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MG Car Repair/MGB acceleration runaway


QUESTION: Hi, I have a 1980 MGB, built for the US. I am in California. All smog equipment is removed, I put in a new SU carb, and the ignition is pertronix. I last drove my car about a month ago. I repainted and restored the interior. Now after reconnecting the battery, I tried to start it. It did start quickly but the engine started to race and when it reached almost 5000 rpm I could only shut off the ignition for fear I would do engine damage. I tried several more times and it kept doing the same thing. I did put a new throttle cable on it but didn't use the throttle while starting. I removed the air cleaner and noticed the main piston inside the carb raising up after the car started and seemed to keep rising as the rpms keep increasing. Because I only have a second or two before the motor would reach redline - I would have to shut off the ignition so I can't really do much in that short a time. I'm stuck. I tried tapping the carb all around it with a plastic headed hammer, it didn't help. Can you help me solve this problem?


I think that the throttle of the SU carb is stuck partially open.

This often happens if the end of the throttle cable has been pulled out of the hole that it is located in.  It can happen at either end.  

See this diagram:

Check where the cable 14 goes into part 15 and where the cable goes into the heat shield.

If it is correctly located then you must check that:

a) you have adjusted the length of the throttle cable correctly, and

b) the tickover adjusting screw on the SU is not holding the throttle open too much.

---------- FOLLOW-UP ----------

QUESTION: Thanks for the quick response. I will check my throttle cable, which is brand new and may possibly be a shorter length, which could result in a partially open throttle.The throttle mechanism at the carb end seems to be completely "off". The return spring is working fine too. The gas pedal end termination also seems ok. I believe my throttle does not behave as a partially open throttle, but a wide open, floored gas pedal...because it is reaching 6000 rpm in only a second or so. Does this symptom suggest a somewhat different problem that I can check? I really do believe the throttle cable and linkage is ok. When I start the car I do not even touch the gas pedal. Thanks again.

Hi Bill.  

With your foot off the throttle pedal and the choke control fully pushed home, the throttle butterfly in your SU should be virtually shut.  It should be just a smidgeon open to allow the car to tickover when warmed up.

If your car revs the moment you start it, then for one reason or another the butterfly is much more open and it should NOT be.  Find the reason why the butterfly is open and you will have found your problem.

If you can tell me what type of SU you have fitted (HIF or HS), then maybe I can diagnose the problem further.

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Barrie Jones


MG Sports Cars from 1949 to 1980 - Including MGB, MGBGT V8, TF1500, TF, TD and modern Midgets. I also specialise in SU carburettors, Lucas wiring, suspension, steering & brakes.


I have owned my 1955 TF1500 since 1966. Technical specialist for the TD and TF with the MG Car Club T Register. Also owned 20 MGBs and currently own an MGBGT V8. Written 3 books on MGs and produced a DVD on how to strip and rebuild the TD/TF gearbox.

MG Car Club.

Barrie's Notes on the 1953-55 MG TF (author). /=====/ Barrie's Notes on the MGB (author). /=====/ Barrie's Notes on the MGB GT V8 (author). /=====/ The Essential Buyer's Guide to the MG TD, TF and TF1500. (author)

M Phil, C Eng, BSc (Eng).

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