WALBRO HD-12 MODS

[Poleman]

Part II of my carb modding today. The first one was of the NO JET ZAMA C3M aftermarket carbs.

I started messing with a Walbro HD 12 today abd got some promising results. I had to do extra tweeting on this also.

Drilled the jet out to .043 from .035-.037 and closed the hole at the end of the H needle again following the lead from the work on the other carb.

The saw started an ran and had a good Hi, but idle was bad and it was starving for fuel. ( Tested on same Dolmar). I couldn't get it to idle right without running out of fuel. Back in I went and put in a weaker needle spring. I did that to alter the pop off so the needle would open sooner. That seemed to do the trick as idle was good, good adjustability on L & H, sounded good reving and slinging oil all over my garage floor.
Again It will need to be tested in wood to see the real truth. I do feel it's promising though. I will want to experiment with other jet sizes to see if a larger dro)ed jet would have any effect on the idle without a spring mod.

I'm finding one thing out....each carb model reacts differently to saw type and brand. The carbs for the Dolmars are way more aggressive with larger jets than what the Huskys can run. It seems each saw has a demand to what will make it run good......and it's different just like saw brands.

Stay tuned......I will be updating this as I do more testing.

 

[Poleman]

Well here's the results of the Walbro 12 today.

This was after getting it dialed in as best I could in garage....weaker spring installed to lower pop off, and give more fuel at/just off idle as it was starving on initial setup.

After initial start up saw was still lean down low. I could adjust the L until it would almost fall out of the carb to get enough fuel. It had a really funky idle and would review and lean out like it would act if it had an air leak.

It would turn 11,1 with a 28 inch bar in probably 40 inch Cottonwood. I ran good but it didn't have the torque as carb #2.

I think this carb has great potential as once dialed in as best I could it was very smooth, good throttle responce. This is a different beast compared to the Zamas.....way more temperamental and finicky to tune. It's more difficult to get set up to do any testing with to even get it in the ball park. I ran this at 14,2 max rpm while testing, and tried to set all carbs the same to get a feel for torque and throttle responce off idle.

I did really like the way this one felt....can't really explain it....maybe the smoothness, I'm not sure. I think this will be a worth while carb to pursue and get figured out better.

 

[Redfin]

Thanks for taking the time with this. What saw is this and what mods were done?

 

[mdavlee]

How many of these carbs do you have Rich?

 

[Poleman]

I was using a Dolmar 7900 because that what I have and fuel requirements are hi on those saws.

Drilled the jet and plugged the H bleed hole, used a weaker spring to get more fuel at idle as it was starving. The way it was may work on a Husky 372 as they don't use as much fuel.

Mike, that was one setting on my bench when I started molding the jet less Zama's......just thought what the hey.....

 

[Poleman]

Walbros or jet free Zamas?

 

[mdavlee]

Walbros or jet free Zamas?

HD 12

 

[Poleman]

Think I have another one....

 

[mdavlee]

Think I have another one....

I can probably have a couple sent to you if you want to mess with them.

 

[Poleman]

After the last test session, I felt the carb was still to lean for a Dolmar saw. I increased the jet size from .042 to .047.....just had a feeling.

This carb started better cold than when .042. Tuning was a little different and a back and forth between the L & H. Once dialed in though it performed well. Idle saw good, throttle responce and H was quite adjustable. Hot start was good on this carb also.

Although this carb performed good, I feel with the one jet it is limited because of the balance between the H & L. This carb is easier to mod than a two jet Zama but I don't think the performance is there.