Dolmar 7910 rebuild. Need some help

[t4driller]

Hmm. Can't upload a video. Never tried to before and I don't use YouTube.

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[SteveSS]

The video worked fine for me.

 

[t4driller]

Compression is 185 right now. Piston and cylinder look good. Gonna try and get a tank or 2 threw it then take it back apart and get some timing numbers and open it up a bit.

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[t4driller]

I got a couple tanks threw it finally. Compression ended up at about 190/195 . Im pretty happy with the way itd running in stock form pretty much, but i want more. So I took it back apart to get some timing numbers and open it up a little. I came up with

Ex. 106
Primary. 133
Secondary 137
In. 80.5

After reading a couple of threads here it seems like they benefit from raising the exhaust and transfers. So I plan on widening and raising exhaust, arising the transfers, widening the intake, muffler mod, and timing advance. Any advice is greatly appreciated. Thanks

 

[t4driller]

 

[huskihl]

Don't raise the transfers much. These like 28° blowdown.
102 130, maybe 104 132. Don't lower your intake more

 

[Miller Mod Saws]

I got a couple tanks threw it finally. Compression ended up at about 190/195 . Im pretty happy with the way itd running in stock form pretty much, but i want more. So I took it back apart to get some timing numbers and open it up a little. I came up with

Ex. 106
Primary. 133
Secondary 137
In. 80.5

After reading a couple of threads here it seems like they benefit from raising the exhaust and transfers. So I plan on widening and raising exhaust, arising the transfers, widening the intake, muffler mod, and timing advance. Any advice is greatly appreciated. Thanks

What size of bar you gonna run and how heavy of a hand do you like to use?

 

[t4driller]

Don't raise the transfers much. These like 28° blowdown.
102 130, maybe 104 132. Don't lower your intake more

I was planning on 102. 130. I'm gonna take my time and go slow. Take a little off and check it.

 

[Miller Mod Saws]

I was planning on 102. 130. I'm gonna take my time and go slow. Take a little off and check it.

Those numbers will work great. 2 details for ya that I think make a difference. Look at your entry angle of your transfers. Both the mains and secondaries are relatively flat. Try to maintain that and the square port shape. But mainly the entry angle.

 

[Miller Mod Saws]

Left to right. Work on lower trans wall a little bit. Square it up kinda like my line and blend it down in.

Thin the bottom of the trans divider. Kind of point it and then blend it up into the tunnel just a little ways.

Gasket match and blend the right side.

 

[t4driller]

Left to right. Work on lower trans wall a little bit. Square it up kinda like my line and blend it down in.

Thin the bottom of the trans divider. Kind of point it and then blend it up into the tunnel just a little ways.

Gasket match and blend the right side. View attachment 19623

Thank you for the info. [emoji2] I spent some time laying out the ports and just looking at what I want to do. On the lower transfers I have it drawn out
almost identical to what you have there. I've learned from doing 372/365s that there's some good gains from nipping the corners and smoothing out the lower transfers.

Do I have to worry about the ring end gap coming down into the intake port if I widen and square off the intake? It looks like it stops above the port enough not to worry about it. I need to take the fuel tank and carb box off to be able to see in there a little better.

 

[Miller Mod Saws]

No. I actually raise my intake as well. Just run your motor to bdc and take a short pencil and trace your intake port onto your piston. You'll see what I mean.

 

[t4driller]

Finally got some time to do some grinding. I have everything roughed in right now. Intake got widened and raised just a tad. Case matched lower transfers. Raised exhaust and widened. Raised transfers. Timing numbers are

In. 80.5
Ex. 100
Trans 128 / 131
Still have to put bevels on. Numbers should go to ex 99. Trans 127/130.

 

[t4driller]

I'm about ready to put this thing back together. I'm waiting on a new piston ring. The oem one I got had an end gap of .023 when I originally put it together. Yes I know it was way to much but new I'd be taking it back apart to port. I accidentally ordered 52mm by 1.5 mm. I checked that ring in it and the end gap was .004. Hopefully the one I'm getting is the same. Lastnight I gutted the muffler and opened it up. For the timing now.... I plan on advancing the timing a little. I also have a am 272 coil. Is it worth it to put that coil on or think I'll be ok just advancing the oem one? Thanks can't wait to put this thing in wood!!

 

[Miller Mod Saws]

I'm about ready to put this thing back together. I'm waiting on a new piston ring. The oem one I got had an end gap of .023 when I originally put it together. Yes I know it was way to much but new I'd be taking it back apart to port. I accidentally ordered 52mm by 1.5 mm. I checked that ring in it and the end gap was .004. Hopefully the one I'm getting is the same. Lastnight I gutted the muffler and opened it up. For the timing now.... I plan on advancing the timing a little. I also have a am 272 coil. Is it worth it to put that coil on or think I'll be ok just advancing the oem one? Thanks can't wait to put this thing in wood!!

IMO for a work saw. The 7900/6400 anything with the 13500 it's not nessisary as they still tune pretty easy. The 12800 coils can be tuned just fine to cut wood but if the saws running good they can be running right on that rev limiter. And can be harder to tune. So do u need it. Absolutely not. Is it nice on the 12800 limited saws. Yes. Yes it is!

 

[t4driller]

Are the 7910 limited to 12.8? Think mine is a 2012 model

 

[Miller Mod Saws]

Are the 7910 limited to 12.8? Think mine is a 2012 model

Yes.

 

[t4driller]

Yes.

[emoji20] [emoji20]

 

[t4driller]

I'll see how it does with the oem coil. To use the aftermarket coil I think I read somewhere that you need to advance the saw 35 degrees. I'm having a hard time with the search locating exactly what needs to be done. Also the coil wire doesn't reach very good. Could I take the lead out of the oem and use in the am coil. What is the am coil limited to? Thanks

 

[Miller Mod Saws]

I'll see how it does with the oem coil. To use the aftermarket coil I think I read somewhere that you need to advance the saw 35 degrees. I'm having a hard time with the search locating exactly what needs to be done. Also the coil wire doesn't reach very good. Could I take the lead out of the oem and use in the am coil. What is the am coil limited to? Thanks

http://opeforum.com/threads/7900-coil-swap.965/
Try this