Large carb on a smaller engine tuning theory

Discussion in 'Performance Mods' started by lazylightning@mail.r, Oct 6, 2016.

  1. lazylightning@mail.r

    lazylightning@mail.r Active Member

    Hi All!

    I have a new engine build and I think I need to do alot of tuning on this new carb. The carb is the largest that should be used on a 66cc engine with a reed valve, and it seems to me that the carb, as it is from the box, is probably not tuned for a small 66 cc engine. I'm wondering what kind of changes in the slide cut, the needle taper and atomizer is common when tuning a large carb for a smaller engine. Please throw me a line here with your recommendations.

    Best Regards,

  2. jaguar

    jaguar Well-Known Member

    just go thru the normal procedure for jetting the carb and then if the jetting seems off when slowly opening the throttle you will need to modify the slide cutaway. If it needs to be leaner you can just drill one or more small holes near its bottom edge. If it needs to be richer then you can lower the cutaway by adding JBWeld there.
  3. jaguar

    jaguar Well-Known Member

    For a 69cc engine that revs to 9000 RPM a 20mm carb is normal, so I wouldn't think of yours as being really oversized.
  4. lazylightning@mail.r

    lazylightning@mail.r Active Member

    Yes, but often these carbs come out of the box for bigger engines. The instructions for my 22mm carb said the carb was for 100 cc to 150 cc engines. Actually I found out that it was a 24mm carb mispackaged in 22mm carb shrink cellophane and cardboard packaging. Funny how I didn't think to measure it with the calipers before using it for a year and not figuring out how to get it tuned right. The 21mm carb is also good for 100 cc engines. I am guessing then, that the slide cut away might need to be extended down a bit with jbweld. I guess the first thing I need to do is figure out if my pilot jets is too big or not. It's a #50 jet like on most all 21mm PHBG's.
    I read somewhere on a scooter forum for tuning the 21mm PHBG that someone was having a flat spot at about 6000 rpm's, before it would finally overcome that flat spot and then continue to accelerate. He remedied it with a W8 needle instead of the W7 needle it came stock with. This solved the problem that no jetting changes could deal with. Mine has a W7 needle also. I am also having a flat spot at a pretty high speed, before it finally overcomes it and suddenly starts accelerating and powerfully continues to accelerate alot more till it winds out really far - too fast. It wouldn't even make it over the flat spot with the #92 jet, but with the #89 it's will at least get over it after a few seconds.
  5. jaguar

    jaguar Well-Known Member

    for about 1000 RPM before it gets "on the pipe" there is a power lag due to the baffle wave interrupting the intake which causes it to be too rich. Compensating for that by making the needle leaner then makes the range before and after that too lean. The boost bottle is a better solution because it leans the mixture for its 2000 RPM band of influence.
    But you can make it better with a leaner needle but just be careful.
  6. Steve Best

    Steve Best Well-Known Member

    My 125 to 380 KTMs all run the same size 38-39mm carbs. I have a 34mm carb I use for where I want more low end response on any of these engines but for the most part all the intakes are dimensioned for 39mm, so I run 39mm on them. The 194cc Blaster comes with a 26mm carb and I have experimented with 28mm, 32mm and 34mm. Not a lot of gain with the larger carbs. I get over 35hp with the 26mm carb and excellent throttle response, which is what you want with a quad. No real gains with the larger carbs, mainly because the rpm and flow is pretty small.

    In order to use a larger carb you have to have intake piping, flow and internal porting sized to use it. As carb and porting cross section go up, flow speeds go down and fuel drops out of suspension. I've have great luck on the Blaster motors by filling the intake with epoxy to keep intake velocity and momentum up. A 14mm carb will power 4hp easily, so until you are beyond that, no need for a larger carb.
  7. lazylightning@mail.r

    lazylightning@mail.r Active Member

    The flat spot I'm talking about occurs "after the pipe". I have great low end power and it seems to get on that pipe pretty quick, all though I must admit it's really smooth and hard to say where "the pipe" starts. When it winds out to about 6000-7000 rpm's, it kind of sputters like it doesn't want to wind out any more, but after a few seconds it gets over that "barrier speed" and starts winding out more, alot more. I don't have a tachometer yet, but I think I'm getting 9000 plus rpms at top speed. So I'm not sure what to call this problem, perhaps an "after the pipe" lag. I don't think it's the low rpm "before the pipe" lag at all.
  8. lazylightning@mail.r

    lazylightning@mail.r Active Member

    Hi Steve,

    I've done alot to the whole engine. I'm not sure how more could be done. I will have an improved torque pipe coming up soon, and that's about it without go stroker. I got over the 4hp barrier a couple of build ago. Not sure what I got now, but I wouldn't be surprised if it's over 10 hp now. It tries to stand up on me unexpectedly at times when starting off. Great low end. It winds out to really high speeds that I never had on the previous builds and it has response and power at those high speeds. I chose the 21mm carb size because it is the maximum compatible size with a 66cc engine and a reed setup. With only one gear, higher rpm/speed-power is something that comes in handy. I posted the changes I made to the engine in this thread. There's some videos there I posted too, in addition to all the photos.
  9. jaguar

    jaguar Well-Known Member

    being "on the pipe" is when the supercharging effect happens from the return baffle wave.
    the return diffuser wave helps at RPM lower than that also which is why you are confused.
    when it sputters is right before it gets on the pipe. Lower your needle or put on a boost bottle tuned to that RPM.
  10. jaguar

    jaguar Well-Known Member

    You can use the gear RPM calculator I've included with my Boost Bottle Calculator, along with a speedometer, to determine engine RPM.
  11. lazylightning@mail.r

    lazylightning@mail.r Active Member

    I think I have added confusion to the description. What I mean by sputtering is four stroking. It is four stroking like a too rich setting will at high range. It did it terribly with the number 92 jet and couldn't over come that high speed barrier. This now only happens at very high rpm's, and it's not like the flat spot that happens before getting on the pipe. My torque pipe and timings are tuned for pretty low end power and it gets on the pipe good at low speed. On my other builds it would have to get up to about 2000 rpm's or so, but with the reed setup and booster port it's pretty much ready to deliver right away.
    When I reach a really high speed,/rpm, it kind of four strokes and the rpm begins to drop for a second, then it gets up again, and the power and rpm will cut out like the engine is dying and then it comes back in and it goes back and fourth 4 - 8 times and then it crawls past that point and slowly gains more speed where it begins to wind out and has good power and winds out alot more. But I wouldn't describe it as being on the pipe at that point. It's a new speed range that none of my other builds let me attain before.
    I definitely want to get a tachometer. I've seen alot of them on ebay now. Tiny and they cost about 15 - 25$. It just has a wire that goes to the plug wire and it senses the sparks.
  12. JunkyardDog

    JunkyardDog Active Member

    Using a carb that is too big for the engine will have serious negative effects at lower speeds, and there is now way to fix it. High CFM carbs are for either large engines, or smaller engines that run only at high speeds. Most drag racers are over carbed for slow speed use, they idle really bad and fall flat on their face if you try to take off at speeds under 5000 rpm. If you want decent low speed performance and torque, use a smaller carb. It will be responsive right off idle.
  13. lazylightning@mail.r

    lazylightning@mail.r Active Member

    I guess I should have named the thread a little more carefully. I don't have a carb that is too big for my engine. The 21mm carb is the largest in the normal size category for the 66cc engine with a reed valve. But the question is in the varying differences of needle taper and slide cut for the different carb sizes. That is from a 14mm carb to a 21mm carb (or 22mm carb for a piston port). I'm going for big power, that's for sure, but surprisingly I haven't lost any low end. It gets on the go right away at nearly a standstill - even with some unexpected wheelies, but now I also have power to wind it out to higher speeds too. The reed valve with the tuned pipe and the massive flow area with a perfect squish all seem to have alot to do with it.
    The concern in my current research is to find out what needle taper and slide cut adjustments would be made for the 21mm PHBG carb on a 66cc engine, as compared to running it on a 100cc engine. It's good to understand which way to go with that in theory first, to make it even better than it already is. I already know what to do with the slide cut. Now I'm hoping to find out more about the needle taper theory, as this has been an issue with the guys over on the scooter forum who drive 70cc rigs and also use the 21mm PHBG carb.
  14. Steve Best

    Steve Best Well-Known Member

    Needle taper is simply blending the jetting from pilot to main thru throttle movement.
    As the needle moves up its orifice gradually increases from pilot to main+ size, with the main jet as a limiter.
    The problem is this relationship is not always linear, so often a double or triple taper is required on the needle.
    In theory, all jetting could be done with a needle, as it is on the SmartCarb.

    Needles generally are sized by taper and size. Most are single taper and have various start sizes. I always like to end up with a needle with the clip in a mid position, so if I am at the end clip, I move up or down a size. Different makes use different size codes. Double taper or progressive tapers are popular too and same system, a code for the taper and a code for the size, like NOZ-A, NOZ-B, etc. You have to read the manufacturers info.

    To troubleshoot needles I tune for the mid-throttle position and then determine what the 1/4 and 3/4 positions are doing for rich or lean. this will tell me needle up or down or different taper needed. This is not a plug inspection sort of thing, it is determined by engine response, crispness or thin/heavy feeling. Sometimes I just file the needle thinner in the drillpress if I just need a little richness at one end or the other.
  15. HeadSmess

    HeadSmess Well-Known Member

    a really good thing to keep in mind when fitting large carbs to small engines...

    if the ports AFTER the carb are smaller than the carb itself...whats the point?

    the venturi in a carb is EXACTLY like an aeroplanes wing. what will a plane do if you stick a piece of timber on the end of its wing? stall. fall out of the sky. too much turbulence.

    then you have to think of velocity through the carb. if the air cant achieve maximum (nonturbulent) velocity through the throat or venturi, it cant suck up fuel.

    making ports larger to suit the carb just reduces velocity, reduces the ability to suck fuel. bernoulli effect or principle or something...

    two stroke engines have different emulsion tubes to four stroke engines, another area to think about.

    *shakes head and goes to bed*
    Steve Best likes this.