Weird carb trouble

[matt993fod]

Hey there folks

Need to pick your brains; the jet on my Moggy's carb was leaking so I swapped it for a nice new replacement from a carb service kit. It now doesnt leak, and everything seems ok in that department.

Problem is, I can't seem to get the damn thing running well. Having put the carb back on the car, I've adjusted the mixture as per the old Haynes manual. Have also set the idle to 900rpm, and had a look at the fast idle adjuster as well, just in case.

The idle, however, is all over the place. If you have been driving along, and stop (for traffic lights, for example) the engine will idle at about 1300-1400rpm, and will only settle down to 900rpm very slowly.

Even more strange, with the engine hot or cold if you pull the choke out, rather than idling faster, the revs die away very rapidly and the engine runs incredibly roughly, dying if you don't ram the choke back in sharpish! If the engine is cold, and you try to start it with the choke out, it won't start at all. Will only fire into life (very reluctantly) when the choke is fully in. This doesnt bode well for winter!

Not a clue what is up. Any ideas would be most welcome!

[callyspoy]

Although most probably not a cure for all problems, have you checked the oil in the dashpot? That can cause all sorts of erratic behaviour (although the choke issue baffles me).
Also, assuming you removed carb to do the jet, have you checked for air leaks at the carb to manifold/spacer?

[mike.perry]

Have you checked that the piston is dropping smoothly on to the bridge?
Is the choke fully closed?
Is the accelerator closing fully?
Is the fast idle screw just clear of the choke cam?
Check that the cables have a little slack

[bmcecosse]

All covered well above...... My money is on tight cable -and 'choke' fast idle screw not set..

[matt993fod]

Hey folks; thanks for the ideas

So far, I've:

Checked the dashpot oil. Seems fine.

Checked that the piston hits the bridge cleanly. Seems fine; gives a nice metallic clunk whether the jet is lowered or fully up.
Fast idle screw seems to be clear of the choke cam. Indeed the fast idle seems to work ok. The revs only seem to drop when I pull the choke out further.
The throttle seems to be closing ok, and both cables have a little slack in them.
I've adjusted the mixture as per the haynes manual.

So I can't really see anything wrong that would explain this weird behaviour. I might have assembled the dashpot 180 degrees out, as I didn't mark it when I disassembled it. Could this explain it? I don't see how. I might not be adjusting the mixture properly. Any recommendations for how to do it correctly?

[bmcecosse]

As long as it rises and falls easily - it doesn't matter.........

[mike.perry]

What carb is fitted?

[matt993fod]

Its an HS2. I think I have found the source of the trouble. For a start, I hadn't set the fast idle correctly. I thought I had, but I was mistaken. Turns out I had given it insufficient adjustment, so that when the choke is out the throttle is still closed. No wonder it ran badly when the choke was pulled out!

As for the erratic idle, I have discovered that the throttle spindle is badly worn. I hadn't realised that it was as bad as it was! It must be leaking loads of air around it. I expect once I get it changed I will be able to set the idle much easier.

[bmcecosse]

You will also need to rebush the carb body......often easier/cheaper to just source another carb - and ideal opportunity to upsize to an H4 or HS4 - or HIF 38.....

[Declan_Burns]

Just dab a small paint brush in petrol and dab the carb body at the spindle at tick over. If the revs increase you are drawing in air.
Regards
Declan