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HS4 SU carbs problem


petegardner_901
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Hi

I have checked the forum but anyone any ideas on the following:

1979 Spitfire with twin HS4's.  They are the waxstat type and as I was having a lot of problems with idle speed I decided I'd do the usual trick of removing the thermostat in the jet - replaced with 2x 1p coins as recommended by many....

I expected to have to readjust the mix but I cannot get anything but a rich mixture on a Colourtune - the jets are screwed almost right up.

Rather stupidly I didn't try checking the mixture before I took out the thermostat but I'm sure it was too rich before anyway.

Rather than put the whole lot back just as a check - any ideas???

Thanks

Pete

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How does the mix look on higher revs? My Spit is the same, twin hs4 with the 2p mod. I can't get it to be anything other than rich at idle if I want it to pull further up the throttle/rev range.

I've tried a few different needles and some are better than others, but fundamentally I can have rich idle and good performance or "right" idle and no guts.

I think the 2x1p coins are too thin to get the jets back to the right starting position. 

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IIRC all SU needles have the same  diameter at the idle "station".

Constantly rich at idle usually means that there is a problem with the float level setting or that the needle valves are leaking slightly, both causing the fuel level to be too high.  The latter can in turn be the result of a repro fuel pump producing excessive pressure.

Nick

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hs4 on mine have a fixed float  only changes by very thin shim washers under the valve seat, i did find flooding due to a fine ridge of wear in the plastic float could jam the needle valve ,  no amount of fine fettling solves it  a new float was the remedy, needs a good magnifier to see the wear lines .

but this is going to be the missed spacer .

Pete 

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