plowin-fire
ArboristSite Operative
Anyone have any reports of using a Walbro 12 HD carb on the 7900? I know a few of you on here have them. Suppose to be a bolt on job. Worth the expense?
Dolmar 7900 Carb upgrade?
- Thread starter plowin-fire
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Why? the OEM carb feeds the saw just fine... The only reason you'd need to swap a higher flowing carb is if you went full blown porting and even then it feeds it alright... you'd also need a unlimited coil to see a larger gain with the saw. why mess with a proven performer? I could understand if it were a choaked off POS saw to begin with...
plowin-fire
ArboristSite Operative
The saw was just ported and MMed.
plowin-fire
ArboristSite Operative
Still at the port shop. Builder recommended different carb...
FATGUY
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maybe I'm wrong here, but as I understand it, if you fed the engine more air, you could feed it more gas too.
correct me if i'm wrong here but if your 4-stroking with the current carb it means you are already feeding it enough of both... Plus the saw is still going to be handy-capped by the limited coil...
If it's a race saw go for it... if it's a solid work saw I'd see if the stock carb would make it 4stroke... JMO...
FATGUY
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I hope someone smarter than me chimes in, but as I understand it, you're four stroking if you're giving it too much gas for the amount of air that's in the engine at that time, feed it more air, you can feed it more gas. I really am not trying to be argumentative, I just have a vested interest in this question.
Just change the jets in the current carb, should be simple and cheaper. If the dude porting advised it, doesn't he have an idea of what other carb???
Not real familiar with the 7900 carb, although I have a 7900 and run the frig out of it. I did change the jets in my 660 after a mild porting and muff mod..
Not real familiar with the 7900 carb, although I have a 7900 and run the frig out of it. I did change the jets in my 660 after a mild porting and muff mod..
I was always under the impression you had a carb big enough when you could 4stroke at WOT after porting. Lets keep in mind that once you put a larger carb on there is only so much fuel you can flow before you start blowing the extra out the exhaust. The blow down period can only be so long to burn "X" amount of fuel. The initial energy generated from a denser charge of fuel and air might be greater but at what cost... Like i said pros and cons... Work saw porting vs race saw porting. The multiple 7900 build threads that I have read always left the carb alone besides tuning.
Eager to hear from builders like Stumpy, Randy, Brad (maybe, i know he hasn't done a lot of dollys), TreeMonkey... etc..
Eager to hear from builders like Stumpy, Randy, Brad (maybe, i know he hasn't done a lot of dollys), TreeMonkey... etc..
Jets only deliver more fuel which may or may not be the case of what he needs here... You need a larger venturi to allow more air so that when mixed with more fuel (from either opening the "H" or larger jet) you still get the proper fuel air mix only in a higher volume.
The real question is above... Will a higher volume matter on a 7900? keep in mind you also have to contend with tuning it with a rev limited ign,...
Just looked at the 7900 specs for carb for 7910, doesn't look like you can change the jets.
mitch95100
Saw Nut
So my ms170 Carb wont fit on my ported 660?? 
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Nik I'm not smarter than you, but you are correct. As long as the carb can provide the engine with the proper air to fuel ratio, the engine can run rich. This is actually quite simple if you think about it. The carb, do to it's venturi size can only allow in so much air/fuel during a given time, make the venturi larger and it will allow more air/fuel in, during the same duration of time. Any time you open up an engine, or change timing even just a little, you're going to flow more air. You can also run into issues like turning out the high speed jet too far. When the needle moves farther away from the seat, a non-linear change in the mixture is a result. I forget who taught me that, I think Tzed250.
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FATGUY
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so if a 372 carb fits on a 7900, and a 385 carb fits on a 372, doesn't it stand to reason that a 385 carb fits on a 7900? :msp_thumbup:
VINIFIREWOOD
I only no what I know
I'm not sure how much if any of what works on a 4 stroke engine crosses over but. I remember back in my days of playing with small block Chevy's. Your average stock to mildly built 350 ran best with a 650 cfm carb. It would run ok with a 750 but was happiest with the smaller carb. Only when you went to a really wildly built engine did you see any benefit fron the bigger carb.
So judging from that I would say there comes a point where you have too much carb and you will start sacrificing throttle response etc.
But truly only one way to find out and that's to bolt it on.
So judging from that I would say there comes a point where you have too much carb and you will start sacrificing throttle response etc.
But truly only one way to find out and that's to bolt it on.
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