Anything and everything Spitfire.

Hello all

Can anyone suggest a cure for a 'hunting' problem on my carbs? Spit MKIV HS2's. Having done a decoke this winter I've sort of got the car running but can't get the carbs to behave themselves. When warmed but still on about half choke the engine ticks over around 1500 rpm (as I'm unable to set the tickover correctly due to this problem), then about every 10 secs it runs up to about 3000rpm stays there for a few seonds then goes down again and keeps repeating the cycle.

The carbs are set up as per the Haynes manual, linkage gaps as stated, jets unscrewed 1.5 turns rather than 2 because then it races at 4000 rpm, similarly the idle screws are wound right out otherwise the 'high' revs are even worse (I don't like the old girl revving too highly)
I've checked the needle valves which are moving freely and cleaned the float chambers. Only difference since before the decoke is I've fitted an inline fuel filter before the fuel pump but I wouldn't have thought that would have made a difference.

Thought it might be an inlet manifold leak but I've re-torqued the manifolds and if it was, wouldn't it be revving high all the time?

Anyone seen this problem before?

Make sure your jet heights really are the same (choke fully in) - you can do this without taking the carbs off the car by removing the tops and measuring with a depth guage.
Then warm the engine up. With the engine fully warm and the choke in set up the idle screws. Do not try to set them up on choke because they don't do anything on choke. Get the idle speed right. If the engine is hunting then adjust one screw. As the carbs become better balanced the hunting slows down and eventually stops. Once you have done this, any further adjustment to get the speed right should be by turning both screws the same amount.
When the engine is cold you can then set up the choke screws. These should come into contact with the throttle mechanism at the same time. Again you may have to adjust one and then fine tune them as a pair.
If you adjust the mixture in the future, say you lower the jets, you will probably have to unscrew the idle screws a bit to keep the idle speed the same.
Regards
Van

Thanks Van, I'll give that a try. I guessed it was something wrong with carburettor adjustment but it's knowing the best place to start from. I've got a vernier which would allow an accurate setting of jet heights so I'll start from there.

What type of air filtration have you got ?
Standard ? K and Ns ?

Mike Ao Syks

Have you had the butterflies out?

I replaced mine and couldn't detect any difference but there is a small chamfer and if you don't put them the right way round they don't close properly.

Just a thought!

Hi Barry

Yes I have had the butterflies out - the decoke was originally done because I'd heard a metallic internal tinkling sound. After this the tickover was high and couldn't be lowered to a satisfactory level. On investigation I peered into the front carb and noticed that one of the butterfly retaining screws had had enough of living in the throttle spindle and wanted a new home in the cylinder.

Didn't know there was a chamfer (I don't think Mr Haynes does either) so I'll take the carbs off again and see what's going on.

many thanks
Chris

Section : Carburettors

Carburettors 'hunting'

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