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MG MGB Technical - Cleaning the carbs

Well, next up on my 73B Roadster is dealing with the intermittantly overflowing HIF4 carbs. We discussed this issue here a few weeks back.

I've picked up a used set of HIF4 carbs on Ebay. Since I like to keep as many truly original components as possible on my car it's my intention to clean up these used carbs and temporarilly put them on the car while I rebuild the originals. I know a lot of you might have something to say about this double the effort plan, but I'm afraid I'm too old and set in my ways to be swayed! This is of course assuming I'm able to get these working properly.

My first question for the process is pretty straight forward. I've bought a gallon can of carb cleaner (includes a dipping basket) in which to soak the carbs. It clearly states on the can to avoid putting in any rubber etc. components.

What should I look at in terms of dismantling the 2 used HIF4 carbs before soaking? I would think there is some sort of gasketing incorporated in them that needs to be removed. I will study this whole process more before proceeding but I thought I'd get a little start up info here if possible.

Thanks,
BH Davis

BH Davis

It's really simple: unscrew the square(ish) bottom lid and empty it from all contents? Assembly is the reverse of removal....

My only advise is to do one carburettor at a time to avoid parts being switched; the piston and dashpot are especially critical.
Willem van der Veer

BH, Here is a SU HIF carburetor rebuild article by the Chicagoland MG club with photos.
http://www.chicagolandmgclub.com/photos/hif_carb/
This article contains lots of information on the adjustments, checking and theory of SU carburetors.
http://www.jcna.com/library/tech/tech0006.html

Clifton
Clifton Gordon

Thanks guys.

Clifton, I had seen the Chicago club article but the other one is new info. I really appreciate your help!

Thanks,
BH
BH Davis

When cleaning carbs, I heartily recommend that you do them ONE AT A TIME, so as to NOT confuse parts between the two. The dashpot assemblies are the parts NOT to confuse as they came matched for carb performance from the factory and if you intermingle the bits, you might just lose that matching . . . if it still exists on a multi-owner car.
Bob Muenchausen

Take a lot of digital pictures. I don't think I could ever do without a cheap digital camera in the shop again! If you think you have mismatched parts (a previous owner mixed up the dashpots and pistons) you will need to do some timed drop tests to get the best and most consistent results. The HS pair on my TD were BOTH marked one and two at different points by different POs.... what a mess!
Dave Braun

Oh, and about the carb cleaner, I use that stuff on Boat carbs, to help clean out the small passages. There aren't too many small passages on our SUs. I've had good luck with spray carb cleaners. If I felt a soaking was a good idea, then I would spray first to get the majority of the grime off, and then soak in the bucket. But I don't think it is essential.
Dave Braun

In my shop we call that gallon of carb cleaner with the basket "can-o-cancer" Please be carefull with that stuff. It does clean the carb bodies really well though. Steve
Steve G

By all means DO the piston drop test as outlined in the EMISSIONS section of the Bentley manual, regardless whether your think the parts were mixed or not. What you may find is that they don't drop at quite the same rate and if too far off on one from the specs given, they simply won't rise quite as they should, screwing up your mixture.

The drop test actually is a measure of how each air piston/dashpot assembly responds to engine vacuum.
Bob Muenchausen

This thread was discussed between 19/06/2007 and 21/06/2007

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