Hello everyone, firstly a confession that my Seven is probably a bit different from those most members have. It is a 1960 Austin Seven (Mini).
I'm seeking advice because I can see that there are members here that have a vast knowledge on SU carbs.
Since last year, on and off I've been working on fitting the car with a 998 Cooper engine and Shorrock supercharger. It is fitted with an H4 carb. All bought from a well known Shorrock specialist, fully overhauled.
Last year after completing the engine build i bench tested it, it started OK, albeit a bit rich.
Likewise after installation in the car, it started, ran, albeit rich.
After a few weeks away I recently got back to working on it. The problem is it's got harder and harder to start, until now it will not start unless I spray carb cleaner or petrol into the intake.
Fuel starvation I here you say ? Not so, the pump is fine and the float level is correct.
I've put fresh fuel in, new plugs! If I squirt carb cleaner or petrol into the intake it'll start instantly, run fine for a few seconds then die.
Today I've taken the carb off and checked that the fuel drilling between the float bowl and the jet is clear. It's fine, the carb is in really good nick. The float level is correct. The jet is centred, the needle is fitted correctly, and every seal and gasket is new. the piston is free and dropping correctly when lifted.
It's crazy because SUs are simple things and I can see nothing that is stopping the carb from working as it should-but it isn't.
Don't really know where to go now....
Any ideas ? All , even unlikely ones , will be gratefully received !

As you say not a real Austin Seven, but 1/ Are you fitting H4 or HS4 carbs.I think Coopers had HS4's. 2/ you say it fires if you spray fuel into the manifold, have you tried manually filling the float chambers before you fire it up? If it runs until the fuel is used up the problem is one of supply. Stuck float needles or internally collapsed feed pipes. 3/ Do the choke mechanisms work.

Location: Piddle Valley

I have had two four stroke lawnmowers that behaved in exactly that way, this year.

I am told it is the way high-ethanol fuel affects the carbs. I do not really believe this, but have no better explanation.

I put a "Fuel system" cleaner chemical in the fuel tanks, and then sprayed fuel directly into the carb to keep the engine running until the system cleared itself.

A two person job, really. Dangerous if you get a backfire. Worked, though.


Simon

Location: On a hill in Wiltshire

If you think it's fuel, it will probably be electrical! Have you tried another coil? Or a leak in inlet manifold gasket?

Location: Wales

I'm with Ian. Make sure the simple things are OK first. Yes it can, of course, be some obscure problem to do with fuel or whatever that has never affected anyone before but there's a much better chance that it's something simple.

Malcolm Parker will tell you a story about a friend of ours who'd convinced himself 'it' was a problem with his new timing gears and, when we found him, was in the process of stripping the front of his engine on a wet and muddy trials hill with his wife/passenger berating the quality of repro parts to anyone who'd listen. Re-setting a distributor that had come loose and retarded itself solved the issue in under a minute........!!

Steve

Location: North Yorkshire

Simple check with an SU take the piston out and ensure the float chamber is filled with petrol to the point where the fuel supply is cut by the float chamber valve. You can now look down into the main jet and see where the petrol sits, of it is overflowing the top of the main jet in the bridge the fuel level is too high, if it is significantly below the top the fuel level is too low, either way the float will need adjusting, and in the case of the latter may explain your difficulties.

Location: NZ

Rings very true that story.
It's confession time. I've got it started and it's in rude health. The carb was set too weak. In my defence I set the mixture nut 12 flats down as per SU spec, and assumed that it would start even if that setting was approximate - but it didn't. My carb needed a quite few more flats down before it started to cough a bit then start. A bit of fiddling with the Colortune and it seems fine.
I suppose it's logical if you think about it: if it's obviously fuel starvation and all else is mechanically fine, the only thing that can possibly cause it is the carb setting.
Moral: as ever don't overlook the obvious.
Thanks all for the help.

There's something not quite right here. The nominal position for an SU jet is 12 flats down, and you shouldn't need to move it more than 2 or 3 flats from there. It suggests that the fuel level is too low. Ok it will run but I think it would run better if you checked the fuel level first.