KZ650 Jet Needle Question

'79 KZ650 B3

So, getting the carbs right was a nightmare, but JCCWall finally got it there there. It's all laid out here:

kzrider.com/index.php?option=com_kunena&...imit=20&limitstart=0

Took WiredGeorge's advice and jetted mains #110 and Pilots #17.5 (pods and 4-1). New Dyna Green Coils, WiredGeorge coil wiring mod, and new Dyna S ignition. Eliminated the smog valve and ran a piece of vacuum hose tube to tube, and plugged where the vacuum lines went to the carb holders.

Bike runs incredibly well. At anything over 50 degrees, the bike starts immediately with no choke. After a 60 second warmup, it runs perfectly. In temperatures as low as 35 degrees, all it takes is about 1/4 choke; turn that off after 15 seconds or so, 60 second warmup, and drive away perfectly. No problem at all with hot restarts, either. They're immediate, with no throttle.

You can pull off from a stoplight at near idle; the low speed performance is perfect. Under full throttle, it pulls perfectly all the way up to redline. Thanks WiredGeorge, you were dead-on on the jetting.

The one concern is MPG. I ride the bike at 55-60 and use a throttle lock on the highway, loaf it on surface streets, short shift it, shut it off at stoplights and on long downhills or when coming up on a redlight, etc., and it still only got 36 mpg with 16/41 gearing. I just changed the gearing to 17/40, and the MPG appears to be better (can't tell for sure yet, gauges are off being rebuilt), but, my estimation is only about 40-41 mpg. I've seen people here and on KZ650.info getting up to 50 mpg, and with my hypermiler driving techniques, I think my bike ougth to be in the upper percental of MPG (although, to be fair, I am 6'5" and weigh 290).

I suspect the issue may be my jet needles. They are non- adjustable, and I suspect the bike is a little fat at mid throttle. The only way I can see to adjust that would be to go to smaller main jets, but I hate the idea of doing that, since it runs so well, otherwise.

How difficult would it be to swap over to adjustable jet needles? Is there another way to lean it out during mid-throttle?

seanof30306 wrote:

'79 KZ650 B3

So, getting the carbs right was a nightmare, but JCCWall finally got it there there. It's all laid out here:

kzrider.com/index.php?option=com_kunena&...imit=20&limitstart=0

Took WiredGeorge's advice and jetted mains #110 and Pilots #17.5 (pods and 4-1). New Dyna Green Coils, WiredGeorge coil wiring mod, and new Dyna S ignition. Eliminated the smog valve and ran a piece of vacuum hose tube to tube, and plugged where the vacuum lines went to the carb holders.

Bike runs incredibly well. At anything over 50 degrees, the bike starts immediately with no choke. After a 60 second warmup, it runs perfectly. In temperatures as low as 35 degrees, all it takes is about 1/4 choke; turn that off after 15 seconds or so, 60 second warmup, and drive away perfectly. No problem at all with hot restarts, either. They're immediate, with no throttle.

You can pull off from a stoplight at near idle; the low speed performance is perfect. Under full throttle, it pulls perfectly all the way up to redline. Thanks WiredGeorge, you were dead-on on the jetting.

The one concern is MPG. I ride the bike at 55-60 and use a throttle lock on the highway, loaf it on surface streets, short shift it, shut it off at stoplights and on long downhills or when coming up on a redlight, etc., and it still only got 36 mpg with 16/41 gearing. I just changed the gearing to 17/40, and the MPG appears to be better (can't tell for sure yet, gauges are off being rebuilt), but, my estimation is only about 40-41 mpg. I've seen people here and on KZ650.info getting up to 50 mpg, and with my hypermiler driving techniques, I think my bike ougth to be in the upper percental of MPG (although, to be fair, I am 6'5" and weigh 290).

I suspect the issue may be my jet needles. They are non- adjustable, and I suspect the bike is a little fat at mid throttle. The only way I can see to adjust that would be to go to smaller main jets, but I hate the idea of doing that, since it runs so well, otherwise.

How difficult would it be to swap over to adjustable jet needles? Is there another way to lean it out during mid-throttle?

It's possible it's running rich. If it starts with no choke, that does tend to indicate it's rich. have you tried checking the plug color after driving ont he freeway and seeing if it is tan? If it's rich, you should see some sooty carbon. If the tip is nice and tan, it's probably OK.

No offense, but IMHO the major factor is probably:

seanof30306 wrote:

I am 6'5" and weigh 290

I drive a KZ-750. I weigh about 170 and my wife weighs 90 pounds. When I used to put her on the back, the difference was like I was dragging a boat anchor. My bike weighs about 540, that makes it 710lb with me, so my wife changes the load by about 13%.

The 650 is a lighter bike and has even less torque than the 750, so I suspect that you are just getting the MPG it needs to pull the total weight of the package.

Another major factor I have found on highway mpg is wind resistance. Bigger body, more air drag. I put an upright fairing on mine and the first time I took it on the freeway it felt like I had lost half my horsepower. Air drag is huge, a streamlined fairing might help.

That's entirely possible, but, during a number of those 36 mpg tankfulls, JCCWall was riding it most of the time, and he's about 5'9", and goes 160 if he's soaking wet.

I haven't done the plug chops yet. I'm off on Monday and the forecast is good, so I'll try it then.

Also, it takes a brave man (or one who hates sex) to refer to his wife as a boat anchor.

seanof30306 wrote:

Also, it takes a brave man (or one who hates sex) to refer to his wife as a boat anchor.

Yeah... to her face:laugh:

Actually, I was likening the effect to the feel of dragging a boat anchor, it honestly surprised me considering how tiny she is.

seanof30306 wrote:

That's entirely possible, but, during a number of those 36 mpg tankfulls, JCCWall was riding it most of the time, and he's about 5'9", and goes 160 if he's soaking wet.

On the wind resistance thing, I recall some people saying that their mpg went up about 10% when they added a streamlined 1/4 fairing compared to stock. Of course, if you add a fairing with an upright windshield it's like an air brake and drags like hell.

bountyhunter wrote:

seanof30306 wrote:

Also, it takes a brave man (or one who hates sex) to refer to his wife as a boat anchor.

Yeah... to her face:laugh:

Actually, I was likening the effect to the feel of dragging a boat anchor, it honestly surprised me considering how tiny she is.

GREAT recovery, sir!

I salute you.

Sean, Sell the 650 and buy a KZ1000; I own a KZ1000E1 for my personal ride as folks who are over 250 lbs look silly on the small KZ650. My wife doesn't like to ride on the shafty as it is too small; we ride two-up on our 88 Voyager XII.

That'd kinda kill the whole economy thing, though.

So, what do you think, George, swap over to adjustable needles, or go back to #107.5 mains?

Won't make a lot of difference. Your economy is about what is to be expected. If you are REALLY interested in economy, start looking at different sprocket sets with a tall rear sprocket... will eat into acceleration but lower rpms for any speed range which is the key to "economy". You might also try and figure out if a windshield would help... one that cuts through the air. The right shield can often give you a couple mpg. Since you are not riding at the limit where max gas flow is occuring, changing jet sizes won't matter a lot... a #110 jet will flow about the same as a #107.5 jet at half throttle.

I've already swapped from 16/41 (2.56:1) to 17/40 (2.35:1). I originally went to 18/40 (2.22:1), but it was too hard to launch, and had absolutely no pulling power in high gear. The slightest upgrage, and it would immediately scrub off 10+ mph. As far as rideability, 17/40 feels a lot better.

I was thinking, since the jet needles go into the main jets, and changing the needle setting essentially lowers or raises the position of the needles in the main jets relative to the throttle position, that changing the size of the main jets would have the same effect, as far as mid-throttle operation is concerned. Obviously, the potential downside is, in doing so, I risk negatively impacting full throttle performance, when the jet needles are fully retracted. What have I missed? Since the nearest option for me to go leaner is from #110 to #107.5, is it that the needle adjustment would be in much smaller increments than that jetting change?

Also, if my carbs did have adjustable jet needles, and the plug chops came up rich, what could adjusting them net in MPG, do you think?