Dolmar 7900 Carb upgrade?

[fordf150]

Sounds like your getting the carbs figured out. Now it's time to move on to learning how to anodize them. Never tried it but supposedly easy to do and no special stuff required for something like a carb

 

[RyanTowry_81]

Rich, have you looked for anywhere else to get that part number idle screw? I did some searching on my lunch break and found one site that had a list of carb models that needle came on. Seemed to be a lot of the c1a series if I remember correctly. I have it book marked at work, I will have to share the link when I get back to work Monday.

 

[Terry Syd]

Here's what I have for calculations on the area of the hole in the various size jets. I can't measure actual flow, so the area is the default measurement. Each drilled jet will have variations and imperfections from the hand drilled process that will affect flow, but these area figures at least create a starting place to try some variations.

.25mm = .049 (millimetre squared)
.30mm = .070
.35mm = .096
.40mm = .126
.45mm = .159

The stock Zama has TWO .25mm jets. That equates to an area of .098.

The 30/40 combination is equivalent to .196

What you need to find is the optimum size for a 50cc saw. Then you can make a linear graph for various size engines. For example, let's say a 50cc saw needed a 25/30 combination. That combination equates to .049+.070=.119

The difference between the 50cc (.119) and the 80cc (.196) is .077

There is 30cc difference between the 50 and 80cc engine.

.077 divided by 30 gives - .00257 for each cubic centimetre increase in displacement over the 50cc engine.

In other words, for a 71cc engine you would multiply 21 (the difference between the 50cc engine and the 71cc engine) times .00257, which gives .05397 (say .054)

Add .119 (the 50cc starting point) and the extra .054 (for the 71cc engine) and the result is .173

A .30mm idle jet gives .070 of area, so you need and extra .103 of area for the 71cc saw (.173-.070=.103)

The .35mm jet is the closest at .096

Thus, a 71cc engine would likely need a jetting of 30/35

It's a starting point...

 

[RyanTowry_81]

Unless you could find a .375mm bit and be at .110 in area and just a tad rich. Or use the .35 and not be dead steady and probably end up with more near a .36 hole. Lol

I do like the idea of the graph of what sizes work for what CC saws. You could even come up with a algorithm for the Fred line and just have an equation to plug in cc's and get your jet sizes out.

 

[Terry Syd]

It's a starting point. We don't know what a 50cc saw needs (let alone a ported one) and the line may not be linear.

Rich needs to get a 50cc saw and see what it needs, then work out a tentative table and see if it works.

Thankfully, there are other various size drills available which will give him some more options for when the size falls between the metric sizes.

 

[Poleman]

Ryan, I haven't checked on any other suppliers for the slow taper idle screw. Guess I never thought about another source.

I'd like to find a drill source that would be in increments to make smaller steps in drilling the jets to maybe bet a better tune. The slow taper screw is more of a bonus as altering the throttle plate archives the same results. End result I'm hoping to achieve is a nodded carb that can be tuned like a stock carb with the benefits of the larger jet size.

It would also be good to have a graph/chart that one could refer to to get jet sizes that work best for what particular saw and cc size. That would be a tuner/modders dream.

Terry I like the idea of doing something with a 50 cc saw!!! That is a very popular sized saw with a lot of interest out there to increase power and performance. I do have some 50cc saws (still 026, Makita 520, still 028 and a Dolmar 5105). The 026, 028 and 5105 have all been nodded and run quite well and would be fun to play with.

 

[fordf150]

Ryan, I haven't checked on any other suppliers for the slow taper idle screw. Guess I never thought about another source.

I'd like to find a drill source that would be in increments to make smaller steps in drilling the jets to maybe bet a better tune. The slow taper screw is more of a bonus as altering the throttle plate archives the same results. End result I'm hoping to achieve is a nodded carb that can be tuned like a stock carb with the benefits of the larger jet size.

It would also be good to have a graph/chart that one could refer to to get jet sizes that work best for what particular saw and cc size. That would be a tuner/modders dream.

Terry I like the idea of doing something with a 50 cc saw!!! That is a very popular sized saw with a lot of interest out there to increase power and performance. I do have some 50cc saws (still 026, Makita 520, still 028 and a Dolmar 5105). The 026, 028 and 5105 have all been nodded and run quite well and would be fun to play with.

Soon as screws show up here I will let you know and send a couple out if your still in need of them.

 

[Terry Syd]

Rich, you won't get the huge gains you got on the BB, but you may be able to add a bit of torque to make the saw easier to use. I don't suppose any of those saws use a Zama do they?

It looks like a 346 would be a good choice if one became available. Perhaps someone has a spare that they might loan you for some free tweaking. I'd suggest putting a 17mm carb on it (I understand the 357 carb is a modder's option), since you are trying to work out a graph/chart for the 17mm Zama.

 

[Poleman]

I think the Dolmar has a Zama but not sure of model. I do have a 350 Husky that has a BB top on it but had the EPA dished piston in it so it was no where near its potential. Been waiting for a 353 piston for threes week on this but it looks like a C3M. I'll get a look at this to make sure. I like these saw as I built one that was a 346 eater. I was never impressed with the 346 and sold mine.

If its set up the same it would be easy to work with.

 

[cgraham1]

Well....worked on the Zama that will go to Randy today...little biwillfferent than the others. This had a double D Hi screw, than I noticed these carb had a hole drilled in the throttle plate. I measured it at .80mm.

Drilled the jets to 30/40, double checked needle height and tested this way. Started easily but was running rich and needed more air. It did have idle adjustment the way it was. I then opened up the throttle plate hole to .90mm and retested.....better. One more try this time to 1.00mm. This seemed to do the trick for adjustability and engine performance. The throttle was a little slow now so I tryed a shim and a spring change....nope not it. Left the shim and put original spring back in....that's more like it!!!!

The saw was still a little rich but I figure after Randy gets finished doing his port work on the 7900 it should be real close...or that's my intentions. This carb has a lot of top end to work with!!

Looking to make these as painless as possible to put on a saw and go!! Hopefully I will be getting some feedback soon as to how accurate settings are and power results.

opcorn:

 

[fordf150]

5105 has a C1Q-DM13B

 

[Poleman]

Clint,

You want me to send you the carb so you can play with it before you send you saw?? You could get some ideas and include it when you send your saw.

Rich

 

[Terry Syd]

I think the Dolmar has a Zama but not sure of model. I do have a 350 Husky that has a BB top on it but had the EPA dished piston in it so it was no where near its potential. Been waiting for a 353 piston for threes week on this but it looks like a C3M. I'll get a look at this to make sure. I like these saw as I built one that was a 346 eater. I was never impressed with the 346 and sold mine.

If its set up the same it would be easy to work with.

Now that would be a great 50cc saw to work with, put in a flat top piston, intake timing to 160 and one of your 17mm carbs. I'd expect that to be a really nice little work saw with a very broad powerband.

 

[Poleman]

Thats kinda what I was thinking, Terry. I think it has everything needed to be able to play with and see what can be made out of it!!! I know what it can do without the carb change....so this could get very interesting with a 17mm carb.

 

[Terry Syd]

I'll bet once you got the 50cc saw worked out, then it would be even easier to dial in a carb for the 372. Mate, you're gonna have the whole range worked out in no time!

Like Ford150 said - "Now it's time to move on to learning how to anodize them."

 

[Poleman]

Lol.....kinda seemed everyone liked the sneaky look better!!??

 

[cgraham1]

Two more weeks... and it's off to Tennessee...

How's that Makita doin', Rich? I just missed another one on eBay.

 

[cgraham1]

Lol.....kinda seemed everyone liked the sneaky look better!!??

Hard to see the carb under all that plastic... What you need is a custom sticker! That's all anyone seems to care about around here... stickers. Just remember, it's gotta look good next to the Mastermind Worksaws sticker.

 

[Poleman]

Clint, I think you right....no one would see the cool anodized carb with the lightning bolt....lol!! The sticker is a way better idea!!!!!
7900 is good!!!! I'm like you will looking for a stock 7900!!

Been waiting for more carbs.....I ordered some of the cheap Chinese ones to play with....don't have the funds for OEM carb as there about $100 each.

I've been playing with different ideas as to modifications and to keep the stock adjustability....or near stock. Still waiting on the slow taper idle screws also.

Since I've had time I have looked at other carbs and their function. I'm thinking any carb can be modified to supply more fuel increasing performance. I've played with a WT, Q1C and C3 so far. I will need to do a follow up and test to see results. Yet to come will be the HD's and possible older Tillotson. Just kinda playing with what I have.

Just need some nonpartisan feedback and evaluations to see where and what weeknesses are and make corrections and improvements.

Rich

 

[psuiewalsh]

Clint, I think you right....no one would see the cool anodized carb with the lightning bolt....lol!! The sticker is a way better idea!!!!!
7900 is good!!!! I'm like you will looking for a stock 7900!!

Been waiting for more carbs.....I ordered some of the cheap Chinese ones to play with....don't have the funds for OEM carb as there about $100 each.

I've been playing with different ideas as to modifications and to keep the stock adjustability....or near stock. Still waiting on the slow taper idle screws also.

Since I've had time I have looked at other carbs and their function. I'm thinking any carb can be modified to supply more fuel increasing performance. I've played with a WT, Q1C and C3 so far. I will need to do a follow up and test to see results. Yet to come will be the HD's and possible older Tillotson. Just kinda playing with what I have.

Just need some nonpartisan feedback and evaluations to see where and what weeknesses are and make corrections and improvements.

Rich

Can you turn the idle screw on a lathe to get a slower taper?
Would powder coat be an alternative to anodizing? You can get small kits of it reasonably? Or Glock 37 can be a good resource.