Dolmar 7900 Carb upgrade?

[Poleman]

Very good question. I see it like this...
From idle through midrange a saw can use more fuel to give it more torque, which in turn feeds the top somewhat. The saw can be richer up in that range to improve performance but the way it needs to be done is by modified the jet size and mixing carituristics, something you can't do with just the needle adjustment alone. By the time the saw is reaching top rpm's the low and mid are also combining with the Hi jet to feed the saw. The changed mixing carituristics in the low circut enhances performance threw out the power band until hi rpm is encountered in which the saw has reached its full fuel air demand. If it's to lean the saws rpm's will increase but heat is also increased because of more friction, more speed and lack of fuel/oil which cools the combustion chamber and maintains proper lubrication. Without the fuel/oil mixture being correct meat to metal contact occurs and seizing and galling occures.

On the other hand to much fuel after optimum will slow a saw down because of to much fuel being delivered and it's inability to use it, thus excess smoke, unburned fuel, and a basic flooding condition with incomplete combustion.

My take on it as I see it. Hope it makes sence.

 

[Chris-PA]

I've been really thinking about all this for a while. It seems we are looking for midrange fueling on our saws. Idle is simple, at WOT if it can 4stroke you are getting enough fuel. I'm thinking out loud now. So my question is this: if we want more fuel in the cut why do we tune to clean up when touching the wood? Why wouldn't running rich achieve the same goal as drilling the carb?

Running rich does not make more power at a steady speed/load. There is a very narrow range of fuel/air mixtures that will run, and you can optimize a bit from the lean to the rich side, but more fuel does not equal more power. There is one correct mixture that gives max power - but these carbs are nowhere near accurate enough to hold that as rpm changes.

The goal here is to improve acceleration, and to accelerate you need to provide more fuel than would be needed at a constant speed. Since these carbs have no accelerator pump system, the only way to do that is to fatten up the mid range fixed jet (since you must go through mid range to get to WOT) - this also makes it too rich at a steady part throttle, but since one rarely runs there it's a decent compromise.

 

[Fly By Night]

I tried it with my little dikes again, it still wouldn't move, I cut a relief to get more bite, still nothing, I cut 2 strait sides on the jet to try to get it to twist to no avail. I wonder if I can drill it out? Someone please tell me you can get just the jet.

 

[Terry Syd]

Don't drill it out, you need the jet for the idle and transition. What I've done on difficult carbs is to take a very small Dremel bit and grind away on the aluminium at the edge of the jet. That will reduce the depth of the hole that the jet sits in and thus reduce the resistance to twisting. When I re-install the jet, I put a small amount of GB Weld around the jet to make sure it is sealed and can't fall out.

 

[mdavlee]

Terry did you do a walbro HD for stihks? I'm wanting to mess with the carb on my 046 soon.

 

[Terry Syd]

No, I just drilled out the low speed jet on a Chinese 460 carb and run that on my 64cc 'Sleeper'.

Here's a link back in this thread where I discussed the HD series and possible drillings. - http://www.arboristsite.com/community/posts/4953791/

If I was going to work up a 046 carb, I'd find some old HDs laying around and start drilling them. Forget modding the HD-12, you might as well mod some other piece of crap that you don't mind 'going too far' with. Near as I can tell, the HD series are basically the same, it's just the drilling of the transition holes that need to be worked out for the displacement. If you can find a crappy emissions carb with the bigger venturi, just drill out the low speed jet and start modding the transition holes.

Whereas the Zama in nicely laid out to work on, the Walbros will require you to remove the Welch plug each time you want to try a different jetting.

For the 046 I'd start with the 460 drilling and go up from there. I'd also try to make the transition hole next to the throttle plate always the smaller hole (like on the 460 carb). What that does is create a more progressive rich mixture as the throttle plate moves across the holes. The result would be similar to what the Zama does with the auxiliary jet coming in after the transition holes.

If you had two carbs to work with you could always keep a 'baseline' to refer to. Mod one carb and see which works the best. Then mod the 'loser' and test again.

The 460 carb has the first hole at .6mm and the second at .65mm. I'd try a .6mm/.7mm next, then a .6/.75, etc.. If the carb started bogging right off of idle from being too lean, then you would have to open up the earlier transition hole. Say, if it started to bog at .6/.75, then you may have to go back and try .65/.7

When you go up to a 385 WJ carb, you can see they have a LOT more flow in the low speed circuit. It may be that the HD will require a fair bit of drilling to get that kind of flow out of the low speed circuit.

 

[mdavlee]

I have a Wj 6 from a 066 but I don't think it will fit on the 046. I do have a velocity stack so I'm sure it could be made to work.

 

[Terry Syd]

Aw, forget the WJ - Lash out and take a HD emissions carb off of a 029 and build a killer carb, you'll be a legend! - Women will swoon at your approach...

 

[Poleman]

I don't believe you can get just the jet...I've looked, maybe not hard enough. I went to a Husky dealer and went through his box of junk Zamas and found a C3 has the jet you would need.....and yes, I have ruined a couple. Thought I'd just take the jet from an A.M. carb....wrong, there slightly bigger than the Zama.
I've had two instances like yours where I DID ruin a jet. They were both older carbs and the jet was set very tight. I did drill mine out just to get it out but I had found replacements in the junk carbs. A slight setback but if you can find old carbs to rob the jet from you'll be good. Seems there's not many junk Zama C3M's out there as the dealer didnt have any so that how I found the jet from the C3 worked.

 

[Fly By Night]

Thanks rich.

Is the air bleed hole by chance directly Below the hole in the jet?

 

[Poleman]

I think I know what your thinking....to drill through the main and go into the air bleed won't work. One you'll need a bigger grill for it then you would have to JB Weld the main and redrill it. Unfortunately the bleed hole is offset toward the edge of the main jet. Actually there rather sporadic lyrics spreed out. Zama's are the worst for consistency where as the A.M. are uniform.

 

[Fly By Night]

My idle mixture screw doesn't seem to make much of a difference in tuning. I have it to where it will idle nicely for 10 or so seconds after I come off the throttle, it will idle up for a few seconds (leaning out) then pretty quickly idle down and die, any thoughts on that? I may just need to bite the bullet and figure out how to get to that air bleed, or tack down some slow taper idle screws.

 

[Poleman]

You will have to get to the air bleed and enlarge that to .55. That's what is causing the issues.
Did you put a hole in the throttle plate and is your idle slowly slowing down until it stops?

I'm trying to figure where your at with it. I've done carbs at first without drilling the bleed home....they just tune harder.

 

[Fly By Night]

All I have touched on it at this point are the the two jets.

 

[Poleman]

So is you idle to slow with the idle screw all the way in? If that's the case you can put a notch or drill a hole ( which I prefer because you can Solder it shut and go back to the last size that worked). That will bring up your rpm's and give you some adjustment on you idle screw. I usually go to where I get to the middle of the screw. You will know if you go to big if you've got the screw all the way out and it idles to fast yet...thus the drill hole...easy to fix with solder.

I'm guessing that's what your issues is....

 

[Poleman]

What size did you go?

Can't remember....your saw ported??

 

[Fly By Night]

I am getting plenty of air, my issue seems to be getting the mixture right so it will idle stably

 

[Fly By Night]

I am at 6,500' I ported it my self so its not as carried away as most. I drilled .30/.35

 

[Poleman]

Ok. That's good for your altitude. What's your adjustment like on you idle screw? It must be pretty good?
Is your idle kinds lopy and irratic? That was an issue without drilling the bleed hole.

You had a problem getting your main jet out didn't you? That issue occurred to two carbs I did....got a couple replacement jets from junk carbs at dealer.

 

[Fly By Night]

Adjustment on the air screw is fine, the mixture screw is unlike what im use to. The idle is fine for a few seconds then gets weird. You are right, I am the one that couldent get my main to budge. I wonder if I can heat it? How much were the Chinese carbs you were playing with?