Stroking a Dolmar 7900

[mettee]

You should have bought one a few weeks ago and you’d have it already.

Slacker.

I have it in my watch list

 

[Fruecrue]

Had a couple minutes so I threw the wheel on her.
Amazing what a millimeter of stroke and a spacer under he jug can do.
Right now I’m reading 102.5 ex., 125 trans., 76 intake.
Compression 175 psig.

 

[Fruecrue]

Con rod length may play a role there as well.
Dolmar stock rod is 65mm center to center.
Waiting on a slow member to let me know solo con rod length, as I was a slacker and never measured mine before I put it in the saw.

 

[Fruecrue]

Also....
Pulled the flywheel and re-mounted 30 degrees andvanced.
Will be fitting an unlimited husky coil when I have the time.
Possibly may go to a friends house and lay down a dozen or so dead ash trees this weekend, it would be nice to have it back together before then.

 

[Wilhelm]

Con rod length may play a role there as well.
Dolmar stock rod is 65mm center to center.
Waiting on a slow member to let me know solo con rod length, as I was a slacker and never measured mine before I put it in the saw.

You did not measure Your rod before using it?!
Blasphemy.

 

[mettee]

Had a couple minutes so I threw the wheel on her.
Amazing what a millimeter of stroke and a spacer under he jug can do.
Right now I’m reading 102.5 ex., 125 trans., 76 intake.
Compression 175 psig.

Do you think you will bring the jug down? A small base gasket would really fubar those numbers wouldnt it.

Do you know measured squish?

I want to see what the timing bump did

 

[Fruecrue]

Do you think you will bring the jug down? A small base gasket would really fubar those numbers wouldnt it.

Do you know measured squish?

I want to see what the timing bump did

1)Unsure of what I’ll decide to do yet. Still looking at other piston options, though I don’t think I’ll go that route. Right now I’m leaning toward swapping jugs and slugs from my 7910 to this saw, cutting the band and leaving the base to run with a standard gasket. Should net me enough compression to raise the exhaust port where I want it.
What’s holding me back is Idea of finding a way to bump compression, have exhaust in the mid 90s and 28 degrees blowdown without touching the uppers. Offset pop up cut on a taller piston may allow all of that. If it comes with wider skirts and ports, that’s very appealing.

2)Squish right now is .023”

3)Me too.

 

[mettee]

How thick is that spacer again, like 40?

 

[Deets066]

You could cut the base about .030, then time the jug again and decide how much you want to cut a pop up or if the trans are where you want just cut the chamber

 

[Fruecrue]

How thick is that spacer again, like 40?

.058” plus a standard gasket in there now.

 

[Fruecrue]

You could cut the base about .030, then time the jug again and decide how much you want to cut a pop up or if the trans are where you want just cut the chamber

If I cut the chamber now my squish would suffer.
The goal would be to leave the spacer and the band. Find a suitable piston to cut and raise exhaust.

 

[Deets066]

If I cut the chamber now my squish would suffer.
The goal would be to leave the spacer and the band. Find a suitable piston to cut and raise exhaust.

If you dropped the jug.030 you would get your intake and trans closer to where they need to be. If you cut the piston your trans would stay the same as they are now.

 

[Deets066]

You cans grind the ex to where you want

 

[Fruecrue]

transfers need to go up if anything, not down.

 

[Fruecrue]

123 / 95 / 81 with 190 psi is the goal.

 

[Fruecrue]

Light bulb.
Sorry, I’m slow.
That would still leave me a bit short on compression me thinks.

 

[Deets066]

123 / 95 / 81 with 190 psi is the goal.

Pretty high trans on a 7900

 

[Fruecrue]

Pretty high trans on a 7900

Right now I’m at 22.5 degrees blowdown, 125 transfers, Runs ok.

 

[Deets066]

Right now I’m at 22.5 degrees blowdown, 125 transfers, Runs ok.

More BD = more gooder

 

[Deets066]

123/ 95 would probly be enough, forgot you were goin that high on ex