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Mastermind meets the Dolmar 7900

nstueve

nstueve

Makita Freak!
Joined
Oct 22, 2010
Messages
4,757
Location
Iowa
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...???
 
dolmarfan

dolmarfan

ArboristSite Lurker
Joined
Jul 27, 2012
Messages
35
Location
Romania
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

Mastermind

Work Saw Specialist
Joined
Nov 21, 2009
Messages
36,453
Age
56
Location
Tennessee
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.
 
cheeves

cheeves

Tree Freak
Joined
Aug 8, 2010
Messages
14,505
Location
Plymouth, Mass. America's Hometown
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!!!
 
Big wood1

Big wood1

Trying to get it right
Joined
Dec 24, 2018
Messages
18
Age
54
Location
Skokomish
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!!! :D



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
 
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