Modding the Dolmar 7900

[parrisw]

Another recent addition to the stable, well used but decent condition 7900 P&C are in perfect condition. Waiting on crank bearings for the 372 so I thought I'd start on the 7900 tonight. Got the machine work done. Machined squish band and cut the base, pretty happy with my new homemade boring bar, works soooooo much better then the other one I was using, no chatter this time, and a nicer finish.

 

[parrisw]

 

[Cleatus]

Subscribed. Anxious to see how your project turns out as I too just picked up a Dolmar 7900. Where in British Columbia are you located as I to live in BC.

 

[tree monkey]

so why do you cut the squish band first?

 

[parrisw]

Subscribed. Anxious to see how your project turns out as I too just picked up a Dolmar 7900. Where in British Columbia are you located as I to live in BC.

Victoria.

 

[parrisw]

so why do you cut the squish band first?

I always set squish before I do the port work and set final timing.

 

[tree monkey]

I always set squish before I do the port work and set final timing.

yes but why cut the sq b4 the base?

 

[Rudolf73]

Cool, I was meaning to ask about this saws build.

How's the new boring bar working?

 

[parrisw]

yes but why cut the sq b4 the base?

Because that's the way I choose to do it. I check stock squish, figure out how much I want to take out of the chamber then do a little math in my head on how much to take off the base after to achieve desired squish, just the way I do it.

 

[parrisw]

Cool, I was meaning to ask about this saws build.

How's the new boring bar working?

Working great, this was its first use. I like it.

 

[tree monkey]

well ok
i cut the base first to get port timing as close to what i want that i can, then cut squish
saves time

 

[parrisw]

well ok
i cut the base first to get port timing as close to what i want that i can, then cut squish
saves time

Right, that makes sense. But if you take more from the base then there is squish how do you check timing if the piston were to hit the CC? Or do you measure timing with a port map? I just use a degree wheel.

 

[Cleatus]

Are you planning on a muffler mod or HD air filter too?

 

[parrisw]

Are you planning on a muffler mod or HD air filter too?

Yes. Muffler mod is a must with a ported saw, well at least I think so. And yes I have the HD filter kit to go on it.

 

[parrisw]

Anyone have any experience with the new slab sided piston for the 7900? Is it worth the money to get? Does it give any advantages in skirt width?

 

[tree monkey]

Right, that makes sense. But if you take more from the base then there is squish how do you check timing if the piston were to hit the CC? Or do you measure timing with a port map? I just use a degree wheel.

i do every thing in dur, you can do ex and tr
to do the in, do port closed and subtract from 360

 

[Cleatus]

Yes. Muffler mod is a must with a ported saw, well at least I think so. And yes I have the HD filter kit to go on it.

Definitely can't wait to see how this project turns out. I'm doing a muffler mod to mine and I have a new HD air filter kit to go on. I also have a Bailey's big bore piston and cylinder kit on the way too but it will sit on the shelf until I figure 100% which direction I plan on going in.

 

[parrisw]

i do every thing in dur, you can do ex and tr
to do the in, do port closed and subtract from 360

Yes, you can, I do it both ways, just to check my numbers, I check timing with the wheel, write it down, then check duration and figure out timing with a little math and see if it matches my first timing check with the wheel, I feel it makes me be a little more accurate that way.

Thanks!

 

[parrisw]

Definitely can't wait to see how this project turns out. I'm doing a muffler mod to mine and I have a new HD air filter kit to go on. I also have a Bailey's big bore piston and cylinder kit on the way too but it will sit on the shelf until I figure 100% which direction I plan on going in.

If you already have a OEM 7900 and the piston and cyl are good, I'd use that, unless you want to play with porting, play around on the BB kit, I did a thread a while back of the same saw but in Makita clothing with the BB kit, you can look through that thread. As for the muffler mod, for these I split the muffler in half and take out the baffling then add and extra port.

 

[Freakingstang]

As for the muffler mod, for these I split the muffler in half and take out the baffling then add and extra port.

that's what i did about 5 years ago... what a PITA. figured someone would have come up with a better idea by now? lol