Still trying to solve an uneven idle due to misses, I noticed the gap
between the point of the rotor and the studs inside the HEI cap is at least
3 mm (1/8). This is 2 to 3 times larger than the gap of the plugs.

Is there some special reason for this? Could it make the spark less strong?
Is there a way, or has anybody ever tried to make this gap smaller ?

(Both the cap and the rotor are rather new.)

Thanks

Rob

I'm curious how you measured this?

Pro, thanks for taking the time.

I used 2 different approaches.

The first is a straightforward measurement of each of the 2 circles. One being the diameter of the inner sides of 2 opposite studs: 11.6 cm., i.e. 5.8 cm radius. The other being the radius of the rotor at the copper strip: 5.5 cm. The difference is 3 mm. in radius.

The second approach. On top of the rotor there is a recess in the middle. This fits over the center of the cap where the electrode sits. When you do this on the bench, you can move the rotor back and forth, or side to side and measure the movement between an edge of the rotor and the inner side of the cap. Of course this shows the same 6 mm in diameter. You will also notice that the copper strip of the rotor cannot hit the stud, the recess will not permit that.

Both measurements assume cap and rotor are truly centered. Perhaps they are not, but I cannot imagine a deviation over 0.3 mm. You can see some marking in the center of the rotor where it touches the electrode of the cap.

This leads me to believe there is at least a 2.5 mm gap between the rotor and the cap when the plug is fired.

Of course millions of cars use a HEI like mine, so I must be chasing ghosts.

Rob

I've never really thought about the gap being an issue. Maybe just dumb luck that it's always been fine. I bought an MSD Street Fire for the Chevelle not too long ago. If I can this weekend, I'll take that measurement. I know it works fine. I've not seen another what else you've tried to do to solve the miss, so I don't know where you are in the process. What brand/age of HEI do you have now? I assume you've done all the "normal" checklist stuff regarding your tune?

Just a couple of quick thoughts. The distance at the cap can't be compared to the gap at the plug,the electrical energy jumps the gap at the distributor before losing energy thru the resistance of the plug wire and the plug. The gap at the plug is much smaller because the electrical energy must jump the gap with cylinder pressures of 160-180lbs/sq.in.

From what I read in M.Maner’s post, the resistance of the gap in the cap (at 15 PSI) is considerably less than the resistance of the gap at plug during compression (160 PSI). This I had not really realized, despite the fact my schooling has been electrics. Thanks for the insight.

To answer Pro’s question.

Engine is a stock 350 crate (some 220 HP), Edelbrock intake with Rochester q-jet. Timing is set at 8 initial, 36 total (3500 RPM), vacuum 19 (starting at 8 inch, ending at 13). Idle is set at 650, vacuum at idle is 15 inch. The car runs on LPG, mixture unit mounted on top of the carburetor feeding both banks the same mixture. Average is 14 mpg.

Engine runs uneven at idle (on gas as well as on LPG). Every now and then (every second on average) the tachometer shows a slight (50) drop in RPM, while the vacuum gauge sometimes shows a small drop (app. 1 inch) at the same time, but not always.

I have done many things. Each bank has it’s own oxygen sensor connected to a gauge, both showing a correct mixture at idle. From the oxygen sensor I would say the left bank sometimes misses a spark, making the exhaust a bit richer. Timing light shows a 2 to 4 degree jump (advancing), with or without vacuum connected to the distributor. The problem is still the same when the vacuum advance is removed and the initial set to the same amount instead.

I have searched for vacuum leaks, and found none. I have done the ‘paper on the valve cover’ test (intake leak). I have readjusted the valves (hydraulic, ¾ turn down).

Spark plugs (AC R43…) are new, set at 0.040. Wires are new. Distributor is not new, but there is no radial play and I have adjusted the lash, made sure the dimple on the gear is aligned with the rotor, new cap, new coil, new rotor, new module (with heat paste) and new pickup.
One thing I could not solve though (apart from buying a new distributor). The mechanical advance dictates the rotor base must be able to rotate with respect to the axis. There is some movement possible before the weights need to move. I have seen this on 2 different HEI’s, which makes me believe it is a normal condition. I expect that with the engine running this movement is nullified by the driving force of the engine. Perhaps this is not true ??

I have not checked the timing of the camshaft. I have not pulled the heads. I have not done a leak down test.

Rob.

This is a tough one. How fresh is the motor/valve job? Does it miss like this when cruising at 2000 rpm?

Pro, to answer your questions: the engine is a new crate, with 15,000 miles. In normal driving I do not notice a miss, nor at cruising speed, nor under load.

The only strange thing I can add is that when revving it in the garage the engine shows a shaky character at 1200 and again at some 2000 RPM. I do not think this is an unbalance, because that would increase with RPM, which I cannot say is happening.

On the other hand Pro, my question concerned the gap in the cap. This seems to be answered. I had no intention trying to solve the miss through this post.

If the ignition is not at fault, I guess I should further investigate a sticky valve. Although I would really love to have the engine idle smoothly, I have no intentions (yet) to open it up as long as it shows no real problems in daily driving, except perhaps the rather poor mileage per gallon.

Thanks for your help.

Rob