high speed miss/ hard start, 1978 b

Did you check the battery? Voltage, fluid?

Hi Hap, The jets you have in the carbs are not standard. The std pilots are 15 and mains 97 or 98. I think whoever cut the pipes put larger jets in because of less back pressure. The blackened plug on number 4 may be from fuel leaking past the enrichener. Did you replace the O'rings when you cleaned the carbs? You may have to lift the needle jets by one notch, but it looks like a fuel leak or maybe the valve seal is leaking in that cylinder causing the plug to foul out aqnd blacken at high speed. Unscrew the plug wires and nip an eighth of an inch off both ends and screw them back on the coils and the resistor caps back on the wires. I believe No. 1 and 4 wire fire the same time so you can switch them to see if the No. 4 plug still fouls or if No. 1 starts to foul. Isn't it fun to work on these? Bob

Yo Hap,
I am having the same problem with my 1976 KZ900A4. You can see the bike in the gallery under KroozR. I bought it new in 76 and until now has been running like a clock. Like you decsribe, I too am getting that high speed loss of power but if I open the throttle it acclerates fine, it just won't maintain a steady Krooz at 50 or 60, as you describe. I haven't changed anything in years, just recently rebuilt the catbs back in 2004 but it's been running fine since then and until now. At first, I thought I had water in the gas so I drained and replaced it. I ran it with Sea Foam, a gas additive, but no change. I've also been using a lead additive called 130 Supreme, (real lead, not a substitute), but I been running this too in my 1963 TR4 Triumph. I can't figure it. I does sound like it is leaning out, but why all of a sudden? It does not make sense to rejet since nothing has changed. All I can think of is a fuel passage is getting clogged and less gas is flowing, hence causing it to "lean" out? Maybe, ...but to re-jet does not seem like the answer since I've been running this set-up with the same exhaust since I bought the bike new decades ago. Please keep in touch with me so when and if you find the answer, I will know of it, I will also let you know how I make out. Many thanks in advance, Dominick (KroozR, kdls@verizon.net)

Forgive me if already mentioned.

A too high fuel level in the float bowl can cause an over-rich mixture.

Would test level using quick and easy clear tube method.

Good Fortune!

took the plugs out and looked in, on number 4 the piston is black and oily looking.
and it was hard to see but i think the intake valve had oil on it above the seat.
so i will be taking off the heads this winter to check it out.

thanks all,
i thought it was the carbs all this time.if not for the help i got here i
would still think it was the carbs.thanks for all the help.

so what exactly did it turn out to be causing the problem?

so what exactly did it turn out to be causing the problem?

KroozR wrote:

...having the same problem with my 1976 KZ900A4...bought it new in 76 and until now has been running like a clock...I too am getting that high speed loss of power but if I open the throttle it acclerates fine, it just won't maintain a steady Krooz at 50 or 60...haven't changed anything in years...rebuilt the catbs back in 2004 but it's been running fine since then and until now...ran it with Sea Foam, a gas additive, but no change...does sound like it is leaning out, but why all of a sudden? It does not make sense to rejet since nothing has changed. All I can think of is a fuel passage is getting clogged and less gas is flowing, hence causing it to "lean" out? Maybe, ...but to re-jet does not seem like the answer since I've been running this set-up with the same exhaust since I bought the bike new decades ago....

Am glad to try helping out within this thread, but creating a new topic about KZ900 carb issues, specific symptoms, and suspected conditions, may likely generate more and better responses.

Where non-carb possibilities as to ignition timing, spark quality, compression, valve clearances, etc., have been ruled out, I would suspect dirty pilot circuits (which may be still having some influence on performance in large Kaw fours at 50~60 mph where throttle may only be barely cracked.

Sometimes Sea Foam isn't enough.

Btw, what are plugs readings, and how long since changed?

Good Fortune!

Hi Patton, I really appreciate your time and expertise, many thanks for your involvement here. You should know that I have owned this bike since I rolled it off the showroom floor in 1976, it was brand new and now only has 18k miles on it. I know the history, it's a well maintained 900. Well, the plugs are brand new. I changed them a couple months age when this problem fistr occurred just to say they were new and rule them out. They were good color all tan in color, dry, but one had a liitle blackish look to it, not excessive, as if that cylinder was slightly rich. I'd say nothing to be alarmed about, none were oily. Those pilot circuits, are you speaking of the choke circuits and what would you recommend should be done about it? Sounds like the carbs should be pulled and cleaned. Damn, I just rebuilt them 4 years ago, they shouldn't be that dirty. And the tank was brand new then too, so it can't be rust. Any more thoughts? Yhanks again!

Would re-clean pilot circuits (see image below), install new in-line fuel filter, and assure fuel level to specs using clear tube method.

The carb choke (enrichener) circuit is a distinct circuit separate from the pilot circuit. It's possible to have a choke plunger not completely closing for whatever reason, but am thinking the issue at hand is more likely within the pilot circuit.

Lucky you -- and me -- for still having, riding and enjoying KZ900 bought brand new way back when.

The oem Mikuni 26mm manual slide carbs are imo among the best most user-friendly carbs ever.

Good Fortune!

[Click on image to enlarge view.]

KroozR wrote:

...plugs are brand new...good color all tan...but one had a liitle blackish look to it, not excessive, as if that cylinder was slightly rich....

Would double-check setting of pilot air screw on the carb feeding whichever cylinder has the blackish looking spark plug.

And might help to turn the pilot air screw out counter-clockwise, say 1/4~1/2 turn, for a leaner pilot circuit mixture.

Incidentally, where adjustments to pilot screw don't influence idle speed, there's likely something wrong inside the carb.

Good Fortune!

Patton, thanks for all the great tips. I'm really glad I have found a decent / knowledgeable source for troubleshooting. I have had my 900 from new and have always performed all the work on it, I even re-laced the wheels with heavier gauge stainless steel spokes, trued and balanced. I rebuilt the bike after a bad crash in 92, (see my posted photos), but never needed to make any adjustments to the Mikunis, even after I fitted it in 1977 with a 4 into 2 Jardine header / system way back when. I rebuilt the carbs in 2004 as I mentioned, to change the gaskets and clean out the 12 years of residue from sitting idle never having been run from the day of the crash until 2004. After the rebuild, it started up like it ran the day before and has been fine up until a few months ago with this hestitation problem. I need to get it resolved, but because of the cold may need to opt for the Spring to get it done. Can you send an email so I can reach you in case you miss one of my posts? (Only if needed to contact you, no hassles dude for sure). Thanks