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Misfire at low revs


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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 by Austin Branson
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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.

 

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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

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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.

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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. 

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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

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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!

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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

 

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