isn't much help either it's just all by ear.

 

[huskihl]

Likely still rich. Turn the H in until the surging stops as soon as you hit wood

 

[Exophysical]

Likely still rich. Turn the H in until the surging stops as soon as you hit wood

Yeah, thats how its set up now.