Large carb on a smaller engine tuning theory

[lazylightning@mail.r]

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.

 

[JunkyardDog]

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.

 

[lazylightning@mail.r]

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.

Hi!
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.

 

[Steve Best]

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.

 

[HeadSmess]

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*