High RPM troubles

I have a 1981 kz750 with Kehien carbs. I just have a header on it. I put in Dynojet needles and 136 main jets, but above 7,000 RPM it starts cutting out.What would be the right jet size for it, or isn't jet related?

Whatever your stock jetting is you go up 2 sizes for a header and up 2 for pods. Then subtract 1 size. If you go up a total of 3 sizes (2+2-1=3), you go up 1 size in pilot jets. So you are running just a header , so you only go up 1 size, {2-1=1) from stock to start with then check your plugs and see how they are burning. I think your 136's are way too big.

You won't find much luck trying to tune with the Dynajet jet needles. Get rid of them and put the stock jet needles back in. Use a #70 main jet and #140 secondary main jet. There are no alternatives for the OEM pilot (slow) jet (#35) but the larger main covers the transition from idle to jet needle so it doesn't seem necessary.

Are jet kits a complete waste of money? I paid $115 for mine and i jot 4 sets of jets,and the needles that came with it aren't any better than the stock ones.

My comments re jetting were for Keihin CV34 with pods and 4 into 1...

As far as "jet kits", they include a wood screw, a drill bit, four "one-size-fits-all" jet needles and at least one set of main jets. The purpose of this stuff is to "recalibrate" your carburetor's air/fuel mix after changing the intake/exhaust. The drill bit is used for drilling a larger return hole in the slide (1/8" drill bit) for CV carbs (not for mechanical slide carbs) and to drill the plug on the pilot mix screw. The wood screw is to stick in the hole and to use as a pry for the plug removal. The jet needles are often the same ones used in dozens of different jet kits. These jet needles have little taper, are thin and have six slots for adjustment. They are typically chrome-look and have no markings. They are generally MUCH richer than the stock jet needles and control mixture for a much longer amount of throttle than a stock jet needle due to their length. The main jets are for wide open operation. There is a lot of conflicting info re Dynajet main jet sizing compared to stock jet sizes. In other words, a #140 Dynajet main jet may not be the same as a #140 Mikuni main jet.

MANY tuning issues that can't be resolved are based in the aftermarket jet needles. Almost invariably, I have better luck tuning with the stock jet needles. Another main downside to jet kits is that they don't provide new pilot jets. They tell you to just adjust the mixture screws which usually will not provide adequate adjustment when you put pods on a bike. The bike will be hard to start and warm slow and may have flat spots off-idle.

When you guys talk about main jet 'sizes' - is that a bump of 5, or 2.5?

So 120 to 130 is 2 sizes?

just a quick question. What if you didn't drill out the slides

I disagree on the notion to scrap the dyno-jet needles!!! a dual taper needle or slightly thinner single taper needle are the heart of the kit itself!!

carb tuning takes patience and experience.

the slide needs to be drilled for altering pressure differences caused by removing the air box.(drill bit)

the E.P.A. restrictor caps get removed so the idle mixture screws can be removed or set. (wood screw)

dyno gives you 2 or 3 different main jets for the different stages of equipment removal or installation, like pipe or pods and camshafts porting etc...

you have the biggest mains in there right? (too big )

I'd check the charging system first.
plugs and wires next


checking the main jet is a little challenging
first ride the bike and get on the main circut==
wide open throttle at or near redline steady high rev you need a load on the engine so you have to be riding it. pull the choke/enrichener that is the test

if the choke causes you to loose power you are too rich
if the choke gives you more power you are too lean

if you do not drill slides the vacuum slides open to early and cause crappy throttle response and trouble every where except slow idle

As it turned out it was because the main jets were too big. I put the 130s in and it pulled good all the way to the redline. I haven't got a chance to drill out the slides yet but that will come soon. Thanks everyone for all of your help

Post edited by: meangreenz28, at: 2007/05/22 15:15