KZ750 4 - 1/8 Throttle Stumble

Quick history, bike was running when I got it but very poorly. Pulled the carbs and the pilot circuits were completely plugged. I've pulled the carbs a few times now and although it runs much better, I still have an issue with some stumbling at roughly 1/8 throttle. This makes it tough to cruise around 35 MPH without riding in 3rd gear and makes cornering really interesting when you can't maintain power through the turn.

These are the version with 3 fuel jets, pilot is capped with a plastic plug and fed through a passage off the main. I covered the air jet and sprayed cleaner though the passage and plugged up the pilot and transition holes alternatively and fluid sprays clearly through the hole that it left open. Floats are set, slides are clean, pass the clunk test.

Shims were checked and replaced a few to get in spec. Vac sync is done.

Here is my thought. Idle is fine, colortuned just inside to Bunsen blue. 1/4 throttle and up it pulls very hard. From my research, it looks like the pilot jet feeds the idle hole at idle but as you open the throttle, you uncover the two transition holes which are also fed by the pilot jet BUT not affected by the mixture screw since they are fed before that. Once you get past 1/4 throttle, the pilot circuit is out of play at that point. Since my issue is at that 1/8 throttle point, I'm thinking the pilot jets need to be bigger (or cleaned better). If I'm careful, I can usually get the stumble to go away by putting the choke on a little while riding (although I prefer not riding with my left arm under my leg). I've also noticed that the mixture screws are out 4.5 - 5.5 turns to go rich on the colortune. I did not record what jets are in there unfortunately.

What do you guys think?

Are you using the stock air box and filter? Ed

Yes, bike is stock. New filter and airbox boots. Intakes are tight. no leaks found with propane test.

What year and model bike ?
Are they Keihin CV34 carbs ?
Early KZ750/4s had Keihin and later had Mikuni

When you had the carbs apart did the diaphragms look like they're in good shape - no holes or tears? Ed

Here is my thought. Idle is fine, colortuned just inside to Bunsen blue. 1/4 throttle and up it pulls very hard. From my research, it looks like the pilot jet feeds the idle hole at idle but as you open the throttle, you uncover the two transition holes which are also fed by the pilot jet BUT not affected by the mixture screw since they are fed before that. Once you get past 1/4 throttle, the pilot circuit is out of play at that point. Since my issue is at that 1/8 throttle point, I'm thinking the pilot jets need to be bigger (or cleaned better). If I'm careful, I can usually get the stumble to go away by putting the choke on a little while riding (although I prefer not riding with my left arm under my leg). I've also noticed that the mixture screws are out 4.5 - 5.5 turns to go rich on the colortune. I did not record what jets are in there unfortunately.

Sounds like you have been really thorough but pilot problems can take several attempts.I think your reasoning is very sound. I have had exactly the same problem and know it can be extremely frustrating

If the carbs are Keihin CV34 then I would suggest first going back to the pilot circuit and removing the pilot jets. Clean each with a nylon bristle or guitar string. If you hold up a pilot jet vertically your may see a hole at the fat end but to the side. I'm not sure if this is to allow fuel from the main jet bleed or to allow one to hook out the jet with a bent pin or pick but I always make sure it is clear. You should also be able to access the main jet bleed pipe and poke

Regarding the transition holes (at #3 in the diagram below) you mention 2 holes. There are 3 in mine and I have had to poke each with a bent pin or wire to make sure all were clear.

Only the combination of carb clean spray, thorough poking with wire or bristle or 50lb nylon fishing all followed by through blowing with the compressed airline at thelocal gas station solved my problem.

Other than that I would suggest looking at the fuel float bowl service level using the clear tube method and checking the fit of the pilot plugs.
Check for holes in diaphragms - long shot

4.5 -5.5 turns out on the mix screws seems huge. I think mine are somewhere in the region of 2 - 3

If your carbs are Mikuni BS34s then best disregard the above

Good Luck

JR wrote: What year and model bike ?
Are they Keihin CV34 carbs ?
Early KZ750/4s had Keihin and later had Mikuni

Sorry, 1982 KZ750 LTD with Keihin carbs.

650ed wrote: When you had the carbs apart did the diaphragms look like they're in good shape - no holes or tears? Ed

Yup, they look great. Slides have been cleaned and polished up. Pass the "clunk" test.

JR wrote: Regarding the transition holes (at #3 in the diagram below) you mention 2. There are 3 in mine and I have had to poke each with a bent pin or wire to make sure all were clear.

You are correct, three not two.


Sounds like I need to go back through them once again. I feel good about previsous cleaning (Pattons method, compressed air, etc etc.) but maybe something is just missed. The carbs were full of water when I got the bike and everything brass was green.

Check fuel levels with clear tube?

The 1/8 throttle stumble on mine was being caused by fine (almost invisible) slits in the diaphragms which made the slide lift a little bit late.

bountyhunter wrote: Check fuel levels with clear tube?

The 1/8 throttle stumble on mine was being caused by fine (almost invisible) slits in the diaphragms which made the slide lift a little bit late.

Yup, levels are within spec. I stretched the diaphragms out and looked them over pretty good the last time they were out. I've posted about this before, the slides were the focus prior. The issue has gotten much better but that stumble is still there.

bountyhunter wrote: Check fuel levels with clear tube?

.

If not already done and if carbs off the bike then why not ? It certainly wouldn't hurt and might even help. It ceetainly helped me with smoother running

Looks like the OP has already done this

jayrodoh wrote:
Here is my thought. Idle is fine, colortuned just inside to Bunsen blue. 1/4 throttle and up it pulls very hard. From my research, it looks like the pilot jet feeds the idle hole at idle but as you open the throttle, you uncover the two transition holes which are also fed by the pilot jet BUT not affected by the mixture screw since they are fed before that. Once you get past 1/4 throttle, the pilot circuit is out of play at that point. Since my issue is at that 1/8 throttle point, I'm thinking the pilot jets need to be bigger (or cleaned better). If I'm careful, I can usually get the stumble to go away by putting the choke on a little while riding (although I prefer not riding with my left arm under my leg). I've also noticed that the mixture screws are out 4.5 - 5.5 turns to go rich on the colortune. I did not record what jets are in there unfortunately.

What do you guys think?

The carbs on my twin work the same way. The second system is called the "pilot bypass" and is fed by the pilot jet but the mix screw does not affect it.

Dumb question:

Have you tried adjusting the needle?

On mine, I actually adjusted that "stumble" away by gradually lowering the needle until I felt the stumble and then raising the needle just enough to get rid of it. On mine, I have five adjustable slots and I have some tiny (thin) stainless flat washers that are 1/2 the distance between slots which gives me a +/- 1/2 slot resolution. The needles will cause a stumble right where the throttle opens about 1/8 because that's where the slide just starts to raise. I would try raising the needle maybe .010" - .020" and see if it improves.

The needle and pilot jet size are "interactive" as they both work in that region. I sized my pilot jet as follows: check running in first gear and very gradually open the throttle as little as you can and walk the RPM's up from 1000 to about 2500 as slowly as you can and verify smooth running. If the pilot Is too small, the engine will run weak and stutter, if too rich it will surge. Just right and you should be able to hold smooth running at any RPM in that range.

NOTE: Stock pilot on mine is 45, but it does run better with 50 even with stock airbox. I think the bikes were set up lean to reduce emissions. Anyway, jets don't cost much so you might buy the next sizes up and shotgun the problem until you find the ones that run best.

The only other idea is maybe to soak the carbs/bowls in cleaner in case something inside is still plugged?

jayrodoh wrote: I can usually get the stumble to go away by putting the choke on a little while riding

That definitely means it's running lean. When mine was new, it also had weak running that got better with a little choke because my California engine came with 40 pilots (the rest of the world got 45) and it was so lean it would never run right. You might try next bigger pilots and see if it improves.