MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Weber to HIF4's

I have a 78 MGB in great running shape. Recently I took off the ZS Carb, cat, air pump etc. and put on a Weber 32/36. Its O.K. but I now want to try the SU's.
Besides the carbs, heat shield, spacers, manifold etc. is there anything else I should do? I read somewhere I should change distributors. Will that make a difference. Thanks for any input.

Mark Neisser
MARK NEISSER

I know the later SU carbs don't support your vaccume advance, so you have to change it to centrifugal advance. I don't have a clue how to do it, my mechanic did it and i know it didn't require any new parts. Also, make sure you get the foam air cleaners with the wire mesh surround; you'll have to make a cleft in the rear carb to clear you brake MC. I know if you can set up your SU's well, you don't have near the problems you might be experiencing with the weber. Can't help ya other than that, but good job on getting rid of the ZS carb!
Dave
75B roadster
David Ryzuk

Mark. Check out the archives as there is a great deal of information on this subject there. Your current distributor used a manifold vacuum take off. So did the HIF equipped cars. This was an emissions thing. You will not have to change the distributor because of the carb change. If you switch to an SU carb and change the exhaust manifold to the older style or headers, you will gain a significant power increase. You might, therefore, benefit from a different distributor advance curve. Probably the one to look at is the 62-67 low compression distributor curve. HIF carbs can be modified to use ported vacuum--the vacuum comes from the carb rather than the intake manifold. The UK HIF carbs came with this provision and the others can be modified to work this way. Once again, the archives contain complete information. You will have some minor problems with the air cleaners fouling the brake master cylinder or servo unit (power brake booster). This, again, is well covered in the archives. There are several options listed and you can use the one best for you. Les
Les Bengtson

Thanks Les and Dave for the advice. Maybe I'll stick with the Weber if I can get it to stop hanging up.
MARK NEISSER

Here is a site that shows the conversion to twin SU carbs on a late model MGB. Note the K&N air cleaners and how they fit around the brake booster.

http://mgbman.tripod.ca/su.htm
Jim Webb

Jim, That's nuts, I'm looking at that picture now and I don't have anywhere near that much room between my carb and my brake booster.
David Ryzuk

I've seen this conversion done. Those K&N air cleaners on Jim's car are a lot flatter than the picture makes them appear. If I remember correctly, they're probably not much more than 1.5" tall...
Rob Edwards

Mark: Weber problems can usually be cured by jet changes. If you are getting a bog on full acceleration you might want to try a larger accelerator pump jet, a 55 or 60 instead of the 50. Go to aptfast.com and to the Performance Parts warehouse site, and to Carbs Unlimited for lots of tech advice on jets and for detailed illustrations and parts numbers. The DGV series carb is very easy to work on. I think that SU's are the first choice for most B's but since you already have the Weber you should try to get it to work right and spend your money on head modifications that will really release the horses. A well set up Weber will match the performance of the SU's.
Terry D

Terry, Thanks for the advice. My immediate problem is that the throttle is sticking. What I mean is that when I let off the gas pedal sometimes, it doesn't go back to idle. Its not the cable, but something on the carb. If I ''kick" it it goes back to idle?
MARK NEISSER

Mark - You might try adding an extra throttle return spring to the weber - I have one connected from a small hole drilled in one of the manifold washers to the swivel where the cable connects to the carb linkage - gives a lot better pedal feel and more positive throttle closing.
Regards, Pete
Pete Dickerson

OK, Mark. A sticking throttle is common with the Weber. I agree with Mark that the spring is one place to look. You can add a spring or get a stronger spring from the local hardware store. But first make sure the nut holding the lever to which the throttle cable is attached is not too tight. This can cause binding. Make sure your throttle cable is long enough- this is hard to describe but the cable should be long enough to have an upwards arch in it between the carb attachment and the fitting it comes out of at the firewall.
I have seen Webers with very little clearance between the throttle lever and the the intake manifold and this can also be a source of binding. The cure is to use a spacer between the carb and manifold. The places I mentioned above all sell these in 1/8" and 1/4" thicknesses. The 1/8 is the one to get as you don't want to raise the carb too much- there's already very little clearance between the air cleaner and the hood (bonnet). Good luck and let us know how things turn out.
Terry D

Terry, I've tried replacing the spring, now I'll try your other suggestions. Thanks for the help and happy holidays to everyone.
MARK NEISSER

This thread was discussed between 15/12/2001 and 19/12/2001

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.