Need help diagnosing tuning issue

[Canyon Angler]

Hey, all,

I have a saw that's acting real weird. I've had it for 12 years or so since new, and it's never missed a beat.

Three years ago, the primer bulb cracked, but I kept using it, using choke exclusively to start it, and it has worked fine for three years using it that way.

This year I opened up the muffler some to try to get more power, and re-tuned the carb, and had my tune checked by a good saw guy, who replaced the primer bulb. Since then, the saw has been giving me fits.

Sometimes it will start fine, and idle fine, and work just as it always did.

Other times, when I start it, it sounds like it's running WAY too rich, as if it was loading up with excess fuel with the choke on (but the choke is off), and it won't come up to speed. BUT if I push the primer bulb while trying to get it to rev up, it will SCREAM (sounds like a "lean scream") ... which leads me to believe that even when I push the primer bulb, it's still not getting enough fuel. And it still won't idle, or not usually. But sometimes it idles like a champ. Temperature of air and chainsaw doesn't seem to affect this.

I should probably note that my saw mechanic said I had the saw too rich, and he leaned it out. I've had good success tuning other saws by ear and with tach – my saw guy said I had my 660 tuned just the way he would tune it after I muffler-modded it – but when I took this particular saw to him to check my tune, he said I had it too rich. (I thought I had it too lean, because if you held the throttle open for 5-10 seconds, the RPMs just kept increasing and increasing, as if it seemed to want to run away, no matter how rich I set the HS screw.)

I ordered a pressure/vacuum tester from Amazon, but in the meantime, can you folks give me any guidance on what to look at? Does this sound like it might be an air leak? Clogged up passages in the carb? (I will probably rebuild carb when I take it apart to test for vacuum/pressure leaks.) The fuel filter is new, the air filter is clean, and I've always been pretty careful to use good fresh fuel with no ethanol, and to run it dry when storing it.

Saw is a top-handle Dolmar PS-3410TH. Thanks in advance...

 

[Chainsaw Jim]

Start with a new carb kit.

 

[Canyon Angler]

Thanks, Chainsaw Jim, will do.

 

[Okie]

A make and model of the saw is always helpful for more precise replies.
SORRY, I overlooked the Model your had already posted. Ronnie Milsap notified me you had it posted.

Sounds kinda like fuel issue. I would check all the fuel lines. 12 years old. I would think about replacing the lines with tygon. See if yours are snug at the primer bulb.
Do you see any fuel in the air box?

 

[cuinrearview]

It's possible to put the primer hoses on backwards. I'm not sure if it would run at all in that scenario though.

 

[Conquistador3]

I don't remember if the PS-3410TH uses a Walbro WT or Tillotson HU carburetor... check what you have before buying a carb kit. You'll find the exact model stamped on the carburetor body.
You can get a Walbro or Tillotson branded repair kit for far less than an obscenely priced kit from Makita and it's exactly the same thing. If you are feeling adventurous, Chinese repair kits are even cheaper and I've had about 80% success with them so far.

 

[Old2stroke]

You have a carb problem so follow the advice above with getting a new kit installed and make sure you understand how to get the inlet control lever set right and make sure the seat is clean. When a saw is running way too rich it is often because the inlet valve isn't seating and the fuel pump is pressurizing the fuel chamber and forcing excess fuel through the jets, then if you press the primer bulb, it temporarily sucks fuel out of the fuel chamber and lets the saw run leaner than its over rich condition. The primer is hooked up properly.

 

[Ketchup]

First, you absolutely need to rule out air leaks. No leaks will mean you definitely have a carb issue. An air leak can cause all kinds of erratic behavior that seems like a carb problem. You could go straight to the carb, but it's much better to know there aren't other factors.

You have a carb problem so follow the advice above with getting a new kit installed and make sure you understand how to get the inlet control lever set right and make sure the seat is clean. When a saw is running way too rich it is often because the inlet valve isn't seating and the fuel pump is pressurizing the fuel chamber and forcing excess fuel through the jets, then if you press the primer bulb, it temporarily sucks fuel out of the fuel chamber and lets the saw run leaner than its over rich condition. The primer is hooked up properly.

If you don't have an air leak, Old2Stroke is probably on the money. Sounds like you are leaning the saw to compensate for a bad inlet valve.

 

[Okie]

It's possible to put the primer hoses on backwards. I'm not sure if it would run at all in that scenario though.

From experience usually only briefly runs. The primer bulb when correctly plumbed sucks fuel through the carb and returns fuel to the tank instead of pushing fuel towards the carb.

 

[Chainsaw Jim]

First, you absolutely need to rule out air leaks. No leaks will mean you definitely have a carb issue. An air leak can cause all kinds of erratic behavior that seems like a carb problem. You could go straight to the carb, but it's much better to know there aren't other factors.



If you don't have an air leak, Old2Stroke is probably on the money. Sounds like you are leaning the saw to compensate for a bad inlet valve.

You must first have a good carb to test for other problems when pressure testing tools aren't readily available.

 

[Canyon Angler]

This all sounds like really great advice, particularly Old2Stroke's comments (sounds like exactly what might be happening). I really appreciate the help!

I'll post up what I find when I get into it...waiting for the V/P tester to arrive...

 

[Canyon Angler]

Just to square the circle, I put a carb kit into it. When I first started it up again, it seemed like it was still behaving the same way.

But after I pulled the limiter caps, futzed with it some and turned up the idle speed, it seems to be OK again ... for the time being. Maybe the diaphragm was a little stiff when I first put it in? Who knows, all I care is that I don't have to restart it every time I want to use it in the tree.

Carb kit did not come with a needle valve, and the metering lever looked about right so I did not mess with that. If it continues to have trouble, I'll open it up again and check for trash under the needle, as well as do a pressure/vacuum test. (Not sure how to P/V test this saw, in part because it has a reed valve box and lacks an impulse line from what I can see...)

Thanks again for all the replies and help.

 

[Huskitoter]

Having to richen up the mixture settings very much from the factory settings likely points to an air leak if the carb is working right.

 

[Canyon Angler]

Thanks, Huskitoter. It's got a muffler mod, too, so I had to richen for that...

 

[Ketchup]

Keep us updated. Sort of a rare saw, somebody may appreciate your writeup when there's gets finicky.

 

[Canyon Angler]

Keep us updated. Sort of a rare saw, somebody may appreciate your writeup when there's gets finicky.

Will do. I probably should have put the saw model in the title, but I was afraid if I did that, no one would read it...