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1990 Kawasaki EX500
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Bike Info: 1990 EX500, 25040 Miles California Spec

Hi folks, need some experience opinions on this issue:

My 1990 EX500 starts up fine, rides smoothly for a good 10 minutes then idle, and throttle response all go to hell. My power basically gets cut in half and I become unable to rev past 7000RPM, Idle then fluxes between 1200 and 2400 RPM then it finally dies after alot of backfiring. Originally I diagnosed it to be a couple of issues which I resolved, had bike running fine for a short while then have same symptoms come back again. (See below) The bike only acts like this when hot. If It dies on me I can just let it cool down for an hour or two I can just hop on it again and it'll start/ride for another 10 min cycle.

Obviously this isn't a great way to commute around town so I'd like to see if this is something I have the ability to work on, or If I should just hand the bike on to someone else. Oddly enough any time the bike wasn't running correctly the symptoms were the exact same. I purchased this bike from a folk that had "fixed" the bike up only to discover that a lot of the work was done incorrectly and have spent months going between riding and wrenching on the bike. I'm genuinely lost, I've done everything I could think of and these symptoms still occur.


What I've done in order:
First Diagnosis: Carbs running rich, fouled plugs
  • Full carb clean and rebuild (Floats, Jets, Everything)
  • Spark plugs (Gapped at .025)
  • Ignition coils
  • Ignition wires (Shortened, unknown length but equal)
Second Diagnosis: Vacuum lines done incorrectly
  • Redo all vacuum lines to OEM spec (Excluding Petcock)
  • Rebuilt carb again (to be safe)
  • New Spark plugs (Gapped at .025)
  • New Petcock (Not OEM, Missing Vacuum port)
  • Vacuum seal in gas cap removed (read somewhere on this forum that it chokes gas flow before)
Third Diagnosis: Magnets in Flywheel/Starter clutch destroyed. (Caused my timing to be off all the time and the bike to hesitate on startup, still have no clue how this happened)
  • Replaced Starter Clutch
  • Replaced Flywheel
  • Rebuilt carbs again to be safe
  • Repaired thread in Crank (whoever worked on my bike before put in a right hand tight bolt instead of a left one |: )
  • New Spark plugs
Fourth (Now)
  • New Air Filter
Bike is back to OEM spec besides the petcock, Plugs were not fouled when I pulled them. I can post videos of the bike tomorrow afternoon. But gotta survive midterms first :p
Also excuse me if I use incorrect terminology I mainly work on cars and this is both my first bike and carb vehicle, so I may need pointers if something is incorrect

- Regards Matt
 

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I don't see a valve clearance check on the list. or a fuel level wet test (to determine if both floats are full correct and even) the key here for me is runs fine (cold) for 10 mins.
this would suggest it's either something that closes down as the metal in the engine warms up (possibly valves) or settings or 10 mins is about the right amount of time for the fuel in the bowls to empty so it appears to run out of fuel also check the flow rate of the aftermarket pet cock some are too slow and cannot keep the float bowls full also happens when you have a restrictive fuel filter.
 

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Could also make a short ride with the tank cap cracked open. These old bikes have fuel tank venting issues.
 

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07 Ducati SS800 '95 Ducati 900SS/SP '19 Honda CBR650R
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Yeah, I'm a bit late to the party with this problem guys but from my experience these bikes are notorious for fuel starvation "sometimes"usually on full throttale and in a high gear... Experienced this so many times in the past with my bike. Open the petrol cap, leave for a minute or so, then start gently. Don't keep pressing the Start button continously, because the float bowls "may" have emptied and the vac tap simply needs to refill with petrol before the engine will start again - which it always will. The other issues for less experienced riders of these older machines is to remember to knock off the choke once you get going. The engine runs a little lean for a mile or so, but rather that than forgetting and then experiencing the aweful chugging, spluttering and sound of a dying engine :)
 
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