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.

 

[Meteor]

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.

Thanks for the suggestions. Piston/cylinder is very clean. Unfortunately I don’t have equipment to test pressure. I guess that may have to be something I’ll have to look into getting.

 

[Vintage Engine Repairs]

Thanks for the suggestions. Piston/cylinder is very clean. Unfortunately I don’t have equipment to test pressure. I guess that may have to be something I’ll have to look into getting.

You’re welcome! If you’re planning to work on any 2-strokes, a pressure and vacuum tester is essential. It will save you a lot of frustration and expense.

 

[Meteor]

You’re welcome! If you’re planning to work on any 2-strokes, a pressure and vacuum tester is essential. It will save you a lot of frustration and expense.

Yeah I ordered one this morning. I’ve got a couple of other saws sitting here that it should help with too.

 

[Meteor]

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.

So I’ve now been able to pressure test the carby and the cylinder for compression. Here’s the results:

- Firstly, the piston and cylinder both look great. Very clean and no scoring/marks on intake or exhaust.
- Compression test came back at 95 PSI
- Carby pressure test came back as fish tank bubbler! Seriously. It would be easier to describe where it’s not leaking. I’ve even had a go at trying a thin bead of gasket sealant near the gasket edges on each surface which helped but no where near enough. I think the plastic diaphragm body is just completely shot.

At this point, I think I’d need to track down a new plastic body or try another carby. The HK’s aren’t made anymore that I’m aware of and I can’t seem to locate a new body anywhere either. I think I’m back at square #1 and wondering if the Walbro HDA-22 will actually fit the 120.

 

[Vintage Engine Repairs]

So I’ve now been able to pressure test the carby and the cylinder for compression. Here’s the results:

- Firstly, the piston and cylinder both look great. Very clean and no scoring/marks on intake or exhaust.
- Compression test came back at 95 PSI
- Carby pressure test came back as fish tank bubbler! Seriously. It would be easier to describe where it’s not leaking. I’ve even had a go at trying a thin bead of gasket sealant near the gasket edges on each surface which helped but no where near enough. I think the plastic diaphragm body is just completely shot.

At this point, I think I’d need to track down a new plastic body or try another carby. The HK’s aren’t made anymore that I’m aware of and I can’t seem to locate a new body anywhere either. I think I’m back at square #1 and wondering if the Walbro HDA-22 will actually fit the 120.

Can you flatten the plastic body carefully on a piece of paper and glass?

Most 2 strokes won’t run on 95 psi compression either. It’s possible your gauge is to blame though.

 

[Meteor]

Can you flatten the plastic body carefully on a piece of paper and glass?

Most 2 strokes won’t run on 95 psi compression either. It’s possible your gauge is to blame though.

I did consider trying to flatten like that but I'm not sure it would work. I may still give it a go but I'm 99% sure it's leaking in other spots internally as well as around the edges.

I only got the cylinder gauge today and also considered it could be off. I'll try it on some other saws as soon as I get a chance.