Dolmar 7900 Carb upgrade?

Arborist Forum

Help Support Arborist Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
Is this a known good carb? Sounds rich to me...

Turn the L (low) in all the way and back it out one turn. That should get you in ball park. Increase idle as needed. I'm assuming the limiters are off on the carb or you will need to remove/trim them to get full adjustment. You will probaly need it with the BIG change you made.

The one I tryed on mine a couple weeks ago dialed in in about 5 minutes. Mine was easy and I knew the carb and that it was good.

Hope that helps!!
 
New out of the box Walbro carburetor. Original Zama would just sputter, rebuilt no help. Saw was run lean and seized when I got it. Early in this thread is when I ordered new carb. How are the limiters removed. I tried pulling off no go, should I just break or cut off.
 
New out of the box Walbro carburetor. Original Zama would just sputter, rebuilt no help. Saw was run lean and seized when I got it. Early in this thread is when I ordered new carb. How are the limiters removed. I tried pulling off no go, should I just break or cut off.
I pulled the limiters off the HD12 I have on my husky 365, it just takes some wiggling with some needle nose. I wanted to just trim them off until I noticed the plastic was stripped out on the high so I just pulled them off.
 
Lol....your pushy arnt you!!! Lol

Been thinking of the same thing but have been concebtrating on the Zama. Haven't studied a HD diagram to get any ideas yet. I'm sure it can be done

Yeah, it can be done, but the Zama is so much easier to work on. If you want to get some of the slow taper idle screws, here's a link - http://www.m-and-d.com/ZA-0031023.html
 
This likely has been said before, but the 17mm venturi of the 7900 Zama carb and the 17.5mm of the HD12 both are obviously too small. The saw brands are sort of pulling "games" on us, regarding where in the design the worst restriction is - carb, engine porting or muffler. What they choose to do has an impact on sound level and emissions + the obvious.

As an exemple, the 385xp/2186 was almost ideally ported - but was extremely restricted at both ends (carb and muffler).
 
Is this a known good carb? Sounds rich to me...

Turn the L (low) in all the way and back it out one turn. That should get you in ball park. Increase idle as needed. I'm assuming the limiters are off on the carb or you will need to remove/trim them to get full adjustment. You will probaly need it with the BIG change you made.

The one I tryed on mine a couple weeks ago dialed in in about 5 minutes. Mine was easy and I knew the carb and that it was good.

Hope that helps!!
Popped the limiters off ( destoyed ) Readjusted L to one turn out and she idles. Needs fine tuning. Poleman was right way rich. Thanks.
 
Yeah, it can be done, but the Zama is so much easier to work on. If you want to get some of the slow taper idle screws, here's a link - http://www.m-and-d.com/ZA-0031023.html
I was just looking back for the part number also to write it down, might just have to order a couple. Looks like it has the bigger adjustment end also, not like the tiny slot on my hd12 on my 365 after I pulled the limiters off. Those little slots are hard to hit and tune while the saw is running and vibrating around.
 
I don't know if it is patience or stubbornness. Crikey, I spent some time trying to figure out how these freaking carbs worked. In fact, One Man Band just sent me a Kart article on carbs that was pure BS - just typical of the crap out there you have to weed through. I'll post a link to the article and you can see what I mean.

If someone can come up with the schematics on the electronic carbs, I'll figure out some mods for those (I spent 7 years as an electronic technician and have modified electronic fuel injection systems).

I've been modding for 50 years now, its become a sick habit that I can't break.

EDIT: Here's the link to the Kart article, see if you can pick all the BS in it - it is chock full of it.

http://nkn.com/nationalkartnews/articles/AnatomyOfWalbro0708.pdf

I read through that article and find it wanting too. I'm curious what parts you didn't like, though I suspect I know. To me the biggest problem stems from his statement in Section C, pg 6:

For our purposes, it is enough to understand that the average pressures in the venturi are proportional to the square of the average intake air velocity (or, effectively, the square of the volume flow rate into the engine). In simple terms, average pressures in the venturi are typically less than atmospheric, and will decrease by a factor of four as the intake flow rate doubles. A complementary, and equally important, relationship governs the metered fuel flow rate. This is based on the same physical laws governing the venturi pressures above. The fuel flow rate is determined by the effective area in the metering circuit, and the square root of the pressure across that circuit. The effective area is the combination of all the restrictions in the flow path, like jet diameters, needle valve position, and cross sections in the flow passages. If the Walbro pressure regulator is doing its job, the pressure across the metering circuit will be held nearly equal to the pressure in the venturi (less the slightly negative pressure maintained in the metering cavity). Therefore, when the intake airflow rate doubles, the fuel flow rate doubles in step. This makes sense since the venturi pressure increases by a factor of four as the airflow rate doubles. The square root of four (relative change in the venturi pressure) is two, so the fuel flow rate doubles. Sounds complicated, but it isn’t if you pay close attention to what is being said here. The beauty of these physical relationships is that the resulting air-fuel ratio remains a constant. This is true despite wide variations in engine speed or throttle position.

This is incorrect, and does not match the known behavior of these carbs – we know that they provide a drastically richer mixture with just a small increase in air velocity (4-stroking), and most carbs in other application have air corrector jets to linearize the mixture.

I can see how he came to this conclusion. The Bernoulli equations show that the pressure at the outlet port is the square of the air velocity. But the characteristics of the flow of the incompressible liquid fuel through the jet is not the inverse of the compressible air through the venturi. This article ( http://en.wikipedia.org/wiki/Carburetor ) contains the following description:

The main disadvantage of basing a carburetor's operation on Bernoulli's Principle is that, being a fluid dynamic device, the pressure reduction in a venturi tends to be proportional to the square of the intake air speed. The fuel jets are much smaller and limited mainly by viscosity, so that the fuel flow tends to be proportional to the pressure difference. So jets sized for full power tend to starve the engine at lower speed and part throttle.

Later in the article he goes on to discuss how he didn't understand why some cart tuners were using the low speed circuit and deactivating the H circuit – it's because the L circuit on that carb actually has an air corrector jet and so can be made to meter correctly. They'd be better off adding an air corrector to the H circuit.

He discusses the L circuit air corrector jet, but misunderstands its function like many do. It is not to improve atomization, it is to turn the incompressible fluid in the jet into more of a compressible fluid, and then the flow is more linear with air velocity as he described.

Other than that, he seems a little foggy on the dynamic function (acceleration). The description of the main fuel diaphragm is decent for steady-state function.
 
Take a look at 'Component 1' - he thinks that the pump diaphragm provides ALL the fuel by 'pumping'. It is a common mistake, but once you make that assumption, any analysis goes down hill from there. For example, take a look at what he came up with for 'Component2' - because he thinks the pump diaphragm is pumping the fuel, he draws the wild azz conclusion that the metering diaphragm is a 'pressure regulator' for the pressure provided by the pump diaphragm.

The pump diaphragm can be removed from the carb and the engine will run. Just remove it, choke the carb and start it. As long as you keep the revs up and don't try to idle it, the engine will keep running. The pump diaphragm and metering diaphragm are used in the idle system and the ramping of the fuel flow off the idle jet.

There's a bunch of other crap in the article, but it is clear he doesn't have a clue how these carbs work. - However, the pictures and magazine layout are very professional, it does make the article look very authoritative.

The best document for how these carbs work is the Mikuni document I linked to in the Chinese Carburetor thread.
 
This thread is about building a better carb for a big bore, like the 77cc 365BB or a Dolmar 7900. However, a person should be able to mod a 372 carb to turn it into a 'torque monster'. I don't have any idea what the jet sizes would be as I wasn't working on a 71cc engine.

Perhaps someone might convince Poleman to start modding a 372 and see what he comes up with. If he ends up going too far in the jet sizes for the 372 while he is testing it, no problem, that carb will become a big bore carb.

I would suggest that a person have on hand a 0019012 metering spring in case it is a bit too rich off of idle. The 0019012 spring has a stiffer spring rate to slow down the ramp. If the pre-load is a bit too much, the spring has 6 passive coils that can be clipped off with a nail clipper to decrease the pre-load.

The spring can be ordered from the same link I gave for the slow taper idle screw.
 
Maybe if I get time before I leave I can throw the zama on my 365 before I big bore it. It might not be the perfect example of performance though as it only has about 120psi compression and i believe it is pretty worn.
 
Take a look at 'Component 1' - he thinks that the pump diaphragm provides ALL the fuel by 'pumping'. It is a common mistake, but once you make that assumption, any analysis goes down hill from there. For example, take a look at what he came up with for 'Component2' - because he thinks the pump diaphragm is pumping the fuel, he draws the wild azz conclusion that the metering diaphragm is a 'pressure regulator' for the pressure provided by the pump diaphragm.

The pump diaphragm can be removed from the carb and the engine will run. Just remove it, choke the carb and start it. As long as you keep the revs up and don't try to idle it, the engine will keep running. The pump diaphragm and metering diaphragm are used in the idle system and the ramping of the fuel flow off the idle jet.

There's a bunch of other crap in the article, but it is clear he doesn't have a clue how these carbs work. - However, the pictures and magazine layout are very professional, it does make the article look very authoritative.

The best document for how these carbs work is the Mikuni document I linked to in the Chinese Carburetor thread.
Yes, I realize that the pump is not always required above idle. Regardless of that however, I do agree with him that the primary function of the metering diaphragm is as a pressure regulator analogous to the float bowl in a normal carb. This is where my view of how they function differs a bit from yours, probably as I am coming from automotive carbs like Webers, where one doesn't mess with fuel bowl levels much as a means of tuning mixture. I have not really understood your terminology of ramping, and how the metering diaphragm provides this.

My preferred approach would be to have the metering diaphragm do its job of providing a fixed pressure differential (as he described), and the extra fuel for acceleration would ideally be provided by a pump. If the carb has been crippled by the lack of an accelerator pump, then the fixed jets must provide it by being set richer than optimum for steady state performance, especially by the richer mixture provided by the transfer outlets as the throttle plate edge moves toward them. You can reduce the spring pressure some so that a fast acceleration will allow the inlet valve to open faster, and then you don't have to set the idle & transfer jets quite so rich, but you can only push that so far.
 
Well Terry....your reading my mind!!!

My next project is to put the 30/35 carb on my Husky 365 BB and see what happens. I also want to try it on my MM ported MAKITA 6401!!

Also want to give more details on carb #2 I was working with. I was trying to adjust out the problem of the Welch plug leak before it was found. Sealing the Welch plug made the carb predictable and I reversed all my changes trying to correct that problem. The carb now tunes and dial in very well.

After modding these carb I find they are very stable and not finiky at all...set and go. That is a BIG plus since no one wants to go cut wood and you have to adjust on your carb for 20 minuted before you can.

Procedure:
Start
Warm up
Cut
SMILE(OR GRIN IF YOU PREFER)

There a lot of power in that trigger finger now!!!!

Rich
 
I think with the possibility and potential of the mnodded carb it will have a good chance!!! It will be giving up a few cc's but if it has the potential Randy thinks it has and it was limited by a carb limitation, this may change everything on the Dolmar 6400-7900 's

Either way it will be a GREAT SAW!!!!
 
Rich, for your business logo I thought a blue carburettor with a lightning bolt through it. I think the lightning bolt should be yellow or gold, either colour should go well with the blue carb.

Oh yeah, the blue carb should be the same colour as the blue anodizing you use on the carbs. The classic aircraft blue anodizing that is used on racing components would really set the carbs off - all the cool guys would be running the carb.

I feel you should put two stickers with your logo with each modded carb. One can go on the saw and the other could go on the beer fridge.
 

Latest posts

Back
Top