KZ750-Tune for redline

I have an '81 KZ750 4 cylinder with Keihen CV34 carbs. Mods to the bike are V&H 4-1 and a UNI foam air filter in the stock air box (not drilled). All valve clearances are in spec, the carbs are balanced (with a mercury manometer), smog parts removed and capped, WG coil mod.
Where I'm at:
With the stock jetting, the bike was lean up top. I drilled out the plugs over the idle mixture screws and set them at 2 turns out. Bike starts and idles great. I replaced the secondary mains (125) with 130s and wow! what a difference. There is noticeably more top end. The bike is perfect for riding around town and cruising on the highway.
The issue I am having is with revving the bike to redline (9500rpm). If I go slow, I can hit 10000rpm in first and second with out a problem. If I whack it, like you would be doing launching off the line, it flattens out at about 8000rpm and sounds like it is not hitting on all 4. Sometimes it will be fine in 1st then flatten out in 2nd or the reverse. The bike will pull over 9000rpm in top gear. 3rd and 4th gear feels like the best for power in the top end. I should add that I am also having popping from the exhaust upon deceleration.
I am assuming that I need different jets to make this happen. What would you suggest? I don't bounce the needle off the top of the tach that often, but it really bothers me knowing that if I get the urge, I will be let down.

cafekz750 wrote:

I have an '81 KZ750 4 cylinder with Keihen CV34 carbs. Mods to the bike are V&H 4-1 and a UNI foam air filter in the stock air box (not drilled). All valve clearances are in spec, the carbs are balanced (with a mercury manometer), smog parts removed and capped, WG coil mod.
Where I'm at:
With the stock jetting, the bike was lean up top. I drilled out the plugs over the idle mixture screws and set them at 2 turns out. Bike starts and idles great. I replaced the secondary mains (125) with 130s and wow! what a difference. There is noticeably more top end. The bike is perfect for riding around town and cruising on the highway.
The issue I am having is with revving the bike to redline (9500rpm). If I go slow, I can hit 10000rpm in first and second with out a problem. If I whack it, like you would be doing launching off the line, it flattens out at about 8000rpm and sounds like it is not hitting on all 4. Sometimes it will be fine in 1st then flatten out in 2nd or the reverse. The bike will pull over 9000rpm in top gear. 3rd and 4th gear feels like the best for power in the top end. I should add that I am also having popping from the exhaust upon deceleration.
I am assuming that I need different jets to make this happen. What would you suggest? I don't bounce the needle off the top of the tach that often, but it really bothers me knowing that if I get the urge, I will be let down.

I should add that I am also having popping from the exhaust upon deceleration.

Mine only does that when the pilot jets are too small. YMMV

The issue I am having is with revving the bike to redline (9500rpm). If I go slow, I can hit 10000rpm in first and second with out a problem. If I whack it, like you would be doing launching off the line, it flattens out at about 8000rpm and sounds like it is not hitting on all 4.

Fuel flow? Are the fuel levels in the bowls right? Also, are you getting enough CFM flow throught the fuel filters?

I replaced the secondary mains (125) with 130s and wow! what a difference. There is noticeably more top end. //// The bike will pull over 9000rpm in top gear. 3rd and 4th gear feels like the best for power in the top end.

I might try the next larger main jet just to see if it improves. Engineers like to turn knobs.. when we turn one and something good happens, we always keep turning it until we see something bad happen.:laugh:

Buy some #140 secondary main jets and #70 primary main jets and the mid-range to redline will be right. There are no alternative slow jets. The ones sold in "kits" by a parts seller WILL NOT FIT or work right. The stock #35s are about it. I have seen #38s in the past but they are rare. With the #70 main jets, the larger main jet will perk up the off idle transition and you won't notice the small slow jets much anyway.

bountyhunter wrote:

... Engineers like to turn knobs.. when we turn one and something good happens, we always keep turning it until we see something bad happen.:laugh:

:lol: Us non-engineers do that too! :woohoo:

bountyhunter wrote:

Fuel flow? Are the fuel levels in the bowls right? Also, are you getting enough CFM flow throught the fuel filters?

I forgot to add to my initial post that I checked the fuel level last year and it was correct. The fuel filter is was purchased at the same time and the tank is clean and I have inspected the petcock screen and it is clean as well. I am sure that I am getting full flow.

wiredgeorge wrote:

Buy some #140 secondary main jets and #70 primary main jets and the mid-range to redline will be right. There are no alternative slow jets. The ones sold in "kits" by a parts seller WILL NOT FIT or work right. The stock #35s are about it. I have seen #38s in the past but they are rare. With the #70 main jets, the larger main jet will perk up the off idle transition and you won't notice the small slow jets much anyway.

Cool. I know that you suggest the 70/140 combo for those running pods and pipes, but didn't know if that combo would be too rich for the stock air box with an aftermarket (foam) filter. The foam filters must flow more air that I thought.

On a side note, it sure is exciting to tune carbs for full throttle riding. Yee-haw!

I can't say how much air an airbox flows with home made element. You are gas starving on your way to redline... The larger main jets will help a lot and I think perhaps larger mains but that is a guess. Never tried your combo of Uni and pipes. If you can pull redline cleanly with the #130s leave them alone and just try the larger main jets.

Thanks for taking the time to respond. To clarify, it is not a homemade filter, it was purchased from Dennis Kirk. Here is the link to what I am taking about.

UNI Foam Air Filter at Dennis Kirk

I was able to get close to redline with the 130s but it wasn't pulling hard or as hard as I think it should.

These carbs are more work to tune than my Mukini's on my FJ. I really like those carbs.

cafekz750 wrote:

didn't know if that combo would be too rich for the stock air box with an aftermarket (foam) filter. The foam filters must flow more air that I thought.

I've got the same kind of foamie in my stock airbox and it runs best with stock main jets. I tend to doubt just changing to foam increases the flow a whole lot.

Not familiar with those filters so I can't say how they will flow but they claim they flow more than the stock element. If you have problems getting to redline with the 130s, try the 140s.

The issue I am having is with revving the bike to redline (9500rpm). If I go slow, I can hit 10000rpm in first and second with out a problem. If I whack it, like you would be doing launching off the line, it flattens out at about 8000rpm and sounds like it is not hitting on all 4. Sometimes it will be fine in 1st then flatten out in 2nd or the reverse. The bike will pull over 9000rpm in top gear.

So, if you rev the motor up slowly, it's fine, but if you yank the throttle open, it falls on it's face before you reach redline?

You realize the only difference between those two scenarios is the rate of fuel flow to the carbs?
Off the cuff example with fictitious numbers :

Opening the throttle slowly uses 1 gallon per hour.

Yanking the throttle wide open uses 2 gallons per hour.

The fuel flow is obviously not enough to handle the two gallons per hour scenario, so you have a restriction between the tank and the carbs. Take the fuel filter off and replace it with a single piece of fuel line and see if that fixes the problem, if it don't, then take the petcock apart and see if it has trash in there restricting the fuel flow.

What's happening is you are running out of gas under rapid acceleration, but not when accelerating slowly, so it's a fuel restriction, not a jetting problem.

steell wrote:

So, if you rev the motor up slowly, it's fine, but if you yank the throttle open, it falls on it's face before you reach redline?...What's happening is you are running out of gas under rapid acceleration, but not when accelerating slowly, so it's a fuel restriction, not a jetting problem.

That is an interesting idea. The way I was approaching the problem from the start was that I did not have the jetting correct.
I rode the bike today to work so on my way home I will try to rev the bike to up to 10000rpm slowly and again by whacking the throttle. This way, I will know for sure that this is what is happening.
I am planning on taking the KZ out of service for the next week or so (waking up another bike from its winter slumber). I will check the petcock screen for trash. I will remove the inline filter (checking it for debris) and replace it with a straight shot of fuel line. I will rev the bike up and see what happens.