Mike's CB550 Project

[Nurse Julie]

Sorry for your loss Dom.

[Oddjob]

You got any vac gauges Mike?

[K2-K6]

As Ted has noted it in another thread,  thought I'd add something to the check list.

When it's "hanging" try gently bringing the choke into operation and see if that's going to slow down the idle.

But also don't forget the basic reality check of all cylinders firing, indication of operational idle fuel jets etc (even if they've been cleaned) do this by stating it cold, run 30 sec, then switch off. Now check exhaust headers for temp and any cold cylinder.  Cheap,and easy to do, a cold one and it'll never have competent idle running, whatever you do or adjust.

As noted in thread too with John's experience, they'll run with camshaft timed out by a tooth, but you'll get problems again getting it to run correctly.

[Oddjob]

Or even better, buy a laser temp gun, dirt cheap these days and compare exhausts temps when it's running.

[Mike_Berkshire]

I thought I would check the mixture screw settings were correct this afternoon as it’s easy to do and doesn’t require removing anything significant. Something possessed me to remove the mixture screws and check they didn’t have any muck in them - they didn’t. I put them back setting them at 1 turn out. The bike won’t run now. It fired on a couple of cylinders and then decided not to fire on any at all. Spark at all plugs but only cylinder 3 plug was ‘wet’ with fuel. I’m guessing I have dislodged some crud and bunged up the carbs so yet again I’ll remove them and give a good clean and re-synch. Plugs were very sooty too.

I don’t have a vacuum gauge

[McCabe-Thiele (Ted)]

Did you fit an inline filter, it can save a lot of work?

[Mike_Berkshire]

I didn’t Ted as I have dual fuel lines from the fuel tap and I thought there wouldn’t be room. Maybe a poor decision!

[K2-K6]

I didn’t Ted as I have dual fuel lines from the fuel tap and I thought there wouldn’t be room. Maybe a poor decision!

If you wanted to do this temporarily even, you can buy more pipe and route down under the carbs, site the two filters there in clear space, then route back up to the carb inlets.

It could be a pragmatic workaround while you deal with getting residual unwanted bits out of the system.

All mechanical things being in order  ..... it's "always" fuel restriction against air supply that gives this hanging effect.

[McCabe-Thiele (Ted)]

I had to fit two of the alloy Aeroline filters, they hang partially under the carbs.

[Mike_Berkshire]

I have a pair of new aero line filters and spare fuel line tubing so when I put the carbs back on I’ll do as suggested.

[Mike_Berkshire]

I didn’t Ted as I have dual fuel lines from the fuel tap and I thought there wouldn’t be room. Maybe a poor decision!

If you wanted to do this temporarily even, you can buy more pipe and route down under the carbs, site the two filters there in clear space, then route back up to the carb inlets.

It could be a pragmatic workaround while you deal with getting residual unwanted bits out of the system.

All mechanical things being in order  ..... it's "always" fuel restriction against air supply that gives this hanging effect.

Can I check the meaning of " fuel restriction against air supply“. Do you mean something is happening to make mixture too lean when throttle is in closed position ie in the idle circuit?

[McCabe-Thiele (Ted)]

Does this PDF file help at all?

[Mike_Berkshire]

Does this PDF file help at all?

Thanks Ted, it all helps. Incidentally mine is. ‘76 K2 and I also have the hollow tip cross drilled air screw in my carbs.

[K2-K6]

I didn’t Ted as I have dual fuel lines from the fuel tap and I thought there wouldn’t be room. Maybe a poor decision!

If you wanted to do this temporarily even, you can buy more pipe and route down under the carbs, site the two filters there in clear space, then route back up to the carb inlets.

It could be a pragmatic workaround while you deal with getting residual unwanted bits out of the system.

All mechanical things being in order  ..... it's "always" fuel restriction against air supply that gives this hanging effect.

Can I check the meaning of " fuel restriction against air supply“. Do you mean something is happening to make mixture too lean when throttle is in closed position ie in the idle circuit?

Yes, exactly that.

The idle circuit, both air passage and it's paired jet act as, effectively, a tiny carburettor within the main carburettor. It gives these carb and engine the fine control necessary to have very smooth tickover and linear response as you modulate the speed at very low rpm.
The passages in tbat circuit are, by necessity, pretty small. Any impairment makes quite significant change in mixture as it's metering such tiny amounts of fuel at idle.
Ultimately any blocking of the fuel jet scuppers that fuel to air ratio that needs to be in place to work. And so, you get very annoying effects from just small or partial blocking. 
I'd expect to see the fuel jets having debris in them here.

In essence if there's air going through the air control route (the screw you adjust and it's route into carburettor vehturi) but low or no fuel being drawn up from float bowl, then it'll run lean and not want to slow down sufficient to give stable rpm. They will, when set correctly, run slightly rich at no throttle idle point. Its only very small amounts of change we are considering here, frustrating nevertheless.

[DomP]

So is that why I have a hanging idle yet also symptoms of running rich like sooty plugs and strong fuel smell?  Sounding like another carb off job