Photos from Phil Matthews's post


Humour me , I need it!

ZXR750H1 has been playing silly buggers for a week. Runs mostly on cyl 1 and 2. have swapped plugs, and coils [same spec] from a running ZXR400- makes no difference, cleaned carbs 3 times in the ultrasonic tank, done the valve clearances they're all as close to middle of spec as possible. only done a cold compression test but the tricky cyl was 130psi, others 135, 140 and 160 from memory. Remember this is stone cold, not run for 5 days, Haynes manual says 139psi minimum but i would imagine this is a hot figure. Was going to check them hot but not got around to it. 3 and 4 cyl are not really getting warm, headers are cool enough to touch. if I pull the plug cap off either 3 or 4 while it's running i can hear it and see it sparking. If I swap the plugs and leads between 2 and 3 [it's a shared coil] then 2 still runs fine and 3 doesn't. carbs are spotless, diaphragms are good, fuel in each float chamber, have run a static fuel level too, all are correct [4-6mm below mark- see pic] carb rubbers are fine no splits. ignore the fuel pump, it's running of an auxilliary tank in the garage which is gravity fed.

So 1 and 2 run, 3 and 4 don't. If I pour additional fuel in to carbs 3 and 4 it still doens't fire properly, but smells rich. Ive set pilot mix at 1 3/4 turns out on all carbs. Tried richening up 3 and 4, no luck

Carb balance is set and very close

ign pickup coil is set at about 0.4mm gap.

So RH coil runs cyls 2 and 3. 2 runs ok, 3 doesn't. LH coil runs 1 and 4. 1 runs great, 4 doesn't.

anyone know much about the ignitors/CDI box. I'm thinking it's failed. Must be something to do with timing for 3 and 4 I reckon and not a coil driver issue, as both coils give healthy spark to 1 and 2 cyl. And it's audible/visual spark to 3 and 4 with plug cap off.

I'm hopefully going to borrow a CDI/ignitor tomorrow

%d comments
  • Get new spark plugs happened to me

  • mmm, but I've swapped plugs between cyls and ruled out it being plugs. I also have 3 complete sets and all run fine in the ZXR400!

  • Let me know what you find.mine is doing the exact thing.. I've done everything you have 3. 4 come in around 4000rpm under a load.. but just cruzin it will start missing again..I'm lost

  • mine is similar on the open road it runs on 4. I had a similar issue with a thunderace motor. tickover to 1500 rpm it would run on 3, after that 4. was a dodgy plug. usually if plugs fail they go at high revs, high load

  • My RG500 kicked into life when under load... it was a dodgy coil! Just saying!

  • Sorted! Spoke to a local guy who has one. The carb inlet rubbers were the guilty culprit. I'd checked for cracking before but they were fine and still easy to fit carbs, so not hard and I assumed they were fine. BUT the head face warps with heat. He advised crankcase sealant, was going to use Threebond until I realised it had gone off so used some Loctite 5920 I had. I tested before hand by spraying easy start near the rubbers and it picked up onto four straight away suggesting the leak. Goes some way to explain why it ran ok under bigger load/throttle. Greater vacuum to override a tiny leak. By eye they looked flat but I took his advice on the sealant and it's sorted. Just goes to show you how the tiniest of leak can cause a week of mischief!

  • I'm checking my tomorrow....cause this is hair pulling bs.. so yours is running good now?

  • yes, seems fine. you want ideally to use crank case sealant, like yama bond, suzuki bond or Threebond. I've used silicoon sealant [instant gasket] for now but if it starts again i know where to look. you're looking at an hour I suppose to get carbs off and sort it.

  • I've had my carbs of allot..kept checking for dirt.. and clogged pilot jets.. .. I hope it's that easy..this had drove me crazy for weeks

  • hope so, let us know when you tried it!