big block ken

I gave the '66 427/390 a tune up, even though it ran fine. Dwell was stable at 30. THEN I had to go and "adjust" the timing! Using a digital advance timing light with rpm, I followed LARS timing procedure and set it at 36* at 2500 rpm with plugged vacuum hose. Idle timing was then at 12. I adjusted the idle mixture on the Holley carb for max rpm. It ran smooth and had instant throttle response. I reconnected the vacuum line and test drove it. But I had pinging under load. So I knocked timing down 2 degrees to 34* at 2500 rpm. I got two blocks away and it back fired in 2nd gear. I immediately turned around. Back in the garage, disgusted. I didn't have any timing problem before I decided to "adjust" it!

The factory original distributor (one family owned car) with 91k miles has never been out or reworked. I noticed about a 1/4" up and down play in the distributor shaft and wondered if that's normal. Again at idle and going through the gears it's fine, but as soon as I get on it, it would either bog under load (at 36*/2500 rpm) or back fire (at 34*/2500). I used to own a '66 427/425 and never had a problem (with TI ignition) finding that sweet spot. But I seem to go through this every time I adjust timing on this car. Any ideas? I don't suspect the carb because again, it ran fine before this tune up.

I have no problem pulling the original distributor and putting it in storage with other original parts saved over the years. But I need it to look 100% stock. Is there a reliable street performance distributor (points or breakerless) that will fit underneath the factory big block shielding? I'll do without the factory ballast resistor on the firewall, but I don't want a 6AL box showing anywhere, since I show the car. - Thanks. Ken

Vetterodder

If the distributor hasn't been reworked the total timing won't be all in until closer to 4500rpm, or maybe higher. I'd expect it to gain another 10º or so past 2500.

MikeM

I have no idea what procedure you used but if it was running okay before you decided to adjust it, how did it run?

There is a procedure outlined in the factory service manual that works pretty well and if you need it, even instructions on how to rebuilt your distributor.

big block ken

It it gains even more timing at higher rpm, that might explain why it bogged or back fired then, but ran fine below 2500 rpm. So if it's at 12* advance at idle (600-650 rpm) I should try backing it down near factory spec of 4* and see what happens.

63 340HP

.

" I noticed about a 1/4" up and down play in the distributor shaft and wondered if that's normal."

No, it's not correct ( but it may be normal for a 50 year old distributor with 90,000 miles ).

Pull the distributor and buy a to adjust the end play between the drive gear and the distributor body with the needed combination of shim/thrust washers (see diagram).

.

" set it at 36* at 2500 rpm with plugged vacuum hose"

The expectation of the all-in advance stopping more advance gain at 2500 rpm seldom occurs with stock advance springs (see diagram). On a first test, it's best to run the rpm well past 4000 rpm to verify the advance does not continue to climb with more rpm. You need to find the rpm that the advance stops climbing with rpm, and then tighten the distributor when that total advance reading is where you want it.

Most of what has been covered for timing is with a SBC, and you have a BBC. In my experience a BBC prefers a little more total advance than a SBC, at 38 degrees (+/- 2 degrees). With a BBC I recommend to start at 38 degrees total (but it must be set with the engine rpm above the all-in advance rpm).

.
"I had pinging under load"

This is a symptom of too much centrifugal advance, and typical for a 90,000 mile distributor that has lost or worn down the advance stop bushing (see diagram). When you pull the distributor to shim the end play, look for the presence of a bushing on the stop pin located underneath the fly weighs and football. Without the advance stop bushing the distributor produces excessive centrifugal advance. In your case you may have 12 degrees at idle, and 36 degrees at 2500 rpm, with a missing stop bushing that allows the centrifugal advance mechanism to rise well past 44 degrees above 2500 rpm (and ping).

The recommended includes a stop pin bushing that usually limits the excess advance (the brass bushing in the kit photos), and you get the spring selection to work with to improve (lower) the all-in advance rpm.

.
You are about $30 and a few hours of tinkering away from correcting the wear in the original distributor, and working past the frustration you experienced today, with the result of restored or better than factory performance.

lars

You need to set the total timing to 36 degrees when the centrifugal advance is fully pegged out. It's not likely that it is pegged out at 2500 rpm with stock springs, so you are likely grossly over-advanced. My paper has detailed instructions on how to ensure that you are setting the timing at the maximum pegged-out point - if you just set it at 2500 rpm, you did not follow the instructions... I don't recall sending you one of my papers - where did you find the paper? If you downloaded it off the internet, throw it away - it's not right... Contact me for a valid version of the paper: Nowhere in my paper do I state to set total timing to 36 at 2500 rpm.

Lars

rnixon

1967 427 390hp w/AC
MSD 8572 vac adv. tach. drive dist. ... 6AL box, tucked almost out of sight.

big block ken

Lars - PM sent.
Ken

big block ken

1966 427/390

That's exactly where I hid my 6AL box in a '71 LT-1. With all of the A/C lines it's not very obvious. But mine is wide open in there with only the Delco repro battery sitting by itself. I'm wondering if it would get too hot if it was mounted under the battery tray? I've heard of them being mounted inside near the heater core box also. I remember how well that engine fired up and ran with that set up though!

Avispa

If you decide to put some additional thrust washers under your distributor gear to clean up the end play, just double check that the end of the distributor shaft doesn't bottom out in the oil pump shaft. Mine needed about 100 thousandths of nylon washers between the intake manifold and the distributor after doing the same thing.

DansYellow66

I think stock FSM timing on a 427/390 is something like 4 or 6 degrees - which is probably conservative. But, like the others have said you probably have a whole lot more total timing in the engine than you think. And then add vacuum advance timing on top of that and your distributor is firing so far in advance of TDC that the cylinder pressure is maximizing before the piston reaches TDC, that it's actually trying to kick backwards - thus the miss/bog/backfire. I suspect if you take it out on the open road and try to cruise it might have a constant stutter from this. Time to pull the distributor, add some washers to cut down on the shaft play, put some lighter distributor springs in with a Mr Gasket kit (but not necessarily the lightest ones) - maybe the two medium ones or a light one and a medium one. I think getting all the centrifugal timing in by around 3,000 rpm is fairly reasonable for the street with a stick.

SWCDuke

Unfortunately, Lars, there's a lot of BAD advice on the Web such as setting total WOT advance at some arbitrary RPM like 3000 or whatever. The first thing one has to do is look at the service manual or AMA specs to determine the OE start and max centrifugal points, and the max varies from 24 @ 2350 for the 365/375 HP 327s to 30 at 5000 on the 427/390.

Then TEST to find the ACTUAL start and max points because the centrifugal may have been MODIFIED at some point in the past. In the case of the 427/390 lighter centrifugal springs should be installed to lower the max point to something more reasonable like no more than 3500, and most will tolerate a more aggressive centrifugal curve without detonation.

The other common mistake is NOT disconnecting and plugging the VAC signal hose. That's why I call it the total WOT advance method. By disabling the VAC you SIMULATE WOT even though the engine has no load.

I'm just pounding my head against the wall.

On a more pleasant note, I found a great craft beer from your neck of the woods - Breckenridge Vanilla Porter. I can't remember if you're a hoppy or malty guy, but I love malty ales, and this is a good one.

Also, I found a Discovery Channel DVD at the library - "How Beer Saved the World". It's must see for all beer lovers. My doctor keeps telling me I should drink less beer, but after watching this video and reading this thread I think I should drink MORE!

Duke

MikeM

When you get in a bind, please refer back to your GM Service Manual to get you back on track.

Once you learn the basics, you can go play.

big block ken

Well after adjusting the timing several times, ending up at the factory specd 4* at idle, and replacing the coil wire, it still back fired and bogged. THEN I started to think of WHAT ELSE I HAD CHANGED besides the cap, rotor, condenser, and points and THE IGNITION COIL came to mind! I also remembered how coils can fail once rpm starts climbing. So I replaced the new one (who I bought from a well known vendor for $90 last month) with my old one. It started right up, and then it ran like a monkey with a scalded *** in every gear!

Thank you for all the suggestions. And I think I'll send the distributor away to be rebuilt over the winter.

DansYellow66

Good deal. Ignition coil failures seem to be really common anymore. Seems the symptoms can vary also. You did good in tracing back over what was changed just before this problem started acting up. Always a good trouble shooting tip.