Carb questions for KZ750H2-idle, jetting, top end

The bike: 1981 KZ750H2 LTD.

The history (very brief): I bought the bike 10 years ago. The seller said it wouldn't idle and he was adjusting the idle with the throttle cable at the handle bars. So I thought "Easy fix once I get it home!". I offered him less than he was asking and he took it. I rode it home, set the idle and wow, the idle shot to 4000rpm and hung there. I haven't been able to get the bike to idle until this spring. It turned out to be the intake boots. The screws for 1, 2, 4 intake boots were loose. I removed all of the boots, used sealer when I replaced them and used lock-tite on the screws. I would really like to finish this bike.

The current symptoms: The bike has a wandering idle. Somewhere between 1500 and 2500rpm depending on how I ride. 1500rpm putting around, 2500rpm after some spirited riding. The idle is not even, you can hear cylinders speed up and slow down. You can sit on the bike and watch the idle increase and decrease.

This spring: I checked the valve clearances, exhaust valves are all at 0.006" and the intake are all at 0.004". Cold compression (had carbs off for valve adjustment) all cylinder were 125psi +/-5psi. Fuel level is 4mm to 3mm from the bottom of the carb body for all 4 carbs. On the carb sync all cylinders were balanced and pulling 20cm of vacuum at 1200rpm. The bike has the stock carbs with stock jetting; the air screws are 2 turns out. I have the stock air box with a UNI foam filter and a 4-1 V&H megaphone system. I removed the emissions equipment. I checked the voltage to the coils and I am loosing ~1.5 volts from the battery to the coils; I would like the link for the coil relay mod. I am planning on checking the advancer this weekend.
Fixing the idle has changed the power band for the bike. It stronger in the midrange now and is not very thrilling above 7500rpm. You can hear it struggle.
Suggestions for jetting changes with this set up? What about pods instead of the airbox?
Thanks for the help,
Joe

Caf,

When you removed the emissions gear, did you plug the vacuum hose connections well? Have you checked everywhere elese for vacuum leaks? Also, do the coil mod ASAP, plus inspect and lube your advancer mechanism. Do you have a fuel line filter? Is the petcock strainer clean? My guess is that one of these things will be the source of your problem.

If not, I would clean the carbs again, and pay particular attention to the condition of your rubber carb diaphrams. They could have pinhole leaks or cracks.

That said, my bikes (same engine and carbs as yours) idle at about 1200 rpm after about a minute or two, but get up to about 1500 rpm when the bike has been ridden a few miles. I do not find this to be a problem. Any idle variation greater than that would be a problem I would try to find.

Does this bike have Mikunis or Keihins? If Keihins, which jet needle does it have... what is the badge number? N01A or N100? It makes a big difference when rejetting. The Mikunis on a KZ750 are a bit more straightforward. You are running out of gas on top and need to rejet.

apeman: thanks for the response.
When I removed the emissions equipment I plugged the air box, bridge the valve cover/reed valves with tubing, and connect the 3 vacuum ports on the carbs to each other. I am planning on replacing the vacuum tubing on the carbs with fresh caps from Z1 Enterprises.
For some reason, I cannot find the document for the relay coil mod. I have put in relays on other bikes for mods done, but never for this KZ. I could figure it out, but hate to reinvent the wheel.
I have an inline fuel filter and I have not inspected the petcock recently but fuel flows well and the petcock does not leak.
I had a '80 KZ750 LTD for 2 years and 10,000 miles. It was a nice bike and enjoyed riding it. My brother-in-law liked it so much he bought it from me for what I paid for it. I have been thinking about acquiring a KZ750E model myself. What do you think about the bike? Thoughts about how it compares to an LTD?

Wiredgeorge: I was hoping you would respond.
The carbs are Keihins. I am not sure what jet needle the carbs have. I went out (I am at work) and looked at the carbs. The #1 carb at the bottom of the carb body has "1200 SC11" and a "29" in a hex indentation on front, upper part of the carb body. Where would I look for the N01A or N100?

Post edited by: cafekz750, at: 2007/06/01 11:35

cafekz750 wrote:

I had a '80 KZ750 LTD for 2 years and 10,000 miles. It was a nice bike and enjoyed riding it. My brother-in-law liked it so much he bought it from me for what I paid for it. I have been thinking about acquiring a KZ750E model myself. What do you think about the bike? Thoughts about how it compares to an LTD?

The KZ750E is sometimes labeled the "sport" version, and has a slightly more aggressive rider position (seat a few inches further back, reach to the bars a little longer and lower). The fork length is shorter as well, so the overall impression to the rider is that your face is closer to the gauges, and you have your head much lower toward the road. The steering is a bit quicker, or at least seems so. The rear tire is an 18", and less wide than the LTD, so this probably adds to the quicker steering. The seat has no rise at the rear, so you can move all around when cornering (good), and slide back when accelerating (bad). The gas tank is bigger, which is also a plus. I like it a lot, but I like the LTD too. It is good to have both. I am looking for a KZ750L, or a 1982 GPZ750 (same bike frame and components, different dress) to round out the herd.

On my ever ongoing third bike project, I am using the LTD frame and fatter 16" rear wheel, but substituting the forks and tripple tree from a KZ750E. Kind of a FrankenKZ750HEstein.

The needle jets for the Keihin carbs are N01A. I am off to check the advancer, pick up new vacuum caps and a relay.

I pulled the ignition advancer and inspected the movement. Everything appears to be fine except on the arms that swing out to advance the timing, there are "inserts" on the arms that contact the cylinder of the advancer when the bike timing is not advanced. One of these was missing from one of the arms. What do these do? How would it effect the bike running?
Fortunately, I have a parts bike with a complete advancer that is currently visiting some WD-40. I am curious if the new advancer will help the idle.

I don't think the missing pad will make much difference in your idle but do replace it with a spare and correct me if I am wrong because I just don't know for sure.

To rejet your bike for pods/pipes with N01A jet needle which provides a relatively lean mid-range compare to the other type of CV34, use #70 main jets and #140 secondary main jets (these are the real main jets; your stockers should be #125s I think and the main jets affect mid-range... these are the little fat barrel shaped jets; your stockers are #62s). There are no pilot jets alternatives. Run the mix screw 1.5 turns out to start with from lightly seated. In fact, replace the orings under the mix screws. You unscrew the mixture screw, pull out the spring, pull out the VERY small metal washer and then the VERY small oring and replace the oring. This improves seal on the pilot circuit and might just smooth your idle out.

Post edited by: wiredgeorge, at: 2007/06/02 15:31

wiredgeorge,

Nice little niche!

I replaced the advancer with the complete one and there was no real difference that I have noticed so far. I have had much of an opportunity to ride since the swap. This weekend I will have an opportunity to put more miles on.
I just remembered :blush: that I have a "exhaust tuning" kit that came with the V&H 4 into 1 that came when I bought the pipes. The reducers are 1.25", 1.5" and 1.75". I checked the bike and I have the 1.5" in the collector right now.:whistle:
So, should I use any of these reducers for the stock configuration until I get around to rejetting since I don't the pods yet? The exhaust has a 2" diameter.

Update:
I pulled the 1.5" reducer out and the bike is all the better for it. It has power from idle to redline and the idle quality is better. The power in the midrange is less but I can spin to redline without spuddering.

Post edited by: cafekz750, at: 2007/06/08 14:53