Which Carburettor?

[bmcecosse]

Why does everyone want to talk about 'over fuelling' ???? What on earth does that mean ??????? The fuel amount is controlled by the jet/needle/spring combo........so it can be wrong no matter what size carb. My 2 litre TR7 engine has 2 X HS6 carbs - they seem to cope just fine with approx 1 litre of flow through each! The HS2 is miserably too small for the 1098 engine...an HS4 (or H4 or HIF38) is adequate with some light engine mods and an HS6 (or H6 or HIF44) will be fine too and will be useful when that 1275 upgrade comes along.

[chrisryder]

i believe 'over-fuelling' is when putting your foot down makes the engine louder but doesnt rev any faster. whether that is actually a case of running too rich high up the rev range (by having the wrong needle) i don't know, you've got a few more years experience over me!

[bmcecosse]

This term 'over fuelling' may come from injection - where too much fuel is squirted in........
If the carb engine gets 'louder' when the throttle is opened right up - but doesn't make any more power/go faster - I suggest it's actually SHORT of fuel! It's gasping in air - but not getting enough fuel to make the power.

[Alec]

Hello Chris,

the beauty of the S.U. is the variable choke which keeps up air velocity at low revs (over the jet) so aiding atomisation. I believe that using an S.U. gives more flexibility to selecting carburettor size, more so larger than smaller. If the engine has an oversize S.U., all that happens is it won't lift the piston fully at full power. (out of interest, compare an E Type Jaguar with 3 x 2" S.U.s and a MK 2 with 2 x 1 3\4" S.U.s, not a lot of difference in power, but a huge difference in choke area)
Ypu see engines with large Webers on sometimes, but they won't run well (particularly on the road) unless small chokes are fitted, which wastes the potential of a nominally large choke.

Alec

[bmcecosse]

I've just had almost exactly that argument on the TR7 forum........ where a lad with a standard engine is planning on swapping (at great expense!) the twin 1.75" SU carbs for 2 X 40 DCOE Webers - with 32 mm chokes in them........ He doesn't understand that he will actually let LESS air into the engine........