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.

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

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.

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.

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.

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.

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.

[K2-K6]

Can you physically see that any choke influence has been removed when you feel it's off?

[paul_D]

Can you physically see that any choke influence has been removed when you feel it's off?

I'm not sure I understand your question, I'll answer it two ways.

1st, with the choke applied after start up the bike starts dieing and becoming unresponsive until the choke is in the open position. The bike will not run with choke closed.

2nd, with the carbs off the bike, choke in open position the plates were horizontal and equal. (minimal interference with flow)

[K2-K6]

Yep I asked that in a bit of a clunky way....you've said what i was getting at though, i.e. are you sure the choke is off when you think it is, so shouldn't be causing these symptoms.

I know what you mean by a weak spark but I've yet to experience that result (weak spark shows as rich mixture) there's always a first time i suppose but I've usually found that if the spark exists and is on time then in general it doesn't show such a severe fault as this. worth checking through to assure yourself of not missing something.

It's a bit more difficult when it's not in front of you, but I'll try to think what else to look at. Let us now what you find with the coils.

[paul_D]

coils are new

But I think poor grounding and connections between the charging system, the reg/rec and ultimately the coils.

[LesterPiglet]

Surely if you lay the plugs on the engine and turn the engine over and get a fat spark that's all you need.
Is the advancer operating?

[K2-K6]

Looking back at your original post, I don't understand "main jet set at 100" what is that referring to?

Also, what position are the main needles set to, i.e. the circlip postion on the needle grooves. Usually five grooves with factory setting ordinarily the centre one.

[paul_D]

Looking back at your original post, I don't understand "main jet set at 100" what is that referring to?

Also, what position are the main needles set to, i.e. the circlip postion on the needle grooves. Usually five grooves with factory setting ordinarily the centre one.

Sorry, main jets were 100's. I've swapped the 100's out for 78's. The carbs are 649a which called for 78 main jets. 627b's had the 100s.

The circlip is in the 3rd. Middle clip.

Looking back at your original post, I don't understand "main jet set at 100" what is that referring to?

Also, what position are the main needles set to, i.e. the circlip postion on the needle grooves. Usually five grooves with factory setting ordinarily the centre one.

New plugs have a strong spark. The fouled plus have a very small blue/purple spark or none.

When I install new clean plugs, each cylinder does fire up and get hot.

[K2-K6]

It seems wierd that we can't collectively get anywhere with this.

So at the risk of "stating the bleedin obvious-teaching grandmother to suck eggs"  and any other number of sentiments, let's state what we have got.

Incomplete burn of fuel only happens (assuming it fires when it should) if you have the ratio of fuel and air at any given point in rpm/load at a miss-match. Black/rich plugs are too little air or too much fuel. As the fuel appears to be sensible, then it must be not enough air. so what causes a restriction?

It didn't have anything stuck in the inlet/carburetors before  assembly did it?
As oddjob says, have you seen the chokes working fully when installed on the bike?
Picking up on what Lester has posted, can you verify if the ignition advance is doing it's job?

[paul_D]

Yep its incomplete burn. I don't know what's causing it. The ignition is set correctly. Advance is working.

I've exhausted everything I could think of. What you could think of. What other forum searches could bring up.

I took it too an old bike mechanic in hopes he can find something I've missed.  Hoping its cheap and simple.

I'll post back with the findings.

[ka-ja]

Hi,
      Any chance you are loosing compression as the engine warms up?    Ken