30MM KeiHein Carb Tuning
 

Hi all,

I upgraded the carb on my Tomoto/Hawk/Whatever to a 28 or 30MM carb. This was about a year ago. I had some consternation with the throttle cable no longer being long enough, and then the longer replacement having too little travel. I got it all sorted out, and have the bike tuned to where I can reliably kick-start it and jettisoned the battery. Since swapping the carb to a larger diameter and attempting to increase the jet size to the maximum that came with the bike, I can not get the thing to put out nearly as much power as I had with the stock carb. I can get up to about 6-7K RPM and then the throttle isn't wide open, maybe 1/2 or 3/4, but if I give it more gas it just makes the engine rpms unsteady, and the exhaust note becomes more cacophonous. If I hold the throttle wide open, I feel jerks as if the motor wants to take off, and then drops back down in rpm's. It's very odd, and I'm not sure if I can get a fix for this but it's bugging me as my top end has really taken a hit due to the change and I was hoping for the exact opposite effect. :hmm:
What do I do to fix this? I've already swapped around the jets and I think I've moved the circlip on the needle around too... I don't like feeling like I'm going slower than traffic and building up a line of drivers behind me, so I've not been riding much since the changeover.

I too, am turning a keihin carb on my BSR. I have a Keihin PZ30.

Look at the spark plugs see if it's running rich after WOT?

I'll pull it and snap a pic, I can't read plugs yet myself.

https://i.imgur.com/niGUT9i.jpg
https://i.imgur.com/6m3BHD4.jpg

Google the spark plug chart. You want to go for light brown color.


Also this guide is great. For a 2 stroke so some of it may not apply to our 4 stroke.


Also it is recommend to tune your main jet first for top power, since no other tuning affects top end but main jet can affect others.

that is very rich, black/darker is rich, white is too lean. you should be aiming for a straw color

I like a nice dark plug, but if the bike runs better with less throttle then you need a smaller main jet.

I'm assuming you have a PZ30 carb? What pilot and main jet are in it now? Definitely way to rich.

It is a PZ30, I had a 120 but swapped the main jet out for a 125 which it has now. The pilot I'm not sure as I've not swapped it out, but since I have a 39 and a 41 in my little baggie I would assume that I have a 40 in the carb at the moment. The idle is just fine so I haven't touched it. So I need to lean out the mixture? I can go down to a 115 main, which is the smallest I have in the pack. Should I adjust that spring clip thing for the needle first? Spark plug is a NGK DPR8EIX-9 Iridim, gap 0.0325"

On a stock engine a 120 will be to big. Not know what mods u have done I would start with a 110 or 115 main. The 40 pilot should be okay. Start with the clip in he middle on the needle. If it doesn't throttle up smoothly then ty raising the needle one notch.

Thank you! I'll do those changes and go to the 115. I don't have a 110.
One other mod I'd like to do is attach 20mm ammo cans on either side of the bike as panniers to use it for some light touring; but I'd have to get more power first. I've thought of re-routing the exhaust or swapping the muffler for something of smaller diameter so that it is flush with the rear frame of the bike. Mounting up the cans as is would require at least the right one to be offset from the frame by 2 or 3 inches, and I'd like to know if there's a mod that would move the exhaust out of the way. Would going under the seat be a bad idea as far as suspension travel is concerned?

Check Timcosby's thread on the anno can mod. http://www.chinariders.net/showthread.php?t=19000

Just swapped the 115 into the bike and it has a lot of oomph at the top end. The low end is a bit sluggish, would that suggest going down a bit on the idle screw? I put the clip one down from the top. Going to take it out for a spin and see if I can get it up to 9k again like before!