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MG Midget and Sprite Technical - Question for those using an HIF44 carb.

My recently finished 1275 has an HIF44 carb from an MG Metro on a Titan manifold. I have carefully rebuilt the carb using a Burlen service kit and it seems in good condition. I am using a BBW needle on the recommendation of Keith Calver, with a short stub stack inside an ITG foam filter.

I'm finding that although the car runs well, I am unable to carry out any sort of tuning. Both the Colortune plug and the "lift-the-pin" method show that the mixture is a little weak, and yet I can screw the mixture screw right in as far as it goes without making any significant difference. Lifting the pin even a fraction kills the engine instantly.

Can anyone shed any light on what's going on? Do I need a richer needle, and if so which one?

Mike
Mike Howlett

Are you sure that you have no air leaks in the manifold?
The other thing I would double check is the fuel level in the float chamber.
Guy

Yes both good suggestions especially the float level. When racing SU tended to run out of fuel at high extended RPM, the easiest solution was to increase the fuel level which richened up the mixture significantly, therefore it is understandable the reverse would be true.

However I think this is a matter of the mixture screw not being attached to the jet tube. I have seen a number of these with similar symptoms where the mechanism for moving the jet has become stuck. In your case (just overhauled) this is not due to age and non use but you may have failed to engage the adjustment correctly

This is simple to check, remove the piston assembly (3 screws) from the top of the car and then you can see the jet, check that it moves up and down with the mixture screw.
Bob Turbo Midget England

Ah yes, good point Bob. And I should have remembered because I did exactly that when I rebuilt mine! Short memory!
Guy

I've checked the jet and it is moving up and down as I turn the screw. I'll have another look when I get a moment and see if there any air leaks. What needles are any of you using in your HIF44s?
Mike Howlett

Getting an answer to the question of 'which needle' really requires a session on a rolling road. The most common needle for a HIF44 on an MG Metro is BDL. When I rebuilt my engine to 1330cc, 270+ cam, high lift rockers I thought I would need a richer needle and fitted a number on a trial and error basis. It seemed to run OK although it seemed a bit rich. After a rolling road session, the diagnosis was that I had fitted a chronically rich needle (doohhh) and the recomended needle to change to was - you guessed - BDL. After fitting this needle the car goes like a rocket and recorded 109 bhp on the rolling road. After several track days it is still going strong and seems like the right choice.

I think the lesson is that every engine is slightly different and the needle when non-standard could be anything (weaker or richer or even standard). I recommend a RR session if you have one near you. Best £70 I have spent!

Chris
Chris H (1970 Midget 1275)

Thanks for that Chris. My carb came with a BER needle. Using the comparison chart here http://www.mintylamb.co.uk/suneedle/ it seems that my BBW is much weaker than the BER and a little weaker than the BDL you mention. I think I'll put the BER back in and see how it is with that, and maybe invest in a BDL as well. Unfortunately I don't know of any R/R near me in SW Scotland.
Mike Howlett

I'm reading the graph that I showed the link for (http://www.mintylamb.co.uk/suneedle/), and have realised that I got it back to front. My BBW is a thinner needle, so will give a richer mixture for any given needle height than either the BER or BDL. Strange then, that I have the appearance of a lean fuelling. More investigation needed.
Mike Howlett

Mike. I'm running a BDL on my HIF44 and I went on the link you posted and the graph showed a more or less straight diagonal line. What exactly is the graph telling us? Or am I just thick!?

Bernie
b higginson

Hi Bernie the graph is demonstrating the diameter of selected needles at 16 differing points along its length. Thus the thinner the needle the richer the fuel ratio.
Bob Turbo Midget England

Thanks Bob. Just me being thick then. Again!:)

Bernie.
b higginson

This thread was discussed between 03/06/2012 and 07/06/2012

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