The 101 on SU carbs

[rtlkyuubi]

Ive just recently fitted a 1inch 3/4 HIF SU carb to the 2litre in my sierra (ready for turbo powars ;D) :



But I'm having trouble setting it up. Ive turned the fueling up and its better but from 3000rpm to 3500rpm it had a massive flat spot. Also I think ive turned the fueling up as far as it will go but splutters unless the choke is pulled out, but then on idle when its warm, it will splutter as too much fuel is getting in.

I'm also using ATF fluid in the dash pot and the carb is off a rover 3.5 v8. Any help on to get it to run properly would be amazing.

and here are some SU's for your troubles:

[BenzBoy]

Hmm, I'm more of a Stromberg man really but the principal is largely the same. What needle are you using? If it's from the V8 then it could be out by quite a way - either rich or lean, it doesn't always follow that bigger engine = richer needle. My carbs from a 2.0 car were too lean for my 1600. Add to the mix that you're using one carb from a twin-carb set-up and there's no telling what kind of profile it has! ;D

Here's a good resource for SU needle profiles.

I would recommend getting hold of a Colourtune tool - it's not all that accurate but it will give you a good picture of what is happening and when - if it's leaning out, going too rich, etc etc. Might be an idea to get an assistant to rev it to 3k rpm so that you can have a look at the colour of the flame.

I was fairly fortunate in that my engine ran lean pretty much across the board. I went and asked for some richer needles and hey presto.
With the SU needle charts that I have, there's a rich / normal / lean spec for each, so once you find out what you need it should be fairly simple to select a needle that delivers more or less fuel as required.

[RetroWarwicK]

try downloading winsu to get a approx needle size. having said that, is a HIF44 big enough for 2l turbo? poss a 2" (HS8) would be better?

[TurboDieselWeasel]

The mixture adjustent is for idle. Everything else is sorted by the needle profile. (of course extreme adjustments have a influence through the whole rev band, but that's not as intended).

So basically: to alter the fueling, you need to alter the needle profile. By either changing the needle over for a suitable one or reprofiling the needle.

I found this book very handsome for SU tuning & adjusting. Also has some handyadvice on reprofiling needles:
www.amazon.co.uk/Carburettor-High-Performance-Manual-Speed/dp/1845840739/ref=sr_1_1?ie=UTF8&qid=1302702713&sr=8-1

You say it's of a V8 - is it a turbo carb? Turbo carbs are a bit different to the normal SUs But they can be converted (have a search on turbominis.co.uk )

[BenzBoy]

HIF44 should be OK for that engine, if I remember rightly the Montego Turbo uses an adapted HIF44.
You will probably need to change the needle again when you come to turbocharge it, but I think I'd be tempted to get a carb from a Montego or Maestro Turbo when you do.

[MrSpeedy]

You may find that flat spots like you're describing could also be down to the damper spring on top of the piston. Try a different one (they're graded by colour, ie green, yellow etc).

I had the same problem on the Sprintfire and a spring change cured it, along with the next richer needle and 3 in 1 oil in the damper

[rtlkyuubi]

Thanks for the replies and the information about the needles! For the turbo conversion (when it happens) ive been following this website:

www.triumphexperience.com/article/how-to-blow-through-an-su.html

And how to convert a normal HIF to turbo power.

The current needle is a BDM. and i think i need the same fuel at idle and more at higher rpm. But I cant figure out the graph on the mintylamb site because I don't know what the values are for the needle i want.

Also if you change the needle, do you change the sleeve it goes into aswell?

[goodoldluke]

oil I use in mine is just standard veg oil for cooking with. worked for me for a good few years.
and +1 for the colour tune they're amazing

could also be that by the looks of it you have nothing on your crank breather pipe. giving it too much air after the fuel in.

[rtlkyuubi]

The crank breather pipe is blocked off what you can see open is the float bowl breather

[BenzBoy]

Apr 13, 2011 17:31:36 GMT rtlkyuubi said:

The current needle is a BDM. and I think I need the same fuel at idle and more at higher rpm. But I cant figure out the graph on the mintylamb site because I don't know what the values are for the needle I want.

Also if you change the needle, do you change the sleeve it goes into aswell?

The jet orifice shouldn't need changing unless it's worn. The needles correspond with a particular jet size - .090", .100" etc. There should be plenty of choice of profiles without changing the jet size I'd imagine.

*Edit*

I've just looked through my SU needle charts and I can't find BDM listed. It could be that the carbs are later ones - my book is quite old!

For the HIF6 on the Rover P6 3500 my book lists two needles (earler HS6 are different, as are US-spec): BBG and BBV. If the specs for those would be of any use let me know and I'll post them up.

[rtlkyuubi]

Just looking on that website you mentioned earlier and comparing the jets:



I guess neddle station 8 is 1/2 throttle? or are they just points on the needle where they measure it? But they could work really well, because at low throttle its fine

[Seth]

Apr 13, 2011 13:53:28 GMT TurboDieselWeasel said:

I found this book very handsome for SU tuning & adjusting. Also has some handyadvice on reprofiling needles:
www.amazon.co.uk/Carburettor-High-Performance-Manual-Speed/dp/1845840739/ref=sr_1_1?ie=UTF8&qid=1302702713&sr=8-1

I can second that recommendation.

Needle stations are diameters as measured at regular lengths along the needle (0.1" I think)

[TurboDieselWeasel]

Apr 13, 2011 21:12:28 GMT Seth said:

Needle stations are diameters as measured at regular lengths along the needle (0.1" I think)

Exactely. The needles are sectioned into steps and for a each step a diameter is given (like a 2D fuel map if you like).

The less diameter, the more fuel passes through the jet - and vice versa. So, basically, the higher the curve, the thinner the needle and the richer the mixture. Or vice versa.

That's why you start with a LEAN needle for profiling

[BenzBoy]

Well, BBG or BBV needles would richen up the mixture at higher revs if that's what is needed.
Trouble is, without knowing what's going on with the fuelling at present you'll be p*ssing in the wind and possibly buying multiple needles. For the price of a couple of needles I would definitely get a Colourtune on there first, just to see what is going on - if it's leaning out or going rich, you can base your decision on that info and get the appropriate needle.
The Colourtune is a useful tool to have anyway, as you can set up your carb and fine-tune, even after you've got the right needle.

[jrevillug]

I agree that a colourtune is very useful for off-load mixture measurement. You can't use it while driving though...

Another way to do it is to look at the plugs - run it in the flat spot for a bit, then find somewhere to stop and quickly turn the engine off before pulling the spark plugs. If the plugs are black and sooty it's way too rich, if they are white it's too lean. Tan is about right.

IIRC, when you snap the throttle wide open at low revs the piston in the SU rises about half-way. So the upper (thicker) half of the needle controls the part-throttle mix, while the lower half controls the what mix at varying engine speeds.

[ettorebugatti]

The Montego Turbo seems to have a BGQ needle.

[rtlkyuubi]

Thanks for the info looking at the bgq needle on the graph, it looks alot more like what its supposed to be i rekon from how the car reacts when driving it.