SOHC.co.uk Forums > CB500/550
Cb500 fouling plugs once warmed up.
paul_D:
Hello
I'll describe my bike setup before the problem.
1972 cb500 (german/ netherland spec.)
649a carburetors
78 main jets
40 slow jets
1.125 turns out air screw
22mm float height (checked with tool and also a clear tube)
original air duct installed
Uni foam air filter (light dusted with K&N air filter oil)
Jardine 4-1 exhaust
d7ea ngk plugs gaped to 0.65mm
The bike starts up well with a little choke (3 degrees celsius today), slowly turn off the choke as it responds. I start riding the bike and when the bike is finally up to operating temperature (2-3 miles) the bike starts back firing/ miss firing and stalls out. Pull the plugs and I get 4 black plugs that are wet with fuel.
I believe the problem is a weak spark, because no matter what I do to the bike on the carb/intake end has any effect. I've had the filter on and off and the results are the same. The main jet set at 100 and the stock 78, no effect. Air screw adjustment does affect off idle performance, thats about it.
Any or all thoughts would be appreciated.
Johnwebley:
how does it run when hot?
can you get 8,500 rpm in 4th?
the standard air cleaner is dry,not oiled.
how is the timing?
is it retarded ??
just a few ideas to check
Bitsa (Ralph Wright - RIP):
Hi,
As with most 4 to 1 in my day ran as rich as hell.You are going to start from basics float height,every air way clear,smaller jets etc.Go find a nice stretch off road give it full bore hit the kill switch with the clutch in coast to a stop pull the plugs and see what colour they are and adjust as nessacary smaller, bigger jets etc.Frackin pain in the arse remember it well also cant remember with the 500 have you got anything blocking the air intake like the 400 vent under the seat?
Cheers
Bitsa
ps Dont know if its me but just once I wish my 750 plugs would look a bit black but they dont with fuels in the 70s lead etc they should look slightly brown impossible today confuses the crap out of me but they are the same colour as my modern xjr1300.
K2-K6:
Has it just started doing this? was there any changes that are relevant? just rebuilt? not been used etc.
An easy statement would be that it has too little oxygen to burn the fuel......if the jets are normal then you would first look at the air supply.......is the choke completely clear when not used? remove the whole airbox and try it, should run a bit lean in that spec.
As John says, have you verified the timing is normal.
paul_D:
I only manage about 1-3 miles before the plugs foul and the bike starts miss firing first at higher rpm and slow miss fires down through the rpm until it dies completely.
--- Quote from: Johnwebley on March 30, 2014, 01:34:56 PM ---how does it run when hot?
can you get 8,500 rpm in 4Th?
the standard air cleaner is dry,not oiled.
how is the timing?
is it retarded ??
just a few ideas to check
--- End quote ---
It fouls when hot. Cant get it up to 4 wide open long enough. Fouls out. Also not road legal right now.
The air cleaner is a foam unit. Foam air cleaners use oil to trap dirt and repel water.
The timing is spot on with a strobe light.
--- Quote from: Bitsa on March 30, 2014, 07:00:27 PM ---Hi,
As with most 4 to 1 in my day ran as rich as hell.You are going to start from basics float height,every air way clear,smaller jets etc.Go find a nice stretch off road give it full bore hit the kill switch with the clutch in coast to a stop pull the plugs and see what colour they are and adjust as necessary smaller, bigger jets etc.Frackin pain in the arse remember it well also cant remember with the 500 have you got anything blocking the air intake like the 400 vent under the seat?
Cheers
Bitsa
ps Dont know if its me but just once I wish my 750 plugs would look a bit black but they dont with fuels in the 70s lead etc they should look slightly brown impossible today confuses the crap out of me but they are the same colour as my modern xjr1300.
--- End quote ---
This bike (German/netherlands carburetors) does have the underseat airduct as well as 78 mainjets. The rest of the world had the same carburetors without a duct and 100s. I've tried it both ways, same result. Even tried 78s with no duct, still fouled.
--- Quote from: K2-K6 on March 30, 2014, 07:17:11 PM ---Has it just started doing this? was there any changes that are relevant? just rebuilt? not been used etc.
An easy statement would be that it has too little oxygen to burn the fuel......if the jets are normal then you would first look at the air supply.......is the choke completely clear when not used? remove the whole airbox and try it, should run a bit lean in that spec.
As John says, have you verified the timing is normal.
--- End quote ---
The bike is rebuilt.
I have the choke open, I've tried it with the air filter and tool tray removed for the maximal intake. Still rich.
I am going to try new plugs again, recheck the timing. And check the voltage drop across the coils. I think the drop is too low, leading to a weak spark that fouls quickly.
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