Guess what the problem is?

I have been having fits tuning a set of VM28 w/pump carbs. I first had a flat spot right off idle and found I had the air screws out too far. I normally turn them out 1.5 turns or so but in this case, I had to turn them out about 3/4 turns. Welllll then I was having a problem with the carbs at the top. The carbs started missing badly at 7K rpm. I FIRST swapped tanks and fuel hoses. Same problem. I had initially selected #110 main jets and tried larger then smaller main jets... SAME PROBLEM... started sputtering at 7K rpm. I stuck the carbs on a different test bike and the problem went away. What could the problem be on the first bike? I have the coil-power mod, Accel aftermarket coils, the ignition advance is clean and lubed and working good and new plug wires/caps.

Anyone care to take a stab at this... I haven't really tried any troubleshooting yet. BTW: The problem was there yesterday when I test a set of BS34s up to redline (a few times as the weather was real nice bhwhwahahaha).

My guess is the ignition advance mech. Had real problems with my KZ900. Checked everything carbs, fuel, coils, points even ignition advance (with strobe). Changed everything. Only when junked the whole ignition system and went to a DYNA electronic did it go away, just like that!!!. Ignition seemed to be advancing. Was last piece of ignition not changed when went to Dyna system. I will never run a points/mech. advance systen ever again. Plugs might fire at lower RPM, but as pressure goes up with higher engine speeds may not be able to fire properly. So I would say either advance mech. plugs or coils.

The advance worked fine yesterday as did the coils. Also, I do have a new Dyna S on the bike. The Accel coils were used and off eBay. I have a set of Dyna coils as spares and may try them. They work fine till it rains and then the cracks cause shorts. I might fill the cracks with epoxy as they work OK otherwise. The reason I don't like the advance is that the problem doesn't occur till 7K rpm. Full advance happens far lower than that and the bike runs perfect till that point... oh well... rather than sit and chat on the internet, I suspect actually working on the bike might help bhwhahaha... Thanks!

Wiredgeorge, I'm pretty sure that I don't have a solution for you, but you say that the Dyna S is new - did the bike ever run ok with the Dyna S? If anyone else reported this problem, then it might be a fuel delivery problem, but I can't imagine you ending up with that type of problem... especially after replacing tanks etc. I always liked points because, in most instances, they could be made to work, electronic ignitions are something else, and I guess they could fail at a certain RPM without affecting the rest of the RPM range.

Full advance should happen at round 3500 rpm perhaps.

Maybe the coils are getting too much juice at higher rpm - bad regulator?

Sorry to hear that you've got this problem, butI'm really interested in hearing what you figure out!

Check that the stator is giving you full out put at that high rpm and the check regulator.as they say carb problems are somtimes electrical problems. good luck

george,
i hope this helps you. i have bought 2 sets of 28's off ebay and both sets had 120 main jets in them. one set went on my 1977 kz1000, the other on a 1976 kz900. they seem to work great. good luck.

did you happen to put another set of carbs on the problem motor? just thinking , wonder if that would make a change

For my own reasons I wasn't going to post here anymore but I thought I might help a friend who has helped me in the past.

WG, it sounds like it could possibly be weak valve springs. The valves will tend to "float" at high RPM's if the springs have lost some tension. Might be worth checking if you run at high RPMs regularly.

mix
timing
spark
compression
flow

sounds like you've eliminated mix

compression is easy to check
spark is easy check
flow, partially plugged exhaust, hand in stream test...compression test would rule out flow problems on intake side unless due to valve float, you've already eliminated carbs.

timing...ignition and valve
if it's misbehaving at 7 grand, does it do this under load as well as sitting in shop out of gear?

if both, I would look at advance with a timing light and make sure it's not hopping around.
I recall reading somewhere that a little bit of twist in the crank will play havoc with advance at high RPM so if advance isn't binding....

valve timing...weak springs on one, chain off a tooth, ???

FWIW. Thoughts from a rank amateur who's only been working on bikes since the beginning of summer.

-Duck

I checked the carburetors on two bikes and the problem with the cutting out only occured on one bike. It isn't the carburetors. I never thought about the valve springs. I don't remember if I measured them when I put them in or not. I do remember that I used KZ1000 valves on the exhaust side and KZ900 valves on the intake. I can't recall which springs I used. I hadn't had this problem prior to yesterday... the bike's cam chain is new and its timing is correct... Will not get to look at it for a few days and will report back.

Yes WG, I hadn't thought of valve springs. However, I think I would swap the coils with your other (test) bike, if compatable, before pulling the head off. I guess a coil break-down at >7k or as mentioned earlier that regulator breaking down.

Come to think of it your alternator rotor could be breaking-up (separating) at high RPM, giving an erratic power supply to regulator.

I'll go for one of the (ex-EBay) coils breaking down!
You might try a quick check in a dark work-shop (at night, no lights)

Any prizes for guessing it right!
RegardZ

Not really any prizes... I just have a couple days of work before I can get to looking at the problem. I hadn't thought of testing the regulator or even the alternator. I hadn't thought about the valve springs... I had thought about the coils... Everytime I buy something electronic off eBay to save a buck, it has pretty much bitten me on the tailbone! Jeff always sniggers at me when I have to call him and buy a new part... Those darn coils cost me over $40!