SU Carbs

[ScottieJ]

The big o-ring should be for the float bowl, on the bottom I think. No O-ring or seal for the jet or dashpot.

[ianedwards]

Tidy, thought as much. All done now, just hope it works, fingers crossed.

[ianedwards]

Evening all - help

Right, I'm having problems with my Sj, I'm thinking it's the carb.

The stupid thing is doing my head in, it's started With the idle speed increasing on its own, now it will only Idle tidy at a high rev speed and if I try and drop the speed down on the screw it will just splutter to a halt, or the revs will just increase on its own . I can't work out what's up with it.
The choke seems OK as when I put it on the revs increase and and the lever on the carb sits properly when off. And oil it fine

It will rev absolutely fine, haven't got clue what's going on, spent ages messing with it trying to get it right this afternoon, and took every ounce of my strength to not smash it with the lump hammer I could see out of the corner of my eye. What ever I do I can't get it right.

Any ideas will be great

[ianedwards]

Hi been having a look at it again today,

I have had the air filter off,

With the air filter off and the idle screw right out, and mixture screw around 1 Turn out( any more and it won't idle at all and cut out) the engine will sit there at high revs, if I lift the piston slightly it will stumble to the verge of cutting out then pick back up or just cut out.

If I stop the engine and restart it, it will start spluttering then the revs will slowly pick up to the faster speed,

When it is running, if I put the choke on, the revs will increase significantly, and then drop back when I put it off.

Any ideas chaps, can't figure it out.

Cheers

[djc]

If you can only get it running with the mix 1 turn out, then the prob must be somewhere else as that is madly rich. Assuming right needle and spring, breather holes clear, approx the right dashpot oil (20SAE but generally quite thin, esp in this weather). Is the vac advance connected right and no air leaks after the carb chamber (carb to mani etc). It sounds like it might be struggling to get air, and then won't run at all at low revs - so revs up and that allows it to suck air in but the piston may be sticking or sticky, breather above it may be blocked.
Maybe set mix where it shoud be (good guide is 4 turns out, some here run 3 but that is rather rich) and set throttle a bit open making sure butterfly opening correctly.
I'd say generally SUs are nice simple things and if you cover the basic checks but also start with mixture about where it should be... it should work nice. You've prob seen this but the SU set-up notes from Burlen are fairly essential I think:

http://sucarb.co.uk/technical-hif-type- ... ning-multi

anyway, hope this might help just a bit

[twiss]

You've got the right spring and correct oil in the carb right?
Sounds like what I had where the piston was opening on low revs due to a crap spring and oil...

I had to have the screw set about 1 turn as well as it was throwing the mixture off

[ianedwards]

Hi, I've had the carb off, and stripped it down, couldn't find any thing obvious. Jet had a tiny bit of play in it so I swapped that for a spare one which was a bit tighter.

It's got the correct oil in it, direct from su. The piston and feels good as you push it up. Nice and smooth. I haven't got another spring to try tho. All the ports and fuel ways, etc are clear. Float etc it ok. Strange one. It was running fine before. Some times it will idle fine, splutter, then come back for a bit, splutter then cut out.

Tested the pump as well seems to have good flow to the carb.

I'll put it back on today, see how if it any better. Cheers

[ianedwards]

Morning,

I've put the carb back on this morning, set it up around 2 1/5 turns out started it with the choke on and warmed her up, it's now running lovely, idles tidy, revs tidy. Spot on. YAY. Got up to temp, fan came on, no problems yay some more..

Then noticed I forgot to put the petrol return pipe back on. So I popped it back on.

Then it started spluttering and cut out. ? Took it back off, started lovely and run tidy again. Put it back on and same thing, it won't run tidy with the return pipe on.

Any ideas what's going on, ??

Lol

[Anton]

Mine vents to atmosphere. I read others (might have been Jordi, but don't quote me on that) had the same trouble when they tried using that as a petrol return pipe.

It's a vent, apparently, not a petrol return pipe. Petrol didn't overflow through it specifically when I put the wrong (way too powerful) pump on my SJ, it just came out of everywhere after the float bowl overflowed.

Some people put a pipe on it that goes under the truck so that petrol doesn't splash on the block *if* petrol ever does come out of it.

[ScottieJ]

There isn't a return from an SU carb, there's a fuel overflow but fuel should only come out of that if there is a problem with the carb.

This needs to be piped up but open, with the end away from anything hot.