Dolmar 7900 Carb upgrade?

[Poleman]

Hey Nate...this better?????

Clint......you have carb #00001!!!! Top of the line jetting for a top of the line saw!!!!

I like the little smaller carb, it was good but had a funky idle. The others I've done with that jet combo didn't have the idle issue. Bad carb I'm thinking. Was AM. carb.

Put the bigger carb on that had the hot start issue after making a couple changes.......WHALA!!!!! Problems solved!!! I was running and cutting and it was just under 95 degrees here today and restarted great!!! Hard to believe less than two weeks ago we had 6-12 inches of snow on the ground.

All came togather good!! Think I will duplicate carb and see if they are consistentwith what I've done.

Clint...your really going to like the sound of your saw too!!!!! Randy ported and installed a huge muffler bearing!!!!!

Rich

 

[Terry Syd]

Rich, I take it after your testing you prefer a .30mm idle jet rather than the .35mm - is that correct?

 

[Poleman]

On most carb yes. The 35 seems to be to rich. I've done two carbs with 35/35 combo and it worked good but the second has been a headache. It runs good just has a really bad idle that I don't like. Have yet to check into it and try to find the problem. Leaky Welch plug??

That 35/35 deffinatly makes more power/torque but a little harder to get just right on a stock saw. It worked excellent on the ported saw I have the opportunity to test on and that's where I was going but the changes to the 30/40 I did changed that carb around and it deffinatly makes more power!!!

Still want to try the .6 air bleed again hoping I could use bigger jet sizes but after the first try with it I'm reluctant.
Have you tryed it with any good results??

 

[cgraham1]

Hey Nate...this better?????

Clint......you have carb #00001!!!! Top of the line jetting for a top of the line saw!!!!

I like the little smaller carb, it was good but had a funky idle. The others I've done with that jet combo didn't have the idle issue. Bad carb I'm thinking. Was AM. carb.

Put the bigger carb on that had the hot start issue after making a couple changes.......WHALA!!!!! Problems solved!!! I was running and cutting and it was just under 95 degrees here today and restarted great!!! Hard to believe less than two weeks ago we had 6-12 inches of snow on the ground.

All came togather good!! Think I will duplicate carb and see if they are consistentwith what I've done.

Clint...your really going to like the sound of your saw too!!!!! Randy ported and installed a huge muffler bearing!!!!!

Rich

Can't wait to run this against the 064...

 

[Terry Syd]

That 35/35 deffinatly makes more power/torque but a little harder to get just right on a stock saw. It worked excellent on the ported saw I have the opportunity to test on and that's where I was going but the changes to the 30/40 I did changed that carb around and it deffinatly makes more power!!!

OK, that is a bit ambiguous, which carb makes the most power on the 7900?

No, I haven't tried the .6mm bleed hole. Heck, I'm just a firewood hack that doesn't get to cut that much, with all the other stuff on my plate I haven't cut anything in a month. Good thing we've got you to keep the ball rolling.

I still haven't tried the 460 carb on my 029 'Sleeper' saw. Hopefully, I will get a chance on Sunday afternoon.

I've thought about modding a 460 carb for the 77cc saw and put in a torque port, but there hasn't been much interest in modding carbs, probably only a handful of people have shown any interest.

I think the torque port would be the way to go on the Walbro. It is a straight forward mod that can be easily reversed with a dab of GBW (unlike drilling the bleed holes) and it would be easier and safer to tune than drilling the bleed holes. There would also be no issues with trying to get the idle right.

 

[Poleman]

Terry,

So far the 30/40 carb on Clints saw is tops. You have to consider the saw is ported also. Its a good combination!!

On a stock 7900 the 35/35 produces more power and its a good alternative to the larger carb. Little more finiky to tune.
The 30/30 is a good step up from stock of you hadnt ran any of the others and very easy to tune. It probably has the quickes throttle responce other than Clints saw.....it goes if you just think about it!!!

It does seem the bigger the jetting the harder to tune but I think that just a part of the beast!!!

Little things I try seem to work so far....just continue to think about the two 35/35 carbs that act so different. I will make up one more and see where it falls.

 

[ncfarmboy]

Guess I need to post to let you know of my interest in these carb mods. This is the very best thread I've read in a loooong time here. Check in everyday to read results. I have 2-7900's and a 681 to do the carb mods to. Already bought micro drills just need the time to do one. I have HD12's on all three. Thank you Terry Syd, Poleman cgraham1, mdavalee and others for posting. Please don't quit. Learning a great deal about carburetion right here @ the school of CARB.
Thanks, Shep

 

[mdavlee]

I'll be starting the tear down of the 7900s after a video or two tomorrow or Sunday. I have one 6400 to test against the 665 and then it will be torn down. I'll pull the carb off mine and pop the one he sent me and try it out.

 

[Terry Syd]

Rich, remember what I said about running the larger idle jet and using one of the stronger metering springs. If you have a metering spring with a stiffer rate (thicker spring, more gap between the coils), it will give you a slower ramp for the fuel. You may be able to get better throttle response as it won't be feeding the fuel in as fast.

I'm using the stiffer springs in the two carbs I've drilled to .35.

I have to laugh at us getting so anal about the throttle response. In the real world it doesn't mean anything, but once you feel how quick you can get it, it seems you can't resist trying to get it as fast as you can.

EDIT: Rich, about those two different .35 carbs. There's two things to check that may be different - the metering spring and the size of the metering orifice. A smaller metering orifice will ramp in the fuel slower than the larger orifice. If one has a shorter or weaker metering spring, it can affect the idle and make it a bit richer. The pre-load on the spring is analogous to raising or lowering the float on an old style float bowl carburettor. If you drop the float, it takes just a little bit more 'suction' to pull up the fuel from the bowl (or in our case, move the needle off the seat).

 

[Poleman]

Yep Terry....one gets hooked on Quick throttle response once you experience it!!!!! Especially when there's power behind it!!!!

Usually when first testing a new carb I try to set it up for quick response but with larger jets I know it slows it down but try to compensate on the first go. If its not there I know that's something I can improve. My main objective is to get carb to adjust and tune it to the saw and get an idea of what's left and where you can go with it, rich_lean so it's adjustable. Then more fine tuning starts if there's something I don't like.

The two carbs that are so different are different....ones a true Zama and the other is AM. I'm thinking maybe I just got a bad one. I did put togather another carb so I can check it out Sunday. Tomorrow were going to Custer State Park with the grand kids for the Buffalo Round Up then to a cabin with relatives near Custer!! Should be a good time...little hot for this time of year tho!!! Lol

Terry, one thing I've been doing on the AM carbs is to remove the membrane under the idle jet. The AM jet is open more compared to the Zama's. Once drilled I've had to go back in twice to remove it after it comes out and plugs the back two holes. So I just take them out.

I still have the one carb I used JB weld on to block off the Hi jet on the wet side. I want to play with it more and get a better understanding of affects of blocking different bleed holes.

 

[Terry Syd]

I have 2-7900's and a 681 to do the carb mods to. Already bought micro drills just need the time to do one. I have HD12's on all three.

I've got an HD-5 off my 029 that I could do the mods on so that you would see how to do it on the HD-12 for the 7900. The Walbro HDs are basically the same so you can mod any of them for different applications.

Actually, I might use a cheap Chinese copy of a Walbro that I have. It at least has the right hardware for mounting on the Husky 365.

 

[LegDeLimber]

A friend of mine ordered these for me right before my knee port & polish job. I'm just now getting around to taking a glance at them.
The blue boxes are duplicate ordered sets and I think the 2 smaller black pouches (left &right ) are also duplicates.
The larger black pouch (center) looks to cover the larger diameter drill range.
They were all from ebay sellers and I think the entire cost was under $35~ish USD.

Looked at a couple of the packaged as "Indian made" pin vices. The threads on them seem decent, actually look quite noticeably better than typical stuff in this price range
when marked as Chinese origin.
The collets will definitely have to be deburred though.
That one diagonal shot is a pin vice lain out in the order of disassembly exactly as received.

Haven't grabbed a magnifier and looked at the bits yet, rear of pouch says China.
Usual problem I find on low price drill bits is relief angle behind cutting edge is bad.
have even seen a few negative i.e. reversed ones, almost like it the grinder set up for a left hand twist but yet right hand helix to the fluting.
The next thing to check is for consistant diameter of fluted area from tip to shank region mainly and the rest of drill too.
Then how is the grinding quality are the flute edges sharp and the overall geometry clean etc. I guess You guys get the ideas that I'm blabbering over.

sorry that the pics aren't among my usual, better (I hope) looking efforts.
The knee job has had some distracting after issues , So I've slipped up and let all of my batteries get low and the camera gets sorta iffy to work with when the cells are on the edge of quitting.

Keep up the postings folks!
hopefully I'll get back to tinkering here and maybe can add something to the pursuit.

Edit: of course it wont be the same saw by any means.
My budgetary options are a very small sampling of the homeowner sort poulan and husky plus whatever redneckery I can inflict upon the weedbeaters.
...funny how much lower the cost runs for the carbs that are in the range for such lawn mania experiments.

 

[Terry Syd]

That's a lot nicer set of drills and pin vices than I have. The cheap 'made in India' pin vice works, but you just have to be careful getting everything lined up.

I just drilled out a 'torque port' on my HD-5 carb. The Chinese clone of the Walbro wasn't exactly a 'clone'. So I drilled the real Walbro to give you guys an idea of what I'm on about. The pictures will be in the next post.

 

[Terry Syd]

Here's a picture of where I drilled the torque port in the wet side of the carb. I used the edge of the relief for the metering pin as my vertical axis. The horizontal axis was measured from inside the carb where I wanted the hole and then that distance was transferred over to the wet side to coincide with the vertical axis. The distance from the edge of the carb was 12mm.

 

[Terry Syd]

Here's a shot of the inside of the carb with the 1mm drill sticking through the hole. You'll see that it is located on this side of the venturi and will open after the last air bleed hole for the idle circuit is opened. I used a 1mm drill, but it could be too small for the GB Weld, you just have to try and see what works. The hole didn't go exactly vertical through the carb, but it still ended up in a good position. A small drill press would have been great for precision work instead of the big-ass hand drill that I used.

 

[Terry Syd]

I should mention that I used a couple of small Dremel ball cutters to trim back the edge of the slot for the metering pin. After I had trimmed back the edge and got the ball down far enough in the carb, it provided a nice depression to start the drill in.

 

[LegDeLimber]

Yeah I had wondered how well the collets will center in the tool body let alone repeat.
took a closer look at the black pouches and realized they are all one set- looks to range from .3mm up to 3.0mm From what I can remember*, this combination of sets Seemed to give the best spread of diameters for that amount of money.
The blue cases are duplicates, ranging number 61 through 80 diam.
Seems like they were about $4~5 including shipping each.

*I'm sorta learning, the hard way, that each round of anesthesia appears to extract its toll from my gray matter faculties.
I have slowly come to hate hearing the words "unique" along with "that so rarely ever happens", when you're talking to a DR.
I cant seem write an entire paragraph or read and process a long one at the moment. lots of re-editing for now.

 

[Terry Syd]

I don't go under anaesthesia if I can help it. I don't like someone else messing with my consciousness. The last time I had a big sebaceous cyst taken out of my back the doctor realised it should be done under general anaesthesia it was so deep. I just told him to go ahead and cut it out. I learned how to 'look at the pain' when I got shot in 'Nam and how to get the endorphins flowing.

They can do a nerve block to your legs. If I eventually have a knee replacement, I'll have the nerve block and stay conscious.

Back to the carbs - I've also got a set of the 61-80 drills, it gives me more options for the size of holes I can drill.

 

[Poleman]

Me Too! I also found a pretty good set at Harbor Freight...They have the collet installed and a colored rubber bishing on the shaft. They go down to .24mm, I've been using them to measure everything!!!

I have the same pin vice with the swival end whis is great but the smallesr collet is hollow and I've found it imposible for me to bet a bit centered...but thats me. It does work on 1mm and larger very well.I do all my drilling by hand'm afraid of breaking a bit off in a jet. Slow as you go....and it dosent take long.

 

[ncfarmboy]

One porter I follow generally mods all the carbs on the ported saws he does. He pulls some massive hp & torque out of his saws. Working on a Walbro C1Q-DM14 from a 5100 Dolmar this AM. Thought I could swap throttle shafts etc. from a 55 Husky C1Q-EL7 both 13.5mm venturi everything matched up til I got to the choke shaft. Back to the drawing board.
Thanks Terry for showing Hd12. They are on for later today & tomorrow afternoon. Want to try and get one done for Mon as I have help on Mon.
Shep