i'm tired of seeing my plugs get so black and I mean as black as it can get while warm up and idle... I've tune the carb to idle crisp and clean it starts instantly and it doesn't burn your eyes or smoke any black smoke at idle.. c23 bbc 588ci brodix hh conv cast intake single 1250 apd carb base timing is at 30* I jet down in the summer to 93 main jets for 100* weather... the nitrous tune is very very lean plugs look clean the first and second pass any more than 2 passes plugs look so dark is wired.... I've seen other conv headed motors run 10 passes on the same plugs and plugs look way better than mine shutting it off clean and pulling the car everywhere
69chevelle
Initial timing low or phasing will cause it to not completely burn, also if 93's are just too big.... But if you make a jet change be aware it will cause the no2 tune to go off the rails
I'm having a simular issue with my 565 on c16. Sorry no help here but definately suscribed to the thread.
^^This.
Changing main jetting will make no difference to idle mixture. All that will do is mess up your WOT ratio and nitrous tune.
Black plugs are either too rich, not enough timing, or too cold a plug for the combination of the first two.
I'm assuming it's got a cold plug in it (as it's a nitrous engine) so a rich idle will be compounded by a cold plug which isn't able to burn the deposits off and keep itself clean at idle.
Lean the idle circuit out until it's too lean. . ie it bucks/surges or it tries to stall when you pull it in gear, or it gets a hesitation when you hit the throttle quickly. Then richen from there to get rid of those symptoms. You want it to idle as lean as it can without hesitating.
You might find you need to go up on the idle air bleeds to get the idle/transition mixture where you want it.
As said above; when the idle is right, it will be cold blooded and require some throttle feathering when it's cold. Idle tuning needs to be for a warmed engine. No good having it perfect for a cold start; that's the same as driving around all day with the choke on.
Make sure it still comes up on the brake clean either testing it or on a motor pass (if you use a transbrake) or do a couple of WOT footbrake pulls on motor after you've made the adjustments (just to make sure you don't introduce a lean backfire situation which could get ugly when you're spraying).
JMO.
Changing main jetting will make no difference to idle mixture. All that will do is mess up your WOT ratio and nitrous tune.
Black plugs are either too rich, not enough timing, or too cold a plug for the combination of the first two.
I'm assuming it's got a cold plug in it (as it's a nitrous engine) so a rich idle will be compounded by a cold plug which isn't able to burn the deposits off and keep itself clean at idle.
Lean the idle circuit out until it's too lean. . ie it bucks/surges or it tries to stall when you pull it in gear, or it gets a hesitation when you hit the throttle quickly. Then richen from there to get rid of those symptoms. You want it to idle as lean as it can without hesitating.
You might find you need to go up on the idle air bleeds to get the idle/transition mixture where you want it.
As said above; when the idle is right, it will be cold blooded and require some throttle feathering when it's cold. Idle tuning needs to be for a warmed engine. No good having it perfect for a cold start; that's the same as driving around all day with the choke on.
Make sure it still comes up on the brake clean either testing it or on a motor pass (if you use a transbrake) or do a couple of WOT footbrake pulls on motor after you've made the adjustments (just to make sure you don't introduce a lean backfire situation which could get ugly when you're spraying).
JMO.
You guys need to tune-up the transition slot mixture by using a restrictor in the gasket face of the body in the passage to the T-slot.
The T-slot in Dominators is too long. T-Slot above the throttle blade is air bleed until the blade is opened up the slot, then it becomes fuel feed. When the blade is low on the slot, at idle and low engine speed (staging, burn-out, return road, etc.) the long slot is too big an air bleed and as the throttle is opened it becomes too big a fuel feed. At low throttle, the too big bleed condition requires a small bleed on top so it will run at all, then as the throttle is opened and the slot ceases being a bleed, it gets too rich because the slot is such a big fuel feed.
The solution is a shorter T-slot (which is available in a custom carb from Bo Laws Performance - they machine the carb body and install an insert with a short slot) or simply put a jet in the passage feeding the slot.
Most single carb Dominators will run well with a .040” idle jet and somewhere close to .070” idle air bleed with a .052” T-slot jet.
Dual carb Dominators need smaller idle jets because there is twice as many of them. Dual carb IJ, .030” or less, with IAB near or smaller than .062”.
The Dominators are not all the same because of random T-slot locations in different production runs of carbs. Because no two carbs are the same but mostly because no two engines are the same, the air bleed and T-slot jet can vary. Again, most single carb Dominators will run well with a .040” idle jet and somewhere close to .070” idle air bleed with a .052” T-slot jet.
Usually, it is best to start with a T-slot jet of .052” and play the Idle Air Bleed between .060” and .090”. If it acts too lean at small throttle, raise the T-slot to .055” and play the idle air bleed up and down again. Rinse - Repeat
The deal is to start out with the idle and low speed-transition circuit so lean it won’t run well and then sneak up on it adding a little fuel at a time until it runs smooth. Just add enough fuel to make it run without being stubborn when it is warmed up. This way you find the minimum amount of fuel, the “lean limit” for low-power running, which will keep the engine clean when it is only making 20 to 50 HP. A race engine does not need a power AFR when it is only running at 10% of its total power.
This is the opposite of the plug-fouling condition you have now. A correct idle circuit tune-up will be cold-blooded. If your carb is rich enough to run the engine without warming up for a few minutes, it is too rich and that is the reason your plugs are fouling.
(Of course, be sure your fuel solenoids are not leaking.)
The T-slot in Dominators is too long. T-Slot above the throttle blade is air bleed until the blade is opened up the slot, then it becomes fuel feed. When the blade is low on the slot, at idle and low engine speed (staging, burn-out, return road, etc.) the long slot is too big an air bleed and as the throttle is opened it becomes too big a fuel feed. At low throttle, the too big bleed condition requires a small bleed on top so it will run at all, then as the throttle is opened and the slot ceases being a bleed, it gets too rich because the slot is such a big fuel feed.
The solution is a shorter T-slot (which is available in a custom carb from Bo Laws Performance - they machine the carb body and install an insert with a short slot) or simply put a jet in the passage feeding the slot.
Most single carb Dominators will run well with a .040” idle jet and somewhere close to .070” idle air bleed with a .052” T-slot jet.
Dual carb Dominators need smaller idle jets because there is twice as many of them. Dual carb IJ, .030” or less, with IAB near or smaller than .062”.
The Dominators are not all the same because of random T-slot locations in different production runs of carbs. Because no two carbs are the same but mostly because no two engines are the same, the air bleed and T-slot jet can vary. Again, most single carb Dominators will run well with a .040” idle jet and somewhere close to .070” idle air bleed with a .052” T-slot jet.
Usually, it is best to start with a T-slot jet of .052” and play the Idle Air Bleed between .060” and .090”. If it acts too lean at small throttle, raise the T-slot to .055” and play the idle air bleed up and down again. Rinse - Repeat
The deal is to start out with the idle and low speed-transition circuit so lean it won’t run well and then sneak up on it adding a little fuel at a time until it runs smooth. Just add enough fuel to make it run without being stubborn when it is warmed up. This way you find the minimum amount of fuel, the “lean limit” for low-power running, which will keep the engine clean when it is only making 20 to 50 HP. A race engine does not need a power AFR when it is only running at 10% of its total power.
This is the opposite of the plug-fouling condition you have now. A correct idle circuit tune-up will be cold-blooded. If your carb is rich enough to run the engine without warming up for a few minutes, it is too rich and that is the reason your plugs are fouling.
(Of course, be sure your fuel solenoids are not leaking.)
Could be too much intermediate fuel, loading up a little in the burnout. Try going up one jet size and go up .020 on the intermediate air bleed.
My flat motor always idle clean with nice looking plugs but when I went to domes, plugs were not clean same carb and heads different camshaft. Nitrous was clean with both.
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