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MG MGB Technical - SU dilemma

On my 79 B the PO changed the Zenith to a single SU HIF44 (3/4") with K&N filter.
I now have the idea to change this to a pair of SU HS or HIF.
The reason is basically to avoid heat problems because of the dual use manifold and maybe get a bit more power.
Considering the costs can anyone tell me if I would really gain anything in power. And on that same note: Does it matter if I use HS of HIF, since I would like to buy a used set and thus have to take what's available.

Carl
C. J. Gaasbeek

If you go by the school of thought of author David Vizard, you won't gain anything but tuning problems by going to two SUs. Realistically, only one cylinder is drawing fuel and air in at a time, so you will have a steadier flow of fuel through a single carb manifold than a dual carb manifold (put into terms that I understand.) This should give better throttle response in typical city driving, at least compared to the log-style twin carb manifold used from the factory. In my opinion, you are already one step ahead of those who have the Weber DGV setup.
Jeff.

Jeff Schlemmer

If you've got a single carb '79, I suppose you have a US-spec car. From reading lots of posts over the years, I believe the actual flow capability of the larger single carb is about the same as the two smaller carbs. Is there a heat shield you could fabricate, similar to the one seen on the dual carb set up for your heat problem?

The SU carb you have now is far ahead of the emissions Z-S, though, as the Z-S had issues with the automatic water choke, plastic fuel lines, etc., etc.

The pinch point for airflow on the late US car is the exhaust manifold, and not necessarily on the intake side. Replacing the late US exhaust with the earlier stock, cast iron manifold is pretty much ideal from a street performance standpoint. There was gentleman who used to post from California who had all this figured out (check the archives).

The other thing to be mindful of is that the performance of the US cars was diminished on the ignition side as much (if not more) as the fuel side. As MG improved the ignition system on the emissions cars, most of the performance degradation of the early emissions cars was erased. Many have had good results using Euro-spec distributors, advance curves and timing.

On the other hand, a pair of SU's looks a good deal better than any single carb set-up. The factory was forced into the single carb solution on LHD cars in order to fit the brake booster - which sits where the air filters want to be. The day-to-day differences between HS4's and the later HIF4's are negligible.

Also, if you go to a real performance rebuild (cam, head work, springs, etc.), then a dual SU (with the larger bore 1 3/4" carbs) set up, or the side draft Weber, would be called for.

Good luck!
John Z

Hi Cj
I just replaced the ZS carb on my '79 B with a HIF44
but instead of keeping the restrictive stock manifold
I put on a Austin Marina manifold and twin down pipes with very good results!I have no heat problems,I just pulled a 4000' mountain at 70mph at 90 degrees yesterday no problems.I have a lot more HP smoother idle You are already have the most expensive parts just get a Austin Marina manifold and an early B downpipe and it is a simple bolt on I have had nothing but compliments on how good it looks.I havent compared it to a '77 B that our club prez converted to twin SUs
but I bet its close.The only small problem is a lack of place to put the egr valve if you need to have your car smog tested If not go for it and you would probably have an easier time finding the marina manifold over there I was lucky and got mine at a swap meet for $50.
Good luck
Pat in Tehachapi
patrick bailey

Thanks all for sharing your thoughts. Since the car is running very well and I'm not doing this for the looks I will try and used Patrick's suggestion and try to find a Marina manifold, maybe in combination with a mild steel exhaust manifold. I have done away with the EGR valve and all airpump related equipment. Once a year I tune the CO down for the anual test and turn it back very quickly after passing.

Pat, Thanks for this wonderfull suggestion. very inexpensive and should be easy to do. Summer looks bright already. One question. do you have a part number from the manifold.

Carl
C. J. Gaasbeek

CJ
No I understand it was off a '74 Marina I also puy an ANSA exhaust on mine and it sounds nice too!Good luck
I put it all on in about 1/2 day I had originally put the SU on the stock amnifold without much improvment but a big difference after the marina swap.
Pat in Tehachapi
patrick bailey

This thread was discussed between 21/03/2004 and 23/03/2004

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