Mastermind meets the Dolmar 7900

[nstueve]

All the Zama carbs I've looked at all have the same jetting - two .25mm jets. The jets can be drilled and the flow increased considerably. When a Zama is used on a bigger engine, they need a lot more flow. I've also tried blocking the auxiliary jet on a Zama and just used a drilled idle jet and with a stronger metering spring it may turn out to be an even better approach.

I'm just starting on modding the HDs. They are a bit more difficult to mod as you have to pull the welsh plug and redrill the air bleed/transition holes (and drilling the body of the carb is permanent, you can't replace the jet like on the Zama if you go too far). Since the HD-12 seems to be a popular carb, it would be great to find out what size holes are used on the carb body, that way someone could drill their existing carb to the HD-12 specs. Although, the HD-12 was used on a 71cc engine, so even the HD-12 might benefit from a bit of tweaking to use on a 79cc engine.

Would seem like walbro probably has a spec sheet on each carb that would provide this info...???

 

[psuiewalsh]

What about a 390 carb?

 

[Mastermind]

Yeah Keith, the 390, or 395 carb will work. A 288 air filter elbow is needed to connect the HD filter....

 

[dolmarfan]

I am a offtopic but here goes:
Tuning dolmar 5000 with a limited coil.Tuning to keep under the rev limiter(13500rpm) requires H screw 3,5-4 turn CCW and my tach is still reading 13200rpm.I dont think I have a leak.Is this ok?
Thanks!

 

[Mastermind]

Probably........

Those small saws tune sorta weird. Less airflow means more turns out on the H screw.

I would find the limiter........then lean it out a little more at a time, checking the tune in the cut. As you lift the pressure it should still burble a little.

Without the rev limiter, that saw would likely still four stroke at 14,500 or so.

 

[psuiewalsh]

Yeah Keith, the 390, or 395 carb will work. A 288 air filter elbow is needed to connect the HD filter....

Good to know. I just put one together and had to dig the epoxy out of the H screw hole. I will test next week and see if it wants more fuel.

 

[psuiewalsh]

 

[cheeves]

Clint- they are the King of the 70cc range. At 79cc, Id have to group them in the 80cc range though- and I think they hold their own pretty well in that class too. Many have considered the 064 to have had the best power to weight ratio in the past, that was until the 7900 came out. It is the reigning "power to weight" king.

I've run a lot of big saws in my younger days, and my 79 is as good if not better than any of them!! Just awesome to use!! Sheer Joy!!!

 

[cbrduckdog]

Update, I still have this saw in this tread. Its still doing very well pulling a 32" skip tooth chain right now. I need to get some video of it since its what 3 years old now or something.

 

[Mastermind]

Hello stranger.

Good to see you here.

 

[cbrduckdog]

Yea been real busy first time back here in a while. Work has me all over the world and the US.

 

[mdavlee]

Nice to see you pop in. You still got that 066 I sold you?

 

[dugide]

Good to know. I just put one together and had to dig the epoxy out of the H screw hole. I will test next week and see if it wants more fuel.

should pressure/vac check, sounds like an air leak. L/H should be 1 and 1/2 turns out.

 

[Terry Syd]

Here's an option on a 7900 carb, check out post #66. - http://www.arboristsite.com/community/threads/dolmar-7900-carb-upgrade.229528/page-4

 

[cbrduckdog]

Nice to see you pop in. You still got that 066 I sold you?

Well a someone ran it an offered a Sig 229 DAK and I could not refuse.

 

[mdavlee]

Well a someone ran it an offered a Sig 229 DAK and I could not refuse.

I would have kicked your tail if you didn't trade for that.

 

[wde_1978]

Nice !

Wish I had any kind of stronger wood to cut to properly check my PS7900 out.

 

[Freakingstang]

Hey Randy, how do these 7900's compare to a good running 064 that you've done? Ever had them side by side?

 

[Big wood1]

Here's a few pics of a build I'm working on.

This is a nice clean saw. It had a Chinese top end and only 140psi...

We're gonna fix that though.



New OEM 79cc jug and new light weight slab sided piston.....yeah baby!!!



It look to me like the designers wanted to try to do something to help the small case size on this saw. There's lots of potential here IMHO.






I'll cut the squish band flat and build a popup in this one.

That's it for now. A bunch of work showed up here today for one of my local guys that held me up a bit. We'll get back on it in the AM though.

Can you tell me where to get that piston please. Thank you

 

[Adirondackstihl]

Can you tell me where to get that piston please. Thank you

7910 piston