Dolmar 7900 Carb upgrade?

[460cixy]

I sort of destroyed the old carb trying to work out what they had done on the low speed side.

 

[LegDeLimber]

Terminally damaged carb. Perhaps a candidate for a viewable cutaway?

 

[460cixy]

It could be fixable maybe I just wanted to see what was under the welch plug where the brass jet was ment to be. Now I don't have any plugs to fit it everything is too big or small. Anyhow road tested the big chingvarna tonight off idle response has improved I haven't changed the metering spring yet so that may improve it more. The top end power is what I'm amazed at much better and can actually be tuned to 13500 now where it should be but I tuned it in the cut for the most power that turns out to be around the 12400/12500 mark this is a chinga copy of a 372xp every thing looks to be identical except when pulling the cyl it has open transfers instead of closed so I don't think it will ever make great power with out some work but it sure does not embarrass its self that's for sure.

 

[Terry Syd]

I drilled that carb as a 30/35 and thought it would be about right for a 71cc saw. I thought it would be tuned a bit higher, so maybe a 30/40 would have worked better, or maybe I should have dropped the main discharge nozzle down further (it could have gone another 2mm).

OR, maybe the powerband on the open port jug is determining the tune. The closed port Husky 365/372 jug has nicely angled ports with the back ports slightly angled upward so the flow characteristics are completely different than that open port jug.

Bugger if I know, that's why a dyno would be so good to dial in the carbs. You could graph the powerband from one change to another until you had a combination that was as good as it was going to get.

As far as that jug being open ported, I remember the first BB jug I got from Matt. It was 54mm bore size, but the Chinese had copied the smaller ports from a 365. I wonder on your saw if they copied the open port 365 jug to make the 372 copy. If so, you may want to try one of the new BB jugs (77cc) that Matt is selling instead of messing around with porting that open port jug. The new jugs (with a little bit of work) flow quite well and have better compression.

 

[460cixy]

no idea terry but it is better i will chainge the spring after another couple of tanks

 

[Fly By Night]

Id like to get some bits coming to do these mods on a ported 7900 that will cut from 6,000' to 11,500' in elevation,. Rich do you have an idea which sizes will work well in that range and not be too rich up high?

 

[Poleman]

If you've got the drills etc. to do the jets, I would probably start small....30 main, 35 aux and do your hole under the main jet to 55. It will be easy to change once that is done because you would only be redrilling the top jets.

My highest working height where I'm at is only around 6000 ft so I wouldn't know how a saw would act up high. That's kinda why to start small. A ported saw can really benifit from larger jets, just not sure what would happen at such elevations.

If saw runs or seems to run normal at those elevations I don't see why bigger wouldn't work. If saw is running rich it's probably not getting enough air for the fuel it's getting.

In your situation tho it may be tough to get a carb dialed in with such elevation changes. Back to the thinking of smaller because of it. A little increase in torque is better than stock. I just think you may be limited with such elevation differences.

I'll be following to see how things shape up for you.

 

[Fly By Night]

Thanks Rich,

I am going to order the drills, probably tonight, I want to make sure I get kits with the sizes I do/may need. Home base is at 6,500' between there 11,000 will probably be my primarily operating elevations, I just want to make sure I don't create something that is unusable up high.

Do you have links for the drill bit kits you use readily available? I just looked some and nothing seems to cover 30-55+

 

[Poleman]

Here's the one I purchased. It has all the sizes you will need. If you need a larger hole in the throttle plate for idling you can just drill two. Than a pliers, side cutter or such to pull the main jet. Might want to get if you don't have a caliper to measure the drill bits to make sure they are truly the size the say...mine came kinda messed up as far as sizes went.

Hope this helps.




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[Terry Syd]

DEFINITELY have a calliper or micrometre with you when you start messing with these micro drills. It is way to easy to get the wrong size drill if you rely upon their position in the box.

 

[Fly By Night]

Thanks Rich, they are on their way to come meed Dolly.

Ebay says that seller has sold 9 of those kits, im guessing most have gone to other AS members.

P.S. Does anyone else think Rich is an ironic name for a guy who has spent a lot of time putting more fuel through carburetors?

 

[Paragon Builder]

P.S. Does anyone else think Rich is an ironic name for a guy who has spent a lot of time putting more fuel through carburetors?

That's pretty clever![emoji106]

 

[Fly By Night]

Rich,

Do you have a part number for the accelerator springs? I just re read most of this thread and cannot find a part number.

If you'd help formatting this, adding arrows and reference numbers to pics ect to post the final findings to the library I'd be happy to assist.

 

[Poleman]

Lol....we were flying by the seats of our pants on most of this.......adding pictures as they occurred and comments like wise.

I'm pretty sure Terry posted the number for the spring. I players a lot with that and found I didnt need it in the most part. I only had 1backwards carb that didnt make sense on tune.....it turned out to be a favorite carb.....thats a whole mother story on that one as I had junked it but somehow found its way in the mix. I had totally tore it down and took the dremel to it in boredom.....

 

[Terry Syd]

The metering spring is used for ramping in the fuel, ie: throttle response. For the most part, you can shift the starting point of the ramp by using the pre-load on the spring - and that will be enough adjustment to get the majority of tuners in the ball park.

Rich and I joked about how anal we could get over getting the throttle response as quick as we could, when in the real world of a bucking saw it wasn't something that you'd even care about while working.

 

[Poleman]

Isn't it AMAZING.....theres more out there like us...... LOL

My kinda people!!!!

 

[Fly By Night]

I will pass on the springs for now per riches recommendation but I think they might be helpful at my altitude. Has anybody else been building these? I do not recall seeing anyone else posting their results.

Looks silly easy for the results you guys are seeing, the hardest part sounds like deciding which pliers to take to the grinder

 

[Poleman]

I'll see if I can find the number but after my testing I don't think you'll need it. Only had one strange setting where I had to really mess with the spring rate. It can help.

 

[fordf150]

I will pass on the springs for now per riches recommendation but I think they might be helpful at my altitude. Has anybody else been building these? I do not recall seeing anyone else posting their results.

Looks silly easy for the results you guys are seeing, the hardest part sounds like deciding which pliers to take to the grinder

May look that way after the fact but I imagine there are to many hours invested in this testing to even count. Rich and Terry invested allot of time and money in figuring this all out and openly share this info.

 

[LegDeLimber]

Hate to say that my drill kits and things are stuffed in my old kennedy top box.
just been a handfull of Dr type #%*# that I've slid on and let ruin my concentration
towards a great many other things around here.