Dolmar 7900 Carb upgrade?

[Gravedigger]

Finally ran full tank of fuel through my Makita 84cc with Walbro carb . That thing was cuzing through 20"+ honey locust with a 24 inch bar. Dozer tuned the carb at PA gettogether . The saw 4 stokes and runs ggreat.

 

[Derf]

Sorry to post this question, but I need a welch plug for my hd12b.

The plugs themselves are cheap, around $0.40 each online, but everywhere wants like $6-$8 to ship me one. Does anyone have one they could send me in a regular #10 envelope with a stamp?


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[acp104]

I'd been following this thread for awhile, and finally decided to give the mods a try on one of these aftermarket carbs URL: http://pages.ebay.com/link/?nav=item.view&id=291047828269 per Poleman's recommendation. I purchased 2 sets of bits for the project because the first set wasn't measuring out very well, and the .30mm bit was more like .34mm. When the 2nd set came in not much better, I ended up drilling the main to 0.33mm (due to lack of a 0.30), auxillary to 0.35mm, and mixing chamber hole to 0.55mm. I set both H & L screws to one turn out, and the saw fired right and stayed running. While letting it warm up, I blipped the throttle a few times and thought throttle response seemed pretty good. After I felt like it was warm, I started to evaluate the idle. It was idling a little fast and wanted to turn the chain very slightly so I first attempted to slow the idle with the idle screw. It didn't matter how far out I turned the idle screw, it would not reduce the RPM. At that point, I started to back out the L screw to richen it up, which did reduce the RPM and stop all chain movement at about 1.5 turns out. Throttle response still seemed about the same (good). Idle seemed a little rough to me but I played with the screw in both directions and ended up leaving it at 1.5 turns out. I haven't played with the H screw much other than to lean it out enough to where I thought it sounded like it was hitting the limiter. Poleman, what are you thoughts on my situation with the idle screw being backed out and still haveing to use the L to reduce engine speed to stop the chain completely? It was 80+ degree last night, and humid here in northern IL, so I assume I'll have to richen her up more come cold weather . Does having the low at 1.5 out sound at all close to what you've had to set yours at on a stock 7900? I would have thought that drilling the main to .33 would have pushed it towards the rich side vs the lean side. Seems I'm not lacking air.

 

[Poleman]

Good report!!! It does happen sometimes where a certain carb/saw combination is a struggle. When you mod these the idle sometimes is a bit funky but not unusable. You did good at richening the L to help adjust idle. That happens sometimes on ported saws sometimes also.
The only thing that doesn't seem right is you are getting enough air without the throttle plate hole and you should be rich with out it.
I'm thinking possibly you may have an air leak. Your carb should be just the opposite.....with your idle screw all the out to fast idle instead of all the way in. In my case if I don't drill the throttle plate I don't have enough screw to make it idle by giving it more air at idle. After the throttle is cracked the hole is not required as air is then supplied through the ventura along with more fuel.
The only time I had a problem like yours was I got in a hurry swapping carbs and forgot to tighten the carb attaching screws......ran great but wouldn't idle down. Another thing to check it the throttle cable....make sure it's seated in its pocket or it will pull the throttle open slightly enough to cause a fast unadjustable idle.

 

[acp104]

What I call the idle screw (large one above L & H) should increase engine speed when turned in clockwise as the taper of the screw moves the throttle linkage/butterfly opened correct? And in my case, backing it out so that the taper is not engaging the throttle linkage is necessary to keep the idle speed minimized. Even then, I need to have the low backed out 1.5 turns to keep engine speed from engaging the clutch and turning the chain a little. I think I'll loosen up the attachment screws this afternoon and re-tighten them a little looser because if anything, I may have tightened them too much last night. I doubt it, but just wondering if being a little too tight could distort the sealing surface and allow it to suck a little air. I'm not hopeful, but it'd be nice if that made a difference. One thing I should also mention is that when first attempting to drill the mixing chamber hole, I started to drill the one at the very edge/corner of the hole that goes down at an angle. I wasn't using any magnification, and saw that hole as the one closest to the edge so I started to drill it. I hadn't noticed that it went in at an angle so I started using the pin/drill to drill straight down like the others. I drilled a bit before realizing it was actually an angled hole and I was just drilling straight down into the casting. I didn't drill through, so was hoping no harm done. I then moved to the vertical hole closest to the edge and drilled my .55 hole. Any thoughts about negative effects of my error?

 

[Poleman]

No I don't believe there are any Ill effects from the drilling error...be Suprised if there was.​

Good thought about possibly over tightening and distorting the plastic flag around the boot.
Is your idle screw clear down to the threads on the screw where there is no taper? Can you see if the throttle plate is cracked open at idle letting in more air? Can you push the throttle arm where the cable attaches down more? Is the plastic cable piece on the carb secured and flat with its tab and squeeze attachment securely into the throttle arm?

Just trying to think of every possible cause.....

Also another tip.....you can JB weld the jets, carb body and redrilling if need be...just let it set over night. I use JB weld to close the Hi jet orifice behind the low .30 jet. I find anything over .40 it needs to be closed or saw will run to rich at wot only getting to about 12,8k or so, and that with the H jet totally screwed in tight. Plug it and it's fully adjustable again. JB Weld is my friend!!!

 

[acp104]

Yes, I believe I've got the idle screw backed out so that the throttle arm is just sitting on the threads with no taper. I will look at it closer tonight and try to determine if any of the things you mentioned are causing the throttle plate to stay opened a little bit. What do you think about me drilling the main jet to .33 since that was the smallest bit I seemed to have? If anything, I gather that going larger with the main would be causing just the opposite of what I'm experiencing, and the need for drilling a hole in the plate to get more air. I did have to push the main jet membrane back in as you cautioned as I must have pushed through. I'm guessing I went too far with the auxillary as well, but obviously didn't have the luxury of viewing it from the opposite side like the main. Hopefully something didn't happen to the membrane under the auxillary causing it to block the jet. Guessing that probably isn't the case either based on some of your other comments in this thread.

 

[Poleman]

No I don't think the .33 is causing the problems your having, in fact you are spot on thinking it should have the reverse effect like you stated. I always use a caliper on the drills before I use them to make sure of the sizes. They are like you say....a .30 is .33 etc. I've found the same issues and do the closest to what I'm wanting.
I glad you caught the pushed out membrain, that will cause a headache if not caught. I've also only had the main do it and not the auxiliary.
I've also noticed the carbs I've been using has the jet sticking out a little higher making it easier to pull and a redesigned jet with a more closed back helping this situation. It seems to be ok if you drill a hole in the membrain just not push it loose as it comes loose and lodges in the back two holes behind the on that gets drilled....least from my experiance.
Hopefully you'll check things over and find something simple, then be able to proceed from there. Your looking for you idle screw to be topped out and not on the threads. Then you just drill a small hole (.8mm-1.2mm) to straighten the idle out...that the usuall.

 

[acp104]

The carb I got is the last one you recommended in this thread that you indicated had a more prominent jet and you liked. I had no trouble pulling the jet with a small thin pair of side/wire cutters. They got a good close bite, and I was able to rock it out with just a couple back and forth motions.

 

[Derf]

Is that how I pull out the main jet on the walbro hd12? Just grab it through the Venturi with some pliers and wiggle it out? I thought I'd have to push it from behind. Any risk of damage grabbing it too tightly?


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[Poleman]

I'm thinking it would be best to tap it out instead of grab it..

 

[blsnelling]

I have not followed these carb threads, but have been reading up the last couple of days. I'm interested as I'll be porting a new 7910.

Help me understand something. I can see how these mods would make a difference at partial throttle poor a very flow transition from idle to WOT. I don't understand how they could make much of a difference of going to WOT instantly. As I understand it, your not altering the total fuel supply at WOT, just altering the fuel curve at partial throttle, as it transition from idle to WOT. Help me understand.

 

[Poleman]

Brad,
I riches the lower off idle through midrange curve giving more fuel to increase torque.....on a stock saw.
On a ported saw with the larger mod .040 aux jet it does affect the H. I find to get adjustability out of the H I need to JB weld the bleed inside on the H jet. That will give you adjustability back as the saw is usually running rich at WOT and need to be leaned but in most cases the H is already totally closed not allowing saw to be leaned out and unable to get it to ideal rpm's.

 

[blsnelling]

I understand that these mods will affect where the H needle will end up, but total fuel at WOT will not change. So back to my original question, how does this mod help if transition from idle to WOT is instant? Your instantly flowing from all jets.

 

[Poleman]

Not sure on that one but, figuring from idle to wot it still has to transition through circut system and probably flowing through all as you state.

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[blsnelling]

BTW, I'm not trying to criticize your work, just trying to understand it.

 

[Terry Syd]

Fuel flow is dependant upon air flow, that's what carburettors do, meter the fuel to the amount of air going through it. - Just because the throttle valve may be wide open, it doesn't mean that fuel automatically goes full on.

There are two circuits in the carb, the high speed circuit through the venturi and the low speed circuit through the straight section of the carb - each circuit has it's own flow characteristics depending upon the pressure differential working on the circuit. The result is that the two circuits (one venturi, the other straight section) have two different fuel flow curves based upon the rate of air flow.

The mod increases the low speed circuit flow so that the engine can run richer when the engine is loaded down, that is, when the air speed through the carb is slower.

It appears that the factories were 'persuaded' to decrease the fuel flow (lean it out) in the low speed circuit in order to reduce emissions. At lower RPM there is a greater tendency for mixture to escape out the exhaust port. By leaning out the low speed circuit they could cut emissions at those lower speeds AND keep the operator of the saw using the engine at a higher speed (because the engine lost low speed torque).

The mod brings back that lost low end torque.

 

[Poleman]

I know....not reading it that way. I'm not sure I totally understand what's all going on with the carb through transitions and operating conditions. I'm just telling you what I do know......and sadly what I dont...lol

 

[Poleman]

Terry to the rescue!!!!! THANK YOU!!!!
Good explanation!!

 

[blsnelling]

Thanks Terry. That's exactly the explanation I was looking for. Makes perfect sense now.