high speed miss/ hard start, 1978 b

just seeing if some one can help with my problem.
it's a 1978 kz650 b
I'm running the vm 24ss carbs that came stock.
17.5 pilots, 110 mains,i didn't change the needle settings.
stock air box with new airbox rubber.
i didn't change the intake manifolds but have tested them for leaks.
the exhaust is stock 4 into 2's, but somewhere in it's life someone cut the ends
off so they have no baffles, going to get 4 into 1's someday.

heres my trouble,when cold it will not start with out starter fluid.
or i have to run it with the choke on for ever,like 10 min or more,
and then i still may have to use the starter fluid.
but after i get it running it runs great, lots of power, good idle.
till i get around 45/50 miles per hour then it starts to miss bad.
if i give it some gas it misses a little less but still misses.
the carbs have been cleaned many time, and i put in the new jets
so i know the main or pilots are not clogged.

thanks for any help

Sounds like it's running lean because the previous owner was a nitwit and gutted the exhaust. Someone on this site may be able to suggest a needle/jet combo that will help or maybe you can find a damaged stock exhaust system that you can transplant the baffles from, at least until you get the 4 into 1 you're considering. Ed

Make sure that there is correct compression, valve adujstments are within spec, timing is correct, fuel level is correct, that you have 12v to the coils, and that the spark advance device is working correctly.

If all of those things are as they should be, then what do the plugs look like, are they black or tan? When you say it misses at 45 to 50 mph at what point in the throttle movement is this. It must be remembered that carburetor jetting is determined by the throttle position, not engine speed. If the engine is having troubles at low rpm (idle to 1/4 throttle), the pilot system or slide valve is the likely problem. If the engine has problems between 1/4 and 3/4 throttle, the jet needle and needle jet (most likely the jet needle) is likely the problem. If the engine is running poorly at 3/4 to full throttle, the main jet is the likely problem.

As to hard starting - there are many things that can casue this - one being poor spark, so make sure the coils are getting 12 v. I know you say the carbs have been cleaned many times but make sure all of the passages are clean.

When it starts to miss, slowly apply the choke. If it runs better...then it is to lean.

I'm not sure about that model? But does it have air mixture screws?

Hard starting: Poor compression. Incorrect fuel levels/jetting. Dirty/clogged choke circuit. Poor idle mixture tuning. Poor spark. Very dirty/clogged air filter.

Good luck.

You'll look funny doing this and your neighbors may laugh and point, but for a test with little to no cost, you can find out if it's lack of back pressure from those hacked mufflers that is causing you grief.

Find yourself two tin cans that are just large enough to put over whatever ends are left of your mufflers. Get two large hose clamps, couple bucks if you don't have some laying around. Put a few holes in the un-opened end of the cans, cut a couple or 3 slices down from the open end if trying to fit over a tapered pipe, and clamp them onto the pipes nice and tight with the hose clamps... All you're trying to do is get the unopened end of the cans up tight to the pipe end. Start off with only a few holes and punch more with a screwdriver you take along for the ride, if need be.

If you find it's simply a lack back pressure that is screwing you over, tis easy enough to make up a set of baffles that you can bolt in, or weld, into what's left of your pipes... and port them as necessary, aka drill more holes in them.. lol

We can help with making some baffles up for your trashed pipes, if it proves to be your problem. Should do until you can afford the replacement pipes you're after.

More fun riding then sitting.

I took it out , got up to about 30 put it in 5th gear and held the throttle at about a 1/4 maybe
alittle less, slowly accelerated when the bike gets to about 45 it starts to cut out,
still holding at about a 1/4 turn. but i open it up and all the cylinders kick in and get up to 80 no problem.
i cant remember but i think when i get up to like 95 ,
no matter how much gas in giving it it starts to cut out again.
i don't go that fast too often so it's been awhile since i hit 95.
it seems not to be the same cylinders kicking out, i hear it on the left and right.
plugs look normal maybe a little Lean.

at low speed (1 to around 40 mph) it runs great, no matter what the throttle is at.

going to get a compresion tester , let you all know how that goes


a little back ground on the bike.my son in law found it in a fence row so he got it cheap.
he got it running, needed some cash so sold it to me.but it acted the same way for him.
but before he got it somebody put in a points plate from another type of bike , the rectifier
was from another type of bike.and the wiring harness was hacked on abit for the other rectifier.

so i changed.
the points plate to make it kz650b again, put in new points
the rectifier to make it a kz650b again
the wiring harness to a not hacked up one, it was from a kz650b too.

so the only thing left from when i got it is the stator and the coils.
i got an advancer when i got the points plate, cant remember if i changed it.
and like i said running 17.5 pilots and 110 mains, never touched the needle.

Thanks all.

hap1968 wrote:

I took it out , got up to about 30 put it in 5th gear and held the throttle at about a 1/4 maybe
alittle less, slowly accelerated when the bike gets to about 45 it starts to cut out,
still holding at about a 1/4 turn. but i open it up and all the cylinders kick in and get up to 80 no problem.
i cant remember but i think when i get up to like 95 ,
no matter how much gas in giving it it starts to cut out again.
i don't go that fast too often so it's been awhile since i hit 95.
it seems not to be the same cylinders kicking out, i hear it on the left and right.
plugs look normal maybe a little Lean.

at low speed (1 to around 40 mph) it runs great, no matter what the throttle is at.

going to get a compresion tester , let you all know how that goes


a little back ground on the bike.my son in law found it in a fence row so he got it cheap.
he got it running, needed some cash so sold it to me.but it acted the same way for him.
but before he got it somebody put in a points plate from another type of bike , the rectifier
was from another type of bike.and the wiring harness was hacked on abit for the other rectifier.

so i changed.
the points plate to make it kz650b again, put in new points
the rectifier to make it a kz650b again
the wiring harness to a not hacked up one, it was from a kz650b too.

so the only thing left from when i got it is the stator and the coils.
i got an advancer when i got the points plate, cant remember if i changed it.
and like i said running 17.5 pilots and 110 mains, never touched the needle.

Thanks all.

Doesn't sound like a carb issue to me. You can run without a baffle via stock jetting, so long as you dont have any air leaks on the intake side of things. The bike will lean out at higher throttle settings, and should be jetted richer, but it would not cause complete cutting out like you describe - usually thats loss of spark or sudden extreme lean/rich condition, which is rare.

Are you sure that you set dwell/timing correctly, and the advancer is fine?
Checked tomake sure coils are recieving near same voltage as battery?

A good compression check, and a peak at the valve clearances never hurts.

30 year old coils are a good suspect too for a high RPM miss.

Also check plug wires and plug wire ends.

UPDATE

I've been riding like crazy since i posted the first post.
I check the plugs after each ride, 1,2 and 3 are a nice tan color.
but 4 is black and sometimes wet, with i think oil, i don't know how dark gas gets in the cylinder
but it don't smell like gas and it looks like oil.

as far as the cutting out, it may just be the right side,
but i am almost deaf in my left ear so it's hard to tell.

if it's a little chilly out it does have a little smoke from the right side.
but other then that no smoke at all.

did a compression test , 160 psi on all 4.

and i checked the advancer, it's got both springs on it and it turns when i open it.
points are new and set dead on

the choke has to be on and chilly for it to have a little smoke from right side