Is that the highest vacuum reading you can get with that cam? I did not think the 268 cam was that large to lose vacuum and why I ask.
Why did you back the timing off from 12* to 10* is it because that is what the factory setting should be?
Well if so right now that is only a guide as the motor is not stock because of the cam and maybe carb?
With the timing light did you check the mechanical, vacuum and total advance on the dist.? This all has to work and then be adjusted for you motor setup. You can use the factory settings as a guide only.

For the miss & cap: When was the last time a full tune up was done, plugs / wires / cap & rotor / fuel & air filters / check timing and adjust carb to best lean idle and then set idle speed.
To lean a mixture can cause a miss fatten it up (make it richer) a little and see what happens. Then again you said with timing light so spark is not getting to the point where the light is hooked up at so cap / rotor / wires as a start. If still an issue I may try the IGN box (because it is easy to swap out) and hold on to the one removed if not fixed, then look at the dist. pick up coil.
If wires are new, at night using a spray bottle of water motor running, spray the wires looking for sparks if so bad wires and can cause a miss as the spark is going to ground before it gets to the plugs.
Have you tried another cap to see if it moves around like yours? They will move a little but not to the point of changing timing or idle. Besides if you set the timing and don't move the cap it should not change.

The back fire is the throttle closing to fast. Some carbs used a dash pot, diaphragm & spring thing, to slow down the closing of the throttle to keep this from happening.
Dave ----

 

Why is your idle speed so high? 1000 rpm idle is a little high. Your idle mixtures screws will have little affect at that high idle speed.

I would also pull the site plugs on the side of the carb and check your float levels. A backfire indicates a rich condition, the excess fuel is lighting off in the exhaust system. If the float levels are high the fuel may be spilling over into the engine when you let off the gas pedal.

 

 

Part of the tuning on a Holley carb is float level then do the idle speed & mix.
If the level checks out then I hole to my story of the throttle closing to fast that would also cause it to be rich because you are putting fuel in then stop cold turkey.

As for timing if the 12* BTDC for starting then ok. I would check to make sure the mechanical advance is returning to base timing, the weights may be getting stuck out.
Look out on Youtube how to check each part of the timing and how to adjust them.
Dave ----

 

Hmm, that doesn't sound right at all. All the cap and rotor do are direct the spark to the appropriate cylinder. There's quite a bit of "slop" in that. In other words, the tip of the rotor is what, a half inch wide? All you need is some portion of the rotor tip aligned with the distributor post, and the spark will get through.

Picture it like this. You're a passenger in a car, tasked with tossing a tennis ball through an open barn door as you slowly drive next to it. It doesn't matter whether the ball passes through the barn door opening at the very beginning, in the middle, or at the very end. Imagine you practiced and got very, very good at this simple task. You had drawn a line in the dirt as your reference point to toss the ball. Next, a construction company came along and moved the barn about one foot to the left or the right as a practical joke, but careful guys that they were, your mark in the dirt was not disturbed. Unless you had previously just been missing the posts at either side of the opening, the ball will still sail through the opening.

The relationship between the rotor and distributor post is like that. The line in the dirt is the ignition timing, relative to the crankshaft. You are the rotor. The ball is the spark. The barn door opening is the post inside the distributor cap.

Judging by your observation with the timing light, showing the spark drops off as the cap is moved, that makes me think you were just catching the edge of the door opening, so to speak. I suspect the spark only drops off when the cap is moved in one direction, not both. If so, your distributor may be off by one tooth. If you can figure out which way it needs to move, reinstall the distributor one tooth over. Then you'd be hitting the middle of the door opening, not the edge. This would give you plenty of wiggle room to adjust the timing.

 

Moving the distributor one tooth is not going to affect the relationship of the rotor to the cap if that is the problem. When he puts the timing light back on it and moves it back to where it was, he will be back where he started before he moved it a tooth.

 

Your carb have a dashpot? Is your emissions system still in play?

 

 

I'm getting a brain cramp, doing all the mental aerobics trying to figure this out, LOL.

Look at post #1 again. He said this all started after making minor timing and carb adjustments. So this is a new problem, caused by something he did. And then he later went on to say he watched the ignition fail (timing light stopped flashing) to certain cylinders as he wiggled the distributor cap. He's got a verified, repeatable fault. I'd certainly want to concentrate on that one before digging any deeper elsewhere.

Does the cap wiggle failure prove my fabulously detailed barn door theory? Heck if I know... But if the rotor/post alignment had been right on the edge before the latest timing adjustment, it makes sense.

I think it would really help if OP could repeat the wiggle test. See if the fault only happens at one end of travel. Remember, it's perfectly normal to have a little bit of play. The manufacturing tolerances aren't that tight. But that normal amount of play shouldn't make the ignition cut out. The fix isn't to weld/glue/secure the cap so it can't move. The fix is to get the rotor/cap alignment back to normal so a small (normal) amount of play won't matter.

Now, would moving the distributor shaft one tooth take care of this? I think so, but I haven't had enough coffee yet to prove it.

Respectfully submitted, drinking my coffee.

 

I am with DaveF, moving it 1 tooth will do nothing because you will have to re-aline the distributor to get it back in time.
Now don't think of the cap moving right now I will get to it.
Say base timing is 10*, we now move it 1 tooth and say the moves it 20*. Now that could add to the 10* and be 30* or move it to -10*.
Either it will not start at 30* or run like crap at -10* So now you put a timing light on it and set it back to 10* where it was and all you did was move where the vacuum can was.
So what did you do other than waste time?

Now the cap moving. All this will do is change the timing just like moving the distributor body that is it and no more.

He has something else going on if he is not getting spark on the wires to them cylinders.
He did not say if when it showed no spark with the light did the motor also miss?
My guess it did not as if it did on all them cylinders it would not run.
Dave - - - -

 

The cap moving will not change the timing. It will change the "barn door" opening where the spark has the opportunity to jump to the correct terminal. There is really no adjustment for this that I know of, short of getting a new cap and rotor. Of course there is some leeway in this system, as the centrifugal weights and the vacuum advance change the timing the rotor does move. Just so long as it stays in the neighborhood of the cap terminal.

The distributor has two jobs; 1. Generate the spark at the right time and 2, distribute the spark to the right cylinder. When you spin the distributor around to set the timing, the spark distribute part on top of the assembly moves with it, it is set mechanically by the keyway in the rotor and the slot in the distributor cap.

There are exceptions to this, and mentioning them may help it become clearer. The EECIII system had a crank sensor. That set the timing, no moving it. But it still had a distributor that had nothing in it but a shaft, rotor and the distributor cap. It still had the spark distribute job, and what was challenging was to twist the gutted distributor around to the correct position to coincide with the crank sensor. I have a 99 tahoe with a vortec 350 in it, and it is the same way, it has a crank sensor, but still has a distributor, and you have to put a scanner on it to get it set right so the rotor is pointing center to the correct dist cap terminal when the crank sensor and the computer generate the spark.

 

I havent made it to studying up on dizzies yet. Continuing with this same thinking, what if the gear on the distributor is loose?

 

The idle would be unstable, you might get a little hunting and variation in rpm. Though a lot of the play would be taken out by the oil pump. The distributor drives the oil pump.

 

I am going to say no & yes on the barn door thing.

The star pickup will tell when the spark is made it just has no place to go till the rotor & cap line up to pass the spark off.
Now if this cap / adaptor can move say 1* based on say a 0 set point. So the spark can happen 0 to 1* based on when the spark jumps from rotor to cap.
So it would be 1* barn door in my book.

Again I cant see the cap moving and no way moving it 1 tooth to have the timing light work on the cly that he listed of not getting spark.
Spark comes from the coil and it gets it's signal from the IGN box that gets it's signal from the pick up coil in the dist.
Dave ----