Dolmar 7900 Carb upgrade?

[Terry Syd]

Using the WJ would likely increase the low speed circuit flow, at least it would be easy to increase. The problem is the WJ mounting holes sit at a 45 degree angle to the Zama/HD carbs. The WJ is also a bit longer than the HD. Somebody would have to take all the parts, manifold, aircleaner, choke and see how it fitted up.

Using a modified Zama on the 7900 looks like the way to go. It is easy to work on and capable of flowing more fuel than the 7900 can swallow. The low end torque increase has to be felt to be appreciated, it turns a good work saw into a great work saw.

 

[fordf150]

I didn't look or think about the mounting screws. I did think about the size difference between the carbs. When I get another carb back in stock I will take a few minutes to see how it will mount up. Likely similar to that 395 carb we fitted to the Makita BB. Custom adapter for the air filter. I know what needs made for that but don't have the tools needed.

 

[Terry Syd]

If I was going to use a Walbro, I'd use the HD from the 460. I expect that it would be a lot easier to drill a torque port in the carb and incrementally step of the flow of the low speed circuit until you had the jetting you like. You would have the same size venturi as the WJ, but you would be able to fine tune the carb to where you wanted it. - And, you wouldn't have to go through all the mounting issues that using the WJ would create.

I suggest you try modding the Zama first (or get Rich to make you one). After you try the modded Zama, I think you'll find it does everything you want. I could get a Walbro and mod it for my 77cc BB, but the Zama on the saw already has enough power to run a 24" bar with 8-pin in Aussie hardwood. I can bog it if I take a really big bite using the dawgs, but I don't have a problem with bogging during normal cutting - and I tend to use the dawgs to keep the revs down and spit bigger chips.

 

[Poleman]

Well just recieved Clint's saw straight from Randy..... Randy said it was a "BEAST" .....I dissagree.........It's a "MONSTER"!!!!! OMG!!!!!

Looking at getting a carb set up for the port work on this saw and hopefully better guestemate on future ported saws. The carb I sent didn't perform like I had intended. Clint was kind enough to let me play with his saw on its trip back home...just a short vaca before going to work!!!

Have intentions of putting on a HD 10 carb to see where it stacks up....just because I'm interested in what it does. Then I will get into modifying a Zama to get the best performance I can for this saw. I have ideas of where to go and how to proceed from Randy's feedback. Also I want to correct the difficult tuning that Randy experianced from the carb I sent. First Gen..... Have some refinements that I believe will take care of this problem. Second will be as I said the performance, in given RPM range since it is limited. So we're looking at increasing torque!! I think and forgot saw was limited to a specific rpm range so I overdrilled the jets on the carb I sent. I will back up a step and see if that is closer...I know the tunability problem will be corrected.

I will give you results of what turns up!!! Clint, Thanks for the use of your saw, Randy for porting and the MAGIC work you did!!! All in the name of "RESEARCH" right!!?? I feel anyone with a Dolmar 7900 stock or otherwise will benifit. Because there there to be had!!

Rich

 

[Mastermind]

Be sure to keep me in the loop.

As you can see, I get big gains with the stock carb, so reworking the stock jetting has never been a priority in my mind. Make a better air pump and you get more airflow.....then you get the additional fuel that is needed by design.

I'm still very interested in what gains you may find though.

 

[Poleman]

I will most deffinatly!!! Your gains are amazing with the stock carb!!

 

[psuiewalsh]

Rich, can you provide info on the carb sent to mdavlee for the other 7900build?

 

[Poleman]

I will. The one I sent to Mike was jetted smaller than What I sent to Randy, it also had the air bleed drilled to clear up the tuning issues. I did run this carb and tune it on a stock 7900 and it ran good!! I will have to go back and check notes to make sure what I sent, I dont want to give wrong info.

Rich

 

[Terry Syd]

Does Mike have a thread of the 7900 he is building?

 

[mdavlee]

No thread yet. I haven't got started on it? I still need a 385 carb boot for the xtorq carb for the other one.

 

[Terry Syd]

So, what are you doing? Building two 7900s, one with the modded Zama and the other with the Xtorq carb?

 

[mdavlee]

So, what are you doing? Building two 7900s, one with the modded Zama and the other with the Xtorq carb?

Actually 3 but yes on the carbs on those 2. Mine will probably be the stock to compare with for now. It's going behind those 2 and one other saw. I just need a 390 carb boot to get here for the one swap so I don't have to rotate the carb and all will fit just like oem.

 

[Terry Syd]

Cool, this will be a real world comparison of performance. I'll be putting my money on the Zama. Maybe the Xtorq carb would have bit more on top (it's 19mm venturi as I recall), but the Zama will likely have better torque down low. The Xtorq carb is after all an EPA approved carb for a 71cc engine so it may be lean on the low speed circuit.

If that proves to be the case, then you might try taking an air bleed hole on the Xtorq carb out a bit and making the low speed circuit a bit richer.

EDIT: Does the Xtorq carb have that damn .25mm low speed jet with the valve? You might be able to just ream it out.

 

[mdavlee]

I don't know about the small low speed jet. Both saws will be the same with carbs being the only difference.

 

[Poleman]

Well...I ran Clints saw straight from Randy......IMPRESSIVE TO SAY THE LEAST!!!!! Clint your gunna LOVE this BAD BOY!!!! I DO!!!!!

I did do the mod's I wanted to do on the carb I had sent that didn't work out. I thinkmit was 30/40 and was unadjustable and rich. I drilled out the air bleed hole and tryed that.....OMG the power this saw produced was scarey. It was now adjustable, had plenty of idle adjustment. Only issue was hot restart....it wanted throttle to be open!!??? Tweeked a little after that.
I put on a 35/35/55 AM carb that was to rich for a stocker and it seemed to like that!! I will get that in some wood tonight and see how close that is to the bigger carb??? It should be close and may go with it if I can't bet the rich/hot start resolved.

This is getting VERY INTERESTING...stock saws you can tell a differance but a ported saw....all I can say is you need to run it to know what I mean as I have NO words to describe how it is!!!

GOOD WORK, Randy on another AWSOME saw!!!! I wish you could run it with the BIG carb....I think it would make you blink!!!!

 

[Poleman]

Also, I have some "TEST MULES" out there to get feedback on from these carbs. This will give me an idea of there tunability/adjustability and ease to use. I will get back treir results, finding, likes, dislikes ets. It will be a no BS tell it like it is report. Thanks for that guys!! What I find out only makes these better...and will/can be used by any one using this carb be it Dolmar or Husky. Some BIG results can be had!!

Rich

 

[fordf150]

I got one.....now to find time to install and run this thing...I substituted a sharpie for the blue anodizing

 

[Poleman]

lol I thought about it!!!

 

[fordf150]

Almost sent the thing back to rich. Wasn't anodized and I didn't even get a sticker!

 

[Poleman]

Proto type.......unmarked before production!!! Lol

Man, I gotta sharpen up to stay ahead of you.....

Hows this???