CX500 GL500 650 Global

Hi, my name is Ross and I'm from Vernon, BC. I need some help before I shoot this bike!

I have been working on a friend's 1979 CX500C for about 8 months now with only partial success. The bike had not run right for about 10 years when he brought it to me, after having been ridden in a heavy rain storm. There's a clue there, but I haven't figured it out yet.

What the bike does is idle well, and will run fairly well at high rpm (though there is an occassional miss), but won't accelerate through the transition from closed throttle to say half throttle. When you "gun it", it will bog, miss, and backfire. The plugs are dark as if running too rich, and there is no oil smoke to speak of. If I physically hold the vaccuum slides in the carbs down while applying throttle, it accelerates much more smoothly.

Also tried a second set of carbs with no success. Compression is good, valve adjustment is good, new fuel and in-line filter.

I have been through the carbs about 3 times, floats are correct, new "cut off" valves installed, carbs synched & cleaned, etc etc. The bike has had a new CDI installed, and timing / advance seems correct. I replaced the resistor in the plug caps with a solid rod and tried new plugs with no change. Spark seems weak but I have not been able to find test specs for the coils.

We finally decided to bite the bullet and check cam timing, and found that the cam chain was badly worn so that the cam was half a tooth out. I said AH HA!, then put it back together only to find that it still does the same thing, though it does run a bit better. Also replaced that nasty little mechanical seal.

My last effort was to double-up the springs in the vaccum slides. That helped, but it still won't run consistently. There seems to be way too much of a vaccuum surge when the throttle is cracked, which pops up the slides and richens it up so much it won't run. Or else the spark is failing under load.

I'm hoping somebody out there can give me a clue or two, or another idea to try. Thanks for any help!

See this thread,

viewtopic.php?f=10&t=335

One bad coil,one bad plug cap can cause these problems.Also if this is a CDI version then the CDI box may be faulty.If so there is a modern Replacement available from a company called,"Ignitech" but leave that until later.

Also check out these threads,

viewtopic.php?f=8&t=84

viewtopic.php?f=8&t=27

Thanks! I'll try the coil test. Am I correct in thinking there should be no continuity from either the CDI lead or the high tension wire to ground? Have already replaced the cap resistors with metal rods.

I'm not sure but for the coil test it's not important as you want to test it in effect out-of-circuit e.g put one end of the meter on the HT side with the plug cap off and the other side of the meter on the LT wire.If it's not somewhere between say 5Kohjs and 10 k Ohms upper or lower then there's a problem.On one of my CX I have an original coil @ 7.5k and a cheap replacement one I think was about 6k but can't remember.

Well, the coils tested reasonable, but I found some low dollar replacements and installed them. Now I have nice fat blue spark but it still bogs heavily under acceleration and seems to be running quite rich. Plug caps are good, high rpm is good, idle is ok, it's just the transition. The slides seem to just pop right up and flood it out. Any thoughts would be appreciated.
Thanks...Ross

The 1st 3rd of mixture control is all to do with the Low speed/Idle circuit in the carbs.At a certain point the Slide and the main jet/s take over.

http://www.carbparts.com/keihin/needles ... r_carb.htm

These are the small brass ended venturi in the top of the carbs.There are four,two hidden by the,"Black Banana".I use a 0.014" piece of guitar string to poke down through them and high pressure air blast after a,"Cold" white vinegar soak.

See the Carb section,

viewtopic.php?f=19&t=140

This is the hardest part of getting the carbs truly clean.If you have access to a Sonic cleaner all the better.This part of your carbs is what I suspect however it can also be electrical and a faulty CDI unit or even Advance and retard mechanism.These differ from CDI engines to TAI(Transistor Advanced Ignition) engines.

Also Air leaks at the carb boots are a common problem as are correct float heights which could also cause the symptom,

viewtopic.php?f=19&t=110

Hi Ross, I am having the same problem, my question to you is did you resolve it and what was the fix?