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there can be a wide variety of culprits 

spark plugs   no R suffix    and plugs life and soul of the engine   many dont last well 

points check the gap

damper oil  top it up use engine oil 

is it over advanced ?

tight valve clearances 

choked air cleaner 

and a must  chech the choke is returning the jet to its adjusting nut give the jet a poke with the finger

followed by as a 1500 do you still have waxstats on the jet   

easy to peen open the case remove the wax capsule, add two 1p peices  and the small metal spacer  refit the case 


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11 hours ago, coxy123 said:

Triumph spitfire 1500

Hot a bad misfire/stutter at lower speeds/revs.

At higher speeds it runs great 

Might there be any clues from the history of the problem? Is this a problem which has appeared suddenly, developed over the course of time or been a constant factor following some other event (e.g. engine work, carb rebuild etc)?

A per Pete's reply there's certainly a rather long list of potential diagnoses. Most of which are non-serious but can be effortful to track down. But to start somewhere I'd opt for simple inspections and measures first e.g.

• Wires to/from coil: check good, not corroded or frail

• Distributor: Inspect points (if applicable), rotor arm, distributor cap for defects. Check vacuum advance line is secure at distributor end and carburettor end.

• Fuel line from pump to carbs: inspect for weeping or leaks

• Carburettors; check dashpots have sufficient oil. Check all linkages, including choke mechanism, all operate and return smoothly. Lubricate in any case. Check heatshield between carbs and inlet manifold is present and intact. Check breather pipes from rocker cover to carbs are secure and free from even the tiniest splits or leaks.

• Check fan. With engine OFF (of course!) spin fan with finger. if spins freely, without resistance, the viscous coupling may have failed.

• Lastly, remove sparks plugs, check are correct specification for vehicle, observe condition and colour of each, check gap.

Siimple inspections might be suggestive of diagnosis, if not then more in-depth measures could be needed.




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Just one more thing, based on my own recent experience, is there any smoke when it is misfiring/stuttering? If so, what colour?

I had the same symptoms with white smoke and it was my head gasket blown, cylinder to water gallery. Ran fine at speed but forced coolant into overflow bottle. 

Edited by cliff.b
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If it's when hot then I'd wager some form of vapourisation. I had a Spit 1500 (years ago) that every garage failed to spot was missing the heat shield that goes between carbs and manifold. It's vital that this is in good condition or the car won't run right on hot days (like this past weekend). Higher speeds are fine since the air flow is sufficient to cool the carbs. It's at lower speeds that issues arise. 

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