one cylinder miss at low throttle

My kz750twin has been running quite well for a few weeks since I got the crabs mostly dialed in, but about 2 days ago it began having a miss in the right cylinder at low throttle positions. It idles smoothly, and pulls great at higher throttle, but misses and sputters at very slow throttle position. Right where you hold it to say run through town at anywhere from 30 to 45 mph, it runs rough and misses a lot. Let off the throttle and it goes to a smooth idle, or give it a bit more throttle and it will smooth right out and accelerate. I don't even know where look for the reason and don't really want to pull the carbs and start from scratch again to get them running right. Any Ideas?

Do you have a service manual? In the manual it shows what you have to adjust for idle, 1/4, 1/2, 3/4 and WOT.

Perhaps carb sync is needed.

Good Fortune!

I will have to look through the digital copy I have of the manual... and borrow my friend's carb sync again. :ohmy: Thanks

Tried re-syncing the carbs yesterday... they were within spec but I got them even closer. No difference
It seems that the miss happens right about where the carb would be just going from the idle jet to the pilot jet but goes away with just a bit more throttle. I'm thinking I might just have a bit of something partially blocking the pilot jet on that carb. Will continue riding the bike as is for a while since I just don't have the time or the patience to tear into the carb AGAIN this soon and clean out the jets., and the miss happen only in such a small throttle range.

fastidiots wrote: My kz750twin has been running quite well for a few weeks since I got the crabs mostly dialed in, but about 2 days ago it began having a miss in the right cylinder at low throttle positions. It idles smoothly, and pulls great at higher throttle, but misses and sputters at very slow throttle position. Right where you hold it to say run through town at anywhere from 30 to 45 mph, it runs rough and misses a lot. ?

1) That is mostly the pilot jets range. Make sure they are clear and I would up size to 50 for the pilots, they run much better.

2) Do a physical sync on the bench: use a very small drill bit to accurately gauge the throttle plate's opening and sync the two together. That is probably good enough for sync, that's always worked for mine.

3) The hesitation you describe can also be caused by a vacuum leak, check the intake manifolds (mounts)

4) On mine, it had a hesitation at low throttle caused by air leaks in the diaphragms which caused them to not lift as quickly as they should. I missed the holes because they were fine slits that I couldn't see until I took them out and held them up to a strong light. The slits were hidden in the folds.

Just a bunch of possibilities. Good luck.

fastidiots wrote: Tried re-syncing the carbs yesterday... they were within spec but I got them even closer. No difference
It seems that the miss happens right about where the carb would be just going from the idle jet to the pilot jet but goes away with just a bit more throttle. I'm thinking I might just have a bit of something partially blocking the pilot jet on that carb. Will continue riding the bike as is for a while since I just don't have the time or the patience to tear into the carb AGAIN this soon and clean out the jets., .

That sounds like it could be the problem:

there may be an easy way to get it without teardown. Get some liquid carb cleaner and some carb spray cleaner.

Take the fuel lines loose at the top fitting to the carb. Take the drain plug out the bottom. Use carb spray to hose down from the top and clean it out. Then put the drain plug in and dump some liquid cleaner in the top and fill the carb bowls. Plug the tops and let it sit a day and then open the drains and hose it down from the top again.

That might clear it out without removal of carbs. The pilot jets and associated paths are located in the bowls on the BS-38 carbs so the cleaner will soak them.

On the 750 twins, you can (pretty easily) drop off the carb bowls without removing the carbs. It takes the right screwdriver, which I made out of parts I scrounged up (see picture). Get a bit that fits the screw head just right, get a 1/4" drive socket that fits it, then I used a couple of drive adapters and some tape to kluge it up. If you are going to keep the bike, you will want to get one of these. Dropping the bowls and cleaning them is part of the standard 6 month maintenance that prevents the kind of crappy running you have now.