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MG MGB Technical - HIF4 Cabr Problems

I have two MGBs one a 1977 and the other a 1979. I have been trying to balance the carbs on both vehicles.
For the 1977 I bought a full service fit with new needles, jetsand shaft seals. Installed them today making sure all the chambers and jet where blown through with the compressor. Fitted back on the car today but they still won’t balance the front one is the one that makes the louder hissing noise . I am using a gunsons balancing tool.
So I moved back to the 1979, this car has had the manifold gasket change with new studs and washers. Also fitted a new heat shield as the old fibrous one was suspect, new spacer blocks and gaskets. But still have the rear one making the louder hissing noise. Have tried to lift the piston on the rear carb and the engine stalls, if the front one is lifted all the way to the top it makes no difference. Changed the needle and jet with the old one from the 1977 car no change.
Would be gratefull for any ideas for either car as we are now experts at removing the carbs but need to move on to other jobs.
a goldup

Hi,

Personally I can't hear the difference in hissing to balance the carbs, so I use a flow meter that I inherited from the PO.

I had no end of trouble getting the mixture right. Turned out to be that one of the jets in the new restoration kit was the wrong size, 0.1 instead of 0.09. Much frustration and wasted time.

Herb
Herb Adler

Hi

I'm sure you did it, but make sure the linkages between the carbs are undone and free.
c cummins

"make sure the linkages between the carbs are undone and free."

Indeed, it's the only way balancing can be done, otherwise the two butterflies will always be at different positions. And what's even more important than getting them balanced at idle (which is done easily with the idle screws when the clamp screws are slackened) is getting them balanced just off idle, which may require a little trial and error when tightening the clamps.

If you can't shut each carb down to nothing i.e. no hissing when the clamp screws *have* been slackened, then something else is wrong, like the butterfly not properly seated in the throat.

If lifting the front carb piston up makes no difference at all then something is very wrong.
Paul Hunt 2010

Also check the poppet valves on the throttle disc. If the spring is weak it allows mixture through. These caused me no end of trouble when trying to balance & tune.

Pat
Pat Gregory

Thanks Everybody for your posts.
Todays post is on the 1979 MGB
• When balancing the carbs I was using a Gunsons balance tool.
• I had made sure the carbs opened at the same time on the spindles.
• The front carb piston can be lifted up all the way with no change.
• The butterfly valves have been soldered shut.

Took the carbs off tonight with no visible sign of gaskets not seating properly on the carb faces or between the heat shield as we used some high temperature gasket sealant.
So went one step further and removed the manifold gasket. Looks as if it might have been blowing on the exhaust side on piston one but on the inlet port the bottom of the gasket on the engine side was yellow and damp on the front port. Have checked the inlet manifold to make sure it is flat appears ok.
Does anybody know if a sealant can be use on the manifold gasket the one we used on the carb gaskets can be used up to temperatures of 250 degrees continuous.

a goldup

But did you balance them with the throttle interconnecting shaft clamps undone?

If you can't get zero suction on each carb by unscrewing its idle screw clear of the stop then something else is holding the butterfly open.

With the clamps still undone can you both increase and decrease the idle speed by adjusting each idle screw in turn?

If lifting the front piston up makes no change to the engine then it isn't doing anything, which is obviously wrong, you will never get them balanced properly, or the engine running properly, until you resolve why that is. Is any fuel getting through that carb? I.e. could the float valve be stuck shut or the jet blocked? Blowing gently in each overflow port in turn should lift fuel up out of the jet.
Paul Hunt 2010

This thread was discussed between 03/07/2010 and 06/07/2010

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