rattle at high rpm

[jimmysisson]

I have a Dolmar 5100s about 3 yrs old only about three dozen tanks through it. Runs great. Lately has started making a light rattling sound at high speed just as you speed up to start into the cut. As soon as it's cutting much at all the rattle stops and the saw pulls just fine in the cut. Starts and idles fine.

The saw seems to turn faster than it did when new. I haven't done anything to the carb or muffler. If it was an old car I'd say it was detonating, pinging like it would under high load/low rpm, though this is a higher frequency and pitch. My old saw is a Jonsereds 80, which runs like a truck, low speed, so maybe I'm just not used to this?

Maybe it's lean? I don't know what 4-stroking sounds like, may be that? Oh yeah, gas is regular, 32:1 with Stihl oil.

Hard to write what a noise sounds like. Something to fix, or is it normal?
Thanks, Jim

 

[GASoline71]

Could be the 4 stroking sound... but I will say that 32:1 is more oil than that saw needs. Especially if you're usin' the full synthetic Stihl oil.

Gary

 

[willsaw4beer]

The carb probably needs to be retuned if you run 32:1. Otherwise it more than likely is running lean. I prefer 40:1 over 50:1 myself but the saw is designed to run a couple thousand hrs on 50:1 so yeah..

 

[Stumpys Customs]

32:1, could be carbon build up, and either spark knocking, or contacting the combustion chamber.

 

[rbmopar]

Could be hammering on the limiter.

 

[Trigger-Time]

Could be hammering on the limiter.

:agree2:


Some coils can make some wild noise's when hitting limiter.
As it hasn't been run all that much the motor may be good and broken
in now and turning a few extra rpm and hitting limiter.

It doesn't sound like you have messed with saws that much.
I would richen the H screw on carb and see if it goes away.
If not I would take it back to the dealer and have them check it out.


TT

 

[jimmysisson]

lean?

You're right, I haven't messed with saws much, though I did work on XL12s for BLM in Fairbanks one summer. In 1968. I think I'll try a lower-oil mix and a little richer turn. The local dealer is really a lawnmower guy so I'll have to find someone else who knows what he's hearing. Thanks for the replies.
Jim

 

[gemniii]

Maybe it's lean? I don't know what 4-stroking sounds like, may be that? Oh yeah, gas is regular, 32:1 with Stihl oil.

Brad, blsnelling just posted a good thread with a video AND SOUND of a saw 4 stroking out of the cut.
/edit

Tuning a chainsaw is often discussed on the forum here. Anytime I use a chainsaw, I listen closely to make sure that it is tuned properly. It's easy to tell by simply taking the load off the saw during the cut and listening for it to 4-stroke. This short video gives a good example of what it should sound like.


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[Eccentric]

If you switch to 50/1 from 32/1 the saw will richen up quite a bit. A thinner oil/gas mix makes for a leaner fuel/air ratio at the carb. These modern leaner running, smaller passage carbs seem to be even more sensitive in that regard than the old stuff.

BTW-with my XL12's and such I can tell a difference in carb tune if I switch oil ratios. Has happened many times. Only requires a small bit or retuning though. Same thing with dirt bike jetting.

 

[056kid]

Sure its just not that "two stroke sound"? after only three dozen tanks, you and the dolmar are just getting acquainted. .

 

[jimmysisson]

I agree

Getting acquainted sounds about right. That's why I want to know what the noise means. The video helped a lot - that's the condition I noticed. If that's four-stroking, or hitting the rev limiter, then I don't have a problem, looks like.
And thanks to all for the mix advice - I will change the mix and check the high needle. A little confused though: Eccentric - if I go to 50:1, you're saying that leans the carburetor fuel mixture, but if I do so it will richen the mixture? Maybe I misread this?
I appreciate all the replies.
Jim

 

[Eccentric]

And thanks to all for the mix advice - I will change the mix and check the high needle. A little confused though: Eccentric - if I go to 50:1, you're saying that leans the carburetor fuel mixture, but if I do so it will richen the mixture? Maybe I misread this?
I appreciate all the replies.
Jim

You read it somewhat wrong Jim. A thinner oil mix (50/1 vs 32/1) makes the saw run richer......not leaner.

People often get into trouble when they think of gas/oil ratios as richer or leaner. That's why I say "thinner" or "thicker". "Rich" and "Lean" should only be used to describe the fuel/air ratio (which is adjusted at the carb).

A thicker gas/oil mixture makes the fuel more viscus, so it moves slower through the orifices and passages in the carb. As a result, more air (oxygen) and less gasoline (the part of the fuel mix that actualy burns) are mixed in the carburetor. The result is a leaner effective fuel/air ratio. The carb may need to be retuned to restore a propper fuel/air ratio.

Also, a thicker gas/oil mixture means there's less gasoline (again, the burnable part of the fuel mix) in a given volume of fuel mix. If you combine the same volume of fuel mix with the same volume of air(oxygen) that you did before (meaning the carb tuning hasn't changed) then the effective fuel/air mixture will be leaner.

The opposite applies if you use a thinner gas/oil mix. Since you're planning on switching to 50/1 from 32/1, you will be running a thinner gas/oil mix. This should make the saw run richer. It may get you back into the sweet spot, or you may have to retune things further. Mix up some fresh fuel at 50/1, run the saw for long enough for the lines to clear out, then retune if needed....

 

[jimmysisson]

Ah..

That makes more sense. I will try the new mix and see how the mixture acts then. Thanks for the help - I'll report back.
Jim

 

[Eccentric]

That makes more sense. I will try the new mix and see how the mixture acts then. Thanks for the help - I'll report back.
Jim

Sorry Jim. Guess my earlier post was as clear as mud. Good luck with the saw!

Edit:

Crap! I just looked back at my earlier post. You DID read it right..............................but I posted it BACKWARDS....Sorry. Too late to edit my first post...:bang:

My second post is correct. This is what I should have typed in my first post. Sorry...

If you switch to 50/1 from 32/1 the saw will richen up quite a bit. A thinner oil/gas mix makes for a richer effective fuel/air ratio at the carb.