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  Chicagoland MG Club:Driveline
Go Brit Baby! - Tech Sessions

Steve Merical Carburetor Conversion
Changing single Z-S Carb to Twin SUs
March 31, 2001

This month I’ll discuss the single Z-S to twin SU carburetor conversion that was done to Terry Gaskin’s ’75 MGB.

First, the club would probably want me to state the following disclaimer “the modifications and procedures presented may violate state and federal laws” blah blah yada yada… Okay, I added that last part but you get the idea. Proceed at your own emissions’ risk.

Second, here is a little MGB history. When the MGB was introduced the car had 95 HP at 5,400 rpm and ran 0-60 mph in 12.2 seconds. Not a screamer but certainly respectable for these classics. Then around 1971-72 this same engine was modified for use with unleaded fuel and BHP fell to 78.5 at 5,500 rpm. By the time all the emissions and the single Stromberg was added, power had further fallen to 62.5 HP and the car clocked 0-60 in 18.3 seconds - a 34% decrease in power.

So, it’s not surprising that some of our rubber bumper friends would be interested trying to get back some of the power that MG originally meant for these cars to have.

Our master mechanic for this conversion was Tom Sotomayer. Tom has done this conversion a few times and was well prepared for the work that would be required to put a nice pair of S.U.s on Terry’s car.

Tom removed the old ZS carburetor, intake manifold, and exhaust manifold. A quick look at the exhaust manifold and it is readily apparent how restrictive this set up is for the engine. Then there were some exciting moments as he and Terry went at the catalytic converter with a sawzall. Hmm, there are probably many MG applications for the sawzall and we should all consider adding it to our tools list…

One bump in the procedure came when Tom noticed that one of the motor mounts was broken. Fortunately for Terry, Tom is also a Moss distributor and simply went into his shop to retrieve some new motor mounts before proceeding further. Tom then added some shims to the motor mounts, thereby raising the engine just enough for the new SU air cleaners to clear the brake servo unit.

Tom installed the appropriate parts for the conversion. A new intake, SU carbs, linkage, and exhaust manifold. Also, now the car has a manual choke to replace the standard blank dash fitting for this rubber bumper model. Tom also swapped the stock 45D distributor for an earlier 25D model. The 25D distributor’s vacuum advance is better suited for the twin S.U.s. More details and pictures on this conversion can be found at the club’s website.

Terry says that difference in power is amazing. The car is definitely quicker and even better is the throttle response. There seems to be more power available and it doesn’t seem to be as sluggish. Before, his car seemed to hit the power wall at about 3500 rpms. Now the car loves 4500-5000 rpms before shifting. This was impossible before with his old Z-S set up.

Lacking access to a dyno to test the before and after horsepower, Terry agreed to do some before and after 0-60 times on our local toll ways. Perhaps the results are a little impaired by traffic and operator error but the 0-60 times are listed below. Please note that Terry had already beaten the stated 18.3 seconds for RBs because a PO had removed a few of the emission components from the car.

1975 MGB Roadster
Carb Set Up0-60 mph
Z-S Single CarbApprox. 16 secs
Twin SU CarbsApprox. 13 secs

Okay, so what’s the cost for this throttle hungry conversion? Well, Terry spent about $60 for the new air cleaners and $10 for a used 25D distributor. The rest of the deal was a parts swap between Terry and Tom. Terry offered up his hardtop for the necessary components from Tom. Giving that a used hardtop can sometimes be had for around $200-250; I’d put the total cost for this conversion at around $270-320. That’s great HP for the buck given that on paper this conversion could mean up to a 20-25% increase in horsepower.

Terry will be bringing the MGB into the state for emissions testing in a couple of months. He promises to give us an update on test results.

Still interested in more power? Well, check out Grassroots Motorsports’ April article on MGB Power Secrets. They gained nearly 10% HP by adding an Ignitor, K&N air filters, and optimizing the timing. Total cost was around $300.

I’m looking for a project for the next tech session. If you have something you would like the club to help out on please give me a call

Steve Merical   630-443-4985

Z-S carb mounted
SU carbs mounted
Z-S carb parts
SU carb parts

See more pictures and notes on line at:

©2001 Chicagoland MG Club, All rights reserved.