SU carb problem

[Mel-S3]

I have a standard spitfire Mk4 with standard SU carbs that i had reconditioned last year by a reputable company. just got round to fitting everything back together after some other restoration work. The carbs have been set on the basic setting to start the car. The problem i have is that, firstly the car will only just manage to start on this setting, then after 3-4 minutes, after it has warmed up, it stops. It seems to be running rich, i have screwed up the jets until they are nearly right up with little or no change, have also checked all the other things on the engine and carbs, as per Haynes manual. Any advice would be greatly appreciated.

Must mention that before the carbs were recon they were running rich, but it did run and drive. Thought a full carb recon would sought the problem out because they were very worn.

[Jonny_uk]

At the bottom of the carb does the jet drop down when you pull choke on fully? Is this sticking?

[//j17]

1) Use the lifting pins (HS2s on a Mk IV should have them) to raise the piston, then let go. Piston should falls smoothly with a 'dunk' sound when it hits the bottom. Do this a few times and it should be exactly the same each time. If it isn't the piston is sticking. If no lifting pins take off the air box and lift the piston with your finger.

2) If that's OK check the jet (red bit on the bottom). It should be sat right up against the carb. (with the choke in) - you shouldn't be able to see any of the copper pipe between red part and carb. (like you can if you look with the choke on). If you can see pipe then the jet isn't centred correctly so you are always running with the choke part on.

3) If that's OK get it running and use the lifting pin (or a screwdriver/etc) to lift the piston 1mm or 2mm. If the engine note goes down => lean mixture. If it goes up and stays up => rich. If it goes up, then drops back => spot on mixture.

4) If it IS rich, remove the damper and dashpot, then lift the piston out (putt it straight up until the needle is clear of everything). Keep it up-right or all the damper oil will fall out and remove the grub screw holding the needle in. At the top (non-pointy) end you should find 3 letters stamped around the needle. For a pure stock Spitfire 1300 they should read AAU. It's possible a PO had some K&N filters on there and has fitted richer needles to match (probably AAQ or AAT) but took them off before selling.

Personally I'd order a pair of AAU needles - they aren't expensive and you can quickly rule out wrong needles/bent needles/incorrectly stamped needles (does happen)/etc.

[Mel-S3]

Thanks for the replies. I have adjusted the carbs better using a colour tune and carb balancer, which has, I think sorted the problem of the car running rich, although I had to screw the jet up nearly to the top on one carb - is that normal - the other is about 8 flats down. Unfortunately this has not sorted the problem of very poor starting and the engine cutting out after about 5 minutes. I have rechecked points, timing (what is the best way to do this accurately, do I make any allowance for unleaded with additive?) and plugs. Any more ideas?

[Jonny_uk]

Sounds like maybe fuel starvation???? IS the filter ontop of the fuel pump clear? Or is it maybe the floats in the fuel bowl

[Mel-S3]

Thanks for the help. In the end it turned out to be a dodgy condenser! The car had a new one only 280 miles ago, but it had been stood for nearly twelve month. There was also a little trouble with one of the Grose jets. Removed these now and gone back to the old needle valves. Fingers crossed all is now well.

[//j17]

Gross jets are an odd one - some people swear by them, other just swear at them. The general mood seems to be Gross jets just gum up and traditional jets with an in-line fuel filter before the fuel pump is the best option.