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1500 carb issues


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I'm having trouble setting up the carbs on my 1500 spitfire, they have been rebuilt and there is no butterfly play all new gaskets etc. The engine appears to over fuel when revved up as you get black smoke and a smell of un burnt fuel. Even if I lean the mixture out as far as I can the smoke still is present. Any ideas?

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Needle valve(s) not shutting off? floats not set correctly?

Have you removed the pistons and dampers (take great care to keep them as matched pairs) and checked the jet height? Start point is normally 2 turns down from the jet being flush.

Have you checked needles correctly fitted in the dampers?

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  • 5 months later...

 I have recently had my HS4 's  refurbished also all new gaskets etc. I have also fitted an electronic distributor which is set at 100    BTDC. My chassis no is FH127081. Engine FMHF.  I always used aprox 2 turns or 12 flats as a starting point . the guy who refurbished them  has said my carbs are late models  and to use 20 flats as the mixture adjustment thread is finer:  has anyone heard this? I have had the engine running after warm up and managed to get a tick over about 1000 RPM  and set the timing as above . The front carb appeared to be lean and the aft one rich. tried to get them the same: Now  It will only start Cold  on full choke and runs at about 2500 revs.  Using a air volume tester I get the same reading for both carbs: which appear to indicate the that the carbs are balanced. Pushing the choke in just a little the engine stops. Any suggestions please. I have a number of tuning guides including Haynes  and following the recommended procedures never before had problems in getting to ticking over smoothly  at 800RPM.

Any suggestions


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The start point turns is a start point ie should make it enoigh to run, when hot you adjust from this setting


You need the top of the jet tube to be around 2 to 3 mm below the throat bridge

Have you centred the jets ?.

do make sure no air box or filters are covering the ports on the carbs front face


if you get it to idle use a lifting pin


you need light fingers to just feel the lift pin contact the piston

then lift it 2 to 3 mm dont yank it up

if it falters it lean

if it speeds up its rich

if nothing about right

you are not looking for monumental changes may be up or down 50 rpm

pointless doing it with the filters off as these affect what you are testing ,

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