Dolmar 7900 Carb upgrade?

[Poleman]

Leo,is that a direct bolt on??

 

[fastLeo151]

You have to do a little fab work. But nothing serious

 

[Poleman]

Interesting.....another carb that will fit!!!!

Does this carb help the upper or lower RPM range? I would think it would loose a little crispness on throttle response but bet great after that!!??

 

[fastLeo151]

Its wearing a mastermind figerported cylinder so I don't know....

 

[Poleman]

Oh WOW!!!!! A REAL SCREAMER!!!!!!! Finger Porting.......in a league of its own!!!!!

 

[Terry Syd]

I'd like to try these mods on my 7900 do you guys have a feeling for what affects altitude has? I live at 6,500 feet and usually am higher when I'm running my saw.

For that altitude I'd expect that pulling the idle jet and drilling the first air bleed hole to .55 would be a better option for getting a nice clean reliable idle. The modded carb is already rich at idle and requires some fiddling with the throttle plate to get it cleaned up. With the extra altitude, the richness becomes worse. With the drilled air bleed hole the carb adjusts easily with the stock idle screw.

 

[fastLeo151]

For that altitude I'd expect that pulling the idle jet and drilling the first air bleed hole to .55 would be a better option for getting a nice clean reliable idle. The modded carb is already rich at idle and requires some fiddling with the throttle plate to get it cleaned up. With the extra altitude, the richness becomes worse. With the drilled air bleed hole the carb adjusts easily with the stock idle screw.

Do you have any pics of this?

 

[Terry Syd]

I'm going to do a carb for my mate's 372. It will have a 30/35 combination and the drilled air bleed/transition hole. I'll get some pictures of what you need to 'pull' the idle jet. It is really more of a 'twist' than a 'pull', you have to get the idle jet twisting back and forth and then it just pops out.

 

[Poleman]

Terrys right....not hard to get out once you get a hold of it, and it is a twisting motion.

Heres some pic's....in the garage now....lol. Sat on my back side all day now ready to do something.....you guys REALLY get me MOTIVATED!!!!

I did have to releave the area to get a grip...little Dremel work is all. The two orfises under the jet....drill out the one closeset to the edge reinstall jet.

Hope these help..

 

[fordf150]

You have to do a little fab work. But nothing serious

Your hilarious! a little fab work to get the carb on....then a lot of fab work to get an air cleaner on it. but please don't post pics of our custom adapter. not until we get around to making one from aluminum and tig welding it

 

[fordf150]

Have you guys gotten the slow taper screws yet? I checked Thursday and they still haven't came in at my distributor.

 

[fastLeo151]

It wasn't that bad...

 

[Poleman]

Nope have gotten any notices or received them yet.....thinking, with Terry's help we've (he) found a way around it and at the same time keeping the carb more stock adjustable. Terry said he like the way this works out better and no need for the slow taper screw!

Just did a carb I will try on a BB365 tomorrow. Went 30/35 with the idle hole opened to .55 under the jet. This will be a starting point for this saw.

I have a 7900 coming in Tuesday I traded my new 2166 Jred for (ouch....this trade hurt) but I needed a stock 7900 to work with as my saws I've been using are a MM 6401 and a BB 6401. I want to see if there are any differences. Besides I'm REALLY LIKING these saws!!!!!!! Slowly trading my Huskys and Stihls off for them....LOL

 

[Fly By Night]

For that altitude I'd expect that pulling the idle jet and drilling the first air bleed hole to .55 would be a better option for getting a nice clean reliable idle. The modded carb is already rich at idle and requires some fiddling with the throttle plate to get it cleaned up. With the extra altitude, the richness becomes worse. With the drilled air bleed hole the carb adjusts easily with the stock idle screw.

. I'll give that a shot, I take it the tapered screw that opens the butterfly doesn't go far enough in to provide the air needed?

I think I would start at .30/.35 and see how the saw runs at your altitude. You will be giving the saw a lot more fuel and after modding the carb the air flow is an issue. The throttle plate will need a notch/hole to give it air at idle.....just don't go to big. Also this changes the H setting as the saw is getting more fuel through the idle circuit. I m not sure if at 6000 ft you can get enough air through so saw doesn't run way to rich. I'm at 4000 ft and can use 35/40 drill size.

I think this will work for you, just go slow to see how the first mod works after that its a 10 minute change to increase to larger size.

I have still to pull the idle jet and drill larger air bleed hole in carb as Terry has suggested to alleviate the need for a slow taper idle screw and/or throttle plate alteration.

You will definatly see/feel a lower power increase at this stage!! This mod is all about torque.....saw wont run faster just have a LOT more usable power in the same power curve and hold RPM in the cut!! I also feel this would help feed a ported saw in its added fuel requirement's giving it what it needs from its alterations also. Only limit is the limited coil on the 6400-7900 Dolmars. It may not run past 13,500 but may be able to run close to that in the cut......who needs it to run more RPM's if it can do that!!!!!

Thanks for the suggestion, I'm ordering my bits tonight! If I go to big I can epoxy the holes and try again correct?

Great info guys, once this all gets figured out I think a new thread should be started with all of the details in the first post so one doesn't need piece together info from hundreds of posts.


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[Poleman]

Correct. And if you make the idle circuit mod under the Idle jet you will have adjustability without modding the throttle plate to offset the rich idle condition. 30/35 will give you a VERY good start. .30/40 will be even richer and a little trickier to dial in. The H will be almost closed.

Just some things to consider.

Ya, I understand it gets hard to follow...lol

 

[fordf150]

I have a 7900 coming in Tuesday I traded my new 2166 Jred for (ouch....this trade hurt) but I needed a stock 7900 to work with as my saws I've been using are a MM 6401 and a BB 6401. I want to see if there are any differences. Besides I'm REALLY LIKING these saws!!!!!!! Slowly trading my Huskys and Stihls off for them....LOL

Red elm is glad you didn't get the one in the trading post. He said it is clean.
Only problem switching to dolmar is your parts supplier is 800 miles away and we all know how bad dolmar dealers are at stocking parts.

 

[cgraham1]

Damn Dolmar dealers!

 

[Fly By Night]

Dolmar dealers don't have any stocking requirements, Stihl dealers have to keep something like 20k worth of parts on hand


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[Terry Syd]

" I'll give that a shot, I take it the tapered screw that opens the butterfly doesn't go far enough in to provide the air needed?"

The tapered screw that opens the butterfly isn't the screw we have been discussing. We've been discussing the LOW SPEED MIXTURE screw. The idle adjust screw for the butterfly remains the same.

NOTE: Rich has noticed a membrane under the idle jet that I hadn't noticed. It may be the two Chinese carbs I worked on did not have the membrane, or I failed to spot it. That membrane may work like the membrane in the discharge nozzle to prevent backflow of air at idle.

Until we get it figured out as to whether or not you need the membrane, leave it alone - and when you drill the idle jet BE CAREFUL when you finally get through the brass jet not to ram the drill through the membrane.

 

[mdavlee]

Dolmar dealers don't have any stocking requirements, Stihl dealers have to keep something like 20k worth of parts on hand


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95% don't keep that stock in saw parts. If I get a 7900 again to mess with I'll mod the carb first on it.