Readers Follow-Up—

July 1, 2011 | By David Coleman

Some Methods for Dealing With Rough-Running Engines

IN THE MAY issue’s Mechanic on Duty section, there was a fellow with a mid- ’60s Pontiac that had a rough idle.

(Editor’s note: That reader has a 1966 Pontiac Catalina with a high-compression 389 cid engine and he said he has “never been able to get a smooth idle.” The engine stumbles slightly and the vacuum is 17 inches at 600 rpm with the transmission in neutral. When he puts the transmission in gear, the stumble goes away but the idle is very rough.)

We found that a hot cam engine would run bestwith18degreesatanidle and 38 to 40 degrees at full rpm(running without vacuum advance).

My brother Bill, Earl Howard and I were the Coleman Bros. Speed Shop of fuel dragster racing in the early ’60s. Back in our “leaning over the fender” hot rod days we saw many Pontiac vacuum advance distributors (mostly HEI era) that had so much vacuum advance, that the spark leaving the rotor tip (going toward one of the eight leads in the distributor cap) would swing through an arc starting from one side of the cap’s terminal all the way across the terminal to the other side of that terminal, and then continue radially to create a large gap from the edge of the rotor to the side of the distributor’s terminal.

Of course, there is always a gap of around .060” or so from tip or rotor to contact surface of the distributor’s terminal, but as this “sideways” gap is added to the .060,” the arc length gets so long that a coil might not be able to fire across this gap, the plug wire’s resistance, the resistor plug, and any corrosion in the spark plug wire terminations.

We used to cut a section out of the side of an old distributor cap, and then look in this hole, while pulling and releasing the vacuum advance mechanism.This would show how much the rotor moved across the distributor cap’s terminal, and how severely the radial gap increased. We sometimes filed the point plate mounting holes and rotated the plate a little.

We would also braze the vacuum advance slot in the “point” plate to decrease the amount of vacuum advance movement available. You could make screw-on tabs, and some of the really crude guys would just jam a sheet metal screw into the slot.

Decreasing the amount of vacuum advance really helped the street race engines (with hotter camshafts) as we could then bring the initial timing up from the book recommended four or six degrees at an idle (no vacuum hooked up) to 12 degrees or more, according to how hot the engine ran and how good the starting system operated.

Another thing GM was faced with was the spark plug wire routing.

We found that a hot cam engine would run best with 18 degrees at an idle and 38 to 40 degrees at full rpm (running without vacuum advance). If one absolutely wanted to run vacuum advance, we would braze the slot to give a maximum vacuum advance of something like 10 degrees or so.

We also used to look at vacuum advance specification sheets for vacuum advance units for trucks and different “family engines,” that would work with lesser vacuum numbers, to try to select a factory vacuum advance unit that would work better with the lower vacuum numbers that our hot rod engines could muster.

I imagine the Pontiac engineers made a compromise in designing the rotor/vacuum advance/point plate positioning that would suit the greatest majority of Pontiac drivers, and could be done using the “parts bin of GM parts.” I doubt if there were many older folks doing wide-open driving with their shiny new Pontiacs.

Don’t hate me if the rotor going too far sideways, and making a large gap, isn’t what is wrong with this fellow’s vehicle (especially as he probably has a point distributor, and we mostly were playing with HEIs), but my info might help somebody with a little later HEI-equipped Pontiac eight.

Problems With Wire Routing

Another thing GM was faced with was the spark plug wire routing. The inductive field from the spark going through a plug wire could generate a sizeable spark in another plug wire laying parallel to the first plug wire. And if the second cylinder was next in the firing order, it would have fuel and air at half the piston’s travel, and that spark plug could ignite the charge way prematurely causing a rough idle such as in the reader’s complaint. As the rpm came up, the cylinder pressure would presumably prevent the weaker induced current from igniting a premature charge.

Back in the 348-409 Chevy days, the factory valve covers had spark plug wire “looms” that spaced the adjacent firing cylinders spark plug wires farther apart. Think #4 and #8 on the passenger side, and #5 and #7 on the driver’s side.

I heard a story of one of the factories making a quartz cylinder so the engineers could observe pre-ignition and the other vagaries of spark propagation inside an internal combustion engine. They reported that at the same load, fuel/air ratio, and spark advance, the pictures to two connective cylinder firings produce dissimilar burn patterns—go figure.

David Coleman Raeford, North Carolina