Makita - Dolmar 6400 - big bore photos

[heyduke]

I test the compression on a cold saw. Leave the switch in the "off" position, pull the choke lever and push it back in (opens the butterfly in the carb), then pull the rope repeatedly until the gauge stops moving.

I believe my 7900 was around 165 last time I checked it (1850 elevation), but I haven't checked since I got it back from Mastermind today.

Here's some interesting reading for you...

http://www.arboristsite.com/community/threads/dolmar-7900-carb-upgrade.229528/page-3

does the 7900 us an open port design like the 6400?

 

[Kegonsa]

Just tried testing the compression but it did not work. The problem is the gauge I have if screwed in all the way so it seals it acts like a piston stop. I know I have two, but I still have not unpacked after moving two years ago so I will see if I can find it or borrow one tomorrow before I go.

 

[Kegonsa]

well I just added a second O ring to the adapter and got just over 125. Looks like next week when I have time I will be checking the squish and seeing if there is a base gasket.

 

[heyduke]

well I just added a second O ring to the adapter and got just over 125. Looks like next week when I have time I will be checking the squish and seeing if there is a base gasket.

sounds like your compression tester may not be accurate. until you can verify it against a know accurate one consider it to be as trustworthy as a politician.

 

[Philbert]

Thanks for restoring and updating this thread Duke!

Philbert

 

[heyduke]

Thanks for restoring and updating this thread Duke!

Philbert

yeah, we lost a lot of valuable content when they couldn't restore the graphics after the hack. seems like the good stuff never lasts. i'm thinking next time i go for a load of firewood, i'll try to make a vid of the 575. it's a strong saw, actually stonger than the 84cc makita.

 

[Philbert]

I am maybe interested in a HD rental Makita, so this is helpful.

Philbert

 

[heyduke]

I am maybe interested in a HD rental Makita, so this is helpful.

Philbert

a really stout chassis, reliable and a pleasure to wrench, they have to be tough to survive rental duty, but hard to tune with the rev limiter, just my opinions.

 

[blsnelling]

does the 7900 us an open port design like the 6400?

No. It's closed port.

 

[blsnelling]

as i said, i live at 6000ft and 140psi is probably close to 170psi at sea level.

Altitude conversion factors

500 0.987
1500 0.960
2500 0.933
3500 0.907
4500 0.880
5500 0.853
6500 0.826
7500 0.800
8500 0.773

 

[heyduke]

Altitude conversion factors

500 0.987
1500 0.960
2500 0.933
3500 0.907
4500 0.880
5500 0.853
6500 0.826
7500 0.800
8500 0.773

yeah, my estimate was close. if i use .830 as a factor a saw capable of 170psi at sea level would show 141psi. a 150psi saw would show 124.5. i do a lot of work above 8000 but don't notice any difference in performance. i think some of the epa saws work better at high altitude because their undersized low speed jets don't lean the saw out as much giving a wider power band.

 

[Kegonsa]

I was able to get my compressions tester setup correctly by removing the extension I was able to get it to thread in and seat properly. The compression came in at 150 psi. Still need to check the squish

 

[heyduke]

I was able to get my compressions tester setup correctly by removing the extension I was able to get it to thread in and seat properly. The compression came in at 150 psi. Still need to check the squish

mine is running without the gasget but it's the 84cc chinese top end, not the oem 79cc. glad to hear you got your compression tester working.

 

[jar944]

This just showed up today so I figured I'd add some pics of the fresh out of the box factory 7900 cylinder.









big bore vs really big bore