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MG Car Repair/idle problem


QUESTION: Hi Barrie,I own a 1965 sprite with SU carbs. The Sprite has a idle problem that has puzzled me for a long time. The idle will drop from 1,100 rpm (eng warm and not choked) to 600 rpm after coming to a stop and I will have to use the gas peddle or the choke cable to bring it back to 1,100 rpm while stopped. It may or may not occur on future stops during the same trip.  On some trips it does not occur at all. I replaced the throttle cable,lubed the gas peddle and peddle linkage. The Carb leakage is clean,does not appear to be worn,and moves freely.

ANSWER: Hi Al.  Your Sprite has two SU carbs and each one has a speed adjustment screw.  The two spindles are joined together, so one of these screws usually does nothing.  It is possible that you have a little slack in the spindle bushes so that when you come to a halt the second SU has enough free play to shut down too far causing the engine speed to drop.

When the engine has warmed up, disconnect the throttle spindle linkage between the two carbs and then set the engine speed so that both carbs are open by exactly the same amount.  The best way to do this is to remove the air filters and listen to them sucking by placing a piece of rubber tube to your ear with the other end just inside the big suction hole.  Once you have got both carbs nicely balanced, tighten the throttle linkage.  You may need to slightly adjust the tickover on BOTH screws so the engine ticks over at about 850rpm.  

The problem should now go away and the engine will run sweeter.  If the problem persists, get back to me.


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QUESTION: Barrie thank you for your help. I accomplished the steps you called out but the problem still exists. I forgot to mention it also happens upon restart( idle drops to 600 rpm)  and is as erratic as the stopping problem. Both problems occur together. On trips that the stopping problem does not occur the restart problem does not occur.

ANSWER: Sorry about the delay.  The system did not tell me you had a follow-up question.

I notice that you are in a hot part of the USA.  I have had several reports lately of problems with fuel.  I wonder if your problem is fuel vaporization, especially if you are using gasoline with added Ethanol.  

1) You might need to lag the fuel lines to protect them from the heat of the exhaust manifold.

2) The AH Sprite originally had quite a good heat shield.  It is essential that yours is still present and intact.  

Here is a stainless steel heat shield on eBay that looks better than the original one:

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QUESTION: Hi Barrie, My sprite has the original heat shield installed. The problem will occur on moderately cool days(around 60 degrees Fahrenheit). When the idle drops from 1,100 rpm to 600 rpm a small push on the accelerator peddle will bring it back to 1,100 rpm. There is no stalling,or acceleration problems with the car when the problem occurs,just a low idle when stopped. Whats your thoughts on the PCV valve as a possible problem.The manual states to test the valve, remove the oil cap and the engine rpm should increase. This does occur on my engine. Putting the cap back on lowers the RPM. Can the valve be malfunctioning causing the problem? It is a stretch but I an getting desperate. I have cleaned the valve and checked the diaphragm.

Hi Al.  I didn't think a 65 Sprite would have any anti-smog equipment.  Yes, it could be your problem.  The valve is supposed to remain shut at idle and low speed.  If it were to open it would cause a drop in the idle speed.  I seem to remember it sits on top of the inlet manifold using a short length of rubber hose.  As an experiment, try blocking this hose with a wooden dowel.  If the problem goes away, then you have your answer.

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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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