31W-00 Carb Jetting Advice Please

[geoffers997]

Hello Forum,
I'm slowly working my way through the bike and its getting better and better, and I'm learning stuff I thought I'd long since forgotten.

I previously chopped the plugs - they were wet and sooty black. I put the needle on 3rd notch and rode it again for about 50 miles.

It goes really well up to about 7000rpm and about 70ish mph. it runs out of puff after that - no backfire / misfire or anything, it just seems to top out.

Another plug inspection and they are dry and a slightly lighter shade of black.

Question: The carbs are 31W-00, and I've made the assumption that the mains are 240 - they aren't marked though. I'll get a new set of 240's but will also get some smaller ones too - would 230 and 220 be a good choice - I don't know how the numbering system works

The ignition is all good - I've redone it all, stator, CDI and coil

Thanks in advance

[hoist1]

Not mixed throttle slides up?

[midlifecrisisrd]

31w same as 31k so 240 main

If you haven't had the emulsion tubes and brass balls out you are wasting your time 😁

Steve

[geoffers997]

Sorry I should’ve said - balls and emulsion tubes have all been out and cleaned - they weren’t grubby but got a clean anyway

[reedpete]

Easy to overlook checking the baffles are clear...had more than one occurrence where someone’s bike just wouldn’t rev out , carbs n stator etc always are first to be blamed but blocked baffle or to be more precise reduced flow due to cokeing up will literally choke it .

[midlifecrisisrd]

Are the pipes derestriced?

If they are 31w too that is

Steve

[geoffers997]

No restriction in the pipes and they’ve been cleaned thru

[abar121]

Are you sure the slides are not in the wrong carbs, as above? That sounds exactly like that.

[geoffers997]

Sorry how do you mean about the slides in the wrong carbs - are they different for each side?

It’s running evenly rich on both cylinders which makes me think it’s the jetting that’s out - everything is the same on both sides.

[stusco]

They are handed they have a slot that lines them up ,the slide bottom has a cut out on one side to allow air flow to the needle jet ,if they are in the wrong carb that doesn’t happen and could cause your problem as its like the throttle isn’t opening fully

[midlifecrisisrd]

If they are in the wrong sides the dimples won't be visible on the windows at fill throttle

Steve

[shaunthe2nd]

Are the 31k and 4L0 throttle sides the same apart from the teflon coating?

[geoffers997]

Ah, ok, thanks, got it - yes the dimples are visible (and equal) in the windows so on that basis I'm assuming the slides are in correctly

[abar121]

You could try running it without the airbox snorkel, if you are sure it's running rich.

Helps a little if anything, and usually requires going up a jet size or two.

[geoffers997]

Interesting as when I got it it didn’t have an air filter in the air box - I put one in to protect the engine.

I’ll add running without a filter as one of my tests if it ever stops bloody raining

If it’s running rich I assumed I’d go down in jest size - standard main is 240 so I thought putting in say a 230 would lean it off or does it work the other way round i.e putting a 250 in would lean it ?

Great advice and thanks

[abar121]

Going up in jet sizes adds fuel, so a 230 would be leaner than a 240 as you say.

They do run slightly rich (safe) in normal conditions, as standard, but should still pull cleanly assuming everything is stock.

Do the power valves do their clean cycle on startup and align correctly?

[midlifecrisisrd]

I've opened a few sets of carbs with 230's in them

Steve