Carb set up

[Matt]

yeah, but you can still advance them too far!

[gert]

Peetee, when I mounted a 1275 A+ engine in my series II (10 years ago), I also tried a 1.5 SU HS4 carburettor from a Marina, only to find out that it didn’t work. Perhaps it could have been made to work properly on a rolling road, finding a suitable needle, but a cheaper solution was a HIF44 SU (as used in the Ital and later Minis that had the 1275 A+ engine). There are quite a few differences between the A-series and the A+ series engine, which is why you will also need a different spec carb.

[aupickup]

i have an a+ital engine with hiff44, and i must say now it is set up it runs nice

[Packedup]

In terms of carb specs there's not much between them, other than throat size, jet size, spring and needle fitted. A larger throat will give less resistance, so higher power with the sacrifice of lower charge speed, a smaller throat will give more restriction, so less top end but better low through higher charge velocity.

A 1275 should easily run fairly well on a HiF38 as that's a better flowing carb than the HS series, and more importantly, the old 1275 factory performance engines were "blessed" with twin 1 1/4" carbs - Effectively meaning the engine was running on a 1.25" carb!

What's needed when messing with the induction side is to take into account the effects the various changes will make. A free flowing air filter will mean more air than "expected", so a richer needle (and maybe even a different spring) might be required. I'd probably ask any A series specialist what they think would be appropriate, rather than fork out for a rolling road.

All this from the man who still can't get the twins on his Triumph set up properly!

[chrisd87]

There's a computer programme called 'winSU' that might be of some help - you put in your set-up and it will recommend a needle for you.

[bmcecosse]

The beauty of the SU carb is that it is variable throat - so a big carb like the HIF 44 will still run fine at low revs. Only the Cooper Minis had the HIF 44, the humdrum 1275s only had the 38. All 1275 Metros had the bigger carb - so that's a good source. A free flowing air filter does nothing until going at full chat - up to that point the engine breathing is limited by the carb size and throttle position - and anyway the carb still just meters out petrol to match airflow. Only problem will come if the piston reaches full height long before the engine reaches full revs - after that the carb will still give more fuel for more velocity over the bridge - but it will be in a fixed ratio, the graduated feed taper effect of the needle having been lost. Then a stronger spring may be called for - but it is unlikely! Most engines at full chat don't even lift the piston right to the top. If it's a standard 1275 engine then pretty much whatever needle comes in the carb (if it's ex 1275 engine) will be near enough ok, certainly a good starting point - if it's a modified engine then go for the BDL needle which was standard in the MG Metro.

[Peetee]

OK there seeme to be some missunderstanding as to my set up. The engine/carb/manifold came together from a running car. I heard it insitue and it was very sweet. So uncompatability is not an issue. In the process of transplanting the unit, something has been disturbed as it no longer sounds so settled or runs as well as I would expect. Finances are zilch at the mo so I was hoping I could delay the rolling road sesh (I intend to I fit a different manifold and carb set up) and see if I could establish if my inability to set the carb mixture sufficiently may point to a problem elswhere that would prove easy to check.

[Willie]

PEETEE.........lateral thinking, is your fuel pump delivering as it should do
before it has warmed up? A half full float chamber could give some wierd
problems. I wonder if you had a 1098 working ok but the new 1275 needs a
bit more fuel when cold which has shown up the problem. (assuming that you
are using an electric pump).