Carbs.... Bike only runs on choke. Next steps?

[Prof Prolapse]

So I thought I'd revive my old '99 CBR600F as I refuse to give in to the temptation to ride at the moment.

It's been sat in the shed since a new engine in January 2017. Fresh oil and fluids before away, and I ran the engine until it died. My theory was this would drain all the fuel from the carbs, meaning I wouldn't have issues later, I may have been mistaken.

To get it going I first gave a petrol soaking to a ceased Chinesium fuel pump in petrol for a few days, removed a failed "Victory" branded battery (never buy one), and replaced some corroded spark plugs it then ran... But only with the choke on full. All fresh fuel of course.

You can bring the revs up, but off choke it just dies. I've adjusted the idle speed, problems seem to persist. But I know very little about carbs, so my only real option now is to remove the carbs and teach myself how to service. Frankly I can't be arsed.

Can anyone more experienced offer a more expert diagnosis?

Something like, absolute guess, but presumably carbs restrict airflow on choke, could I perhaps have a poor seal somewhere on the carb to engine side letting too much air in?

[Krikkit]

Sounds like a partially clogged jet - as you say, when the choke is reduced it'll lean out the mix which will cause it to stall.

Sitting for 3 years I'd bet the pilot jet is gummed up with crap old fuel. Remove and soak/clean the carbs up, if you have a compressor give the needles a blow through to clean them up if you can.

[black-k1]

Krikkit said:

Sounds like a partially clogged jet - as you say, when the choke is reduced it'll lean out the mix which will cause it to stall.

Sitting for 3 years I'd bet the pilot jet is gummed up with crap old fuel. Remove and soak/clean the carbs up, if you have a compressor give the needles a blow through to clean them up if you can.

This, almost definitely.

[Prof Prolapse]

Really appreciate it guys. Having read a bit more that seems a good starter for ten, but sadly doesn't negate the need for a full clean out. Hardly seems worth cutting corners once it's out though. I may as well clean them all up and balance them in a oner.

I'll look at the Haynes big book of lies and see what it looks like to take apart all four carbs. I do have a compressor.

Sadly combined with three substantive coolant leaks, one oil rocker cover gasket, one questionable generator seal repair, one frame spacer needing a "special tool", and with every single panel in need of replacement I'm tempted to cut my loses on the spark plugs and leave it until at least winter. Or retirement.

That's before we get to the suspension and front calipers needing rebuilt....

You really could drive yourself daft fixing old bikes couldn't you?

Edited by Prof Prolapse on Tuesday 28th April 15:30

[Krikkit]

Funny you should say that, I'm looking at my garage thinking I need a project to sink loads of time into

[s1dew1nd3r]

Just get the carbs off and get them cleaned/sorted should be reet.

[julian64]

a) If you are needing to have the choke on to make it run, its because its likely running too lean, and adding the choke just brings the mixture within tolerable for the engine to run.

b) If i were jets then it'd still run lean due to little fuel so they would be similar.

The difference would be opening the throttle with the choke on. If it revs up then likely to be a) If it dies fairly immediately with throttle then likely to be b)

If a) its air getting through a an old dry gasket or such downstream from the butterflies. If (b) the carb needs to come apart for jets/sticky float needle etc.

[Steve Bass]

Prof,

As above... Pilot jets.

Remove the carb bank, turn upside down and remove 1 bowl at a time.
The Pilot jet is the one that isn't the Main jet that the needle jrt lowers into. They really are that simple.
The choke is the sprung brass barrel in the top of the carb body up by the slide tube.
If you can, get a set of jet wires/brushes and a service kit (bowl gaskets etc) Doddle to do and you'll see/find a couple of the Pilots looking blocked with a varnish type material. Poke it out, make sure everything else looks tickety boo and reassemble on the bike.
Start the bike, enjoy the bike and say thank you

As an aside, remember that carbs are a cumulative device. The fueling from each jet adds to the previuos. The Pilots come in first and actually provide far more fueling than most realise. Then it's the needle jet and finally the Main jet for the WOT areas. (Normal carb, not specialty items like a Keihin VJ 22 carb...)

[Rubin215]

Steve Bass said:

As an aside, remember that carbs are a cumulative device. The fueling from each jet adds to the previuos. The Pilots come in first and actually provide far more fueling than most realise. Then it's the needle jet and finally the Main jet for the WOT areas. (Normal carb, not specialty items like a Keihin VJ 22 carb...)

Farkin 'ell!

Finally someone else on here who actually knows how carbs work!

[rigga]

I do ...... I do



But then 40 year old 2 stroke and you kind of have no choice.

[crofty1984]

Rubin215 said:

Farkin 'ell!

Finally someone else on here who actually knows how carbs work!

My bikes are 51 and 49 years old - no idea on these modern fancy pants constant velocity carbs with their lah-de-dah chokes. Just tickle it then kick it.

[Turn7]

Nice big can of Carb cleaner and strip them down.

Carb cleaner is brilliant for manky carbs, but its also phenominal at making you aware of EVERY.SINGLE.CUT on your hands.......

[julian64]

Rubin215 said:

Steve Bass said:

As an aside, remember that carbs are a cumulative device. The fueling from each jet adds to the previuos. The Pilots come in first and actually provide far more fueling than most realise. Then it's the needle jet and finally the Main jet for the WOT areas. (Normal carb, not specialty items like a Keihin VJ 22 carb...)

Farkin 'ell!

Finally someone else on here who actually knows how carbs work!

cringe

[Prof Prolapse]

Thanks all. Two last questions if you please...

If I've understood, and the reason the choke is still allowing the engine to run if the jets are blocked, because the fuel-air mixture is sufficient to combust... The fuel pump is notably a weak spot here. Would that not cause similar symptoms?

Secondly, I've looked at some carb cleaning tools on ebay but they're all stainless steel rods. Are the jets not brass? Will it not scratch the bejezus out of them?

[Krikkit]

Prof Prolapse said:

If I've understood, and the reason the choke is still allowing the engine to run if the jets are blocked, because the fuel-air mixture is sufficient to combust... The fuel pump is notably a weak spot here. Would that not cause similar symptoms

I prepare to stand corrected by the carb gurus, but you barely need any fuel flow at all to keep it idling, if the bowls are full(ish) it'll keep running.

But yes your thoughts on the mix are correct.

Prof Prolapse said:

Secondly, I've looked at some carb cleaning tools on ebay but they're all stainless steel rods. Are the jets not brass? Will it not scratch the bejezus out of them?

Sounds like they could do, unless you're very delicate with them.

[Moulder]

These are excellent for carb. cleaning, and come in a range of jet sizes.



I would also check the fuel tap as for me (on older bikes) this has caused a fuel restriction meaning they run better on the choke.

[Steve Bass]

Moulder said:

These are excellent for carb. cleaning, and come in a range of jet sizes.



I would also check the fuel tap as for me (on older bikes) this has caused a fuel restriction meaning they run better on the choke.

The stainless jet brushes are perfect, as long as you don't do a psycho with them.

Re the running, the choke will provide fuel to run under no load. if you try to rise it it just won't pull. it might be the fuel pump but consider that the amount of fuel into the carbs is determined by the pump. The amount of fuel into the engine is determined by the jets. If it runs on choke there's a good likelihood the restriction is in a carb or two.
Secondly, if the bike is carb'd, the fuel pump is a low pressure item not 3 bar + as in an EFI system. If the motor runs on choke then there's probably plenty fuel getting into the bowls.
Also check the integrity of the slide diaphragms, as a split or tear may also cause similar symptoms.

Let us know

Edited by Steve Bass on Wednesday 29th April 11:14

[julian64]

Prof Prolapse said:

Thanks all. Two last questions if you please...

If I've understood, and the reason the choke is still allowing the engine to run if the jets are blocked, because the fuel-air mixture is sufficient to combust... The fuel pump is notably a weak spot here. Would that not cause similar symptoms?

Secondly, I've looked at some carb cleaning tools on ebay but they're all stainless steel rods. Are the jets not brass? Will it not scratch the bejezus out of them?

I've never put a drill, wire or anything down a jet for fear of buggering it. A long time ago I bought one of those ultrasonic cleaners and just fill it full of de-gunk/carbclear, or when desperate petrol. it turns them back to showroom.

Years ago when I thought I knew better than the manufacturers I drilled jets to enlarge them but invariably the machine was worse

[Steve Bass]

Krikkit said:

Prof Prolapse said:

Secondly, I've looked at some carb cleaning tools on ebay but they're all stainless steel rods. Are the jets not brass? Will it not scratch the bejezus out of them?

Sounds like they could do, unless you're very delicate with them.

No, they're fine. You need something strong to dislodge the varnish that can form from dried out fuel. And if you look at the jet rods, they're made of differing diameters so they fit nicely into the jet bore. And to damage brass with these you're doing something mental.
FWIW, I cleaned out the carbs on my CB400 Bol'Dor with a set bought from Webike and they are excellent.
The dental brushes can be too flimsy to clean out a really gummed up or completely blocked jet but are fine for gum or soft muck.

[Prof Prolapse]

Cheers Mr Bass.

I'm still looking for some between work and other jobs, but only place I have found so far is fleabay and, same as everything else taking forever at the moment, it's looking like mid-may/June before delivery. That "Webike" seems to be in Asia somewhere, so perhaps no faster.