Mastermind meets the Dolmar 7900

[Mastermind]

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.

 

[Stihl n Wood]

Master mind, what type of piston is that? Dumb question but are you making your own windows in that piston? Or do dolmars not have windows? I'm not familiar with dolmars what so ever as you can see. The jug looks like it has alot of potential though. Lots of meat by the looks.. happy shaving!!

 

[Mastermind]

Master mind, what type of piston is that? Dumb question but are you making your own windows in that piston? Or do dolmars not have windows? I'm not familiar with dolmars what so ever as you can see. The jug looks like it has alot of potential though. Lots of meat by the looks.. happy shaving!!

That's a factory piston for the 7900. It's the latest design for that saw. Lighter and with a different shape.....no windows.

 

[chopperfreak2k1]

that's a weird lookin piston. can't wait to see what ya do here Randy!

 

[bryanr2]

I'm watching this one extra close as I have the Makita 6401, the oem kit in a box, the HD filter, and black cover. The saw needs a sticker.

 

[Terry Syd]

Good looking piston to get around all the shrouding of the lower transfers. - I never figured out why guys would put ports in pistons when the ports would be above the bottom of the cylinder at BDC.

 

[Mastermind]

Good looking piston to get around all the shrouding of the lower transfers. - I never figured out why guys would put ports in pistons when the ports would be above the bottom of the cylinder at BDC.

I don't get that either Terry. In order for a windowed piston to be of use the lowers would need to be moved way the heck up the walls.

 

[Andyshine77]

Looks like a fine test subject. I'd say it's an 05 or 06 model. The oil lines will be black and the filter cover won't have the prefilter mesh screen. She will likely leak oil with the old black lines, but as long as it oils the bar properly it's fine. I personally haven't seen any performance difference with the HD filter. Don't go crazy with your porting numbers, blow down is your friend as well as intake duration.

Have fun with the build.

 

[Andyshine77]

Good looking piston to get around all the shrouding of the lower transfers. - I never figured out why guys would put ports in pistons when the ports would be above the bottom of the cylinder at BDC.

I don't get that either Terry. In order for a windowed piston to be of use the lowers would need to be moved way the heck up the walls.

Precisely.

 

[bcorradi]

I'm curious to see how it works out for you. All of the good builders that I know say they can get more out of a 440 and a 372 than they can out of a 7900.

 

[Andyshine77]

I'm curious to see how it works out for you. All of the good builders that I know say they can get more out of a 440 and a 372 than they can out of a 7900.

I'd say it's easier to build 440/372 than a 7900, but in the right hands the 7900's can really come alive.

 

[bcorradi]

I'd say it's easier to build 440/372 than a7900, but in the right hands the 7900's can really come alive.

Ya your probably right vs stock. However, I would think it would be at least a 1/2 step ahead once ported vs the 440/372 and that isn't the case from the people I talk to. It is a cool/formidable saw in stock form nonetheless.

 

[Andyshine77]

Ya your probably right vs stock. However, I would think it would be at least a 1/2 step ahead once ported vs the 440/372 and that isn't the case from the people I talk to. It is a cool/formidable saw in stock form nonetheless.

Most of the time you're likely right, but the EC 7900's seem to be quite strong.

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[bcorradi]

Thats a good running saw. Heck who knows maybe mastermind will come up with a recipe that will turn out great and I wish him the best.

 

[Terry Syd]

I figure one of the limits on power of the 7900 is the dinky 17mm carb. Probably why Andy made the emphasis on intake duration.

I've got four C3M carbs sitting here and I just might try butchering one up to see if I can open it up to 18-19mm. If it worked, I could use it on my 365BB, but it would also work on the 7900. Sure wish I had a lathe, using the Dremel is gonna be down right ugly.

 

[bcorradi]

I figure one of the limits on power of the 7900 is the dinky 17mm carb. Probably why Andy made the emphasis on intake duration.

I've got four C3M carbs sitting here and I just might try butchering one up to see if I can open it up to 18-19mm. If it worked, I could use it on my 365BB, but it would also work on the 7900. Sure wish I had a lathe, using the Dremel is gonna be down right ugly.

Ya you could be right, but one of the guys I know had no problem drilling out carbs and experimenting with different carbs, etc. I'm not saying that there isn't very high potential...i'm just relaying info from people I have talked to. It would be awesome if a person could come up with a good recipe on the 7900 because it is a great saw.

 

[Terry Syd]

I think I may bite the bullet and go for a WJ carb on my 365BB. I can clock the intake manifold to fit the carb and then use some parts from a 385 to make up a filter for it. That would give me a 19mm carb off of a 066. I wonder if the 7900 would be able to do something similar.

 

[Andyshine77]

I figure one of the limits on power of the 7900 is the dinky 17mm carb. Probably why Andy made the emphasis on intake duration.

I've got four C3M carbs sitting here and I just might try butchering one up to see if I can open it up to 18-19mm. If it worked, I could use it on my 365BB, but it would also work on the 7900. Sure wish I had a lathe, using the Dremel is gonna be down right ugly.

If I recall correctly the Walbro HD 12 carb that came on the 372 will bolt right in as well, not sure about the venturi size though.

 

[Terry Syd]

The Walbro is actually a bit smaller than the Zama. Walbro is 16.67mm and Zama is 17mm.

I would prefer a straight bolt on mod, I'm not keen on cobbling up things up to get them to fit. However, it is just a firewood saw not a production saw, so if things rattle loose I can always walk back to the shed.

 

[Andyshine77]

The Walbro is actually a bit smaller than the Zama. Walbro is 16.67mm and Zama is 17mm.

I would prefer a straight bolt on mod, I'm not keen on cobbling up things up to get them to fit. However, it is just a firewood saw not a production saw, so if things rattle loose I can always walk back to the shed.

I was thinking the HD carb may actually have a smaller venturi, it's been some time since I looked. I agree a real work saw needs to be reliable, so a bolt on is preferred in this build I'm sure.