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.

 

[pioneerguy600]

Tillotson HS222 would be my choice.

 

[stihltech]

Throw the compression gauge away! Internal condition is best done by holding saw by starter handle. If it stays up, OK. If not, tear it down. Most likely rings at that age. Piston if worn.
There, ready for maters.

 

[Meteor]

Tillotson HS222 would be my choice.

Thanks. Very curious why you recommend the 222 rather than the Walbro HDA-22? It looks like getting one of those two will be my only option now. It’ll cost me around AUD$130 for the Walbro inc. shipping but double for the HS-222. (Hence why I’m wondering what the benefit of the Tillotson is.)

 

[Meteor]

Throw the compression gauge away! Internal condition is best done by holding saw by starter handle. If it stays up, OK. If not, tear it down. Most likely rings at that age. Piston if worn.
There, ready for maters.

Funnily enough, I got a refund and did just that. It was useless. I tested 6 or 7 saws (some of which only have 5-10 tanks through them) and EVERYTHING was coming back in a narrow range of 90-105. I’d guesstimate it was reading a good 30+ low. The piston and cylinder are immaculate. In fact the whole saw is really very clean. I’d be surprised if it’s had even a couple of dozens tanks run through it. (Which is why I was so surprised initially about the compression test.)

 

[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 pulled the carb again yesterday and remembered that there’s several locating pins on each side so a flat piece of glass is out. I realised also that it’s not just the plastic diaphragm body that’s out but also the main body as well. I reached the point of realising the thing is toast and decided to get drastic and pulled out a honing stone.

I honed both the plastic and main carb body face on the diaphragm side. (Fuel pump is fine.) I also tried a different gasket sealant/maker which seemed to go better. I tested it 24hrs later and incredibly my “bush fix” approach reduced the leaks by 80-90%. But it’s still leaking. I’ll give it one last shot over the next few days and call it. It’s also leaking somewhere inside the main intake which I assume is a jet. I did test the main needle when I had it apart and it didn’t seem to be leaking so not sure what’s going on there yet.

 

[pioneerguy600]

Thanks. Very curious why you recommend the 222 rather than the Walbro HDA-22? It looks like getting one of those two will be my only option now. It’ll cost me around AUD$130 for the Walbro inc. shipping but double for the HS-222. (Hence why I’m wondering what the benefit of the Tillotson is.)

Sorry for your difficulty in procurement of a Tillotson, they are much easier for me to get them here and I have always used them whenever I can, have spares for them in my parts dept. Use whatever you can get in your part of the world.

 

[pioneerguy600]

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.

Tom, the Tillotson HK carbs are very prone to the metering valve seat wearing out in them, so it gets almost impossible to tune the carb. I have tossed many of them in the bucket and just replace them with a different model that will work. The plastic cassette was a bad idea for longevity, they worked fine til they don`t and then it is about useless to try a repair on them, the valve seat cannot easily be recut or resurfaced smooth enough to get a good tight seal, even the bore the needle rides in can become ovaled out. I tried repairing them but never achieved a satisfactory result ,better off just to find a different model of any manufacturer to replace the HK.

 

[Meteor]

Tom, the Tillotson HK carbs are very prone to the metering valve seat wearing out in them, so it gets almost impossible to tune the carb. I have tossed many of them in the bucket and just replace them with a different model that will work. The plastic cassette was a bad idea for longevity, they worked fine til they don`t and then it is about useless to try a repair on them, the valve seat cannot easily be recut or resurfaced smooth enough to get a good tight seal, even the bore the needle rides in can become ovaled out. I tried repairing them but never achieved a satisfactory result ,better off just to find a different model of any manufacturer to replace the HK.

Thanks for that. Pretty much confirms my suspicions. That plastic cassette is such a bad design choice that I can only assume it was a deliberate measure to introduce “planned obsolescence”.

 

[pioneerguy600]

I have a great affinity for the Sachs Dolmar chainsaws as they were my main saws for about 7 years for commercial logging and other wood harvesting. The 116 up to the 120 were my main saws, have them in most configurations including the I and Super models. There is a thread over in the Stickies covering them.
https://www.arboristsite.com/threads/sachs-dolmar-chainsaws-thread.143945/

 

[Meteor]

Just returning to this thread to provide a final conclusion.

Long story short… it was definitely the Tillotson HK carb that was the whole problem. My new Walbro HDA22-1 carb arrived and I installed it today. The saw popped on the 2nd pull and was running BEAUTIFULLY after a couple more. No tuning issues so far and everything seems to be running smoothly. Compression is certainly not an issue. The only difference with the HDA carb is that the low and high screws are slightly shorter than the Tillotson screws so don’t quite make it to the rubber case mount. It won’t be an issue though and it can still be tuned fine. The air filter, gaskets, etc all match up perfectly with the new carb.

I also recorded it so will turn it into a YouTube video at some point. Hopefully this thread helps others somewhere down the track. Thanks for everyone’s help in troubleshooting along the way too.