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Discussion Starter · #1 ·
hi guys my names luke have a k reg gpz500 in that im having some issues with.
bike will on start with cylinder one ht cap plugged in if i attach the ht lead to cylinder 2 all i get is an all mighty backfire out the exhaust and bike refuses to start i have hooked up a compression tester and im geting 130psi on both cylinders so i ruling out a bent valve any ideas what could be causing this thanks in advance.
 

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hi, it's impossible to say without a lot more information as the history of bike is unknown.
how long have you had it, when was the last time it ran ok, how much work has been done on it, what was the last few jobs undertaken. age and mileage that sort of thing. could be something simple like the coil leads are the wrong way round or the flywheel woodruff key sheared. or something far more complicated.
you say it's a "K" reg so puts it @ a 1992 gen 1. these models have their own unique issues not found on later models.
without more correct info anything suggested would just be a guess and a stab in the dark. sorry.
 

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Discussion Starter · #3 ·
sorry yeah i have just acquired the bike as a non runner sadly I've just changed the flywheel and have doubled checked its not sheared the woodruff ive currently just put the carbs in the ultrasonic but ive just noticed as i have removed the pilot screws that the springs are stuck in place and no rubber o ring was present on the pilot so in thinking that its hopefully the pilot screw causing the issue does that sound plausible.
 

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with a non runner and unknown history anything is possible. but it does sound ignition related to me, you say you changed the flywheel why broken magnets ? what did you replace it with same sort or gen 2 did you convert it to run on a gen 1.
 

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The pilot screw O-ring is probably still in the carb. You can pull it out with a "hooked" paper clip.
 
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Discussion Starter · #6 ·
carb rebuikd kit turned up today ive gone through the carbs and set the new pilots to 2.5 but still having the same issue with the right ht plugged in it just back fires with only the left ht cap plugged in it starts straight up and runs and idles but only on the one cylinder
 

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Discussion Starter · #8 ·
ive just checked again both plugs are sparking nice and strong woodruff key isnt sheared ive just had the flywheel back off to check when running on one cylinder if i attach the ht lead to cylinder 2 its as if you have hit the kill switch bike shuts off could this be a short somewhere in the wiring causing this
 

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possible but I doubt it or it wouldn't spark at all on that cylinder, this bike has a 180 degree crank, 4 stroke engine that is designed to fire 2x per cycle, once on the compression stroke and again on the Exhaust stroke.

a backfire is caused when for some reason the inlet charge is not ignited in the cylinder on the power stroke, the unburned charge sits in the cylinder through the cycle until the exhaust stroke then the unburned fuel passes the valves down the exhaust pipe as it is forced out. now if the plug fires at this point it ignites all the unburned fuel and detonates in the pipe this is the backfire.
if both of your plugs are sparking either they are not sparking at the correct time or are not in the right order,
this is why I asked about your new flywheel and the woodruff key. as both issues would cause this scenario.
if the flywheel is not cut correctly or part of the trigger ring is left it makes one of the cylinders fire at the wrong time Coincidently 90 degrees early (just when the piston is half way up the stroke stopping it dead in it's tracks.

you can test (but not alter) the timing if you have the old school strobe light handy take out the centre and top flywheel case covers. turn the engine over by hand (clockwise) then mark both the "F" line and the "C" line as well as the pointer on the case. fit strobe between HT and plug on #1 cylinder (left side nearest flywheel) start engine or at least turn it over (ignition on) look at the strobe the "F" mark should be in line with the pointer. if it is swap the leads to the right #2 cylinder do the same test this time the flash should be in line with the "C" mark.
if either are off you have a timing problem caused by either the flywheel or one of the two triggers in the case.
Note. this test will also establish if each plug is firing every time it is supposed to.
EDIT. because you should get a flash on both "F" and "C" mark (on both cylinders) as the engine is revolving.
 

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Pics of the wiring and ignition coils would be helpful. If the flywheel has been swapped, 1) is it a dedicated gen1 upgrade unit, or 2) a gen2 unit that has been modified? A gen1 ignition uses only a single trigger for two sensors. If a gen2 flywheel, are you certain that the trigger(s) have been ground away and that only a single trigger remains on the flywheel? There is a thread here detailing how to modify the gen2 flywheels for gen1 use.
 

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hi, I still think you have an ignition issue does it not run at all on the right cylinder even if you disconnect the lead for the left one.
it could be the sensor for the right one has been damaged when the magnets came flying off, try swapping them over to see if the issue transfers to the left cylinder. if it does you know it's the sensors.
otherwise the next logical step is a faulty igniter, while these units are pretty much reliable they can go bad been a few over the years.
 
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