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Engine bucking/hesitating/backfiring


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Hi All,

After 10 months of working on my MK3 spitfire I finally got it through it's MOT last month! Woohoo! The car has been treating me well ever since. However, an issue has now developed that I can't work out.

On Sunday I finally got round to replacing the carb jets after discovering one of the previous owners fitted different sized carb jets (they were about 15mm different in length). The carbs are twin SU's (HS2).

I tuned the carbs accordingly, and have been having issues ever since. The engine runs smoothly and revs nicely when it not under load, however, under-load is a different story. Essentially, they car is suffering from a loss of power through extreme bucking/hesitation accompanied by an occasional backfire through the carbs. Initially I assumed this was due to ill-tuned carbs, however, after tuning a number of times, I am convinced it must be something else.

I have checked the obvious; spark plugs all good, fuel in both bowls, jets do not appear to be blocked.

I have tried applying the choke when driving but its not changing the running of the engine at all.

Its also important to note the issue is consistent at all temperatures.

Could the timing need to be changed?

Any suggestions/ideas would be greatly appreciated.

Many thanks,


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Hi Guys,

Thanks for the responses.

The engine was timed about 2 months ago to 6 BTDC as per the manual. Is it better to have it set a 10 in that case?

With regards to the ignition, its not electric; standard dizzy. The condenser, rotor arm, and coil were replaced around three weeks ago, and had been running fine up until the point the jets were replaced so I assume it is unlikely to be any of them.

Nick, I thought the same thing initially hence why on multiple occasions I pulled the choke to see if it would make a difference. Also deliberately setting the carbs to run lean off-choke, but with no change.

There is the correct amount of oil in the dashpot.

In the meantime, I shall wait on a friend to borrow his timing gun again.

Many thanks,


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Changed out the condenser and rotor arms for the old ones, and she ran beautifully 🙂. I will aim to find out which was the issue (I think the condenser) in the coming days.

I would still like to replace them both as I think they will be coming to the end of there life soon. I will be buying a red rotor arm as suggested by Tony. With regards to the condenser, has anyone got suggestions where to buy decent quality ones?

Thanks for the help!


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It is so easy and quick to refit the suspect rotor arm that I would do so just to be sure which part was the cause of the trouble. Condenser life is unpredictable. Some seem to have very long lives. Others are not worth fitting their lives are so short. So if the old condenser is not the cause of the poor running don't replace it just because it has some miles and years on it.

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