Stroking a Dolmar 7900

[Fruecrue]

Any pictures of the air filter and chain catcher

And here’s a pic of the chain catcher.

 

[Fruecrue]

What do you think about putting a 394 carb on it?

I’m not sure if it’s configuration. The other two should go on without much trouble.

 

[huskihl]

I’m not sure if it’s configuration. The other two should go on without much trouble.

The rwj and 394 carb are the same configuration.

 

[Terry Syd]

Poleman already demonstrated that the HD-12 was inferior to the modded Zama twin jet (big time!) on a 7900. However, I know you have been working with the HDs - eliminating the EPA circuit and the like.

I spent a heap of time dialing in the HD-5 on my modded 029. The HD can be done, it just a bugger of a bunch of work. The Zama is a piece of cake compared and it will still have a better idle circuit than an HD.

The HD looks like it has the potential to flow more than the Zama. Having said that, most of the restrictions in the intake tract come down to the bottle neck of the air filter.

If you can get two HS-12s, you could block off the EPA circuit on both, then start opening up the auxiliary jet. First one at .35mm and the other at .4mm. See what works best and then go from there.

 

[Marshy]

The rwj and 394 carb are the same configuration.

I just bought a NOS Walbro WJ70.

 

[mettee]

The modded zama is gonna be hard to beat.

 

[Terry Syd]

Yeah, it's a great carb to work with. When I go by a saw shop I will often stop in to see if they have any old crappy Zamas sitting around in a bucket. I got my last one for A$20. The dealer probably thought he had done a great deal...

 

[mettee]

Only thing better is free.

 

[huskyboy]

I bought one of the new wj71 394 carbs they were selling on eBay for 40$ a while back and put a rwj top plate on it for impulse (straight swap), and a few mods to get it to work on a 385/390. It tuned very well. Should work on a 7900, might have to mod the throttle cable stuff/turn the boot?

 

[Marshy]

I bought one of the new wj71 394 carbs they were selling on eBay for 40$ a while back and put a rwj top plate on it for impulse (straight swap), and a few mods to get it to work on a 385/390. It tuned very well. Should work on a 7900, might have to mod the throttle cable stuff/turn the boot?

I'll compare later but looks like throttle cable placement is perfect. I'm hoping for direct swap on the WJ70.

 

[Redfin]

I’m not sure if it’s configuration. The other two should go on without much trouble.

The rwj and 394 carb are the same configuration.

Tilly 298 is plug and play Dave.

 

[Fruecrue]

I remember reading somewhere that the RWJ Venturi is larger than 385/390 carbs.
Anyone know how a 394 carb stacks up?
Seems I need one now....for science.

 

[Lightning Performance]

Made it to page five. Too nice to sit here longer today. So many good threads on engine specs/builds here. Looking forward to building up and retooling a shop for bench work again. You guys keep my head in the game.

 

[huskyboy]

Venturi is 18.25 in the wj71 and 18.45 on the wj39. Rwj is like 19 I think. The jetting is richer on the 394 and 385/390/395 carbs. Wasn’t a fan of the rwj on a ported 385, the tilly behaved better.

 

[srcarr52]

Tilly 298 is plug and play Dave.

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.

 

[Fruecrue]

Venturi is 18.25 in the wj71 and 18.45 on the wj39. Rwj is like 19 I think. The jetting is richer on the 394 and 385/390/395 carbs. Wasn’t a fan of the rwj on a ported 385, the tilly behaved better.

Jetting can be changed. I’m thinking move as much air as possible and match fuel to that is the way to make power.

 

[Fruecrue]

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.

Good info thanks Shaun.

 

[Deets066]

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

 

[Intoxsawcated]

Has anyone messed with an RWJ-5 over a RWJ-4?

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[huskyboy]

Has anyone messed with an RWJ-5 over a RWJ-4?

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Looks like the same carb as the rwj4 but internal impulse right?