V 'n H exhaust + K 'n N filter= Jetting conundrum

First off, let me say hello. I joined no more than 20 minutes ago because I just found this forum.

Secondly, I have a 1981 kz750 E2 (4 cyl) with a Vance and Hines 4to1 exhaust and I'm looking to out on a K and N high flow filter.

My problem is that right now, with only the exhaust, the bike runs through a lean spot. The only way I can give the motor enough gas is when I'm accelerating or on the highway, other wise acceleration must be abrupt or the engine sputters (an extra blip with my wrist lets more gas in).

The question I'm getting to is how much should I up-jet for the pipe and the higher flow air filter (trying to preserve the looks of the bike, pods dont do it for me.)

Other than that the bike runs great and starts like it warm every morning, I love tearing away from toll booths etc, the acceleration is intoxicating. And if anyone happens to have an '81 e2 gas tank in red (factory paint) they're willing to part with, I'm interested.

The bike shouldnt encounter a "sputter" with only an exhaust change. Usually a simple air/mixture screw adjustment takes care of that.

I would suggest tearing into the carburetors and discovering what jet numbers/needle position your running now. If someone has changed them, your solution could be just returning things to stock.

The pilot jet should be a #35
The Primary Main should be #62
The Secondary main should be #125
The needle should be a: N01A

How informed are you of how carburetion works?

GL.

I'm fairly new to the scene, (I'm only 16) But I consider myself fairly intelligent with technical details and workings. I understand the difference between flatslide and CV carbs, and most of the vacuum effects going on.

I just had the carbs cleaned by a pro mechanic (ultrasonic machine etc), he replaced a few o-rings and stuff. The bike had sat for 2 years before I bought it and the carbs were gunked, it wouldn't even start.

I haven't touched any screws (even the idle, it idles great and drops down immediately from revs). But my old gs400 had a mixture screw/idle problem but never sputtered like this. I guess I could try messing with those are they different from the pilot screws or the same one on the carb? My mechanic also suggested moving the needle clips a notch or two until I can fully rejet with the filter.

supmonkeyface wrote:

I just had the carbs cleaned by a pro mechanic (ultrasonic machine etc), he replaced a few o-rings and stuff. The bike had sat for 2 years before I bought it and the carbs were gunked, it wouldn't even start.

I haven't touched any screws (even the idle, it idles great and drops down immediately from revs). But my old gs400 had a mixture screw/idle problem but never sputtered like this. I guess I could try messing with those are they different from the pilot screws or the same one on the carb? My mechanic also suggested moving the needle clips a notch or two until I can fully rejet with the filter.

I had the same set up for a while.
The needles that came stock in the USA do not have clips or notches. You can shim the needles, but I found that it did not get me where I wanted to be.
I drilled out the plugs covering the pilot screws and set them to two turns out.
I bumped the secondary mains from 125 to 130.
Then I synchronized the carbs.
Nice throttle response, no popping with good spark plug color.

cafekz750 wrote:

I had the same set up for a while.
The needles that came stock in the USA do not have clips or notches. You can shim the needles, but I found that it did not get me where I wanted to be.
I drilled out the plugs covering the pilot screws and set them to two turns out.
I bumped the secondary mains from 125 to 130.
Then I synchronized the carbs.
Nice throttle response, no popping with good spark plug color.

There ya have it

If your engine and carbs are clean and good, then finding the "close to correct" spot with the mixture screws is easy with the Honda method (from thier FSM).
Basically, with the engine running, turn all 4 mixture screws in/out 1/4 turn. You will immediately hear a difference in the engine, the rpm will audibly rise/fall. you'd have to be deaf not to.
Then just keep turning all 4 at same time in or out, till you find the highest idle point, then thats good enough for now. You need a sensative external tach/vac guage to set any better.

A good vacuum manometer sync is considered necessary maintenence with these bikes, and they dont run well until its done.

GL!

Thanks a bunch guys. I guess I'll have to give that a try, but where are the screws on the carbs and what do I have to drill to get to them?

(yes, I'm getting a manual soon.)

Thanks a bunch guys. I guess I'll have to give that a try, but where are the screws on the carbs and what do I have to drill to get to them?

(yes, I'm getting a manual soon.)

I am not sure if you are in the USA, but in order to comply with the emissions laws, the pilot screws were covered with a lead plug on the USA bikes (along with non-adjustable needles). The lead plug and be drilled out or removed with a punch as long as care is taken not to damage the screw beneath. This plug is located on the top of the carb, between the engine and the top cap that covers the slide.