The Official John Lee Jr Thread

[Sharps40]

So, New engine, would not run well below 18 degrees BTDC and I did not know my mechanical curve in the dizzy nor the vacuum advance rating. Fortunatly, I got some good advice here and figgured those out....18 mechanical + 10 on vacuum advance. That particular thread didn't go well, was particularly frustrating for lack of documented/actionable data points for a stock motor with minnor reliability upgrades and I deleted the info and will attempt to clarify how I located what I believe to be a safe and adaquate engine tune here.

I started over several weeks by driving on hi test only and examining all functions in the engine as time and work allowed. Its a daily driver and as such a tune what ya got proposition.

The first fix implemented was to return to stock jetting on the Weber....The first engine ran well there with both points and electronic dizzys and I knew it was adaquatly rich to keep me out of trouble.

I then corrected the pcv system with a new valve installed with the arrow pointing to the source...no more uncontrolled vacuum hole there....still wouldn't run well below 18 BTDC initial.

I corrected smaller leaks at the 3, 4 runner and at the adapter to manifold junction. Now it would run well at 16 BTDC...tweaked the jetting IAW Mr. Langdons recommendations. (Factory idle circuit jet, slightly leaner on the main, much richer on the secondary)...engine ran great, thought I was done.

But, I was at 16 BTDC and "knew" the factory inital was 4 BTDC. I never considered that 4 was for points.......still didn't know if an HEI was different and didn't know the mechanical or vacuum advance.

Followed some advice, between the frustrating moments and mapped out my Dizzy, finding it brings 18 degrees mechanical to the table and the vacuum can brings another 10 degrees.

Considering how well it was running as newly rejetted (18 + mpg smooth and powerful) I was still concerned about being at 16 to 18 BTDC.

So, I lowered the initial advance in increments. First 14 BTDC, ran better and it stopped kicking back against the starter as it would occasionally do on hot restarts at 18 BTDC.

Then tried 9 BTDC (Still didn't know there was a difference in factory inital timing between points in the 60s and electronic in the late 70s) and it ran great but was weaker at low speeds, especially cold.

Then went to 4 BTDC, the factory points initial advance recommendaton for many 250s thru the early 70s. Ran okay but was weak across the entire rpm range and sucked fuel like a BarPig on cheep wine every Payday Saturday Night.

So....now suspecting a range of 9 to 14 was more correct for the engine as currently outfitted I reset to 10 BTDC. (10 initial + 18 mechanical + 10 vacuum brought 38 degrees total to the table and it was comeing all in by 3000 rpm as indicated by movement of the marks on the balancer under the timing light....figgured this out with a bit more advice after weeding back thru a rather frustrating thread on engine tuning.) The truck ran great here.

How wonderful it was to finally receive confirmation from a member in the form of a photo (from a haynes manual) of the timing specifications for a Chevy truck 250 with a stock GM HEI.....factory recommendaton for my late 70s engine and similar era HEI was in fact 10 degrees BTDC! Now we're getting somewhere, actionable and documented information for a daily driver stocker with HEI, confirmation, not frustration, not remachine to race engine set up, not timing is easy so close the hood and enjoy the ride....

Comfortable now, I increased the timing a small step.....9 to 14 seemed about right from driving and performance, 10 was factory and drove/felt well, so I selected 12 degrees BTDC. Went out for a drive. Made it to 80+ mph before I backed off. Couldn't get it to ping. The engine seemed alive...pulling hard on the main and harder on the secondary in all gears and without rattle, clatter or any hesitation. Plugs, though hard to read, showed some indications of tanning up and none of the black or silver flecks that I am told are a sign of detonation.

So I have set my engine 12 initial + 18 mechanical + 10 vacuum advance for a total timing of 40 by 3000 rpm. With the T5 transmission, I tach 2800 rpm at 75 mph. Its unlikely I'll ever break 3000 rpm. It appears we are going to remain in the 18 mpg range for this set up.

I'm about 200 miles down the road now on this setting...plugs still look fine and I am pleased with the performance and fuel mileage.