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MG MGB Technical - Misfire at hi-RPM


A '73 MGB with twin HIF exhibits strange behavior. Driving at a contant speed, on a hi-way, at 3,500-4,000 RPM, engine backfire once in a while. Every 2 minutes there is that backfire, which is annoying. I thought it was a carb. synchronism problem. I bought that German synchroniser and carbs. are synchonized at idle. Does someone have faced that problem and what is the solution?


J. G. Catford

Are you on electronic ignition or points? Have you noticed the tachometer at the misfire point?
Allan Reeling

Hi Allan,

It is original 25D and vacuum appears to be functional and advance-dwell-gap are in specs. I replaced vacuum hose which was plastic type with plactic adapters, as I do not like this set-up. I checked oil level in carb. and it was low. After adding oil, problem was less frequent but still present.


Jean G.
J. G. Catford

You have balanced the carbs at idle but now you have to check that the linkages are starting to open both carburettors at the same time
You can do this by eye without the engine going
Simply pull on the throttle cable and check both carb throttles start to move at exactly the same time and if not adjust one of the arms on the cross shaft until its all good
Also check plug gaps as wide gaps will cause high rpm missfiring max-28though
William Revit

You haven't answered Allan's question, which is fundamental to problems of this type.

Is the tach steady throughout or flicking with the missfire?

A backfire in the exhaust when running (as opposed to when on the overrun) usually indicates missing sparks, and the next combustion ignites unburnt fuel that has been pushed into the exhaust.

"you have to check that the linkages are starting to open both carburettors at the same time"

This is also fundamental - it's far more important that the air-flow is balanced when the throttle is open than when at idle. However I believe that checking the balance with an air-flow meter again, this time just off idle (not mentioned in the Manual for HS carbs) is preferable to simply gauging if both butterflies start moving at the same time.
Paul Hunt


Sorry, as Paul mentioned, I forgot to answer that basic question. The answer is no, tach is steady. I adjusted carburetters synchronization with wire gauge and inspected visually when it starts to open both butterflies. It seems OK, but may be problem lies there...

Thanks you

Jean G.

P.S. I will try to check synchronism at 2,500RPM.
J. G. Catford

Synchronisation of the carbs should be done while butterflies are being operated by the throttle cable., not on the idle stops. This can be done using the pedal stop screw.Synch at idle is done after balancing "on the cable pull".
BUT the regular misfire is this on a trailing throttle?
Allan Reeling

"Synchronisation of the carbs should be done while butterflies are being operated by the throttle cable., not on the idle stops"

Book says both - with throttle spindle clamps slackened while doing so at idle first as the carbs need to be independent. Then tighten the clamps, then check off-idle, readjusting the clamps as required.

At least the Workshop Manual does for the HIF, neither that nor Haynes mention off-idle for the HS. Major omission.
Paul Hunt

We each have "our way". I do it by first (engine at running temp), winding out the pedal stop to lift throttles off the idle stops 1200 - 1500 rpm. second wind ONE idle stop down, third, slacken that linkage clamp. Then balance, adjusting with the one idle stop.
You usually have to put this one carb you are using as "balancer", a touch "over" to compensate for, taking up slack up and slight twist in the linkage, so that when you clamp the linkage and screw out the throttle stop, that carb falls back to balance.
Once balanced on the cable, screw in the stops and unscrew the pedal stop, THEN balance using both sops, to you chosen idle speed. THEN pull out choke to fast idle position and balance again using the choke "quadrant" screws.
Worked of me for 40 odd years.
Allan Reeling

Thaks all for your input,

I will proceed next week-end as suggested.

J. G. Catford

Is the mis-fire on a trailing throttle??? If so this can suggest a diagnosis.
Allan Reeling


I am not shure if it is trailing throttle, but lets say I am at 70 mph, constant speed, on a flat highway. It may be near trailing throttle as depression of gas accelerator pedal is constant.

J. G. Catford

have you checked your rotor arm, dissy cap and ht leads
Nigel Atkins

This thread was discussed between 27/09/2014 and 03/10/2014

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