Just recently I noticed a miss at part throttle. I pulled all the spark plugs and they all looked good. I did notice that the #5 spark plug boot (Accel wires) was burned through where it sits close to the exhaust manifold. Last night, I had the engine idling, in the dark, but I didn't see any arcing in that area. I couldn't actually see the plug boot but I didn't see any light coming from the area. I will replace that boot, but do you guys think that's the problem?

 

pull the plug wire while the car is idling,no difference,thats the problem,bigger miss ,not the problem

 

If the miss is noticed just a part throttle (less load)
it probably isn't the plug. Too much timing advance
maybe??

 

blue427 - I will try pulling the plug wire, and try to avoid the shocking.

LT4 - I have been playing with the timing lately, but I don't hear any pinging.

 

Replace the plug wires with a higher quality set and road test....

 

 

If the miss, or feeling of a misfire, occurs only at part throttle cruise you're going to have to do some diagnostics before you start throwing money at it.

But first, when did the problem show up and what did you do that may have caused it?

The easiest test is for over-advanced ignition is to drive the car under the conditions where the problem occurs to confirm that it is a predictable part-throttle problem. Pull the vacuum advance line off the distributor can and plug the line. Duplicate your driving routine where the problem showed up last time and see if it goes away. If it does go away, you have too much ignition advance. Vacuum advance is highest at part-throttle cruise. The feeling it gives you is a surge, like you're slightly manipulating throttle position.

You could also have a lean condition in the carb. The surge feeling is about the same since it shows up as a lean random misfire. The affected carb circuits are the idle and transition. Under light throttle, you're probably not in the main circuit, or at least not by much. It really depends on how you define light throttle cruise. I'm referring to throttle position just a crack above idle. If it is a lean misfire, you may be able to cure it by fattening up the idle circuit by turning the adjustment screws out to the point where the idle starts to faulter or the idle vacuum dips. Then turn the screws in just enough to recover the engine.

If it's in the main circuit, then you'd have to address this with modifications on the main circuit like step-up spring, metering rods or main jets. And, yes, sometimes your carb is just dirty and needs a good, thorough cleaning.

Secondary ignition defects will show up under a load. The ignition isn't being taxed under idle and cruise since cylinder pressure is very low. If the problem isn't showing up under heavier throttle application, then it's probably not going to be solved by throwing money at plugs, wires, and new components.

 

Q

 

Guys - Thanks for the suggestions so far.

Gerry - I am not using vac advance, so we can rule that out. As for the carb, it is a Qjet rebuilt by Lars about 1,000 miles ago. I do have another Qjet, in good condition, that I will try after I replace the boot and try pulling plug wires one at a time.

 

NEW wires,, why fool around,, you have one damaged,, change them and make life easy. 99 Nassau Blue

 

I can't argue with the suggestion - the only reason I'm hesitating to change the wires is that right now I have stock exh manifolds with the wires routed through the original shielding behind the manifolds to the plugs. (I installed most of that when the engine was out.) This winter I plan to put headers on it which will change the wire routing.

 

Problem solved - Big G & Jimcork were right - problem was in the plug wires. I changed the two burned plug boots, for a temporary fix, and the miss is gone. Thanks to all who replied.