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1978 kz400B. 9100mi. all stock. last November it started dying as i pulled in the clutch and wouldn't idle without me giving it throttle. if it would idle it was very poor and has intermittent dead spot. took carbs off and cleaned twice (found something in right cylinder idle circuit). still nothing changed. tried all 3 positions on petcock and nothing changed. i have let it sit but today i fired it up and it had same symptoms. i took it down the road and got it up to 70mph the let off to turn around and it didn't die, it was idling at 3K. so i revved and turned idle down to around 1500. revved again and it dropped way down. started ride back to house, pulled in clutch to hit driveway, and it still didn't die like usual. idle was low so i revved it and stopped. bike idled around 1800 then down to 1000. parked it.

known:
1) idle screw 1-1/4 turns out
2) synced carbs with homemade manometer early last year
3) good air filter
4) tank clean
5) newer coil (less than 1500mi on it
6) newer points and condenser (replaced with coil)
7) good, clean gas
newer slide diaphragms (replaced with coil)
9) rubber idle circuit plugs new
10) good plugs, gapped correctly

unknown:
1) guy who originally worked on carbs set float height (i haven't checked lately but don't know how they can randomly change on their own)
2) timing

other symptoms:
1) left cylinder seems to have random dead spot at idle, i can feel right cylinder fire with no problems by putting hand near exhaust
2) left cylinder exhaust header is yellowing, right side is still silver
3) used to start with one touch of starter button and no choke. now need full choke AND give some throttle while warming it up

it's riding season and i love this bike, i need help PLEASE!

Sounds like you got a leak around the intake.

i should have stated that i just sealed the intake boots with "shoe goo repair". read a post that it works but maybe not. haven't sprayed boots to check for leak yet though. would a leak between airbox and carb cause that much of a problem? or would it only be between carb and motor? because the airbox side of the carb is a little sketchy. rubber is 35yrs old

I would definitely try to find replacements for that old of a rubber, most likely they are showing hairline cracks or even pin hole cracks. Yes, if the boots are that old you can have leakage and mess up the fuel air flow mixture.

Try eBay.

Mike

that's what i was thinking. i tried that shoe goo stuff because it's basically a rubber sealant but who knows how well that works. would this have anything to do with the float level being set wrong causing not enough fuel to be in bowl?

i sprayed boots between carb and motor, boots between carb and airbox, top of carb where slide diaphragms seat in, and sprayed petcock air line. nothing happened. anything else you can think of?

ok so this is embarassing, the left cylinder spark plug gap somehow became completely closed. so i opened it back up to .035 and she runs perfect. amazing how i went over the basics and missed the basic of the basics. thanks for all the help!!!

That's weird. Did you check your spark plugs or something? If not, check to make sure there's nothing in the cylinder that could have hit it and closed it.

neverdie38 wrote: 10) good plugs, gapped correctly

Wrider wrote:That's weird. Did you check your spark plugs or something? If not, check to make sure there's nothing in the cylinder that could have hit it and closed it.

I agree with Wrider, that is very strange. If you are absolutely sure that you gapped it correctly and didn't drop it electrode side down on the concrete garage floor before you installed it, there's no logical reason why it should be bent closed.

Is the spark plug the correct one? If it is too long, the piston could've just barely kissed it but it would do that again once you regapped it so that is unlikely; and just about any piston contact usually results in grenading. So there could be a debris in there like Wrider said that might've hit the plug.

The other explanation I was thinking of is one that I've seen (albeit rarely) firsthand. When you put a spark plug down the hole, if the rubber of the plug socket doesn't hold it, the plug can drop and hit the electrode on the side of the hole, closing the gap.