KZ750 Dead Cylinder & Backfiring Out Carbs

I have a 83 KZ750 H3 (Four cylinder) that I have been struggling with. I bought it not running. It seems the more I work on it, the further backwards I go.

I started out, after a carb clean, with cylinders 1,2,4 firing, but the bike would backfire out the carbs. #3 had almost no compression and would not fire, but the bike would sort of run.

I pulled the carbs off for another clean, got #3 to run, but then lost #4.

Compression was low, valves way out of spec. I adjusted the valves, and now have stellar compression; 120-140 from dead cold.

#4 still doesn't run, and it still backfires out the carbs. I am going to detail everything I have done to eliminate stuff I have already tried. Which is significant. The only thing I haven't done is pulled the carbs off for a 4th-cleaning. I don't want to pull them off unless I have a specific thing to check...

My definition of cleaning is removing all jets, float needle, etc. and thoroughly cleaning and inspecting them, then fishing around as best I can with dentists picks in the passageways.

- Cleaned the carb and slide 3-4 times.
- Polished slides
- Checked float levels (all spot on)
- Checked fuel flow and needle jet seal by removing bowl and testing(great)
- Checked slide movement (great)
- Checked rubber diaphragms (perfect)
- Checked choke operation (great)
- Manually triggered spark, checked all four (looks OK, all four are the same)
- Swapped Coils (no difference)
- Swapped Plugs (no difference)
- Swapped coil-ends (no difference)
- Pulled plug wire off #4 (Runs exactly the same)
- Re-did plug wire end (no change)
- Insulated wire entirely from spots it could be grounding out, and watched for arcing (nada)
- Held onto plug wire to try and find a 'leak' (None)
- Checked 300 times for vac leaks (NONE)
- Pulled idle mix needle, cleaned and reset all (no difference)
- Tried a really fast idle to see if the mains or other jets were flowing but the idle wasn't (Nope, still ice cold)
- Checked air jet passages in the top of carbs, ONE does seem to have an obstruction, but not the non-running cylinder.
- Quadruple checked cam to ignition timing
- Nothing obstructing airbox
- #1 Runs GREAT
- Checked for choke or throttle linkage interference (None)


Here is what I know:
- Plug seems to be wet with gas
- All slides are wet with gas, including the dead one
- I can not find ANY difference in spark strength between any of the cylinders
- Air jets in the top of the carb on the dead cylinder are probably the cleanest of any of the 4.
- Slide moves super freely on the dead cylinder
- All visible surfaces of carb on dead cylinder are spotless
- Have quadruple checked all boots, all are snug and I can feel\hear\see no leaks.
- Backfires, often violently, out the carbs... I really thought this was the intake valves being way out of adjustment, but it didn't go away.

Reminders:
#4 DID RUN EARLY ON, but stopped PRIOR to the valve adjustment.

Help! I am getting super fed up, and am out of things to try! I have ZERO confidence that pulling the carbs off four a fourth cleaning is going to solve anything!

OF NOTE: The advancer is busted in half. Only half the advancer is riding around, the other half was shattered into pieces behind the timing cover when I got the bike.

In searching around, I am thinking this may be a big part of the reason it backfires out the carbs? But I guess it doesn't explain why #4 is cold.

wb, have you checked cam timing? back firing thru carbs is rare, Probable causes, cam timing, bent/burnt intake valves, Ign. timing. I'm guessing you have cdi, which have 2 faults, 1 no spark & 2 ign, timing random. if cam timing is correct, compression and leakdown help narrrow field. luck G

Cam timing is correct, I have checked it a dozen times. Also, it was backfiring out the carbs BEFORE I did the valve adjustment, so I know I didn't mess it up.

As I mentioned, compression is now 120-140psi (Specifically 125-120-120-140)

Check with Jeff at www.z1enterprises.com and see if he can get an ignition advancer for your bike. Are the pick up coils in good shape or does your bike have points?

Another idea is maybe the igniton coils may check good with the plugs out of the head but are too weak to fire under compression. A new set of coils may be what you need.

I didn't think the spark looked very good at first...

I just checked that all the wiring from the pickups to the coil was correct (it is.)

I just tried again with a 100% charged battery, and all the plug wires connected, the bike is throwing BIG BLUE sparks. Very fat. So I am going to rule out coils for now.

I then realized the bike does NOT backfire out the carbs when cold\first starting, it is only after the starter has been ridden a little bit to get it going... When I started it dead cold a few minutes ago, with the fully charged battery, I got no backfiring. Granted, it was also refusing to rev up at all. Usually when the backfiring is happening, it will rev up. My guess is the spark is getting weak when the starter is ridden; maybe the stator doesn't have enough power to throw a good spark on its own. I hate electric start only bikes!

There is\was an advancer on eBay for about $30 that looked really good.

Granted, NONE of this is getting at why #4 isn't combusting...

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I just tore into the carbs again... The #1 primary main had rattled loose, and was sitting in the bottom of the carb. That probably explains why off-idle wasn't so hot. The jets did not look perfectly clean in the other three carbs; some crap is still getting in, or was not cleared out the first time I guess. The tank is rust free and I am using a filter, and the screens on the petcock were rust free, so I am not sure where the junk is coming from... Perhaps the old gas that was in the tank; there was only a small amount of gas in it, but since the tank looked so clean I never bothered to drain it. That may have been a mistake. I think I will do my best to drain off the old stuff after I really, really carefully and thoroughly clean the carbs...

Without the advancer, all I want is to see it idle!

WBPDX wrote:

Cam timing is correct, I have checked it a dozen times. Also, it was backfiring out the carbs BEFORE I did the valve adjustment, so I know I didn't mess it up.

As I mentioned, compression is now 120-140psi (Specifically 125-120-120-140)

I wager the backfiring is the broken advancer since that could cause the timing to shift upwards of 20 - 30 degrees and tht could cause it.

I think many of the problems are attributable to the advancer, and my cam timing may be off because of that. I realised this morning I drew a picture when the cams came out, but when I put them back, I followed the manual, and I think they were slightly (As in one link) off from where they came out. Perhaps the advancer threw the timing mark off a smidge, and then I put the cams back in off slightly.

I think I need to replace the advancer and recheck the cam timing.

Pin was sheered\pulled off behind the advancer. It DOES look repairable, as there is a 'hole' to put a new pin in.

Looks like that was my problem; appears I was 10-20 degrees off, but not because the advancer was busted up, but because of how the advancer was located on the crank.

WBPDX wrote:

Pin was sheered\pulled off behind the advancer. It DOES look repairable, as there is a 'hole' to put a new pin in....

Recalling a recent thread about this subject, and that oem pins are available through Kawasaki dealer and other sources.

Good Luck!

Here's the other thread mentioned in above post.

Sheared advancer pin

Thanks! That thread describes exactly what I considered my two options; hardware store or order, as I saw it was available on bike bandit.