ncfarmboy
Addicted to ArboristSite
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Shep
Dolmar 7900 Carb upgrade?
- Thread starter plowin-fire
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Shep
Poleman
Addicted to ArboristSite
Well did the carb mod with the knock off and that turned out great!!!! One time shot....so I figured I'd start with the other knock off for the 7900 Dolmar.
Made the same mods 30/35 jets but drilled out their bleed hole to .60 instead of .55. I figured this saw could handle more air.
Couldn't get the saw right at these setting and couldn't figure out why.....I've done bigger jets for other saws and they worked fine.
Took the drilled idle jet back out and found the membrane had come out and was blocking a couple of the holes in the back. Took that out and replaced the jet and tried it again....worse. It was so rich both H & L were closed. Tryed a 1mm hole in the throttle plat better but still rich. Removed the jet again...JB Welded the .60 hole I had drilled and closed off the jet on the H. (Jet in the back on other side of needle). Re drilled bleed hole to .55, crossed my fingers and retryed........ Not sure what went on but the saw liked it!!!! Dialed right in, lot of adjustment all the way around. It even wanted more fuel on the H side!!!
I will run this like this and see if it holds and if I think more power can be obtained. I like the performance the way it is....pull good and excellent throttle response. Very smooth to operate and fun!!!!
Just another chapter....not sure what was causing the super rich condition but was able to reverse it and get a good mod that the saw liked!!!
Rich
Made the same mods 30/35 jets but drilled out their bleed hole to .60 instead of .55. I figured this saw could handle more air.
Couldn't get the saw right at these setting and couldn't figure out why.....I've done bigger jets for other saws and they worked fine.
Took the drilled idle jet back out and found the membrane had come out and was blocking a couple of the holes in the back. Took that out and replaced the jet and tried it again....worse. It was so rich both H & L were closed. Tryed a 1mm hole in the throttle plat better but still rich. Removed the jet again...JB Welded the .60 hole I had drilled and closed off the jet on the H. (Jet in the back on other side of needle). Re drilled bleed hole to .55, crossed my fingers and retryed........ Not sure what went on but the saw liked it!!!! Dialed right in, lot of adjustment all the way around. It even wanted more fuel on the H side!!!
I will run this like this and see if it holds and if I think more power can be obtained. I like the performance the way it is....pull good and excellent throttle response. Very smooth to operate and fun!!!!
Just another chapter....not sure what was causing the super rich condition but was able to reverse it and get a good mod that the saw liked!!!
Rich
Terry Syd
Addicted to ArboristSite
Rich, good work on exploring what works and what doesn't. I've used that .55mm air bleed hole on both a 64 and 77cc engine and it seems to work well with a nice reliable idle. I'm going to continue using it for the time being.
As far as the High side tuning, once you block off the jet on the back of the discharge nozzle, the needle then has to provide all the fuel for the High side. The High speed needle may go from being closed to two turns out.
The various Zama carbs (and their clones) have different size jets on the back of the discharge nozzle. You may be able to get by on a carb from one saw that has a smaller jet, but on another carb with a larger jet you will need to block it off. You won't know until you fire it up that you have to go back in and block the jet on the discharge nozzle.
Now that you have the High speed circuit turned down to where you can tune it, you may find that you can go up to 30/40 on the Low speed circuit and pick up some more low end torque.
As far as the High side tuning, once you block off the jet on the back of the discharge nozzle, the needle then has to provide all the fuel for the High side. The High speed needle may go from being closed to two turns out.
The various Zama carbs (and their clones) have different size jets on the back of the discharge nozzle. You may be able to get by on a carb from one saw that has a smaller jet, but on another carb with a larger jet you will need to block it off. You won't know until you fire it up that you have to go back in and block the jet on the discharge nozzle.
Now that you have the High speed circuit turned down to where you can tune it, you may find that you can go up to 30/40 on the Low speed circuit and pick up some more low end torque.
Poleman
Addicted to ArboristSite
Kinda what I'm hoping for Terry. I think there is room for improvement. I think I'll try .35 on the idle circuit and leave the auxiliary alone and see what that does. The one thing I like now is the crispness of the throttle now.
Still trying to decide if the membrane was the issue or the .60 hole instead of .55. I have a habit of doing to many things at once then can't figure out what, what change did...
Still trying to decide if the membrane was the issue or the .60 hole instead of .55. I have a habit of doing to many things at once then can't figure out what, what change did...
Terry Syd
Addicted to ArboristSite
The 35/35 works on my 365BB. It may be that the 35/35 works better than the 30/40, but I don't know. You would have to have two carbs to try back to back to see what combination works the best.
The .60 hole should have leaned out the idle even more than the .55 hole, so the extra richness you were experiencing was likely from the rich High speed circuit (which you have now adjusted by blocking off the nozzle jet). If you go to the 35/35 combination, you might drill out your GB Weld and try the .60 hole again. I'd like to find out if it does work better, I'd go back in and do mine.
Oh, and if you do try the 35/35 - wait until you see how the saw runs before you go back in and try the .60 hole - like you said, one thing at a time. As far as the 35/35 combination, when you open up the idle jet you may find you have to add a bit of pre-load (washer/s) to the metering spring to lean the throttle response back to where it was.
EDIT: The larger you go on the idle jet, the more fuel you will get in that first 1/4 of throttle. You will probably be OK on your 77cc saw, but on a smaller engine you may have to use the stronger spring I referred you to earlier in the thread. The stronger spring will allow a slower ramp in of the fuel to allow for the smaller displacement.
The .60 hole should have leaned out the idle even more than the .55 hole, so the extra richness you were experiencing was likely from the rich High speed circuit (which you have now adjusted by blocking off the nozzle jet). If you go to the 35/35 combination, you might drill out your GB Weld and try the .60 hole again. I'd like to find out if it does work better, I'd go back in and do mine.
Oh, and if you do try the 35/35 - wait until you see how the saw runs before you go back in and try the .60 hole - like you said, one thing at a time. As far as the 35/35 combination, when you open up the idle jet you may find you have to add a bit of pre-load (washer/s) to the metering spring to lean the throttle response back to where it was.
EDIT: The larger you go on the idle jet, the more fuel you will get in that first 1/4 of throttle. You will probably be OK on your 77cc saw, but on a smaller engine you may have to use the stronger spring I referred you to earlier in the thread. The stronger spring will allow a slower ramp in of the fuel to allow for the smaller displacement.
Poleman
Addicted to ArboristSite
I will try that tomorrow.....I promise...1 Change at a time. I will do the idle jet to .35 and see then do the idle hole to .60.
I really thought I was at a point of no return when both Hi & Lo were all the in.....sure glad it came around. I really like it now!!!! Just want to see if I can get the killer torque and still stay the same in other aspects!!
I really thought I was at a point of no return when both Hi & Lo were all the in.....sure glad it came around. I really like it now!!!! Just want to see if I can get the killer torque and still stay the same in other aspects!!
Poleman
Addicted to ArboristSite
Well tryed the .35 for the idle jet and that worked pretty good. Not really any changes too the saw. Throttle response seemeda little slower tho.
Next I did the air bleed hole to .60 from .55. Now needed some adjusting and again throttle response slowed. Best setting was with L about 1/4 turn open. I could adjust for more power in wood by giving it more H....seemed to really like the H opened up!! I was also running 1 washer under the spring.
This is a good setup but I think I prefer the 30/35/55+1 for its throttle response and quickness with still good pulling power in wood.
The .35/35/60+1 was good just a little slower response and a torque power curve.
Now it seem one can taylor for speed or power and any combo inbetween. Don't get me wrong when I say slower....its still seems faster than stock response.
I do feel the Big Bore 84cc Dolmar kit makes more torque than the stock 7900, I guess which it should. But neither are peaky, they both have a good power band. I could still have the bar buryed in a cut, stop it let the saw idle then throttle with no bog or hesitation...it would just start throwing chips again. Thats the kinda power/torque I like!!
This was all conducted with a 24" bar and Oregon 72lg chain.
Rich
Next I did the air bleed hole to .60 from .55. Now needed some adjusting and again throttle response slowed. Best setting was with L about 1/4 turn open. I could adjust for more power in wood by giving it more H....seemed to really like the H opened up!! I was also running 1 washer under the spring.
This is a good setup but I think I prefer the 30/35/55+1 for its throttle response and quickness with still good pulling power in wood.
The .35/35/60+1 was good just a little slower response and a torque power curve.
Now it seem one can taylor for speed or power and any combo inbetween. Don't get me wrong when I say slower....its still seems faster than stock response.
I do feel the Big Bore 84cc Dolmar kit makes more torque than the stock 7900, I guess which it should. But neither are peaky, they both have a good power band. I could still have the bar buryed in a cut, stop it let the saw idle then throttle with no bog or hesitation...it would just start throwing chips again. Thats the kinda power/torque I like!!
This was all conducted with a 24" bar and Oregon 72lg chain.
Rich
So I must've missed it, Rich... Are you testing this on the 7900 or the Big Bore Makita, or both at the same time?
I really appreciate your enthusiasm, but I think you may be addicted to modifying chainsaw carburetors. You should seek mental help before it's too late.
I really appreciate your enthusiasm, but I think you may be addicted to modifying chainsaw carburetors. You should seek mental help before it's too late.
Poleman
Addicted to ArboristSite
Clint....it is to late for me....I'm past help!!!
I'm working with a stock 7900...sorry if I didn't mention that. I've worked with the BB and have a baseline for it now I've got a stock 7900 thats in like new shape. I've also been working with the knock off carbs with this. From what I can tell there just as good as the Zama's.
Some of the mods have somewhat changed and been made to make carbs more adjustable and more like a stock carb but only better!!!
Hope this clarity's things a little. I was kinda thinking out loud with my thoughts on the 7900 compared to the BB.
Rich
I'm working with a stock 7900...sorry if I didn't mention that. I've worked with the BB and have a baseline for it now I've got a stock 7900 thats in like new shape. I've also been working with the knock off carbs with this. From what I can tell there just as good as the Zama's.
Some of the mods have somewhat changed and been made to make carbs more adjustable and more like a stock carb but only better!!!
Hope this clarity's things a little. I was kinda thinking out loud with my thoughts on the 7900 compared to the BB.
Rich
Terry Syd
Addicted to ArboristSite
I just got the 460 carb and it is an 'emissions' carb. Crikey, the idle/low speed jet is only .25mm! Even on my 029 with the 64cc jug on it I will have to open it up to something like .40 or more.
I noticed in another thread there was discussion about the 461 having a butt-load of torque compared to the 460. The same thread mentioned that the 461 carb had a bigger low speed jet and a smaller high speed jet.
I'll have to play around with this carb a bit and see what the air bleed/transition holes need to be to get a good idle when I open up the low speed jet. Unfortunately, each time I go in to change the transition hole I will have to pull the welch plug.
The Walbro low speed circuit is much different than the Zama. On the Walbro emission carbs the low speed jet is a LIMITING jet to the low speed circuit (ie: emissions reduction). As far as modding this circuit, I could just drill out the jet to 1mm and use the low speed needle to set the flow in the low speed circuit (which I will probably do).
I also note that Walbro put a bit of material behind the main nozzle in order to reduce turbulence. With the larger venturi, the offset nozzle and the material behind it, this carb is likely to flow better than the Zama.
Although this is a Chinese knock off, the second air bleed hole is drilled up towards the back of the venturi so I won't have to be redrilling that bleed hole.
All in all, this looks like it is going to be a good carb, I just wish I had a selection of Walbro metering springs to play with.
I've been gone a few days, did Randy ever try Rich's carb on the ported 7900? If so, can someone provide me a link?
I noticed in another thread there was discussion about the 461 having a butt-load of torque compared to the 460. The same thread mentioned that the 461 carb had a bigger low speed jet and a smaller high speed jet.
I'll have to play around with this carb a bit and see what the air bleed/transition holes need to be to get a good idle when I open up the low speed jet. Unfortunately, each time I go in to change the transition hole I will have to pull the welch plug.
The Walbro low speed circuit is much different than the Zama. On the Walbro emission carbs the low speed jet is a LIMITING jet to the low speed circuit (ie: emissions reduction). As far as modding this circuit, I could just drill out the jet to 1mm and use the low speed needle to set the flow in the low speed circuit (which I will probably do).
I also note that Walbro put a bit of material behind the main nozzle in order to reduce turbulence. With the larger venturi, the offset nozzle and the material behind it, this carb is likely to flow better than the Zama.
Although this is a Chinese knock off, the second air bleed hole is drilled up towards the back of the venturi so I won't have to be redrilling that bleed hole.
All in all, this looks like it is going to be a good carb, I just wish I had a selection of Walbro metering springs to play with.
I've been gone a few days, did Randy ever try Rich's carb on the ported 7900? If so, can someone provide me a link?
I got an email from Randy a couple days ago saying the port work was done, but he hadn't had time to get to the carb testing quite yet. That's the last I heard from him.
Terry Syd
Addicted to ArboristSite
I just drilled out the Walbro low speed jet to .35mm as an experiment to see how much this 64cc engine wants. I'll take it up in increments and maybe do some mods to the transition holes as I go. Maybe I'll learn something by taking it up in increments.
Oh, I made another observation about this Chinese knock-off of a Walbro - it has a diaphragm cover on it that has the word ZAMA cast on it, LOL!
Oh, I made another observation about this Chinese knock-off of a Walbro - it has a diaphragm cover on it that has the word ZAMA cast on it, LOL!
SawTroll
Information Collector
Have anyone looked into the possibility of putting the RWJ-4 of the 372xpXT (less divider) on the 7900?
It is possible on the 372 (non XT), 385 and 390 at least, and should be a nice upgrade with a 19mm+ venturi. You need some 385/390 parts to do it on the 372 though - and I don't know about the Dolmar...
It is possible on the 372 (non XT), 385 and 390 at least, and should be a nice upgrade with a 19mm+ venturi. You need some 385/390 parts to do it on the 372 though - and I don't know about the Dolmar...
SawTroll
Information Collector
SawTroll
Information Collector
Not a bad swap and it seemed to help. I'm beginning to think a lot of saws could use more carb.I more or less expected something like that.
SawTroll
Information Collector
I am sure you are right, I understand the carb size often are on the small side to keep the noise level down.
That may be part of it. It would be nice if there were more carb choices in the 17 - 20 mm venturi size.
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