Dolmar 7900 Carb upgrade?

[Terry Syd]

If you want to see a visual representation of what I believe you did, take a look at this thread with Chadihman's dyno. There are several power curves there that have a 'dip' prior to peak power. My post at 519 is my take on the dip.

http://www.arboristsite.com/communi...lie-ms-461-is-king-over-ms-660.258040/page-26

It looks like you have filled in your 'dip' with the extra fuel flow in the low speed circuit and may have extended the power band after peak torque when you leaned out the High speed needle and flattened the fuel curve.

EDIT: I don't think you can improve much on the 30/40 combination and there is a bigger risk of going too far at this point. When you had to back off the High speed needle that much, I expect that was a strong signal to look for when doing this mod.

 

[Poleman]

Terry, I think your absolutely right!! The extra fuel give the saw a VERY WIDE power band. It accelerates like a rocket and pulls like a train is about the best way to describe how it runs.

I also think the 30/40 is the limit for this saw and any more would cause more issues. The only other thought would be for a ported saw with squish cut to get around 200lb max compression. I think things could be opened up to the max because the carb could feed it. I understand that the carb issues or lack of performance from carb limited the porting of the 7900. This may change all this and could be a big boom on ported Dollars in the future.

I know I sure like how these changes have made mine perform!!! Still have porting and dropping base gasket in mind and possible timing advance in the future.

The second carb I spoke about earlier did turn out to be a leaky Welch plug. Pulled the cover, pressurized and added a squit of leak finder(kids blow bubbles) and sure enough it was leaking bad all the way around it. Applied sealer, let it dry then tryed on saw again....whala...a whole different acting carb. One more little change to make, and retest. I gained more adjustment but this carb is still on the hi side of the idle screw. I wish I had some of the slow taper screws to work with so I wouldn't have to massage the throttle plate anymore.

Was another good night in the garage and things are looking good!!!

 

[Mastermind]

You guys must have loads of patience.

 

[Terry Syd]

Here's my opinion of where to go next. I look at cost effective solutions, doing the carb was a cheap mod, why not follow the same philosophy for the rest of the tuning.

To put your carb in perspective, I have a 50cc saw with the equivalent of a 19mm venturi, you have a 84cc engine with a 17mm venturi. You just aren't going to be able to feed that cylinder if you raise the cutting speed with radical port work. It will likely make the engine less efficient. If you want more cutting speed, then using the torque and going to an 8-pin seems more logical.

Removing the base gasket will bump the compression and give more torque. Increasing the intake timing to 160 degrees will help feed the existing port configuration with the small carb. I'd leave the exhaust and transfer ports alone (except for cleaning them up).

Your saw is based upon the 64cc Dolmar, the mounting location for the coil is based upon the 64cc cylinder. The 84cc cylinder has a lot wider combustion chamber and may need the timing reset to handle the extra distance for the flame to travel.

If you did those three things you may decide you didn't need anymore - and those three things won't cost you any more money, just your time and effort.

 

[Poleman]

Lol...and you don't Randy????

What do you think about this? Do you think this could make a BIG difference on feeding a ported saw and make it perform optimally? It would sure give a thirsty saw what it needs!!

 

[cgraham1]

Well there's only one way to find out, right?

 

[Terry Syd]

You guys must have loads of patience.

I don't know if it is patience or stubbornness. Crikey, I spent some time trying to figure out how these freaking carbs worked. In fact, One Man Band just sent me a Kart article on carbs that was pure BS - just typical of the crap out there you have to weed through. I'll post a link to the article and you can see what I mean.

If someone can come up with the schematics on the electronic carbs, I'll figure out some mods for those (I spent 7 years as an electronic technician and have modified electronic fuel injection systems).

I've been modding for 50 years now, its become a sick habit that I can't break.

EDIT: Here's the link to the Kart article, see if you can pick all the BS in it - it is chock full of it.

http://nkn.com/nationalkartnews/articles/AnatomyOfWalbro0708.pdf

 

[Poleman]

Never thought of it that way Terry...
Going to test the 8 pin sprocket this weekend and see what I think. Still weighing other options. Always want to improve and move forward...hate to ruin an already good thing...

 

[Mastermind]

I'm playing chess right now........I'll need to read through carefully......or get a cliff notes version.

 

[Poleman]

Clint.....that's what I'm thinking too!!!!

 

[Poleman]

Randy, here's the cliff note version.....

You port saws.......a Dolmar 7900 is scheduled to arrive in September......I send carb for you to test against stock and see if this is a viable solution to feeding a MM ported Dolmar. You do your as usual, test stock then test modded carb to see difference...report results.

 

[Mastermind]

Randy, here's the cliff note version.....

You port saws.......a Dolmar 7900 is scheduled to arrive in September......I send carb for you to test against stock and see if this is a viable solution to feeding a MM ported Dolmar. You do your as usual, test stock then test modded carb to see difference...report results.

Cool.

 

[Terry Syd]

OK Poleman, how much will you charge to mod someone else's carb. You have spent some money on the appropriate tools, picked up a fair bit of knowledge, experience and most importantly the 'feel' for the mods. Other guys don't have that, so you're sitting on a gold mine.

Someone sends you a carb that you rebuild and mod and then send back to them - I wonder what the market will bear for that kind of work.

 

[Poleman]

Good question Terry. I lack the knowledge you have about tuning and altering these carbs. You gave me your specs and knowledge of what to do and was open and VERY helpful to get me where I am with all this. So what do I owe you??

I just like saws and making them perform to their full potential. I like this site because of people like you and many, many others. I've learned a lot about porting, cylinder clean up muffler mods from guys like Mastermind, Terry Landrom, Brad, Stumpy and whomever shared their knowledge over the years. I just want to contribute and repay what I have learned.

What do I charge???? I want to be fair, I want to help others because so many of you helped me with sharing your knowledge. I would use what you have found and tryed in my own applications. Besides it's something I like doing!! A job you like doing is like a paycheck. This is my fun and hobby, and for the most part keeps me out of trouble.

It's a very good question and I don't have an answer.

 

[Mastermind]

I'll be the first to admit that modding carbs is an untapped area for me. I'm sure there are good gains there.

I'm all about seeing someone make money by doing something they enjoy.

Zama carbs have frustrated me so many times........

 

[Terry Syd]

I can't charge anything for what I've contributed, I've already gotten enough back from other members to more than compensate me for my ramblings. If it wasn't for that spirit of contributing about what each of us knows, I wouldn't be here.

Although... to bad there have been some azzholes that have driven away some of the old timers that probably already knew what I figured out about the carbs. Heck, you don't think some of the ol' timers figured this stuff out when the 'emission' carbs started coming in, they have probably been sitting on this information for years.

 

[Mastermind]

I've done some drilling on those Zamas........but never had enough time to really get it down to knowing how much was just enough.

Normally I find a Walbro or a Tilly and throw the Zama in the can.

I'm all ears here.......believe that Terry.

 

[Terry Syd]

OK Poleman, now that you've got your head around modding carburettors, ya wanna try modding that HD-12 and see if it is possible to get it even better than the Zama?

It's basically the same concepts, but a different approach to get the results. You'll need a fist full of welsh plugs for that puppy.

EDIT: I got a better idea. Let's find the crappiest EPA approved HD and turn it into a HD-12 for the guys that really like that carb. Then we'll see if we can mod it further to compete with the modded Zama.

 

[Poleman]

Lol....your pushy arnt you!!! Lol

Been thinking of the same thing but have been concebtrating on the Zama. Haven't studied a HD diagram to get any ideas yet. I'm sure it can be done

 

[Gravedigger]

I just got my new Walbro HD12-1 and bolted right in my Makita 6401 Weedeater big bore. Started first pull set on fast idle. Won't idle off fast idle. Need help adjusting carbs. I'll travel for lesson.