Practically Classics Forum

Hi all - great to find the forums are back again, just a shame about all the old info that's been lost.

Just a quick question - can you mix carburettor dashpots and pistons for SU carbs? I.e. can you use a dashpot (suction chamber) and a piston from different original assemblies together as long as they pass the 'drop test'?

Any answers greatly welcomed!

Cheers all, Bolin.

All the workshop manuals I have used for SU's say you should ensure the piston and dashpot stay together as a matched pair. This suggests that selective fitting was used during manufacture - this was before modern maching techniques remember, which produce components to much closer tolerances.
You should get away with it IMO, if the piston falls freely with no tendancy to bind.
Obviously If it is excessively sloppy or stiff you could get a variety of problems including serious mixture variations, backfires or lack of accelleration.

^^^WHS^^^

Plus I definately wouldn't do it on a twin carb engine, you could have a right royal game getting them nicely balanced.

You don't say what car this is on?

Hmm, OK so best to keep the matched pair I have.

I have one mixed set, which does the 'drop test' too quickly (approx. 4 seconds but works OK when fitted), and one matched pair that passes it perfectly (six seconds). Thing is, the piston from the mixed set has fewer scores on it that the piston in the matched set, which is why I was thinking of swapping it into the dashpot of the currently matched set.

As the matched set piston passes the drop test fine then it must be OK to use, I was just wondering about using the piston that is in better condition. I haven't actually tried the better piston in the matched set dashpot, and won't bother trying it now.

It's an Allegro 1300 with a single SU HS4.

Many thanks for the advice guys, I'll use the matched set as it is, despite the extra piston scores.