Tuning a Dolmar 7900

[Gregford]

Hey guys,

can you tune a Dolmar 7900 carb by ear or do you really need a tachometer? The local Dolmar/Makita agent insists that a tachometer is necessary.

I'm wondering because this one seems a bit different to tune compared to the average chainsaw. The high-speed mixture screw seems a lot more gradual than a Husky's, and if I set the mixture so it's only just 4-stroking, it seems to be running too rich according to the plug colour.

2 reasons I ask is that the last owner stuffed a piston and barrel by running it far too rich (severe carboning scratched up the bore) and I'm not all that impressed with the saw. On paper, it has marginally more hp than my 288, yet there is no way it compares. It hasn't run in yet, but it has excellent compression (perhaps a tad more than the 288), so I was expecting great things from it. Perhaps the old adage that there's no substitute for cubic inches applies here...

Thanks!

 

[rms61moparman]

This is a good video of tuning a 7900.

 

[Gregford]

This is a good video of tuning a 7900.

Hey cheers for that. Looks like a good way to do it.

 

[angelo c]

is that bouncing off the limiter or Stroking ?

 

[rms61moparman]

The saw in the video is 4 stroking.


Mike

 

[angelo c]

Thanks Mike,
It doesn't sound like my BB kit when stroking. It also doesn't sound like riching up the mixture is having much effect.

 

[rms61moparman]

Thanks Mike,
It doesn't sound like my BB kit when stroking. It also doesn't sound like riching up the mixture is having much effect.

It probably would, IF he was richening up the mixture!
It started out quite rich and he kept going leaner 'til it hit the sweet spot!


Mike

 

[Andyshine77]

Yup started rich and ended up clean and right on the limiter, aka just right.

 

[angelo c]

Yup started rich and ended up clean and right on the limiter, aka just right.

Thanks Andy and Mike,

I always wondered what the difference in sound was between stroking and bouncing..I guess my follow up question is can you have one without the other or do you just lean till you get consistant bouncing ?

is there an easy way to tell if the Makita has a rev-limiter ?

 

[rms61moparman]

Thanks Andy and Mike,

I always wondered what the difference in sound was between stroking and bouncing..I guess my follow up question is can you have one without the other or do you just lean till you get consistant bouncing ?

is there an easy way to tell if the Makita has a rev-limiter ?

Angelo,

The difference in sound is really hard to tell.
That is why Andy tunes them in the cut, to take the rev limiter out of the equation. The sound when hitting the limiter is usually a coarser sound if that makes any sense. Huskies are easier to hear on the limiter for me than the Dolmar/Makitas.

Yes you can have one without the other! That is what makes the tune on these so critical.
If it four strokes before it hits the limiter at 13,500 that is ok for the most part as long as it is close.(Way too rich can cause excessive carbon which can cause damage too)
But hitting the limiter and being too lean is a sure recipe for disaster, that is what causes the dreaded LEAN seize.
The easiest way to tell if the Makita has a rev limiter is to ask! Yes they do unless the ignition has been changed out for an unlimited one.


HTH
Mike

 

[angelo c]

Angelo,

The difference in sound is really hard to tell.
That is why Andy tunes them in the cut, to take the rev limiter out of the equation. The sound when hitting the limiter is usually a coarser bound if that makes any sense. Huskies are easier to hear on the limiter for me than the Dolmar/Makitas.

Yes you can have one without the other! That is what makes the tune on these so critical.
If it four strokes before it hits the limiter at 13,500 that is ok for the most part as long as it is close.(Way too rich can cause excessive carbon which can cause damage too)
But hitting the limiter and being too lean is a sure recipe for disaster, that is what causes the dreaded LEAN seize.
The easiest way to tell if the Makita has a rev limiter is to ask! Yes they do unless the ignition has been changed out for an unlimited one.


HTH
Mike

Thanks Mike,
that all makes sense. I guess the best thing to do is to tune your ear TO the tach and confirm one off the other. I've always just leaned till it stroked and really didn't concern myself with the limiter. I agree I don't have too much uncertainty tuning my Huskys or the Stihls, my Mak seems "smoother" to me.

 

[FlyLow]

As the video was ending he made one last adjustment, I am thinking just a touch leaner?

 

[ncfarmboy]

Thanks Andy and Mike,

I always wondered what the difference in sound was between stroking and bouncing..I guess my follow up question is can you have one without the other or do you just lean till you get consistant bouncing ?

is there an easy way to tell if the Makita has a rev-limiter ?

Another way to tell if your saw is rev limited (if you have a tach) your tach will go crazy when it's bouncing off the limiter.
Shep

 

[Andyshine77]

As the video was ending he made one last adjustment, I am thinking just a touch leaner?

No I was actually richening it up just a bit.

 

[stubnail67]

be cool to get this video back....

 

[Barcroftb]

I agree this thread is useless without the vid!

 

[blsnelling]

Andre doesn't post much anymore, so here it is.

Dolmar 7900 tuning.:

 

[Barcroftb]

Thanks brad!