Topic: CB500 Fouling Plugs

[jkminer]

I have a CB500 k0 that started fouling plugs about 2 weeks ago.  I read through the FAQ about fouling plugs and have done everything short of tearing off and rebuilding the carbs.  My bike is newly on the road, I pulled it out of a barn in january, and got it on the road in july.  It started recently fouling plugs in cylinders 1 and 3, so i dont think it is a timing or coil issue.  I have replaced the plugs, tried hotter plugs, leaned out the fuel mixture to 7/8 or a turn from off on each carb, removed the air filter and still foul plugs after about 10 miles.  Nothing has changed on the bike from when it ran well to now that it doesn't, I do have to run on ethanol blended gas in MO and i dont know if that has an effect or not. 

I just noticed that the bike doesn't charge apparently, i got back from a 10 mile run around town and the battery was dead when i pulled back into the garage, it wouldn't turn it over fast enough to start the bike.  Would this affect my plugs fouling

Oh the plugs are not oil fouled, no smoke from tail pipes, they are fouled with a black sooty susbtance that smells like gas and can be cleaned off with carb cleaner and the plugs reused.  Also mmy exhaust was butchered by the previous owner, 4 into 4 straight pipes with no mufflers, the piped end about 6 inches behind the drive footpegs.

thanks for any input

[number13]

Hello -

The plug fouling you are experiencing is probably caused by the carbs running rich.
Adjusting the idle screw will not do anything for this problem
Taking the carbs off and doing a complete rebuild is the only cure, and an official Rite
Of Passage for any old bike enthusiast. Do it.

Charging issues could be any number of things including a worn out battery, but
a poor electrical system could contribute to plug fouling.

best of luck!

[Deltarider]

leaned out the fuel mixture to 7/8 or a turn

That's not exactly what I would call lean out. Standard is one turn out ⁺/_- ¹/₈, so ⁷/₈ out is the richest position and 1¹/₈ is the leanest position within Honda's specs.
500's and 550's foul plugs easily at idle. Probably that's the only reason they have standard "7" plugs and not "8"s like the rest of the family.
Since yours is a K0, I suspect you have pilot screws with solid tips. Using a magnifying glass you may check to see if they're undamaged.

[Spanner 1]

Low battery voltage as you ride the bike with h/l on and no charging..'battery dead after 10 miles '....yes it will drop-out cylinders as the coil primary voltage gets lower and lower....probably solved with a working charging system........does it run on 4 cyls. with a charged battery ??

[jkminer]

It runs like a champ and then starts to pop and backfire as the plugs start to foul.  When i got back in yesterday i kick started it and it was only putting out 9 volts to the battery at idle with low beam on.  Could it be that the coils only have enough juice to fire one plug per coil and that's why 1 and 3 are fouling?

[Spanner 1]

Absolutely, none of your 4 plug-caps have exactly the same resistance and with an already low voltage to the coils the secondary voltage will be reduced enough to fail to produce spark on one or more cyls......

[jkminer]

ok, thanks, i'll go through my charging system with a multimeter and see where it's failing.  Is there a good walkthrough of that on this site?  I have a clymers manual for that and i figure that should atleast get me started.  Thanks for all the help

[IndyFour]

I have a cb550 that had the same issue for a short time right after I got it.  I ended up getting new coils and caps (mainly because the plug wires had been hacked up so bad by the PO) and that solved it.  I also did the mod where you bypass the coil power going through the kill switch by installing a light relay (it's in the FAQ section).  I don't know if it really made any difference or improved voltage to the coils all that much, but i thought it was worth a shot.  It's run like a champ ever since doing that work with no fouling.

I dont think anyone has asked yet....is your air filter clean?  If you are running a dirty filter, that will make the bike run rich and will contribute to fouling as well.  That's probably the first thing I'd check before doing anything else.  Good luck!

[jkminer]

I actually ran it without the air filter to see if that was the problem and it still fouled the two plugs.  I have new dyna coils and wires on it, the original wires were shot on it, new condensers as well.  The electrical system on the bike is rough to say the least, i have already had to rewire a fair part of it, so it looks like the charging system is next, go through it all clean everything and put it back together.  Thanks for all the help