Dolmar 120 is not holding tune.

[Meteor]

I recently picked up a nice Dolmar 120 which was hard to start so I did a total clean and rebuilt the carb and replaced the fuel lines, filter, etc. I’ve been able to use the saw and it works great when running but find it “detunes” between uses and on different days. Yesterday I tuned the saw again and it ran great but today was hard to start and impossible to keep running again.

I’m wondering if it’s just down to the Tillotson HK carb and it’s plastic diaphragm body (top arrow). I’ve heard they can become problematic over time and difficult to tune but I’m not sure how common that is or if I may not be considering another option. I’m seriously considering replacing it with a Walbro HDA-22 instead as I know these saws were also made with them. Does anyone have a 120 with the Walbro carb? And maybe pics? My main concern is whether the idle/L/H screws will be in the same place with the different carb and work with the rubber piece around them (bottom arrow).

Or could there be another issue I’ve overlooked that’s unrelated to the carb?

 

[Vintage Engine Repairs]

First I’d suggest pressure and vac testing the saw.

I’m not familiar with that specific carburetor, but if it were mine, I’d also pressure test from the fuel line through the pump sidr to check for air leaks and needle sealing. Then, I’d take a spare metering side cover, drill a hole, and fit a barb with epoxy. Before sealing it up, I’d remove the diaphragm and pressure test the metering side. If the model has a bypass with a check valve, use vacuum instead of pressure.

This process will help identify if the carb is warped and leaking. If it is, you might be able to carefully flatten it on a piece of glass with abrasive paper. If it’s not warped, you’ve saved money by avoiding a replacement, and you’ll also have a reusable cover.

Swapping parts rarely fixes the root problem, and the repair can quickly turn into a money pit.

Have you:

1). Done a pressure and vacuum test on the saw?
2). Checked the piston for scoring through the exhaust port?



I’d recommend checking all these before spending any money.