Stroking a Dolmar 7900

[Intoxsawcated]

Yup, I havent found much on it other than it's similar to the 4 with a splitter.

Sent from my SM-S767VL using Tapatalk

 

[Time's Standing Stihl]

Good info to have.....

For science and stuff....

 

[Terry Syd]

Good information, but it is a bit simplistic. Some carbs work better on a certain size displacement because of the carb configuration. The bigger venturi sounds good, but may not allow the carb to have the necessary fuel curves to get the best fuel delivery through the powerband - and that is the primary function of a good carb.

Brewz and I ran into that using a 460 carb on the modded 029 (64cc). That carb had a bigger venturi than the stock HD-5, but it didn't have the fuel curves the engine wanted. It appears that the 460 carb worked well on a 76cc saw, but just couldn't provide the necessary fuel curves on a 64cc engine. Perhaps it may be a good carb to consider modding for a 7900.

EDIT: The 460 carb also has a main discharge nozzle that doesn't stick out into the venturi to restrict the flow. I comes in at an angle and stays close to the edge of the venturi. The low pressure response through the nozzle, I expect, is much different than other carbs in it's class. Not only is the air flow through the venturi increased, but the fuel response to air flow is different. It could be the carb to mod for the bigger engines around 76cc+. I don't know, never done it...

 

[Fruecrue]

Only one way to find out.
I’ll start testing soon.
Fitted a homemade, 5 minute chain catcher this morning by sleeving a 1/4” bolt in 3/8” copper through existing holes in spikes.
@Canadian farm boy Thank you for taking the saw to Connecticut and I apologize for not having a chain catcher on it, overlooked that point completely. Lesson learned. Luckily Mike had chaps on and can take a punch!

 

[mettee]

This saw runs a 10 pin is that correct? I don't think that is known.

 

[Fruecrue]

This saw runs a 10 pin is that correct? I don't think that is known.

I’ve run 8 and 9 on it, don’t own a 10.

 

[Marshy]

I’ve run 8 and 9 on it, don’t own a 10 yet.

Fixed.
It's in your future, I can tell.

 

[Fruecrue]

Ran it today in a small log that is pretty consistent with the modified Zama. I didn’t do the carb work, not sure what was done but it runs good.

First cut was to get rid of some dirt and nasty stuffs, doesn’t count. After that;
2.69
2.63
2.53
2.60

 

[Fruecrue]

Crap, should’ve done it without the air filter.

 

[Redfin]

If the HS298 is plug and play then the 394 based carbs (WJ39, 70, 71) are going to have a throttle shaft that sticks too far out. The throttle pull is about 1/4" further out on the 394 carbs than the 395 carbs, this causes the throttle cable to run at too much of an angle and it gets caught up.

I've adapted a 394 carb to work on the 395 but I had to recess the carb body for the return spring to shorten the throttle shaft enough.

If ya go with the HS 296-298 or RWJ you want a 390 boot and ring I believe

I stuck with the oem boot. Mild pucker on sides from retainers. Not sayin this is the best option.

 

[mettee]

I’ve run 8 and 9 on it, don’t own a 10.

Did someone put a 10 on it at CT gtg?

 

[Fruecrue]

Did someone put a 10 on it at CT gtg?

I’m not 100% certain but I don’t believe so.

 

[Deets066]

Did someone put a 10 on it at CT gtg?

You thinking of the 661 stroker?

 

[mettee]

Not sure, usmcbuckwild was there and mentioned it, said something about ten pin, maybe he meant the stihl not the dolmar.

 

[Canadian farm boy]

The 661 stroker was running a 10T in the cant.
Dave’s 7900 stroker was running a 9T with the stihl bar.
I’m 100% sure of this. The 10T sprocket on the 661 stroker was mine and 9T on the 7900 stroker is what Dave sent the saw with

 

[mettee]

Very nice

 

[Fruecrue]

Swapped to an HD-12, found some gains.

2.4
2.44
2.53
2.37
2.22

 

[mettee]

Yep its cookin

 

[usmcbuckwild]

Not sure, usmcbuckwild was there and mentioned it, said something about ten pin, maybe he meant the stihl not the dolmar.

The 661 stroker was running a 10T in the cant.
Dave’s 7900 stroker was running a 9T with the stihl bar.
I’m 100% sure of this. The 10T sprocket on the 661 stroker was mine and 9T on the 7900 stroker is what Dave sent the saw with

My mistake, i thought we established it was wearing a 10 pin when we flipped it over on your tailgate. I get confused sometimes.

 

[RI Chevy]

Too many cervasas. [emoji16]
I'm not talking Busch beer either.