Jet Needles / Main Jets / Gas Mileage

Each slot is .5mm wide. if the spacing was .75mm each divider would only be .25mm. I have never seen them that thin.

Yea, I think 1mm is right. I probably eyeballed it crosseyed.

If you have to move clip positions all the way up or down your needle to get it to run right then you need to obtain the next richer or leaner needle or go up or down one size on the jet that the needle is overlapping at the problem throttle position.

I changed the shims under the jet needles from 4 mm. richer than stock, down to 1mm. richer. The bike was definitely too rich before. It runs much more crisply now, still with good power. I still have the Main jets at 2 sizes richer than stock. Pulls really hard at WOT.

The only thing that puzzles me, is that the gas mileage is still the same, at about 43 MPG, as it was with the 4 mm. shimming. I have made the same run at least 50 times, and can usually predict the mileage within 5 MPG. The way I was riding, it was averaging in the low 50s before with stock Main jets and jet needles in the original single clip position. And I was riding easy - not over 1/4 throttle. I am surprised that 1 mm. hurt the mileage that much. Maybe there is something else going on, but I don't have a clue what it might be. Any ideas out there?


Thanks for all the replies.

WABBMW wrote:

I changed the shims under the jet needles from 4 mm. richer than stock, down to 1mm. richer. The bike was definitely too rich before. It runs much more crisply now, still with good power. I still have the Main jets at 2 sizes richer than stock. Pulls really hard at WOT.

The only thing that puzzles me, is that the gas mileage is still the same, at about 43 MPG, as it was with the 4 mm. shimming. I have made the same run at least 50 times, and can usually predict the mileage within 5 MPG. The way I was riding, it was averaging in the low 50s before with stock Main jets and jet needles in the original single clip position. And I was riding easy - not over 1/4 throttle. I am surprised that 1 mm. hurt the mileage that much. Maybe there is something else going on, but I don't have a clue what it might be. Any ideas out there?


Thanks for all the replies.

If the airbox is still on the bike and the exhaust system is stock, you shouldn't need to go up 2 on the main jets.

You might try leaving the needles where they are and going back down to the main jets you had before.

The thing with fine tuning jetting is you have to try and see what's best.

Did you change the pilot jet? I thought it was responsible for cruising fuel supply.

The airbox is completely stock, and has been all along. The boots are in good condition and not leaking. The air filter is almost new.

I changed the pilot jets several years ago, from 37.5 to 40.0, and the gas mileage still averaged 53 to 57 MPG afterward.

I modified the exhaust slightly, but this was a couple of years ago. Gas mileage both before and after this modification stayed the same 53 - 57. The motorcycle did seem slightly lean after the MOD. All I did to the stock 4 into 2 system, was to weld in a smooth "Y" fitting between the pipes from cylinders 1 & 2, and also between 3 & 4. The original had an almost right-angle intersection where the pipes meet. Also the pipe exiting the "Y" toward the muffler is about 1 3/4". Mufflers have stock baffeling.

My gas mileage with the BS32 carbs always seemed to be higher than most other KZ650s, and I attributed this to what I thought was very lean jetting when stock. Then with the exhaust work, I decided to go 2 jet sizes richer on the Mains. This may have been a mistake, but I would think that it would not even come into the picture until about 3/4 throttle and above.

I still wonder if the larger Main jets are impacting the mileage and making the overall mixture richer. I have deliberately kept the throttle open no more than 1/4 at any time during this testing.

More comments and opinions are very welcome. Thanks.

The air/mixture screws can have a generous effect onmileage, have you played with them/richened them up in order to obtain a better idle?

GL!

Although, now that I think about it, since my most recent rejet where I dropped the needle clip 2 slots and went up in the mains by 2 sizes (similar to you) my fuel economy has kind of suffered, I get around 6.3 litres/100km now where I used to get around 5.something... (20 % worse) but my power is way up...maybe it is the needle after all...i digress, listen to those who know what they are doing

No, I synched the carbs and set the idle mixture about 5,000 miles ago. It idles so smoothly that if I have ear plugs in, and come to a stop, and there is any noise around such as a truck, I have to look down at the tach to see if it is still running. Haven't touched the mixture screws for a long time.

Sorry if already mentioned, but would also check fuel levels in float bowls using the clear tube test method.

An incorrect fuel level will impact otherwise perfect jetting and fuel metering.

Good Fortune!

raisng the needles, by lowering the clip(richer), and the pilot jets, screws will affect the cruseing speeds fuel milage. the main jet should have no afect.