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Dolmar 7900 Carb upgrade?

Discussion in 'Chainsaw' started by plowin-fire, Feb 20, 2013.

  1. Poleman

    Poleman Addicted to ArboristSite

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    Hi, all...just a little update on the funtionality of the Walbo change. I've used the saw all week without a glitch. Startes, idles and runs AWSOME!! It had good power, excellant throttle responce everything that I look for. It will be hard to find a better carb combo for this saw.

    Still waiting on the micro bits so I can MOD the Zama. Terry said its a good Mod and excited to give it a whirl. I find the advice here on AS to be GREAT!!

    Terry, was wondering if when drilling out the jets and only having 1/4 to 1/2 turn of idel adjustment left, what would happen if one would drill a hole or grove the throttle plate to give it more air. I had to do this on an Echo 8000 for idle adjustment after porting the saw and adjusting. It had little or no idle adjustment left so I did alter the throttle plate to draw more air at idle...solved the issue I had. Now saw tunes and seems to have all stock type settings back to normal...just has a VERY nasty disposition toward wood now. Just thinking!!??

    Also Terry, you say you can alter throttle responce with the springs? Been thinking about this and trying to digest it....stronger spring faster responce??? Thus the spacer washers and/or differant springs??

    Any way just wanted to update and tell my findings thus far.
     
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  2. Terry Syd

    Terry Syd Addicted to ArboristSite

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    Yep, you can take a little off the throttle plate (on the side opposite to the idle cutaway) to lean out the mixture, just make sure you have sufficient idle screw adjustment to turn the idle back down after you trim the plate. If you go too far, just solder the edge back up and start again.

    Throttle response is a combination of a few things; spring pre-load, spring rate, size of the metering orifice and the low speed jets. For all practical purposes, we just use the spring pre-load (otherwise known as POP) to tune the throttle response.

    If the metering system wasn't in the carb, then as soon as we started to open the throttle we would get a full hit of fuel from the jet/s. So the metering system is used to 'ramp' in the fuel flow. For a saw carb, that ramp mostly occurs in the first 1/4 of throttle movement (unless you are using a really strong spring).

    If you slowly open the throttle you can feel how the engine is responding to the fuel flow. If it is bogging with extra fuel, then an increase in spring pre-load will delay the ramp a bit and make it leaner. If you need more fuel, then a decrease in spring pre-load will start ramping in the fuel sooner.

    A higher spring pre-load will also lean the idle mixture a bit, so that is another method you may be able to use to get a little more idle adjustment.

    If the modded Zama turns out to be a stronger running carb on your saw, then I'll work out a further mod to the transition holes so that the carb has at least one turn of adjustment for the idle screw. It would be easier if slower taper idle screws were available, but they aren't (I waited 6 months for Zama to fill a back order and finally gave up). If you have a 044 carb handy, that idle screw has a slower taper and will double the adjustment.
     
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  3. Poleman

    Poleman Addicted to ArboristSite

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    Terry,

    Thanks for all the GREAT information. I do have a 44 carb handy if need be.

    Really interested to see how the Zama acts with mods and get further instructions and input from you.

    Thanks
     
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  4. Poleman

    Poleman Addicted to ArboristSite

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    Well got the Zama jets drilled out to .30 and .35. Still a little cross eyed from the procedure.....wow those drills a small. Carb has been rebuilt and a stihl 044 slow ramp idle screw installed.

    Will head out in the morning and cut some larger wood (40+ dbh) to get an idea of Walbro bottom pull. It has been great in up to 24" pine. I will make it work then do a field carb change and try the Zama. Possible further mod's may follow. I am starting out conservative and can go from there, per Terrys advice.

    Learned so much so far and was intimidated by the drill work.....not that bad once I got into it. It turned out to be easier than expected. Terry advised to measure the drills before drilling.....VERY good advice as my drills were mixed up a little upon receipt of them....besides it's just makes good sense to take your time and do it right the first time. Patients Grasshopper!!! Lol

    I may take a 28" bar along and run that also since my normal bar I use is a 24. Little more pull in the wood.

    I'll report back after testing and may request more thoughts on further mods after describing results. Terry can analyze and give me more tuning tips.
     
  5. MCW

    MCW Somebody's talking crap here & it ain't the tree!

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    Interesting thread. The rev limited coil is not an issue as mentioned. Even methanol piped 3120's generally won't be pulling over 13,500rpm in the cut. Unlimited coils can help with tuning but proper tuning should be done in the cut anyway.
    Anybody who doesn't think a stock saw can use more fuel and air really needs to see a properly modified and bored out carby on an actual stock saw before and after the mod :)
    I've seen it and was absolutely blown away that it was the same saw.
     
  6. Poleman

    Poleman Addicted to ArboristSite

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    :)k..here it is.

    I cut another tank with the Walbro in big wood. It pulled very well, had good throttle responce as noted before. A very good carb for this saw, very sensible, easy to tune. I would probably leave this carb on and would be very happy with it if I didn't have othere options. I used it and tryed it because I had one and becaus of this thread. The Zama that was on this saw was running irattic and wouldn't stay in tune, it ran good when it did...very conparable in performance and responce as Walbo...but it had problems.

    Now for the Zama... after drilling out per Terrys instructions and setting everything to factory spec, new kit etc. I changed carbs on site after one tank was run through with Walbro. Had H & L 1 turn out to start with and idle screw just touching. Had to do quite a bit of tinkering to get it to fire and run. I found I had to totally screw in idle screw to get it to stay running.....making progress. Finally got the low somewhat dialed in and able to back off idle just a bit...less than 1/4 turn. Getting it to the point of tune. It would stay running but seemed to load up and slow down a little at idle.

    Now for the GOOD news...the throttle responce is REMARKABLE....it actually blew me away. It was lighting fast from idle to top!!! I could stop the saw in a cut let it idle and just pull the trigger and it would just flat GO!!!!! No bog or sputter just flat start cutting with the bar buryed....my kinda saw!!!

    Like I said the Walbro is GOOD by this Zama just makes this saw scream with its mod's. I think one thing I will do is put a grove in the throttle plate to give it more air and try to regain some idle adjustment. It was starting and acting good by the time i quit but earlier the only way I could get it started was to pull the chock out to set fast idle then start....reasoning to think it needs more air.

    Power wish I'm not sure there was a differance. The Walbro was turning 10,500 to 11,300 in the cut and the Zama about 10,800. I was pushing both pretty hard and almost feel the Zama wouldn't bog as much dogging in. I may not have been turning as fast but I feel it pulled better.

    What I REALLY like about the Zama is the throttle responce. It still has some bugs but it's close. I don't know about needing any more size in the jets. I'm at 30/35 now per Terry and would hate to loose throttle responce. I am so astounded by it it's unreal...I'ts like a ported saw.

    The walbow again is really good, after the swap back to the Zama I would full heartedly recommend this mod!! The only difficult part is to get the saw started to tune in once you get it to run. The drilling process wasn't hard, but get a pair of those magnifying glasses...you'll need them. It makes it much easier to see the tiny drill bits your using

    Saw had a 24 inc bar and Oregon 72L chain with a 7 tooth sprocket. I was running Stihl ultra at 40/1 with straight 91 non ethanol. I forgot the 28 inch bar in my hast this morning...oooppsys.

    I hope this wasn't to long winded but I'm rather anal about things. I hope to get more tuning tips and recommondations.:)
     
  7. makguy

    makguy ArboristSite Operative

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    Just to clarify, these carb mods were on an otherwise stock 7900?
     
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  8. cgraham1

    cgraham1 I feel old

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    I'm pretty sure it's an 84cc big bore on a 6401 Makita. ;)
    image.jpg
     
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  9. Poleman

    Poleman Addicted to ArboristSite

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    No. They where on saw pictured in Clints post. I purchased it a couple weeks back. It's a 6401 With I think a Baileys 84cc Big Bore. Im going to get the carb nailed down then pull the base gasket and possibly port. Then go from there.

    I did put a notch in the throttle plate this afternoon a little smaller than on the jet side. I know have TOTALL adjustment. I put orininal idle screw back in and had to keed backing off to just touching to make it idle where I wanted. I was then able to richen the L to where I wanted it now because of low idel issues before. I sure the little change will increase low end tourqe......it was good before. It also retained the lightning fast throttle!!!!

    Sorry for any confusion I may have caused.
     
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  10. Terry Syd

    Terry Syd Addicted to ArboristSite

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    Poleman, good write up. I agree about nipping a bit off the throttle plate to try and get it to further lean out the idle, I did it to my carb and it worked to give me 1/4 idle adjustment with the stock needle. If you are only getting 1/4 now with the 044 needle, then reworking the throttle plate should give you more adjustment.

    What you call throttle response in the cut is actually the torque off the bottom of the power band. I've found that kind of effect in the carbs I've done.

    The 30/35 jetting may be the hot set-up or it may be that a 30/40 combination would give you a bit more off the bottom. As far as top-end power, you may be able to use the 30/40 and then lean out the high speed to give a flatter fuel curve and get more speed in smaller wood.

    If you had a second Zama to work with you could drill it to 30/40 and compare it to the 30/35 carb. That would be the ideal way to determine which jetting works best.

    The problem with the idle adjustment can be fixed with drilling a larger transition hole/s. I'll have to get my butt in gear and mod a carb. Pulling the idle jet out of the carb to get to the transition ports will be the biggest hassle, but do-able - I just wish Zama made screw-in jets.

    Edit: I see you posted while I was writing, great to hear the throttle plate mod worked.
     
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  11. Poleman

    Poleman Addicted to ArboristSite

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    Yep....I see a second Zama in my very near future!!!!!!!!!!
     
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  12. MCW

    MCW Somebody's talking crap here & it ain't the tree!

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    Hi Terry.
    Are your carby mods still on a stock 372XP (;)) BB end or have you ported it by now? The improvement I saw on Neil's saw (same as yours) with a simple muffler mod yet stock top end were huge. That was a bored out and modified 395XP carby - it showed me just how much fuel and air a stock saw can utilise when done properly.
     
  13. Terry Syd

    Terry Syd Addicted to ArboristSite

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    My 365/372 BB is essentially stock timing. I took the intake timing out to 160 duration, but exhaust and transfer are stock. Neil gave me a 395 carb to try on my saw, but there were issues with the mounting and choke lever. Since this is just a cheap Chinese bucking saw that is only running a 24" bar with 8-pin, I still have the 395 carb sitting on the shelf. The modded Zama carb gives me all the torque and power I need.

    As I was working out the mods, I tried a 35/35 combination. The .35mm idle jet causes more issues with the idle adjustment, that is why I recommended that Poleman start with the 30/35 combination. I'd suggest that if someone was modding a Zama for a stock 372 or other saw that they don't go larger than .30 on the idle jet.
     
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  14. Terry Syd

    Terry Syd Addicted to ArboristSite

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    LOL, Now you're hooked! Not bad for a $10 investment in tools. You may even start a business of 'Poleman's Carbs' - for the discerning wood cutter.
     
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  15. cgraham1

    cgraham1 I feel old

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    It is an NWP kit from Bailey's. I found a Dolmar 7900 while I was messing with this saw, that's the only reason I sold the Makita to Poleman.
    image.jpg
     
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  16. Poleman

    Poleman Addicted to ArboristSite

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    Yep....I'm addicted!!!!

    I've learned so much and have put you research to an applied aplication.....with good results!!!!

    I don't know about that but I do now how to mod the Zama. I will get another carb and go another step and see what happens the way it is then maybe port this saw and again experiment.

    Terry, Thanks for sharing all your finding on what you've already done.....I couldnt have done any of this without our help.

    Big THANKS, Rich
     
  17. one.man.band

    one.man.band ArboristSite Guru

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    2 cents: drilling jet holes/butterfly throws initial screw settings out the window, main nozzle check valve (unadjustable) becomes an issue/limiting factor

    needed this adjusted: see attachment photo
     

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  18. Poleman

    Poleman Addicted to ArboristSite

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    Your absolutely right....stock setting are out the window since it is no longer stock. With any modifications that is an understanding. To improve is to change. Let's face it here on AS we're all looking to improve the cutting ability of our saws.....and who like stock anyway??? Once bitten it becomes habit forming to change and modify.....CAD's is the results.

    Have some more carbs coming in to test. Will try the 30/40 and see how that works and make a judgment as to which works better on the BB kit. I think the 30/35 would be killer on a 7900!!

    One word of caution....I played with the notch on my throttle plate some more and it seems I'd you go to much you will gain more adjustment but loose some of the throttle snap off idle. It still runs and cuts the same just loose a bit of crispness.

    I may try to play with some on the washer Terry had mentioned in another thread to see if that can be gained back what I lost.
     
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  19. Chris-PA

    Chris-PA Where the Wild Things Are

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    I think Terry's mods make sense for the particular Zama model he's working on. The idle and idle transition (part throttle) circuits of the various carbs work differently, and he's using those particular Zama carbs because they allow him to modify the operation at part throttle the way he wants to.

    That's a complicated portion of the carb function, as it must provide a correct steady-state mixture, but also must provide a properly enriched mixture under dynamic conditions (acceleration/throttle response). The Walbro and Zama designs are different, and even the various models of a given brand are different. Carbs with accelerator pumps have an even more different approach (which is my preferred method), supplying the fuel for acceleration from a totally separate system rather than requiring the fixed jets to do it.

    I find carbs to be quite interesting and enjoy reading about the mods by people who really understand them. Sometimes they take different approaches to how I would do it, but that doesn't mean they don't work.

    I'm not sure why the check valve should be a problem? Ultimately the flow through the main nozzle cannot cannot be changed too much or the main mixture would be grossly rich, and I would assume the check valves have at least a little overhead? The purpose of drilling/filing the throttle plate is to move the mixture screw so that it is more centered on the taper - if you are at the ends (either just off full closed or where the point is withdrawing from the hole) then you have very poor control.
     
  20. Terry Syd

    Terry Syd Addicted to ArboristSite

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    In regards to the 'snap off idle', perhaps you can move the throttle a few times and see if the 'off idle' transition is going 'rich' or 'lean'. If it is going too 'lean' you may want to try a shorter metering spring. The shorter spring will allow the 'ramp' (slope) of the fuel to start sooner. Don't cut the spring, see if you have another spring from another carb you can try.

    After you try the 30/40 combination, there's more you can do to the Zama. Although the designer of the carb flowed the venturi on a bench, the freaking middle management probably save $.23 production costs on every carb by using two halves in the mould that come together at the SMALLEST POINT IN THE VENTURI. Crikey, the area of fastest flow through the carb and they put a line of casting flash 360 degrees around the inside of the carb, brilliant. The designer of the carb was probably physically ill after he saw what the management wonks had done. Anyway, you can clean up that line of casting flash and smooth out the flow.

    AND, as soon as you have got the Zama sorted out for your saw, you may want to try modding the Walbro HD. What if, like, what if a modded Walbro could be made to work better than the Zama????
     
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