Carb Jetting Question

I have a 1982 KZ750 Spectre. I picked the bike up in the spring, did some minor cleaning and tweaking just to smooth things out. The bike came with an aftermarket 4 into 1 exhaust. The bike starts and runs good. It is still strong and seems to pull fine.

My questions is, should the carbs be jetted different for this type of exhaust. When I am running in 2nd and 3rd gear, it seems to backfire a fair amount on the down shift. It seems like it has excess fuel it is burning off.

Also, the bike runs excellent up around 6000 - 7000 RPMS, at the lowere rpms it seems to hesitate a fair amount on the upshift.

Any help and advice would be great.

Yes, time to buy a jet kit.

What would be the recommended jets for this type of application? Will I have to resync the carbs and adjust them after I jet it differently?

Rather than rehash the same things, check out this thread . We are covering everything there, you are welcome to join in. I even included a link on how to tune your carbs. Enjoy.

koolaid_kid wrote: Rather than rehash the same things, check out this thread . We are covering everything there, you are welcome to join in. I even included a link on how to tune your carbs. Enjoy.

I read the link provided and it is contrary to what Mikuni suggests for tuning a carb. Granted, my resource is for the HS series of carbs, but they suggest tuning from the "bottom-up" rather than from the "top-down" as suggested in your reference. Why would your system start with the main, rather than the pilot circuit as described by www.zodiac.nl/instructions/234862.pdf
Curious minds want to know!

You tune VM carbs from the bottom up. You tune CV carbs from the top down. which do you have? Stock your bike came with CV carbs. The document you linked to is for smoothbores, which are VM carbs.

Thanks so much for that tidbit of knowledge. I'll be putting that in my bag of tricks. I used the referenced link when dialing in a mikuni HS 40 on one of my flattrack bikes. Yamaha 500(ish) single. It was my first time setting up a new carburetor from scratch and found it very informative and educational.

I have VMs on my RD, Keihin CR Specials on my Suzuki. My wife's little gpz has the first set of CV carbs I've ever had apart. Thanks again for clearing up the difference. Paul

You are quite welcome. I am curious; what year RD do you have? I had 2 RD350s (73 & 74) and both had straight slide carbs.

My RD is a 78 and I only say they're vm s because they look like the vm I have on another Yamaha 500 single. Screw on caps and round slides?

O.K., you have the pre-VM carbs. I call them a slide carb because they are just that; screw on top and then you pull out the slide and needle, which has a long spring holding the cable in the slide.
The VMs were a strange animal; the top was sort of oblong front to back and quite complicated. They were actually the reason I sold my 78 GS750E after only one year of ownership. I was used to my RD where I would slam the throttle open and point to the sky. The GS would fall flat on its face because it needed vacuum to pull the fuel out of the bowl, and when you did that there was no vacuum, hence no fuel, hence no acceleration. That is why the CV came along; when you slam the throttle open, the butterfly valve opens, but the slide opens more slowly, with the vacuum through the hole in the slide pulling the diaphragm up. Much better. I still miss pointing at the sky, though.

Sorry to have hijacked the thread, but I always thought that "your" vms for the multiple cylinder bikes... you know gs kz etc were modified to accept the linkage to tie all the cylinders together. Hence the need for different tops. Thanks again. Paul