Stroking a Dolmar 7900

[Fruecrue]

You gonna mod the carb to poleman spec?

I’m not familiar with this mod.
Please advise.

 

[Deets066]

Some drilling and epoxying may be involved

 

[Terry Syd]

Wow, you lucked out - you got an original Zama 'twin jet' carb. Either mod that sucker yourself or ship it off to Poleman to rebuild and mod - you won't be disappointed.

 

[mettee]

I’m not familiar with this mod.
Please advise.

This is from pole man.

http://opeforum.com/threads/dolmar-64xx-73xx-79xx-series-and-smurfs-thread.13540/page-44

Terry and poleman are the carb masters, I took that from polemans post with his permission.

A good set up:

.30 main
.30 to .35 aux
.55 mixing chamber

 

[Terry Syd]

I think for the displacement of this saw, Poleman would go for a .40mm auxiliary jet. It would be best to check with him.

 

[mettee]

I got lucky, another twin jet

 

[mettee]

I think for the displacement of this saw, Poleman would go for a .40mm auxiliary jet. It would be best to check with him.

Terry if you think you may not get a clean hole when you drill, do you think it best to go smaller in that case?

 

[Deets066]

What’s the bore and Venturi on these twin jets?

 

[redline4]

@Fruecrue if you need to drill the jets and dont have the bits, Im pretty sure I have my tiny bit index and pin vice laying around from when I was messing with Tecumseh carbs I can send to you.

 

[Terry Syd]

The first run through the jets is the hardest. Going from .25mm to .30mm takes a lot longer and more patience. However, after that the jets ream out they are very easy.

If it is the first time you do the mod, you might get a bit of wobble during the first part of the jet, but it will go go through the bottom cleanly. Just don't keep turning the drill/reamer after you punch through.

I use an arm chair that I can rest my elbows on to help stabilize my hands. The carb sits on the side of the table so that it can lie flat. Just go slow and easy the first time.

Getting the idle/transition (referred to in the link as the 'main') jet out will be a hassle the first time you do it. Everybody (Poleman, me, Randy, ....) chopped the top of the jet off. You need to make up a special tool to grip the jet down low and twist the jet as you pull it. Once you can get the jet to twist you should be good - pretend you are 'unscrewing' the jet from the carb.

After you get the carb running, you might tweak the throttle response with some shims under the metering spring. It doesn't change the jetting any, but you can get the carb to work like a light switch. It won't make you cut wood any faster, but it sure feels impressive.

 

[Fruecrue]

@Fruecrue if you need to drill the jets and dont have the bits, Im pretty sure I have my tiny bit index and pin vice laying around from when I was messing with Tecumseh carbs I can send to you.

Thanks Ed, I received a PM from a respected builder offering to mod the carb at no cost.
I really enjoy doing things myself but this is an offer I can’t turn down.
Good things to come!

 

[redline4]

Yeah, you dont turn stuff like that down!

 

[Terry Syd]

What’s the bore and Venturi on these twin jets?

The venturi on the Zama is deceptive. They apparently designed the carb for the best fuel metering and the venturi looks weird. The reason is that the venturi was designed on a flow bench. The area around the base of a nozzle can create 'eddies', that means that some of the mixture is actually going backwards in the carb. By filling in the area next to the base of the nozzle the overall flow of the carb increases even though it looks smaller.

When the Husky engineers built their factory ported saw, they removed the HD-12 and put on a modded Zama. They also gutted the high top filter and used foam for the filter element in order to increase the flow. The carb flowed just fine for the engine, it was the filter that held back the flow.

 

[mdavlee]

What’s the bore and Venturi on these twin jets?

Pretty close to a HD16. Less than a mm difference if I remember correctly

 

[mettee]

Thanks Ed, I received a PM from a respected builder offering to mod the carb at no cost.
I really enjoy doing things myself but this is an offer I can’t turn down.
Good things to come!

So, uh, like, where/who do I send MY carb to [/Beavis]

 

[Terry Syd]

Check with Poleman. He's got a lot of experience with different applications. He'll ask you a bunch of questions and then jet the carb for that saw.

 

[mettee]

I'll try not to bug Rich.

I really want to build this saw and port it and do all the mods myself, that's my goal at least.

I've lined up a lathe thats the hard part.

 

[Deets066]

These must be fairly common

 

[Terry Syd]

These must be fairly common

Not anymore they aren't. We used to be able to get Chinese copies for less than $20. You could buy a couple and starting modding. Open up a jet on one, then go a bit larger on the other carb, etc. - then eventually use the one that worked the best.- A modded carb with all new parts.

The Chinese source has dried up and now you have to find an old original lying around somewhere.

 

[Deets066]

Not anymore they aren't. We used to be able to get Chinese copies for less than $20. You could buy a couple and starting modding. Open up a jet on one, then go a bit larger on the other carb, etc. - then eventually use the one that worked the best.- A modded carb with all new parts.

The Chinese source has dried up and now you have to find an old original lying around somewhere.

I’m Gunna try my hand at modding it. Then run it against a larger Tilly and see how it stacks up