HS2 or HS4 carb

[Rust bucket]

Just fitted a 12g940 head to my 1098 engine with alloy inlet and standard exhaust manifold using a HS2 carb. Can't get the carb adjustment right by fiddling with the bottom nut, idle not constant and no extra power at the moment, what needle would you recommend?

[amgrave]

For that head you need a HS4 carb, you won't get satisfactory results with the carb you have fitted.

[philthehill]

Both HS2 & HS4 carbs will work with the 940 head.
The HS2 carb was fitted to the 1622cc Morris Oxford so flow and capacity can be somewhat ignored for the application in hand.
If you have not got a HS4 carb to play with try a different needle - it may be a lot cheaper than finding a good S/H HS4 carb
The 1098cc Minor with the AUD13 HS2 carb was fitted with a AN needle as standard - fitting the richer H6 needle may be the solution.

http://sucarb.co.uk/carbspec/carburetto ... s/id/1907/

[Rust bucket]

H6 needle arrived yesterday for my car with the 12g940 head fitted, not had a run out in it yet as it's being painted but started well and best of all ran smoothly, so looks like the carburetion is about right now many thanks.

[Mark Wilson]

Sounds like a result, well done!

Sorry for slight hijack of thread, but what needle would be a good starting point for an HS4 on a Midget 1275 (metro type cooled inlet manifold, adapted 1098 exhaust) ? Carb was given to me by Roy a couple of years ago, haven't opened it up yet, but probably set up for his rally driver tendencies!

Mark

[Monty-4]

Roy always recommended AAA as a 'starter'. I think that'd be a good shout. AAU for leaner, AAC for much leaner still.

[liammonty]

I'll be criticised by some for saying it, but the only way to get it right (unless you are amazingly lucky) is a rolling road - unlike tuning other types of carb with jets of different sizes, where you can tweak the idle mixture, or top end etc, the SU is basically infinitely variable. So it's not only a question of where a needle is 'a bit richer' or 'a bit leaner', but also whether it's at the correct part of the rev range for a particular engine, all of which have different requirements. It's possible to get close if you have a lot of patience and are prepared to spend a bit on buying various needles and road testing, but I find that a rolling road session with somebody who knows what they are doing with SUs is well worth the money.

[philthehill]

I totally agree but some do not have access to a rolling road and rolling road sessions can be prohibitively expensive when considering the cost / benefit.
The H6 needle will be a good start and the needle requirements can be reviewed if necessary.
If not rich enough at the top end a fine file lightly run down the top half of the needle may be all it needs.
If too rich put the original back in and again lightly run a file down the needle until the needle gives the right amount of fuel for a particular rev range.
Road testing will determine if the fettled needle is working or not. Slow and steady is the way to go with the needle.
Before the advent of the internet and good supply channels fettling/filing the needle and road testing was in most cases the only way to get things right and I have used that method on many occasions to get the engine running as I wanted it.
Modifying SU carb needles to suit is one of the various modifications I have undertaken on the SU carb including remote float chamber - but that is another story.

[StillGotMy1stCar]

Phil, that is what my dad used to do to fine tune the mixture with great results, except he used fine wet n dry to re-profile the needle.
Regards John.

[philthehill]

I am glad to hear that I was not the only one to modify the SU needle to get the mixture right.
Whether fine file or wet & dry the end product is the most important.
I found that if a flat instead of a general reduction in roundness is formed the flat should be placed towards the air inlet as it helps with dissipating the fuel droplets.

[Mark Wilson]

Thanks for the replies to my hijacked bit, very useful indeed. After a mere six years should have the engine in sometime in August (wood goes on next week ), so look out for more mechanical queries soon!

Mark