11 replies to this topic

[bradderz]

1989 Mini 998cc

the engine is completely standard and i am wanting to get twin carbs and a stage 1 kit.

what size twin cabs do i need?

i have seen some on ebay with the whole lot for around £150-£200 including the manifold.

will i need to split the fuel line into 2?

do i need the whole stage 1 kit, because i cant find one for a twin carb?

[BennerW]

I believe twin carbs might be abit overkill on a stage 1 998 engine, Not sure tho, A carby expert will be along shortly

Ben

[Cooperman]

The idea of twin carbs is to get more miuxture into the cylinders. However, that assumes that the head can accept the extra mixture by virtue of its design or state of modification. The problem here is what the term 'Stage 1' actually means as it's not a technical term, just an expression used by the sales companies. If it includes a good gas-flowed cylinder head with a slightly higher compression ratio then a twin carb set-up on a decent inlet manifold combined with the right exhaust system, ideally a 3-into-1 manifold and a 1.625" exhaust pipe vis an RC40 or maniflow silencer, then twin HS2's, that's a 1.25" SU carb x 2, will suit it quite well. Although not as well as if a better cam was fitted. If the head is standard and there is not also a better exhaust, then no appreciable increase will be noticed as the useable mixture will be governed by the inlet ports in the head and the exhaust tracts in the head and exhaust system.
I hope this helps to clarify things for you.

[bmcecosse]

I think you are confused brad! A 'stage 1' kit is a better inlet manifold with NO hot spot - for the standard 1.5" single carb you already have, combined with a decent air filter and a good exhaust system. Fit this first - a better cylinder head is the way forward next - with larger valves and improved breathing. Even now - your single carb is plenty adequate for your engine - 'twin carbs' are NOT the way to go on an A series in the early stages - at least - not until way up the power range after cam change and very much larger inlet valve head has been fitted........And then you can consider 2 X 1.5" carbs or even 2 X 1.75" - although they are normally only for a 1275 engine........

[samsfern]

id have thought twin 1 1/4s will be ok for it, after all early spridgets ran them, and they werent 1275.

[bmcecosse]

That was just because 'Sports Cars' were expected to have 'twin carbs' - otherwise no-one would have bought them! The single 1.5" carb on a decent manifold actually passes more air into each cylinder than the twin 1.25" carbs. Each cylinder only breathes through ONE carb - so it's better if that carb is 1.5" rather than 1.25" !! There is no 'cross flow' through the ghastly balance pipe on the standard twin carb manifold - in fact - it ruins the straight air flow from carb to head - manifolds with NO balance pipe (ie like Weber) flow better!

[AndyMiniMad.]

This seems to be a popular question.. And always with the same answer.. Twin carbs or big hiff 44 carb on a 998 are in my oppinion a total waste of money. Unless as stated already you have already modified your cylinder head and exhaust system. Which you say you have not done.

[Cooperman]

The 997 & 998 Coopers all had twin HS2's as standard, that's 1.25", and with the open-chamber head with its slightly larger valves and 3-branch manifold they went well in their day. Even the original Cooper 'S' had twin HS2's. IMHO twin 1.5" carbs are not good for a 998 as they are just too big. The only 1000 cc engine I've used twin 1.5's on was a 970 Cooper 'S at full competition spec which revved to 8000 and needed the bigger carbs to get the mixture in at those revs. It gave 88 bhp at 7000+.
If you can get the 998 engine to a 998 Cooper spec, then twin HS2's would be good so long as you have them set-up properly. The key is the head and cam plus the exhaust system. Don't bother using the term 'Stage 1' as it really doesn't mean a lot.

[bradderz]

i definately want to put a stage 1 kit on, but not sure about the head as its a bit pricey and i am only a novice in car mechanics. so looks like i will keep the origanal carb and just get a stage 1 kit

[bmcecosse]

Good plan!

[samsfern]

The 997 & 998 Coopers all had twin HS2's as standard, that's 1.25", and with the open-chamber head with its slightly larger valves and 3-branch manifold they went well in their day. Even the original Cooper 'S' had twin HS2's. IMHO twin 1.5" carbs are not good for a 998 as they are just too big. The only 1000 cc engine I've used twin 1.5's on was a 970 Cooper 'S at full competition spec which revved to 8000 and needed the bigger carbs to get the mixture in at those revs. It gave 88 bhp at 7000+.
If you can get the 998 engine to a 998 Cooper spec, then twin HS2's would be good so long as you have them set-up properly. The key is the head and cam plus the exhaust system. Don't bother using the term 'Stage 1' as it really doesn't mean a lot.

[bmcecosse]

Again - the Cooper only had 'twin carbs' to sell the car - in those days they were derigueur - or the cars wouldn't have sold. The S had twin 1.25" carbs - so that for competition they could be swapped for 1.5" carbs and still be in Group 1!! Most Mini folks now use either a single 1.5" HS4 or HIF38 - or a single HS6 or HIF 44 to get plenty of air flow into the engine. Twin HIF 44 work well for very high power - or a Weber 45 DCOE - which is effectively twin carbs in one body..... The main advantage of twin carbs is that it is possible to get a very straight/smooth air flow into each port on the head - but only with a good inlet manifold - and that's NOT the BMC one with the horrible balance pipe!

Edited by bmcecosse, 18 October 2010 - 05:49 PM.