Honda ST50 (like CT70) running rich or weak spark?

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Sea

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I have a UK Honda ST50 1970 it’s very similar to a CT70 K0 I believe (hope that’s welcome here!) it’s been stored for a while but started straight up, everything’s original on it.

At first it wouldn’t idle which was the idle circuit blocked, at the same time I cleaned everything in carb and fitted new seals and gaskets and set float height. It’s got the stock jets at 62 and 35.

Problem I’m having is the bike will run, but will eventually cut out and have black sooty spark plugs. If you keep trying to kick it over after it’s cut out you will get a bang from exhaust. Take plug out and clean the plug or give it a while and it will start again.

originally the bike was running without and air filter element so I added that, also new seals on intake manifold.

I think it’s running to Rich and flooding but can’t figure out why as everything seems correct?

My other theory is that the spark might be too weak and not burning turning the fuel well enough - but I have nothing to compare to, it’s never a snapping blue spark more crackling around yellow jumping round the end of plug I will get another plug today and confirm this.

I have fitted new points, condenser, wiring harness and also I had another coil which gives same result, also made sure the ground was good. Checked points gap. Also new spark plug cap after I cut a bit off to give fresh wire. I have a timing light coming today so I can get the timing bang on.

Could the coil on the stator be bad? Is there any way to test?

Just looking for ideas if anyone can think of something I’ve missed.

Thanks in advance.
 
A timing light will bear great help! Opening and closing the points will adjust the timing.

Have you checked the valve clearances?

I was initially going to say condenser but you replaced that.

How's your air filter?
 
I haven’t checked valve clearances I will have to dig out some instructions as never done that before!

The air filter is the standard box, without the inlet tube that goes up into the frame. It has an OEM air filter element that came pre oiled. I squeezed it and there wasn’t any excess. Originally the bike had no element or inlet tube on the air box and the previous owner says this is how it always was, he had it since late 70s but it hadn’t been on road since 80s.

Also got the tool to check compression on its way.
 
NGK plug heat range go like this, low numbers are cold. 6 is stone cold 7 is factory and an 8 would probably burn off the soot and keep you in business. Make sure the carb needle clip is in the middle to start the test and you can raise the clip one notch to lean out the low speed and idling and test again, it should clear the plug up as well as long as it runs clean thru the revs..
Buy a stock CR7HSA and one step hotter CR8HSA and test.
 
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So slight update - I put in a CR7HSA, started straight up as usual. Got it idling nicely, pulled off to ride down the road and it cut out and wouldn’t start again. Took plug out and it’s black again.

Still haven’t used the timing light as I need to get my hands on a 12v battery
 
For anyone interested -
I checked compression and it’s showing 125psi.
Both valve clearances were tight, smaller than 0.002. so adjusted to 0.002.
Confirmed the cam chain hadn’t skipped as this all lines up with the flywheel.
Adjusted the points again.

It’s running much better and I managed to take it for a spin down the lane.
Although it doesn’t seem to have much power (not that it should) but I could barely get above 20mph.

Anything else I should be checking? Might the clutch need adjusting? Do the carb screw adjustments have any effect on full throttle?

I will make sure the timings spot with the light when I have it.

Thanks for advise
 
when taking for a spin down the lane, if you move the choke flap slowly open or closed do you gain any more power/ run different?

as for the clutch adjusting does the kick start lever "slip" when kicking over?

2 screws on carb adjust are main idle and air/fuel mix. should not effect full throttle, what effects that is the needle clip position in the slide
 
Time to take the carb cap off pull the spring up out of the slide and remove the slide off the throttle cable to see where the needle clip adjustment is... You want it in the middle or one step higher to lower the needle in the seat.. Less fuel, =s, less soot.
 
Thanks for everyone’s help so far. I will check the suggestions above out but in the meantime I have found another issue.

I’ve realised I’m getting spark even with the battery disconnected, which includes the black jumper wire.

I’m confused as I thought the jumper wire completed the ignition circuit but I’m still getting spark with it unplugged!

The key switch still functions and when turning it off it kills the engine.

I did fit a new TB parts wiring loom previously but every seemed fine.
 
I have figured that that’s the way the new harness is, put a fresh plug in and it ran well enough to check timing and it’s spot on but after a while fouled the plug again.

Just checked oil dip stick and there is fuel in the oil, so will change that and try again.
 
if you have fuel in the oil then your float level is too high or your needle and seat are bad. Either one of these issues would cause a fouled plug as gas will just run into the intake and cylinder.
 
Thanks - if the float or needle seat were bad would it overflow the fuel into oil even if not running? Providing petcock was on?
 
yes, with a non leaking bowl gasket and high float level or bad needle/seat. the gas will flow through the main jet, into the intake, down the cylinder past the rings and into the crankcase. The only time it would not flow through the intake is if the intake valve was closed and had a 100% seal.
 
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