Hallum Posted August 18, 2015 Share Posted August 18, 2015 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,Harry Quote Link to comment Share on other sites More sharing options...
Velocita Rosso Posted August 18, 2015 Share Posted August 18, 2015 If backfiring then you will probably have a timing issueJust undo the dizzy clamp and try turning the dizzy until the engine runs smoothHowever its best to set up a timing light and use the timing marks on the cover and pulleyYou should be somewhere around 10BTDC Quote Link to comment Share on other sites More sharing options...
nang Posted August 19, 2015 Share Posted August 19, 2015 Replace the dizzy rotor with a red one. A lot of of after market replacement parts are rubbish.Tony. Quote Link to comment Share on other sites More sharing options...
Jonny-Jimbo Posted August 19, 2015 Share Posted August 19, 2015 Do you have electronic ignition fitted? I had a Lumenition unit fitted that gave very odd behaviour. In the end I junked it off and fitted a new old stock set of Lucas points and condenser... ran fine after that.However, as has been suggested, I'd advise checking timing first. Quote Link to comment Share on other sites More sharing options...
Velocita Rosso Posted August 19, 2015 Share Posted August 19, 2015 Quoted from Jonny-Jimbo Do you have electronic ignition fitted? Lucas points and condenser... ran fine after that.. A dicky condensor can also give the same symptomsIt has been know to fit the wrong coil (6v/12v) and condensors can burn out very quickly Quote Link to comment Share on other sites More sharing options...
Nick Jones Posted August 19, 2015 Share Posted August 19, 2015 Sounds like it's now running lean under load though I'm surprised that pulling the choke out doesn't help.Did you check/change the needles at the same time? Is the engine modified at all (air filter/exhaust manifold etc)?Is there oil in the carb dashpots?Nick Quote Link to comment Share on other sites More sharing options...
Hallum Posted August 20, 2015 Author Share Posted August 20, 2015 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,Harry Quote Link to comment Share on other sites More sharing options...
Nick Jones Posted August 20, 2015 Share Posted August 20, 2015 If still running points, check the gap. They don't make points like they used to and the gap can close up incredibly quickly. Condensers can be rubbish too (though normally act up when hot) and as for the rotor arms..... 😲 Nick Quote Link to comment Share on other sites More sharing options...
Velocita Rosso Posted August 20, 2015 Share Posted August 20, 2015 Timing set .......but the dizzy clamps are notorious for being loose ....when you think they are tightDid ours and it came loose within 200 yards...allowing timing to go way out Quote Link to comment Share on other sites More sharing options...
Hallum Posted August 22, 2015 Author Share Posted August 22, 2015 Well,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!Harry Quote Link to comment Share on other sites More sharing options...
drofgum Posted August 23, 2015 Share Posted August 23, 2015 Harry,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. Regards, Paul Quote Link to comment Share on other sites More sharing options...
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