Running on three

[gary123]

The initial start up on of my 500/4 went well, firing on all 4 but running a bit rough and petrol dripping from an overflow pipe. So I took carbs of and double checked float height's again.
Dont know why but one was way off (3). Replaced carbs, air box etc inc new filter and now No 3 wont fire at all.
If I take the air box off and put my hand over No 3 intake it strikes up till I take my hand away, do you think this is still a float height issue.

Ive double checked tappets, points, timing, plugs and leads.

[mickwinf]

possible blocked idle passages or air leak?

[K2-K6]

Sounds initially like the idle circuit jet is not flowing fuel on that carb from your description.

When you put your hand over intake it should just draw fuell through the main jet as that's it's only route to fuel, if that makes sense.

If you've got any easy start to spray down that csrb when running to see if it'll also chime in, this would confirm it's a lack of fuel through that idoe circuit too and give you a double/cross check.

[K2-K6]

Crossed over with Mick there, both thinking along same lines though.

[gary123]

Cheers guys, Ill get the easy start out the shed.

[gary123]

Well the easy start brought the cylinder in, any advice on what to do now will be gratefully accepted.

[JezzaPeach]

Hi just a couple of easy checks, maybe swap the plug with#4 and also drain the float bowl and measure, mine are all about 50ml. I used a cheap plastic water dispenser cup marked at 50ml, the petrol melts everything else plastic!
Ash’s excellent tip correctly says to drain tubes first for absolute accuracy, but a straight drain still gives an approx measure.

[K2-K6]

Seems to confirm no fuel flow through that idle jet, which ultimately means clearing it. These can unfortunately (similar for any small flow jets on most carbs) require a few times cleaning when getting them going again. Frustrating, but often typical as any small amount of debris from anywhere in the system can get stuck in such small openings.

You could try firstly winding the air adjustment screw fully inwards while running it (gently) until it has no air supply (essentially the same as putting your hand on the carb, but just for the idle circuit) to see if anything soft that's restriction in there will get going again.

Otherwise it's pulling at least that jet and also blowing through the associated air passage to bring it back to fully operational.

[K2-K6]

To be clear, closing the airscrew is just a measure to encourage the vacuum to draw any obstruction from the fuel jet on that idle circuit, if it supports running then it can be set back to desirable adjustment and monitor for running then afterwards.

[gary123]

Hi just a couple of easy checks, maybe swap the plug with#4 and also drain the float bowl and measure, mine are all about 50ml. I used a cheap plastic water dispenser cup marked at 50ml, the petrol melts everything else plastic!
Ash’s excellent tip correctly says to drain tubes first for absolute accuracy, but a straight drain still gives an approx measure.

Ive tried swapping plugs and leads, made no difference, Ill drain and measure.

[gary123]

Seems to confirm no fuel flow through that idle jet, which ultimately means clearing it. These can unfortunately (similar for any small flow jets on most carbs) require a few times cleaning when getting them going again. Frustrating, but often typical as any small amount of debris from anywhere in the system can get stuck in such small openings.

You could try firstly winding the air adjustment screw fully inwards while running it (gently) until it has no air supply (essentially the same as putting your hand on the carb, but just for the idle circuit) to see if anything soft that's restriction in there will get going again.

Otherwise it's pulling at least that jet and also blowing through the associated air passage to bring it back to fully operational.

OK Ill try all that. Thanks

[JezzaPeach]

I think my 3 and/or 4 were slow filling up due to a combination of gummed up tank cap breather (due to over filling 🙄), a slope up in the fuel pipe, and maybe float height. After steady running it seemed like the fuel level was dropping so it struggled to reach the idle jet, being higher than the main jet, and wanted to stall, so it would run on the main jet with more throttle if I stopped, and then the fuel level ‘caught up’. Also while running, after running a bit, and changing down, blipping the throttle seemed hesitant.

I’m hoping I’ve sorted the tank cap by putting a rubber tube tight against the small hole and blowing plus squirting carb cleaner into the hole.

[gary123]

Drained the bowls and there was an identical volume of fuel in each. So now Ive got the idle jet out . The hole that runs right through has some kind of restriction in it, is this how it should be, it looks like its meant to be there so I dont want to be ramming something down it.

[McCabe-Thiele (Ted)]

Try  a wire out of a wire brush there is no restrictor just a tapered hole.

[JezzaPeach]

There’s a whole debate about that, stand by! I didn’t see anything in mine as some were bunged up with deposits, and had to clear them out very gently using a kit of stainless slightly abrasive rods from eBay. If doing it again I’d use my digital vernier to measure the right rod. Some kits come with tiny brushes that are useful for the openings they fit in plus in my case the inlet. But best wait for more experienced comments.