76 KZ 900 / Fouling Plugs

Just finishing up my restoration of a 76 KZ900, however having some running issues. When I got the bike it had a 4 into1 exhaust, an pod filters. Thankfully the previous owner still had the original air box and all the parts. Bike has good compression, valves adjustment in spec. It has a Dyna S ignition and Dyna 3 ohm coils. Timing is dead on and the coils test out within spec. I replaced the exhaust with a new reproduction 4 into 4 exhaust, and reinstalled the stock airbox. Rebuilt the carbs with Keyster kits , going back to stock jetting , 115 mains, 17.5 Pilot, and needle clip in the 3 slot down as stock spec. Float height set and checked with clear tube. Mixture screws set at 1 3/8 turns out.
During the initial fire up, the bike started right up, sounded great , however after warm up and test drive, it seemed to start running poorly . Hesitation , surge off idle around 2500 RPM, runs good under acceleration. When I got back, pulled plugs ( stock NGK B8ES) and the were trashed,,,, soot black carbon fouled. I then tried a set of Hotter B7ES and some improvement . Plugs still showing some soot but a marked improvement. So what to do now? Perhaps try a set of hotter B6ES? or do I look at reducing pilot jets down to a 15 , or perhaps moving the needle clip?

With the bike previously having pod filters do you know if the carbs have had the matching air corrector jets fitted in the mouths of the carbs at the needle jet air bleed?
If they have they need removing when using the stock airbox as this will over richen the off idle mix..
It's along shot as these rarely got to the states but it's the first thing to check.
Unless you are doing plug chops at high speed the plug colour only really relates to the fuel circuit that was active at the time of shutting off the engine so that and the 2500 rpm issue would indicate the possibility of an over rich pilot circuit and I would start with trying a size down on the pilot first.
I wouldn't try to ease the symptoms by going hotter on the plugs.

I've found the Keyster kits to be the best ones sold but I've had trouble with their replacement choke plungers. They don't have the point on them (on the bottom) and that's what controls fuel flow through the choke circuit. I have a set of '76 carbs and I had trouble synchronizing them until I changed out the plungers. If you installed them you should try going back to the original plungers. If you used the original plungers then check to see that they function properly and are not introducing fuel during normal riding.

Other than that thought, have you checked your timing advancer?

I did not replace the original plungers, but certainly a good check point. Should have the smaller idle jets in the next couple days, then we will see what we have.
Yes, advancer unit working correctly, checked both static , and running with a timing light, all good.
Thanks

1 3/8 turns out is the recommended number but is that the point where the bike idles best? If the 17.5 pilots are too big and the fuel/air mixture is too rich then you might solve your problem by turning them further out.

Well finally got my Mikuni #15 Pilot Jets from bike bandit (paid $7.95 for shipping that took 9 days! ) and installed them. Fresh set of B7ES plugs. Ran great for a few miles then progressively worse , finally misfiring and barely run. Of course plugs again were soot black trashed. It gives all the signs of , like running with the choke on . So I guess the enrichment system is messed up, There is a gent in England that has new Mikuni plungers, or can refurb old ones with new viton seals. Anything else in that enrichment system that can fail?
Pretty frustrating,,,,, carbs need to come back off.

I'm the seller in the UK with the new plungers and the re-seating service.
If you decide to go that way then PM me here direct as they are around 10% cheaper without Ebay and Paypal fees etc.

Plug fouling 101
Fuel mixture Questionable
Ignition Probably OK
Basic motor Good compression ( good ) VALVE GUIDE SEALS were they replaced ?????

Valve guide seals get old and allow oil to be pulled into intake ports and foul plugs in a short period of time .
Not saying other things are not wrong .
Larry Cavanaugh used to preach on this site dont short cut looking for why a motor is not running perfect
Go thru the WHOLE list when diagnosing a running / tuning problem and the grand daddy statement is Dont assume anything . Check it
Dave

Rob,
Once I get the new choke plungers installed , I'll be putting every thing back to stock with the 17.5 pilots. The needle clip is currently in the 3rd slot (stock). I live at near sea level and wonder if I should move the clip to the second slot to lean things out little?

One thing at a time. Turn the engine so you can see the valves as they open and look for oil on them.
Steve

mrmach5 wrote: Rob,
Once I get the new choke plungers installed , I'll be putting every thing back to stock with the 17.5 pilots. The needle clip is currently in the 3rd slot (stock). I live at near sea level and wonder if I should move the clip to the second slot to lean things out little?

You should leave it in the middle position .
Only change one thing at a time otherwise you won't know what works and what doesn't.

OP,
Don't want to lead you on a wild goose chase, but have you confirmed full battery voltage at the coil primaries? The ignition switch & associated circuit can introduce significant voltage drop.

www.wgcarbs.com/index.php/using-joomla/e...-categories/89-COILS


Good Ridin'
slmjim & Z1BEBE