Austin Branson Posted August 15, 2020 Report Share Posted August 15, 2020 (edited) Greetings, Oh Sage Ones! Here we go again. It’s a supercharged CF, and the lumpy tickover led me to install a 123TUNE. A bugger to set up, but after it ran sweetly. My wife said ‘it’s making a funny noise when you accelerate’. My hearing is pretty poor, and I worried that it was pinking. Long story cut short, it now pulls really well above about 3,000rpm, but misfires horribly below that, cuts out on tickover, difficult to restart. I’ve changed the fuel filters, reset the timing a dozen times, stripped the carburetor (SU HD8), and closed the plug gaps from 30 to 25 thou. Plug gaps had the biggest improvement. Anyone got any ideas? Regards, Austin Edited August 15, 2020 by Austin Branson Quote Link to post Share on other sites
RogerH Posted August 15, 2020 Report Share Posted August 15, 2020 Hi Austin, Look at the plugs. Modern plugs do not seem to last any time at all. On my 4 pot I have given up on NGK and am now using Champion. On my 4A with SU's it would misfire at low RPM with a lot of throttle. Roger Quote Link to post Share on other sites
John McCormack Posted August 16, 2020 Report Share Posted August 16, 2020 12 hours ago, Austin Branson said: Greetings, Oh Sage Ones! Here we go again. It’s a supercharged CF, and the lumpy tickover led me to install a 123TUNE. A bugger to set up, but after it ran sweetly. My wife said ‘it’s making a funny noise when you accelerate’. My hearing is pretty poor, and I worried that it was pinking. Long story cut short, it now pulls really well above about 3,000rpm, but misfires horribly below that, cuts out on tickover, difficult to restart. I’ve changed the fuel filters, reset the timing a dozen times, stripped the carburetor (SU HD8), and closed the plug gaps from 30 to 25 thou. Plug gaps had the biggest improvement. Anyone got any ideas? Regards, Austin The electronic ignition. Quote Link to post Share on other sites
Mike C Posted August 16, 2020 Report Share Posted August 16, 2020 +1. Do your plugs fire in the air with a crisp, blue spark? Quote Link to post Share on other sites
Peter Cobbold Posted August 16, 2020 Report Share Posted August 16, 2020 Austin, Refit the original disy - I suspect the electrickery too. Peter Quote Link to post Share on other sites
astontr6 Posted August 16, 2020 Report Share Posted August 16, 2020 5 hours ago, John McCormack said: The electronic ignition. HT leads! and coil s/b checked! Closing up of the spark plug gaps is a clue for me? Bruce. Quote Link to post Share on other sites
Tim D. Posted August 16, 2020 Report Share Posted August 16, 2020 On my SC setup (holley rather than SU) I had awful trouble with low speed misfires. Not ignition, but weak mixture. Had to fatten it up loads the it ran great. But without an air fuel ratio meter it is hard to track this sort of thing. Tim Quote Link to post Share on other sites
Austin Branson Posted August 16, 2020 Author Report Share Posted August 16, 2020 1 hour ago, Peter Cobbold said: Austin, Refit the original disy - I suspect the electrickery too. Peter Hello Peter, I did that, and the problem was still there. I git the dizzy from Kai, so I’ll drop him a line and ask if he has any ideas. Stay well, great to hear from you. Quote Link to post Share on other sites
Austin Branson Posted August 16, 2020 Author Report Share Posted August 16, 2020 4 minutes ago, Tim D. said: On my SC setup (holley rather than SU) I had awful trouble with low speed misfires. Not ignition, but weak mixture. Had to fatten it up loads the it ran great. But without an air fuel ratio meter it is hard to track this sort of thing. Tim Hello Tim, I have a UEGO gauge, and it says it’s OK, but I shall try richening up the mixture. Thankyou. Quote Link to post Share on other sites
Peter Cobbold Posted August 16, 2020 Report Share Posted August 16, 2020 58 minutes ago, Austin Branson said: Hello Tim, I have a UEGO gauge, and it says it’s OK, but I shall try richening up the mixture. Thankyou. Austin, If its misfiring the UEGO will read leaner than the carb is delivering. So maybe the carb has gone over-rich. Has the float sunk, or supply over-pressured? Plugs sooting up ? Peter Quote Link to post Share on other sites
Austin Branson Posted August 17, 2020 Author Report Share Posted August 17, 2020 On 8/16/2020 at 1:05 PM, Peter Cobbold said: Austin, If its misfiring the UEGO will read leaner than the carb is delivering. So maybe the carb has gone over-rich. Has the float sunk, or supply over-pressured? Plugs sooting up ? Peter Hi Peter, the plugs are very sooty, and wet. It’s always ran rich, but I left it because I worried about pre-ignition. Now I’m worried that I will not be able to identify the real situation. Help! Quote Link to post Share on other sites
Mike C Posted August 17, 2020 Report Share Posted August 17, 2020 The sooty plugs could be the cause of your misfire. Quote Link to post Share on other sites
Peter Cobbold Posted August 18, 2020 Report Share Posted August 18, 2020 13 hours ago, Austin Branson said: Hi Peter, the plugs are very sooty, and wet. It’s always ran rich, but I left it because I worried about pre-ignition. Now I’m worried that I will not be able to identify the real situation. Help! Hi Austin, Do new plugs cure the low speed misfire? if they work OK up until they foul with soot, that points to mixture rather than spark ( assuming the spark timing is OK) Also, did the misfire appear suddenly?-if so I'd suspect sparks more than the SU. Since neither disy cures it, maybe it is the 12volt feed from the loom or plug leads. Check the fuel level in the jet with the dome/piston/needle removed and ignition on, which will tell you if the float or fuel pressure is faulty ( assuming electric pump). Is there a bypass valve in the Vespertini blower? ( however I cant see one in your photos: https://supertrarged.wordpress.com/2013/05/14/austin-bransons-tr6-verspertini/ Check also for air leaks at blower/head joint, or a part-open back-fire valve Is the blower belt slipping? but you would see that as lack of boost on the gauge. Or hear the screech. Peter Quote Link to post Share on other sites
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