SawTroll
Information Collector
Yes, it is of course not the only reason - and that would have been nice. One option is if course to open the 17.5mm carbs more, but you have to know what you can and cannot do.
Dolmar 7900 Carb upgrade?
- Thread starter plowin-fire
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Yes, it is of course not the only reason - and that would have been nice. One option is if course to open the 17.5mm carbs more, but you have to know what you can and cannot do.
ncfarmboy
Addicted to ArboristSite
Builders have been doing that for a looong time. Esp 385 Tillotson HS296 for 372 carb upgrade before the RWJ4 came into being.
Shep
Shep
SawTroll
Information Collector
Yep, but that Tilly has just a 17.5mm venturi - same as the HD-12.
Terry Syd
Addicted to ArboristSite
I got a chance to fire up the 029 with the modded 460 carb and things are looking good. First off, the idle was fine and the throttle response was acceptable. The throttle valve was almost all the way closed, so I may have to put a bit of solder around the inside of the cutaway on the valve to reduce the airflow. I can then drill a hole (if needed) on the other side of the valve to lean out the idle.
I ended up with the High speed needle all the way in, so I will have to go back in and block off the auxiliary high speed jet. Thankfully this carb has a removable jet, so unscrewing it and blocking it won't be a big deal. I may use solder to block it off, that will give me the capability to drill the solder to a smaller jet size if the High speed needle ends up too far out.
I should note that the idle/low speed 'jet' appears to be thin copper (easy to drill, its not brass) . - OK, my gut impression is that I've seen this before in lots of EPA approved cars, trucks and motorcycles - an easily modified EPA restriction. It may be that the carb body is drilled proper for the 76cc 460, but they put in the restricting 'jet' to pass the tests. Being a thin copper piece, it allows the operator to pop out the restriction and run the carb the way the engineers designed it.
I've got a few other things on my plate at the moment, but I thought you guys might want to know these first impressions. I won't be able to do some cutting with the saw for a week or more.
I ended up with the High speed needle all the way in, so I will have to go back in and block off the auxiliary high speed jet. Thankfully this carb has a removable jet, so unscrewing it and blocking it won't be a big deal. I may use solder to block it off, that will give me the capability to drill the solder to a smaller jet size if the High speed needle ends up too far out.
I should note that the idle/low speed 'jet' appears to be thin copper (easy to drill, its not brass) . - OK, my gut impression is that I've seen this before in lots of EPA approved cars, trucks and motorcycles - an easily modified EPA restriction. It may be that the carb body is drilled proper for the 76cc 460, but they put in the restricting 'jet' to pass the tests. Being a thin copper piece, it allows the operator to pop out the restriction and run the carb the way the engineers designed it.
I've got a few other things on my plate at the moment, but I thought you guys might want to know these first impressions. I won't be able to do some cutting with the saw for a week or more.
Terry Syd
Addicted to ArboristSite
I just took the idle jet out to .4mm (then .7mm) and tried running it again. It required the same low speed needle setting. In other words, the drilling in the carb body is controlling the idle mixture. So, you can't just pull the idle jet, you also have to remove the welch plug and drill the transition hole.
I also drilled out the idle jet on a HD-5 I had and found that there was a 'membrane' behind it. OK, what is going on here?? Both the Zama and the Walbro 'emissions' carbs have a .25mm idle jet with a membrane valve behind it. I got a feeling that this .25 jet with valve is an EPA mandate. As far as it being functional, on the Walbro with the wet side of the carb on top, the valve is always off the seat. Anyway, with it all drilled out it works just like the earlier carbs that didn't have such a jet and valve. I don't know what it is for, but the carb works fine without the jet and valve.
Whereas the Zama has a separate idle circuit from the low speed circuit that doesn't affect the low speed circuit fuel flow, the Walbro is a combined circuit. With the Walbro, if you adjust the idle with the low speed screw, you are also adjusting the WHOLE low speed circuit fuel flow.
OK, to tune the carb what I'm going to do is find a log and keep opening the low speed needle and turning in the high speed needle until I find a combination that works good. I'll have to turn the idle screw up to keep the engine running and the chain will be turning. A bit dangerous, but I'll just have to be careful.
Once I find the combination I like, then I don't touch the mixture screws - that's the setting I want for cutting. I then have to go into the carb and open up the transition hole and maybe the throttle plate to get the saw to idle. Then, I may have to look at the pre-load on the metering spring to clean up the throttle response. - Wish I had a dyno to set the carb up, it would be a lot quicker and safer.
Now about those transition holes - these sizes are as close as I can get it using my drill set to measure the holes. They should be considered approximations, close, but not exact.
On the HD-5 which is used on the 54-64cc 029-039 series the holes are .6mm for the first hole and .5 for the second.
On a 044 carb (Zama, non twin jet) for a 70cc saw the first hole is .75mm and .6mm for the second.
On the 460 carb for a 76cc saw the first hole is .6mm and the second is .65mm.
Of the three carbs the 044 is the only non-emission carb. Admittedly the configuration of the carb is different (especially the venturi), but I'm only looking at the idle circuit bleed holes so it likely has some relevance to this discussion.
Using the 044 carb as a baseline for non-emission carbs we have a .75/.6 combination - for a 70cc engine. Those are larger bleed holes than for the 460 carb at .6/.65 - on a 76cc engine. I would love to find out the size of the bleed holes on the 461 as the guys running a 460 might want to drill their carb to those specs.
Here's what's cool about putting the 460 carb on my 64cc saw, it looks like it is a perfect match .
HD-5 at .6/.5
460 at .6/.65
044 at .75/.6
The 460 bleed holes are a bit smaller than the non-emissions 044 carb for a 70cc engine, which makes the 460 carb about right for my 64cc engine.
I'll get around to testing it and dialling it in, but I expect it to be close. Here's another cool thing, I can drill out the second bleed hole on the HD-5 to .65 and it should work like the 460 carb - which means that all the guys with 039/390s could drill out their carbs and not have to buy another carb.
I also drilled out the idle jet on a HD-5 I had and found that there was a 'membrane' behind it. OK, what is going on here?? Both the Zama and the Walbro 'emissions' carbs have a .25mm idle jet with a membrane valve behind it. I got a feeling that this .25 jet with valve is an EPA mandate. As far as it being functional, on the Walbro with the wet side of the carb on top, the valve is always off the seat. Anyway, with it all drilled out it works just like the earlier carbs that didn't have such a jet and valve. I don't know what it is for, but the carb works fine without the jet and valve.
Whereas the Zama has a separate idle circuit from the low speed circuit that doesn't affect the low speed circuit fuel flow, the Walbro is a combined circuit. With the Walbro, if you adjust the idle with the low speed screw, you are also adjusting the WHOLE low speed circuit fuel flow.
OK, to tune the carb what I'm going to do is find a log and keep opening the low speed needle and turning in the high speed needle until I find a combination that works good. I'll have to turn the idle screw up to keep the engine running and the chain will be turning. A bit dangerous, but I'll just have to be careful.
Once I find the combination I like, then I don't touch the mixture screws - that's the setting I want for cutting. I then have to go into the carb and open up the transition hole and maybe the throttle plate to get the saw to idle. Then, I may have to look at the pre-load on the metering spring to clean up the throttle response. - Wish I had a dyno to set the carb up, it would be a lot quicker and safer.
Now about those transition holes - these sizes are as close as I can get it using my drill set to measure the holes. They should be considered approximations, close, but not exact.
On the HD-5 which is used on the 54-64cc 029-039 series the holes are .6mm for the first hole and .5 for the second.
On a 044 carb (Zama, non twin jet) for a 70cc saw the first hole is .75mm and .6mm for the second.
On the 460 carb for a 76cc saw the first hole is .6mm and the second is .65mm.
Of the three carbs the 044 is the only non-emission carb. Admittedly the configuration of the carb is different (especially the venturi), but I'm only looking at the idle circuit bleed holes so it likely has some relevance to this discussion.
Using the 044 carb as a baseline for non-emission carbs we have a .75/.6 combination - for a 70cc engine. Those are larger bleed holes than for the 460 carb at .6/.65 - on a 76cc engine. I would love to find out the size of the bleed holes on the 461 as the guys running a 460 might want to drill their carb to those specs.
Here's what's cool about putting the 460 carb on my 64cc saw, it looks like it is a perfect match .
HD-5 at .6/.5
460 at .6/.65
044 at .75/.6
The 460 bleed holes are a bit smaller than the non-emissions 044 carb for a 70cc engine, which makes the 460 carb about right for my 64cc engine.
I'll get around to testing it and dialling it in, but I expect it to be close. Here's another cool thing, I can drill out the second bleed hole on the HD-5 to .65 and it should work like the 460 carb - which means that all the guys with 039/390s could drill out their carbs and not have to buy another carb.
billyj
ArboristSite Operative
intresting reading, id love to try a modded carb on my 7901
Terry Syd
Addicted to ArboristSite
I had a chance to check the transition/bleed holes on a WJ for a 385. There were three holes -
First - .83
Second - .77
Third - .63
That is a lot of air to add to the idle mixture, I expect that the low speed circuit flows a lot of fuel.
Those figures should give an indication that to mod a Walbro HD for a 77-84cc saw you will need some serious drilling for those transition holes. Perhaps even put in a third hole like the WJ.
First - .83
Second - .77
Third - .63
That is a lot of air to add to the idle mixture, I expect that the low speed circuit flows a lot of fuel.
Those figures should give an indication that to mod a Walbro HD for a 77-84cc saw you will need some serious drilling for those transition holes. Perhaps even put in a third hole like the WJ.
Mastermind
Work Saw Specialist
I'm interested.
But does anyone care.......no.
But does anyone care.......no.
Nice. I hope he can figure out a good fix on the stock carb for more power.Just an update for everyone interested...
Mastermind has finished the porting on my 7900 and it is currently on it's way to Rich for some fine tuning.
Poleman
Addicted to ArboristSite
I DO TOO!!!
Have made some changes for the better since that carb was modded to make them more user friendly!!! With the way that one was modded ( Like the BB carbs) I know there is a happy medium. Have a good set for a stock 7900 that works well now to see what that does on Clints saw and go from there. I'm thinking it may be as easy as drilling the air bleed hole out under the idle jet to bring it to usability.
Rich
Have made some changes for the better since that carb was modded to make them more user friendly!!! With the way that one was modded ( Like the BB carbs) I know there is a happy medium. Have a good set for a stock 7900 that works well now to see what that does on Clints saw and go from there. I'm thinking it may be as easy as drilling the air bleed hole out under the idle jet to bring it to usability.
Rich
That would be nice. I need to get a set of those micro drill bits.
Poleman
Addicted to ArboristSite
Mike...Get a pair of magnifying glasses too when wou get the drills...it helps cuz there small.....
I got 20/15 vision. I need a set that will go up past .062" also. I got something I want to try on another carb.Mike...Get a pair of magnifying glasses too when wou get the drills...it helps cuz there small.....
Poleman
Addicted to ArboristSite
Good!!! Then you'll have them after your done!!! I get the cross-eyed syndrom!! LOL
I think my sets run fro .o3omm to 1mm by half steps...another set is decimal which fills in sizes between some of the others of the metric set. Harbor Freight has some that start at .5 and go up...dirt cheap and work good also. If you happen to have one close by to stop and pick up. Just make sure you get a pin vice or if you have a Dremel to get the small collet set to hold it. There kinda like putting a hair in a drill chuck.
I think my sets run fro .o3omm to 1mm by half steps...another set is decimal which fills in sizes between some of the others of the metric set. Harbor Freight has some that start at .5 and go up...dirt cheap and work good also. If you happen to have one close by to stop and pick up. Just make sure you get a pin vice or if you have a Dremel to get the small collet set to hold it. There kinda like putting a hair in a drill chuck.
I found a 49 piece set on eBay. Should work fine. Up to 3mm.Good!!! Then you'll have them after your done!!! I get the cross-eyed syndrom!! LOL
I think my sets run fro .o3omm to 1mm by half steps...another set is decimal which fills in sizes between some of the others of the metric set. Harbor Freight has some that start at .5 and go up...dirt cheap and work good also. If you happen to have one close by to stop and pick up. Just make sure you get a pin vice or if you have a Dremel to get the small collet set to hold it. There kinda like putting a hair in a drill chuck.
Poleman
Addicted to ArboristSite
AWSOME.....lots of them out there to do the job!!!
Terry Syd
Addicted to ArboristSite
After thinking about the transition holes on the WJ and what it would take to mod a HD carb, I'd try using a 'torque port' (like the one Zama added to their carb) on the HD. I mentioned the concept in the Chinese Carburettor thread where I'd put in an extra jet to increase the low speed circuit fuel flow on a Walbro.
Adding air requires big holes to flow, but adding fuel is easy. When you have mixture ratios of 12.5 to 1 BY WEIGHT of air to fuel, it is easy to see that a little extra fuel flow goes a long way.
OK, say you take a 460 carb, drill out the idle jet and set up the idle. THEN, you drill a separate hole through the wet side into the carb body to a point at about 1/4-1/3 of the throttle movement. Take a dab of GB Weld (GBW) and smear it into the hole from the inside of the carb, put a piece of tape over the hole, turn the carb upside down and let the GBW set. Now you can easily drill a jet hole through the GBW to make a torque port.
Keep drilling out the port until you go too far, now you know the size of jet you want. Clean out all the GBW from the hole and reapply a fresh dab. Now drill the hole to your desired jet size.
Or, you could just get a Zama with the jets nicely laid out for you to work on.
EDIT: If the GBW tended to get eroded by the passage of the fuel, you could make a nylon jet for the port. I used some nylon jets for Mikunis years ago and they worked fine, you just couldn't apply much torque when tightening them.
Just take a bit of nylon fishing line and GBW it in the hole, then drill through the nylon.
Adding air requires big holes to flow, but adding fuel is easy. When you have mixture ratios of 12.5 to 1 BY WEIGHT of air to fuel, it is easy to see that a little extra fuel flow goes a long way.
OK, say you take a 460 carb, drill out the idle jet and set up the idle. THEN, you drill a separate hole through the wet side into the carb body to a point at about 1/4-1/3 of the throttle movement. Take a dab of GB Weld (GBW) and smear it into the hole from the inside of the carb, put a piece of tape over the hole, turn the carb upside down and let the GBW set. Now you can easily drill a jet hole through the GBW to make a torque port.
Keep drilling out the port until you go too far, now you know the size of jet you want. Clean out all the GBW from the hole and reapply a fresh dab. Now drill the hole to your desired jet size.
Or, you could just get a Zama with the jets nicely laid out for you to work on.
EDIT: If the GBW tended to get eroded by the passage of the fuel, you could make a nylon jet for the port. I used some nylon jets for Mikunis years ago and they worked fine, you just couldn't apply much torque when tightening them.
Just take a bit of nylon fishing line and GBW it in the hole, then drill through the nylon.
I was just getting ready to ship a couple parts out and ran across this. dolmar concrete saws...PC-7414 come with a walbro wj126 carb. 17.46mm Venturi. while searching for specs on this carb I ran across a metering spring assortment pack in an organizer. part # 400-606 for anyone that is interested.
Additional for anyone that doesn't know. The concrete saws share a lot of parts/ design with the chainsaw. PC 7414 is a 7300.
Additional for anyone that doesn't know. The concrete saws share a lot of parts/ design with the chainsaw. PC 7414 is a 7300.
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