Practically Classics Forum

Hi,

I have a 1976 1500 spitfire that is running twin SU HS4 carbs. I recently had a problem with some bad running and so I converted it to electronic ignition and serviced the carbs including replacing the needle valves etc. The car then went to the garage for a tune up, timing adjustment etc. It is now running much better however it still seems to have a flat spot / stutter occasionally. This stutter tends to happen when the car is warm and at around 2,500 - 3,000 revs, it runs perfectly when the engine is cold. I have noticed that the stutter is worse with warm weather. Whenever the car stutters there tends to be a noticeable smell of unburnt fuel. I have had a look under the bonnet and have noticed when the engine is rev'd occasionally fuel will spray at high pressure from the vents / overflows on the float chamber lids. I assume that this is the cause of the fuel smell and I also assume that this is related to the slightly dodgy running as the stutter and smell of fuel tend to go hand in hand.

The garage tell me that they can't find a problem but I think that this is because it is fairly intermittent and they are not driving / testing it for long enough. Does anyone have any thoughts on what may be causing the problem/s?

Thanks

Phil

better fix that soon..might catch fire . Old rubber fuel lines/joining bits hate the modern petrol, could perished bits be blocking/jamming the float needles...intermitantly? Fuel tank vent blocked?

Yes, float needles and seats, fit a pair of unsinkable floats while you're in about at it. If your car doesn't have the float chamber lids with overflow hoses that attach to stubs on there and on into the small bowl that clips on under a breather with two stubs, that fixed to the o/s of the bulkhead. This measure also maintains the mixture as any unburnt fuel vapour is breathed in by the engine when it would otherwise be running too lean.

If your car had, for example, the carbs from the later 1500cc Spitfire or any 1500TC engine without the associated breather pipework, then setting the idle mixture correctly would give you too lean a mixture further up. That would cause your flat spot.
Are your HS4s intended for waxstats? If so, then they must be run with the full breather system in place.

Ordinarily, I'd have condemned the ignition system first, but unless yours uses a variation on the old blue modules from Britpart, such as SimonBBC ones use, then it should be reliable. If you have the blue trigger module try a red one direct from Britpart at under a tenner.

I agree with new floats and needles recommendation and also calibration of the float height - see manual. The needles are shutting off too late at high demand, either because they are worn, the floats set too low or the floats and pivots are cream crackered.

Thanks for all your help with this problem so far. I've spent the last few weeks trying various solutions without any real luck! I should point out that it is just the front carb float chamber that is flooding. I've tried the following to sort it:

Replaced needle valves and seats (still front chamber floods)
Replaced floats with stay up floats and adjusted as specified in the manual (still floods)
Adjusted floats so that the gap between the chamber lid and top of the float with the valve closed was much bigger than suggested in the manual (still floods)
Tried swapping the valves and seats around (still the front one floods, not the rear)
Made sure the air vent on the float chambers were clear (still floods)
Tried fitting grose jets (both float chambers flood and more often than with needle valves)
Made sure the fuel tank vent was unblocked (still floods)
Replaced the fuel pump and filter (still floods)

I'm running out of ideas and getting a little frustrated with it! Can anyone think of anything think else that may be causing the problem?

Thanks for your help

Phil

Is the fuel pump a standard mechanical one?

Yeah, both the original pump and the new one are standard units.

That IS baffling indeed.
Are you using solid needles or the neoprene tipped type, I think Burlen only supply the latter and they fixed my twin HIFs.
Could it be a cracked or porous casting on the front carb
An original SU mechanical pump has pressure regulation built into the design to prevent over pressurisation of the float valves is yours a genuine SU

I've got the tipped needles direct from Burlen.

The lid of the float chamber looks fine. I did consider that it could be leaking and so fitted a small washer between the needle casing and the float chamber lid which didn't help! I'm inclined to think that it is not a leak as the fuel comes out at high pressure when it floods. I'd imagine that if it were leaking that it would be more of a dribble out of the top of the overflow but I could be wrong? Maybe I need to buy a new float chamber lid and try that. It is the only part I haven't replaced!

As for the fuel pump, I think that the one on the car before I replaced it was the original pump, although obviously can't be certain. The new pump is an replacement part bought from Moss. I looked on the SU website but it seems that Spitfires were not originally fitted with SU pumps and they don't have them for sale.

The fuel pump should stop pumping when the fuel line is up to pressure. It sounds as if yours isn't doing this, so I'd investigate the fuel pump again - in spite of having a replacement!