Hesitation on accelleration - cleaning primary jets without removing carbs?

Hi all,

I recently did a lot of work on my kz750 (clutch, head gasket, valves, rebuilt brakes ) and also got married in the middle of all that work, so the bike was sitting for a few months. Everything is back together and riding pretty well *except* I have pretty bad hesitation when accelerating from low rpm. I'm pretty confident this is the pilot system not working as expected.

Now I probably should have dismantled the carbs while they were off, but since there was a lot of other stuff that needed doing, I wanted to make sure I actually finished it, and if it was running badly then I'd have a better idea what it might be.

Now reattaching the carbs was one of the worst parts of reassembly. I have large hands, and while the carb boots don't leak ,they're pretty hard and very difficult to get back into place. It took me almost an hour just to fit the boots. So I'm trying to avoid that. I'm pretty confident that I could take the float bowl off the bottom of the carb and take out the jets and put them in the ultrasonic cleaner to clear them out, but is it likely that the fuel channels and the pilot screw could be gummed up too? Is it even worth doing to just clean the jets, or is this something that really required the full set of carbs to be removed?

Or I might be completely wrong, and it's something else wrong with the carbs - what else should I be looking for? For now I've put some injector cleaner in the fuel and I'll run a few tanks through - it's improved, but still not what it should be.

You can try that but the proper way is to take the carbs apart so you can clean them thoroughly.
If new rubbers are available then they can make your life much easier. Well worth it IMO.

Of course you are aware, it is possible adjusting the mix from the bottom, if the 'US regulation" caps have been removed(if you are in the US). Mine have been removed, and to adjust mix, I use 1/2 leg cut from a straight edge hair twezers

As GPzGuy said, all this does not replace proper cleaning, but...……….I own a '91 Zephyr 750 which was a 'retro' repackaging of the older KZ750s. The frame may be a tad different, but I have been able in the past to access and remove needles/diaphragms from the top, and also changed jets in the bottom, removing the bowls, without taking the carbs off the bike. You can also check the float height physically.

Now, I have some pluses;.... I do have a manual camchain tensioner, instead of the bulky wedge type stock automatic tensioner. The stock tensioner would not have allowed access to the carb #3 from the bottom, unless removed first. Also, long time ago, I changed the carb bowls screws from phillips to allen making turning them in tight spaces easier.-

The other thing is I used bits from a jewelry JK-6089 set and 4 mm socket, with 1/4 ratchet driver to turn jets grooves and allen bowls screws. ....all of these made it relatively easy working from the botton with very little room.

I have pretty bad hesitation when accelerating from low rpm.

What year kz750 ? 4 cylinder ? what carbs? what rpm?

I had a pretty bad stutter/hesitation at about 2 - 3000 rpm caused by blockage at the pilot bypass - 3 small holes at #3 in the diagram below. Took me months to find the problem. Took a small wire bent at 90 degrees to poke the holes, lots of carb cleaner fired through the pilot jet and compressed air to blow it all out..

IMHO you would need to remove the carbs to do this. Like That GPzGuy said new rubbers are well worth it


I have a 1980 KZ750 E1 with Keihin carbs

Well, if you have trouble getting those big hands into small places, you definitely will not enjoy trying to clean the carbs on the bike. Also, if the rubber boots are hard, replace them, and if the rubber carb holders are hard, they most likely are leaking too. It is impossible to clean the carbs on the bike. You also need to clean and inspect the diaphragms, and blow out all of the passages. Anything half done will be half A$$ed, and you will end up up with the same problems as before you did all that work.

I usually route my throttle cable and hook it up prior to sliding the rack into position. There is a procedure in the manual for carb R&R with the air box.

From '76 onwards Kawasaki used more substantial air intakes, making it more difficult to remove or install carbs. One of the pleasures of working on a Z1 is how easy it is to remove the carbs. It's so easy that I removed the stock air intake on my B4 and replaced it with a Z1 intake.

Thanks all, sounds like I'm better off taking the plunge and taking off and cleaning the whole lot. The injector cleaner has made an improvement, but I've travelled 10k on this bike so far without even touching the carbs except taking them off and back on again - it's probably time for a full clean. Everything else on the bike needed some tlc when I got it.

The early KZ 4 cylinder bikes used Keihin carbs which are a little more difficult to work on compared to Mikuni CV carbs because of the choke mechanism. I've done a bunch of sets of them though and once you figure out the idiosyncrasies they are pretty simple. Key thing is to take them all the way apart, and that includes unganging them. Do your ultrasonic cleaning job and blow through all the passages. Don't forget to remove the bleed pipes and pilot screws. Then reassemble with new O-rings. I've got all the sizes determined in case you get stuck trying to find the right sizes. Z1 sells some of these O-rings but not sure if they have them all.