Large carb on a smaller engine tuning theory

[lazylightning@mail.r]

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,
Paul

 

[Deleted member 12676]

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.

 

[Deleted member 12676]

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.

 

[lazylightning@mail.r]

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.

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.

 

[Deleted member 12676]

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.

 

[Steve Best]

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.

 

[lazylightning@mail.r]

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.

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.

 

[lazylightning@mail.r]

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.

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. http://motoredbikes.com/threads/cheap-reed-vavle-setup.48662/

 

[Deleted member 12676]

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.

 

[Deleted member 12676]

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