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MG MGB Technical - needle selection

Hi All,

I just put a Peter Burgess tuned 18V engine acquired from a B racer in my restored B with the following spec:

1. Stage 3 head;
2. Balanced bottom end with standard bore;
3. Peco HDR exhaust;
4. HIF4 carbs with AAA needle.
5. K&N filter (thick one).

But it's running lean and wonder which needle would be the best starting point?

Thanks in advance!

Ennio
Hong Kong

Ennio Wong

Hi All,

The cam is bp285.

Ennio
Ennio Wong

How do you know it's running lean and when is it running lean?

useful su needle chart:
http://www.teglerizer.com/suneedledb/index.html
Willem van der Veer

Willem,

I set up with 2.5% CO at idle with the help of a 4-gas analyzer. But running the car at 3K rpm, the engine was not very smooth and the plug was very white at the tip after 20km run.

Ennio
Ennio Wong

AAA seems to be one of the richer needles but does taper off just after mid-way. The only two I can see that are richer in the 2nd half are AAM (slightly) and ABY (a bit more so). However whereas AAA is still getting richer right to the final stage, AAM stops getting richer one stage before the end, and ABY at two stages.
Paul Hunt 2

Hi,
I don't know explain very well, the difference is subtle, the SU HIF are not a good carbs, use HS4.
Regards
michel

Ennio,

Are you ABSOLUTELY sure that the balance between the carburettors is spot-on; i.e. are all plugs "very white at the tip"? As Paul says, the AAA is quite rich as it is.

If the balance is correct maybe you should try 3.5% CO at idle?
Willem van der Veer

Do you need to meet 2.5& CO? If not just set the carbs up as originally intended using the lifting pins. My V8 doesn't run well at 3.5% CO, which is the limit it has to meet on the annual test.
Paul Hunt 2

Hi Guys,

Thanks for all input!

All plugs are white making me believe that the carbs balance should be OK.

I've tried 4.0% CO at idle, but the plug (NGK BP7ES) fouled in the traffic, so set to 2.5% CO.

The temperature in HK is about 34 deg C and very humid, I daren't to use a hotter plug (BP6ES) as the cooling system is very marginal in the traffic.

Michel,

I only have 3 pairs of HIF4 and 1 pair of US spec HS4 which has no inlet for breather tube.

But I have 3 HD8s from a Jaguar E-type, has anybody tried this on a B-engine?

Many thanks!
Ennio

Ennio Wong

I *would* use a hotter plug. It won't have any effect on the engine temperature or cooling system at all. In fact the engine will run fractionally *cooler* with a hotter plug, as less heat is transfered from the plug into the head and hence the coolant. All a hotter plug does is get hotter at its tip to burn off any fouling because there is a more restrictive path for the heat to travel from the top to the body.
Paul Hunt 2

If you want to prove a point! Does the car run better with a little choke? Are the needles correctly located in the piston? You can try putting the needle deeper into the piston, this will richen the mixture proportionaly at all throttle openings. Hotter plugs means the tip runs hotter because the heat transfer area from tip to body is smaller, therefore it runs hotter, only consider hotter plugs if they are fouling or oiling up. Check also that the pistons rise and fall freely, take the damper out and the filters off, push both pistons to the top, let them both go together, if they "clunk" on to the venturi together, all is well, if not one or both may be sticking. Also check you have the correct piston springs and dampers. Let me know the SU No. i.e. AUD............ or in one case FZX.............. and I will advise>
Allan Reeling

Hi Allan,

The HIF4 is FZX1229 which is used pair with extremely low mileage.

They clunk on to the venturi together with damper off.

But this remind me that it's very hard to push both pistons to the top with damper and oil which is very different from the set of HS4 on my 70BGT.

I will try your suggestion this weekend.

Thanks!
Ennio
Ennio Wong

Ahh - is this a needle problem or a plug problem???

I run a similar specification, Burgess Fast Road Big Valve, HR285, Peco, etc with HS6s and have similar PLUG problems.

BP6ES run too hot on the open road, splitting the ceramic after a while. But BP7EV run too cold in traffic and foul pretty quickly. The 7s are VERY expensive (£25+ for four) and just about impossible to clean so fouling is a costly episode. You have to REALLY want to use them

Unfortunately, I therefore change the plugs as appropriate to the drive I expect to have!

Just a thought, you haven't 'wrapped' the manifold have you? This seems to lower under bonnett temperatures but significantly raise head and plug temperatures. With a wrapped manifod, my engine used to eat BP6 plugs for breakfast!

And yes, it has a modified distributor and has been set up on a rolling road.

You may need to set richer and run BP6 plugs. IMO, fouling of the BP7s is about temperature not mixture.

Neil
Neil22

You can always file a few thou off of a needle, however you cant put it back on so easily.
Stan Best

"You can try putting the needle deeper into the piston, this will richen the mixture proportionaly at all throttle openings"

That's the same as lowering the jet, and it's easier to get both needles flush with the bottom of the piston then try to get both recessed by the same amount.

The pistons should also clunk on to the bridge with the dampers *in*, and that is the test you should do, as that is what they are doing in real life. The dampers hsould only delay movement of the piston *upwards*, they should fall smartly.

Easy to taper one needle a bit more, less easy to taper two to the same profile. And 'file a few thou'? Rolls Royce RB911 then? :o)
Paul Hunt 2

The only FZX's I have in my "bible" are 1001. but it is a mid 70's book, so is probably behind in manufacturing numbers.
Standard needles are ACD, springs are red, (the colour on the end coil)damper AUC 8114.
Also worth checking that the over-run valves on the butterfly discs are seating properly and not damaged in any way.
The pistons should be difficult to push up when the dampers are full of oil. What oil are you using? Rover say use multi-grade engine oil; SU, SEA 20 or 30; SOme, including me, use Red-ex, alone in winter, mixed with engine oil in summer.
Allan Reeling

Hi Allan,

The damper oil is just ATF and the butterfly discs already been changed to plain one without poppet valve.

Thanks!
Ennio
Ennio Wong

Needle filing was an accepted Abingdon Special Tuning technique, they even had a system of recording what was done and marking the needles so they could identify them, including the fitter who had done the work.
Stan Best

Poppet valves not sealing just simulate a throttle butterfly sticking open i.e. high-idle, they can't in themselves cause any change in mixture that the butterfly itself won't cause.

Damper problems usually cause a weak mixture when attempting to accellerate (fluid too thin) or a richened mixture for too long (fluid too thick) but this 2nd is much rarer. The crucial point is it only affects things when opening the throttle, not steady state running.

Needle filing: I know, but a few thou? There is typically only 30 thou from max to min, but the comment was only in jest anyway.
Paul Hunt 2

Also, another non-PC invasive program you can use to compare needles is this one: http://www.mintylamb.co.uk/suneedle/
Bob Muenchausen

Maybe we all know this, but it does not matter if you do not keep the needle round, on HS carbs anyway. To a good aproximation the extra fuel is directly proportional to the removed cross sectional area,and I wil also re-direct you to my original post, it is easier to remove metal than to replace it.
Stan Best

This thread was discussed between 12/09/2007 and 17/09/2007

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