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MG MGB GT V8 Factory Originals Technical - SU carburettor mixture - rich


On my V8 roadster I have copied the factory setup with a Clive Wheatley plenum chamber and a cut down Rover inlet manifold. The carburettors are HIF6 as were used on the factory V8.

I have done about 3,500 miles now and am having trouble with fouling spark plugs. They can get very black and the cylinders supplied by the right side carburettor 1,4,6,& 7, are the worst. The engine is all new and the needles were selected after experimenting on a dyno with an experienced operator who likes SU's (very rare).

I have been back to the dyno after trying to sort out why I am fouling plugs myself without success.

The short story is that there is variation in the carbon monoxide emissions.

The fuel bowls are getting too hot and giving inconsistent emissions (carbon monoxide).

The dyno operator has a new device that puts a laser spot where pointed and measures the temperature at that point. He can compare each exhaust port and any other part of the engine which may be beneficial.

The right hand side carburettor has a hotter fuel bowl than the left side and I believe this is because the heater tap is mainly underneath it. My engine does not overheat and the temperature range at this time is about midway on the dashboard guage.

We were getting inconsistent readings over a period as he was making adjustments and I believe that the bi metal strip which influences the jet in the fuel bowl may be causing this problem.

My first thoughts are;

1. Relocate the heater tap.

2. Insulate the main fuel line where it comes above the gearbox in the standard fashion.

3. Recirculate some fuel. I believe the factory V8 did not have a fuel return, but the Rover car did and I could plumb a return pipe which may assist to keep the fuel cooler and perhaps 'steady' the bi metal strip?

4. See if there is an alternative to the bi metal strips or find out if they can be faulty.

I am determined to retain my SU setup and would appreciate any advice.


Ian Buckley
Ian G Buckley


I see that you're in OZ, therefore hot?
I've installed some of that silvered fibre mat heatshield material under my carbs. It was a scrap piece left over - just the right size, and possible to fit without dismantling the inlet tract (been there, done that). I did it with a view to reducing the likelyhood of vapourisation. I was out on Sunday in hot UK weather and it was fine.
Are you on a standard fuel pump? no excessive pressure?
Float valves OK? Overflow pipes in place and not dripping fuel?
Recirculating fuel I've also considered - you can drill out carb 2 and fit a return feed with a restricted flow, but you'll also need to fit a higher pressure pump with regulator.
Any other solution will cost............

Dave Wellings


I have some of that insulating material I think I will try that.

Its winter here at the moment, but even in summer my car does not get overly hot, that is why I am surprised these bi metal strips are changing the emissions. I have a fuel regulator set at 2 psi and I use premium unleaded which is 96 octane. I have 10.5 to 1 compression.

The only time the overflow pipes look wet on the end is after the motor is turned off. I don't think any fuel drips, probably the heat sink effect after switching off.


Ian Buckley
Ian G Buckley

I would suspect a problem with one of the carbs, particularly if varying mixture on one of the carbs is the problem, at tuning when hot should overcome any fixed differences between them. Have you compared one with the other using a Colortune? On the V8 you can do this, unlike the 4-cylinder, as the intakes are entirely seperate.
Paul Hunt

The valves in the SU are well known for dribbling and it used to be common practice to up-grade them. Like Paul, I suspect a carb problem and would start here.

Yes - it does get hot in Europe and the principal difficulties with SUs are fuel consumption in traffic or under heavy acceleration - temperature would be an unusual issue !

All this apart, the OE manifolding was never perfect and some cylinder burned a little 'brighter' than others - not a serious problem - part of the character of the car.


I have a set up on my v8 like yours, and i'v also had my car on a dyno to find the right needles for the SU's. With standard BAK needles CO 4.5 on idle the mixture was too poor on high rev's (1.1).
Now it has BDP needle's and the CO now is 4.2 on idle too 6.8 at 5000 rpm.
The dyno operator is very expirienced with SU's but found that the CO level was varying a bit and suspected the heat of the inlet manyfould coursing this as the float chambers are very close to the manifold ( (2-3 mm of space).
My heaterpipe is not that close.
Haven't ever checked my sparks but engine runs perfect and give's 134 HP at the wheels and torque of 258.
Maybe this helps you a bit further,


Great!! I thought I was alone trying to use a British dual carb setup...only, I'm trying to use HS-6's on the stock Triumph TR8 manifold instead of Clive's 'laid back' setup...everybody tells me I'm gonna lose power at top end over a Holley or Carter but I think I'll be equal to them as I'm leaving the HS6's directly over the top of the intake, not behind it....
Tony Barnhill

Thanks Dave, Paul, Roger, Arno and Tony,

That is all helpful, but perhaps I could add a couple more factors. The carburettors have had new everything, but I took them back to the specialist and he replaced the choke O rings as I had heard fuel can by pass them and cause a problem. He said they looked OK but replaced them anyway. He also double checked the float levels.
I have 130 bhp at the wheels at 4,500, we didn't take it any higher. Forgot to note the torque.
Paul, I thought of colourtune and might try that.
Yes Roger, I have noticed the plugs aren't consistent despite coming from the same carburettor. Apparently I have to strike the best average.
I hope I am not barking up the wrong tree, but I still suspect the bi metal part that holds the jet. Is there an alternative?cheers


Ian G Buckley

Well, it is fixed and the plugs are almost lilly white.

Obviously the mixture needed to be set by an expert with a gas analyser on the dyno. The traditional lift the piston and listen system is out the window for me.

I did 150 miles on Saturday, out through the Adelaide hills. 28 degrees C, 14 days into Spring sounds promising!

The car ran perfectly and gave 25 mpg. When I removed the plugs I was very surprised how clean they were. They were in fact cleaner than when I cleaned them before the car was dyno tuned.

Speaking with the main SU dealer in OZ, the bi metal strips don't usually give a problem, but the O rings on the chokes on the HIF can allow extra fuel to bleed past them.

Still might recirculate some fuel and relocate the heater tap.


Ian Buckley
Ian G Buckley

This thread was discussed between 03/09/2002 and 16/09/2002

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