Dolmar 7900 Carb upgrade?

[Mastermind]

Birds of a feather.

 

[Poleman]

Flock of turkeys gotta stick togather!!!! Power in numbers.......

 

[Chris-PA]

This works here because of the way these carbs are made, but won't work the same on other carbs. A shot from the Dolmar tutorial shows what happens:


Notice the fuel outlet in the center - it is fed from a fixed jet, and so its mixture is only a function of the pressure in the metering chamber, while the H and L are fed from the adjustment needles.

If you use a weaker spring, the fuel pressure in the chamber will get closer to atmospheric (i.e. it will rise), and the mixture will get richer. You can then adjust the needles to get it back on the H and L, but the center jet stays richer. So at part throttle, and as the throttle plate moves through there during acceleration, the mixture will be richer.

This won't work on something like an HDC, as the transfer outlets are fed from the idle circuit and are adjusted by the L needle:

 

[Mastermind]

Excellent post Chris......

What you are saying makes perfect sense.

 

[Mastermind]

There's a lot going on in a little area in these carbs. lol

 

[LegDeLimber]

Just a quick freehand sanp, not my usual, more fussily lighted crap.

WT-285 body: "L" needle regulates the total flow into the chamber
undeneath the Welch (Not Welsh ) plug.
Red is inlet from "L" needle.
Green are transition ports
Yellow is the idle port
Black is the supply to the L needle.
Throttle bore (not the Venturi) and plate, are seen in the bottom of the frame.

Makes ya wonder how much the metering lever motion influences the idle
by it's pulsing right over the inlet.

 

[Derf]

Can't you just clip a few winds off a spring instead of buying a spring kit?


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[LegDeLimber]

Some people do and seem to get ok results.
But springs work better with a few, unseperated turns at the ends.
I've read some of the reasoning and seen a couple high speed vids of how springs move
and the way that cutting those ends off or using a different winding rate at hte ends affects them.
Using this one from a WYA-155 (from EBZ8500 Redmax) as an example.
I would not cut more than about 2.2 ~2.5 turns off of an end.
A turn meaning starting from the yellow line and counting the loop.

 

[Terry Syd]

Just modded a carb last night and ran it today.....really bad bottom and transition. I put in a lighter spring (Walbro 98-530) and it straightened right out. Fast throttle and responce!!
I'm finding every once in awhile a weaker spring is needed for some unknown reason. All mods are the same as other carbs that work fine with stock spring.
Just put this out there in case others have problems tuning a carb on a modded saw or if using a modded carb.

Springs can make a difference!

Rich, it may be that the carb had a smaller metering orifice than you're used to. You might check it and get back to us.

Remember that the C3M carbs are put on 50-80cc saws. It may be that there are optimum ranges of metering springs and metering orifices that correspond with engine displacements. For example, I was given four C3Ms one time and there were three different size metering orifices in that batch of four.

 

[fordf150]

Can't you just clip a few winds off a spring instead of buying a spring kit?


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i would prefer you buy a kit....http://performanceoutdoorequipment.com/products/walbro-metering-lever-spring-kit-400-606

 

[Poleman]

You can clip the winedings but its harder to duplicate if you want to do so.

The kit is good and am glad I purchased it. Makes things easier. On wish is that it would have a spring rate listing. Its still easier than cutting winds.

 

[Poleman]

Well finally.....

I know a few of you have been waiting for this and others just want to know/see how it's done.

First off this is for the Zama C3M on Dolmar 6400-7900 and will also work for on Husqvarna 365-372's old style.

View attachment 412930 .
Tools you'll need.
Micro Drill Bits ,Caliper to measure and make sure of drill size being used.
Pin vice to hold drill bit for drilling
Modified pliers to pull main jet
Hammer and punch to reinstall main jet

Also if eye sight is poor a set of magnifying glasses. And a small magnet.

I drill totally by hand so I can feel what the bit is doing and when it goes threw the jet. I do this because there is a membrain beneath the main and auxilery jet. I can see no adverse results or efrecommendyou drill threw it.


View attachment 412941
Here is the carb lay out, with jets identified.
I drill the main jet first then move to the auxilery jet.
I then use the modified pliers to get ahold of the main jet (firmly but not crushing). I find it will come right out when using a rocking motion. Again firm grip so it doesnt slip. You will probably deform the jet slightly wher you grab. Its just cosmetic. You can sand it smooth again but reinstalling it with a sharp rap will usually make the edges lay back down.

Next the mixing chamber hole under the main jet is drilled to .55 mm. This is the one constant jet size on all carbs. This helps ideling caricturistics.

View attachment 412942
Drill the hole closest to the edge. Again very easy to drill by hand. Be patient.
The jets drill slghtly harder since they are brass.

Now look at the underside of the main jet you removed...is the membrain still in place? If it is you can reinstall the main jet...if not take your small drill bit and work it back in. The membrain can come out and block the mixing holes and cause tuning problems. I have not seen this happen with a true Zama only Aftermarket carbs.
This is why I drill both jets first so when I remove jet I can check it.
Reinstall jet with the hammer and flat punch.

At this point reassemble and set carb and install in saw for your first test run.

On a stock saw I would reccomend
.3 for main jet
.35 for auxillary
.55 for mixing chamber-this will always be the same.

Start saw and tune. If there are problems having enough idle screw to get saw to idle but saw is running good otherwise remove carb from saw as a hole will be needed in the throttle plate to give the saw more air it is requiering from the introduction of more fuel it is recieving

View attachment 412954
Drill it on the top of the throttle plate away from the jet on the bottom. With stock I would recommend 1mm hole to start. Reassemble and retest saw, see if there is enough screw to set a satisfactory idle with more screw to spare.

Run and enjoy!!!!

Now for the brave and builders......LETS GO BIGGER!!!
This will be for the ported and Big Bored saws.

Jet sizes:
Main .3 to .35
Auxilery .40
Mixing chamber .55

When you go with bigger main and auxillary it also affects the top end fuel. You may find that the H screw is totally closed and you need more rpm's as the saw isn't reaching the rpm it should.

View attachment 412956
To regain the high speed adjustment you will need to JB weld the high speed jet closed. Clean carb with a shot of brake clean and let it dry. Mix a very small amount of JB weld and use a toothpick to place a small amount in the hole shown. The carb will need to dry over night at this point.

This should give you the high adjustability back an H screw will be totally adjustable.

The carb should be close to adjusting like stock when finished. If you find you went to far on the main or auxillary just JB weld them shut and redrill to the size that worked better.
Like wise on the throttle plate, if your getting to much air take the plate off carb, clean and solder shut and redrill smaller.

I have found you can correct things with JB weld if you go to big on any of yor drilling or if you just want to try bigger.

I hope this helps all interested and makes things easier to follow compared to the earlier thread.

Feel free if there are any questions.

Thank you, Terry Syd for your encouragement and your wisdom......this is you Buddy!!!!!

Thought there would be more interest in the how too.....

 

[fordf150]

Don't worry...it will get looked at and used. This is still the experiment/question thread

 

[Paragon Builder]

Well I read it through carefully and I'm about to start looking at some of my saws. I'm sure I'll have questions!

 

[Poleman]

It's pretty easy once you see the layout of the carb and t you'll be doing. Hardest thing to do is probably removing the main jet without destroying it....small steps there, and the right tool! I suggest ordreing a couple am carbs to practice on as there not as expensive as full blown Zama's......and seem to work good for me. You have to watch the membrain on those tho as they come out quite easy when your drill drops threw when you drill. Little more open design than the Zama jet.

Any questions let me know!!!!

 

[Fly By Night]

I was able to get my aux punched out to .35 last night, it went really smooth. I couldn't get the main jet to budge, I have some small old side cutters that get in there well but will not twist the thing! I put some penetrating oil on it overnight. Ive thourght about maybe epoxying a nail or something to that jet to remove it.Also my 3 smallest bits all measure right around .35 on my calipers, I was going to go to .30 on my smaller jet has anyone else found that with their drills?

 

[Poleman]

I have too..... I find if I bear down on the caliper the measurement will change.....a little.
On your side cutters can you gently just pry the jet up so it pops out? I've never used those but just a thought.

 

[Terry Syd]

You may be able to put a small notch in each side of the cutter jaws using a Dremel wheel. That will give the cutters a better grip on the idle jet so that you can turn it. Once you get the jet to turn, it can be worked back and forth as you pull it out.

 

[Fly By Night]

I have too..... I find if I bear down on the caliper the measurement will change.....a little.
On your side cutters can you gently just pry the jet up so it pops out? I've never used those but just a thought.

I tried rocking it side to side and front to back with no luck. I thought about cutting a relief. I really don't have any cutters I want to grind on.

 

[Paragon Builder]

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?