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  • Hello, I have a 1964 1/2 with a 4 barrel carb 289. Its had work done to it but I'm not entirely sure what has been done, looks to have a new cam that's larger than stock, and has been bored out 6 thousandths of an inch. It also has a new edelbrock carb. After checking the compression I came up with a low of 170 and a high of 180 psi. I was wondering if this is normal compression for this engine. It also has a constant miss in one of the cylinders, and it doesnt go away even at higher rpms. I've tried adjusting the timing and carb idle screws but it hasn't changed, what could the problem be?
 

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Couple dozen things it could be...bad distributor cap, rotor, wire, plug, points, condenser, ground, a broken valve spring, a bad or sticking lifter, a head gasket blown between the narrow area between cylinders or a coolant passage, an unseated or broken piston ring, a cracked head or block...on and on... any blow-by? Steam? Lost or dirty coolant? Milky oil? Pulled the plugs and read the color?
 
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Couple dozen things it could be...bad distributor cap, rotor, wire, plug, points, condenser, ground, a broken valve spring, a bad or sticking lifter, a head gasket blown between the narrow area between cylinders or a coolant passage, an unseated or broken piston ring, a cracked head or block...on and on... any blow-by? Steam? Lost or dirty coolant? Milky oil? Pulled the plugs and read the color?
I originally had a HEI distributor with an internal coil, which I changed out for one with an external msd coil and had no change. Both times I was using low resistance high shield msd plug wires and new spark plugs. I believe everything is grounded well and I've taken the valve covers off to inspect them and nothing was broken with no carbon build up, looked like it was rebuilt in there. There also hasn't been any leaks, smoke, or any change in coolant or oil and when I've removed the spark plugs on multiple occasions they werent coated with with much of anything. The car does seem to run rich, I'm not sure if that could be a possible problem, my fuel pressure is higher than the carbs recommended, could that possibly be flooding it and causing it to miss?
 

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Ok, most carbs don't like more than 5-6 PSI. You don't mention your psi? Too much pressureI can force the float off it's seat and causing flooding. If it was flooding, it would show up as blackened plugs? I have you checked the OHMs of each wire? They should be consistent.
 

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Ok, most carbs don't like more than 5-6 PSI. You don't mention your psi? Too much pressureI can force the float off it's seat and causing flooding. If it was flooding, it would show up as blackened plugs? I have you checked the OHMs of each wire? They should be consistent.
I havent checked the OHMs of the plugs since they are new, I dont remember the spark plugs being blacked much but is has been a while. My carbs recommended psi is 6 I believe and my fuel gauge has been reading peaks of around 7.
 

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I havent checked the OHMs of the plugs since they are new, I dont remember the spark plugs being blacked much but is has been a while. My carbs recommended psi is 6 I believe and my fuel gauge has been reading peaks of around 7.
Do you detect any miss at idle?
 

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Yes, it misses in idle and when it's at higher rpms
Ok, with the engine off, pull each plug boot and place it back onto the plug, but, "not" fully seated. Fire the engine, while idling, pull each plug boot (use an insulated tool) noting any RPM drop. If you pull a plug boot and there is "NO" change in the idle RPM, you have found a bad plug, a leaking plug wire or bad distributor cap contact point?
 

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Ok, with the engine off, pull each plug boot and place it back onto the plug, but, "not" fully seated. Fire the engine, while idling, pull each plug boot (use an insulated tool) noting any RPM drop. If you pull a plug boot and there is "NO" change in the idle RPM, you have found a bad plug, a leaking plug wire or bad distributor cap contact point?
Alright I will try that tomorrow, I dont think it's always missing but its definitely missing very often
 

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Ok, with the engine off, pull each plug boot and place it back onto the plug, but, "not" fully seated. Fire the engine, while idling, pull each plug boot (use an insulated tool) noting any RPM drop. If you pull a plug boot and there is "NO" change in the idle RPM, you have found a bad plug, a leaking plug wire or bad distributor cap contact point?
Which cylinder is missing?
 

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What about just verifying the firing order is correct for the cam you have? I should have mentioned earlier.
 

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On an older 221/260/289/302 with a constant miss the first thing I would suspect is "inductive crossfire". This occurs on engines where 2 adjacent cylinders are also back-to-back in the ignition sequence (firing order). The this case, cylinders 7 & 8. Somewhat like when the coil makes a spark, what happens is that when cylinder 7 fires the electrical charge flowing through the spark plug wire creates a magnetic field. When the spark is "finished" and the magnetic field collapses, it collapses across the nearby wire, causing a spark and prematurely igniting the mixture in the following cylinder which is coming up on compression, causing a miss. When the next cylinder fires, there is nothing left to ignite and you have the miss.

When routing plug wires on the SBF with the 1-5-4-2-6-3-7-8 firing order, on the driver's side loom the #7 wire should be at the front and #8 at the rear so the plug wires go 7-5-6-8 and that wires 7 & 8 are kept as far away from each other and, if they have to cross, do so at a right angle to minimize the distance they are somewhat parallel.

Other than that, such a constant miss could also be a bad spark plug or wire, which can be checked for resistance, a cracked distributor cap, worn distributor cam lobe, etc.
 

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Discussion Starter #15
On an older 221/260/289/302 with a constant miss the first thing I would suspect is "inductive crossfire". This occurs on engines where 2 adjacent cylinders are also back-to-back in the ignition sequence (firing order). The this case, cylinders 7 & 8. Somewhat like when the coil makes a spark, what happens is that when cylinder 7 fires the electrical charge flowing through the spark plug wire creates a magnetic field. When the spark is "finished" and the magnetic field collapses, it collapses across the nearby wire, causing a spark and prematurely igniting the mixture in the following cylinder which is coming up on compression, causing a miss. When the next cylinder fires, there is nothing left to ignite and you have the miss.

When routing plug wires on the SBF with the 1-5-4-2-6-3-7-8 firing order, on the driver's side loom the #7 wire should be at the front and #8 at the rear so the plug wires go 7-5-6-8 and that wires 7 & 8 are kept as far away from each other and, if they have to cross, do so at a right angle to minimize the distance they are somewhat parallel.

Other than that, such a constant miss could also be a bad spark plug or wire, which can be checked for resistance, a cracked distributor cap, worn distributor cam lobe, etc.
Alright thanks I will try moving wire 7 and 8 away from each other and see if it fixes the problem.

The distributor, coil, spark plugs and plug wires are all new, but I was wondering if it's possible that they arent compatible with each other causing a weak or miss spark?
 

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What carb are you running? Is the choke closing and opening when it should? Take it back to baseline and on a hot engine work your idle mixture screws in or out equally, make sure float is set correctly and jet/seat/needle the carb as its appetite desires. Too many people associate a higher CFM carb with streetable performance.
 

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Discussion Starter #18
What carb are you running? Is the choke closing and opening when it should? Take it back to baseline and on a hot engine work your idle mixture screws in or out equally, make sure float is set correctly and jet/seat/needle the carb as its appetite desires. Too many people associate a higher CFM carb with streetable performance.
I have an edelbrock 1406, I've set the choke and idle mixture screws to what I believe is correct, the idle screws are almost as far as they can go in without dropping rpms. I havent messed with the jets or float height. (The rpm at idle is set to 600-650 when its warmed up)
 

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I have an edelbrock 1406, I've set the choke and idle mixture screws to what I believe is correct, the idle screws are almost as far as they can go in without dropping rpms. I havent messed with the jets or float height. (The rpm at idle is set to 600-650 when its warmed up)
Typically, the inability for the idle mixture screws to make any significant difference is caused by the throttle plates being open too far at idle, causing the transition slots to be partially uncovered, causing the carburetor to be on the transition circuit as well as the idle circuit.
 

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Another candidate for the Edelbrock Tuning Kit with step-up springs, rods and jets !
 
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