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MG MGB GT V8 Factory Originals Technical - HIF6 Float level valves

Help me out guys and bear with me if I ask a lot of questions. Last night I found petrol coming out of one of my carbs overflow vents (factory V8, original carbs). I assume that it is just a question of replacing the float valve (though the fuel pressure may have suddenly become too high). I have had plenty of older SUs apart but never one of these new fangled HIF things. Whats the best way to go about it. Is it best to leave the carbs alone and remove them complete with manifold? Are there any pitfalls which I should look out for or anything else which it makes sense to check/replace at the same time? Are the standard valves OK or should I go for Speedwell (easily available?) or Grose jets (takes longer). Are the jets the same size/ thread size for all SUs (I would expect to be able to get them off the shelf at my local parts shop). I have a high pressure punp and filter king pressure regulator which has been OK up to now though it does "tick" rather a lot - not that lack of fuel is my problem here! How can I check the pressure without a gauge; perhaps one of you has a rule of thumb i.e. so much fuel in so much time? I have been having intermittent difficult starting problems recently,. both cold and hot, could the two issues be connected? Is there any other advice you can offer?

Ian Thomson
ian thomson

Hi Ian,

Take the carbs and extension manifold off together then you won't have to reset all the linkages.

STD valves should be OK, I've had no probs in 5 yrs. I'd replace the float chamber gaskets at the same time.

Other stuff to check

Check the heights of the floats in the float chambers are the same

Check the amount of casting flash in the valley manifold where the two main tracts branch out. On my 1st V8 one of the tracts was significantly restricted - BL build quality again!

Check you've got the same neeedles and springs on both carbs (a DPO this time) STD needles are BBU. If you've got K&Ns you'll need richer needles (can't remember the ### offhand but can find out if you want)The coding paint will have gone from the springs, check they're the same height and stiffness using a small weight (I used a set of feeler guages and found that although they were the same height, one spring sat a 1/4" lower than the other.

Check the resistance of the two oil dampers, you can do this by feel or using weights and a stopwatch. They should be the same but usually aren't. Try doing a mix & match to get roughly equal times. This feature of SUs controls mixture enrichment on acceleration

Although the V8 has twin carbs there's no balance pipe so they can't be balanced as easily as a 4 cyl MG. The best way I've found to balance them is to unscrew the idle adjust screws until both throttle plates are just closed and then screw them both in exactly the same amount to get 800rpm. I know that in theory the flow meter or "hiss" methods should still work but the lack of balance pipe seems to desensitise them.

Anybody else found a better method?

Any parts you need try and consider getting some of their SU dashpot oil. SU's were designed to use single weight oil on their dashpots not multigrade.

Hope I'm not trying to teach gandma to suck eggs. I've just realised I've spent far too much time on this over the years!!

Happy burbling


Philip Shingler

Before fitting new valves I would try and find out what caused the problem in the first place. Disconnect the fuel pump and run the engine till the float bowls are empty. Reconnect the pump and the resultant rush of petrol through the wide-open valves may well clear the dirt or whatever was keeping them open. If it does, and stays clear, all well and good. If it doesn't clear it then you do need to change the valves - I would do both. If it clears it, but happens again, then dirt is getting through and so new valves will probably be affected in the same way. If you have a fuel filter replace it. If not consider fitting one.

Can't imagine any reason why the fuel pressure should suddenly increase if the pump hasn't been changed, it is spring-pressure that pushes the fuel through, not the solenoid.

While you have the carbs off check the condition of the short hose that feeds fuel from the LH carb to the RH (looking forward). Consider replacing this if it is hard or cracked, even at the expense of upsetting the linkages, I always fully setup my carbs after working on them anyway.

I haven't had a problem balancing the air on HIFs (using a gunsons) but when balancing the mixture the lifting-pin only gives a very small difference in response between rich, correct and weak - you have to listen carefully. If you have to meet MOT CO limits then use a Gunsons Gastester and adjust both carbs by the same amount in the same direction to get the correct figure. However my Gunsons is about 2% pessimistic. To be on the safe side I set it to 4% before an MOT, then if the MOT print-out says 2% I reset back to 6.5% on the Gunsons.

Paul Hunt

Thank to you both. Paul your trick worked. I turned off the pump and ran the engine dry. On subsequent restart it was OK. I have new valves for it now anyway, (and a fuel filter)just in case it happens again. The whole incident warned me about a potential problem as the overflow pipes were pouring fuel on a hot exhaust! They are now replaced, extended and relocated out of harms way.

Ian Thomson
ian thomson

Glad to hear, Ian. Mine came without any overflow pipes at all, which I didn't notice till mine overflowed one day! Fortunately the 'neeners' were not required.
Paul Hunt

This thread was discussed between 28/05/2002 and 30/05/2002

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