I'm a newsgroup junky. :/ This is my first real try at a website forum.

But I like nothing better than poking sharp sticks up-at those inflated guys that talk down to me and others. It's like a hobby of mine.

I'm at least 3/4's scot maybe that's the reason? :/

I don't even like hearing myself talk down to others. :/

I made a mistake not snipping your original post, the one you're so upset about.

I liked the part about making changes to the ignition system timing parts tho.

It was the "stock in a piston company" part and disagreeing with Bear 45/70's #6 post that was the only part the "Beavis&Butthead painful cry" was intended to be about. Believe it?

Pull off and plug the vacuum hose from your distibutor's vacuum advance and drive around, and get back to me about what you find out, ok? That's your homework assignment.

Alvin in AZ
ps- was a C-Span junky too, had to quit, C-Span, libertarianism and high blood pressure don't mix, can't poke-up at them elected official (gas bags) with a sharp stick

 

Hey bud; on a 1970 Motor's Auto Repair Manual the spec's for a 68 390 auto trans 4-barrel, centrifugal advance at dizzy rpm's starts 1/2 degree @ 350 rpm's, full 14 degrees @ 2,000 rpm's. Vacuum advance to start plunger 5" vacuum, max 12 1/2 degrees @ 25".We are speaking dizzy rpm's here. I picked 68 as a good year before smog and crap retardation era.

All specs even if the dizzy is spun are only general ball park numbers, this is where a good engine person begins then pinches vacuum and centrifugal to the limits before detonation, rewards are great should you work with the motor. No two motors will take the same tuneup specs as well your driving habits, load, heat, humidity altitude and fuel grade in your area.

.....=o&o>.....

 

The 390 ran 10° intial and I was using the 13° slot in the mechanical advance for a total mechanical advance of 36°. The CJ's dual point used the 10° slot in the dist for 20° advance and the initial was 18°. To get the added total we had the dist.'s points unbalanced. The first set of points was set to give the 44° and when I flipped the switch on the shifter the second set of point came in and changed the dwell to give the 38°. On the 460 I again used the 13° advance slot for 26° of advance. I tried to use 12° initial for a total of 38° total advance but that created a slow overheat situation with the camper in the truck and either of my trailers behind it (no problem empty, but that pretty much defeats the purpose of having a truck). I backed off to 10° initial and it helped but didn't cure the overheating issue. So 8° was used for a total advance curve of 34° and it has produced a nice increase in fuel mileage and no overheating, even in the heat of summer.

 

No arguement from me about the factory specs being a ballpark idea. With todays fuel, unleaded preminum is a crap shoot at best when dealing with the old "leaded fuel" era engines. This is where these high advance numbers really get scary. The numbers you quoted total 26 1/2 degrees at 2K(4K crank) with 25" of vacuum. This is about right. The initial could be established by setting max at the conditions as listed, and allowing the initial to fall where it will. Then noting that initial # for future settings. As a base line, a 4" stroke will be .100 deep in the hole at about 16 degrees BTDC. so 32* would be .200 and 48* degrees would be .300 deep when firing. This is where the problem lies, and when the wrist pins get hammered, and pistons wind up with holes in them. I am curious how the 85 460 truck, camper, and trailer's timing was recorded while going down the road? Unless changed, the cam is retarded several degrees, (I don't remember exactly how much), so maybe it needs that much lead time to make the mixture explode prior to actually quenching it all the way. Or maybe he is refering to "cam degrees", not crank degrees....(48*= 24*)...I am at a total loss to understand how that much timing doesn't detonate like crazy. I know even with the preminum unleaded I can't get much over 25 total advance without pinging like crazy.....I see others here are having simular detonation issues with about 1/2 of that advance, How? Very low compression? Or the retarded camshaft? Those numbers defy reality even for the unleaded engine, but crank those numbers in an earlier engine and it has to hammer like crazy.........aMP

****Note: The numbers "in the hole" are an approxamation, as the ratio isn't constant due to crank arm angle.........

 

I don't have a distributor in front of me but let me run this by you guys anyway.
Using those 68 specs that were given if we take the distributor plate and advance it
with vacuum 12 1/2 degrees this is with the shaft running at under 350 rpm we pull
the plate 12 1/2 degrees, now we speed up the distributor to 2000 rpm which throws
the cam in an advanced direction 14 degrees how much additioal advance have we
added? By my figures it works out to one and a half degrees. This seems to be what I remember from the days I ran a Sun distributor machine. The advances are not compounded but overlapping. In this case the maximum would be 28 deg. crankshaft plus the static timing of whatever.

 

If you were lucky to get 25" vacuum 12.5 degrees @ dizzy or 25 @ crank with light engine loads or in neutral and 2,000 rpm's with 14 degrees mechanical at dizzy or 28 degrees and 4,000 rpm's at crank that would add up to 53 degrees then add the 10 degrees initial timing and you have 63 degrees total timing advance.
You must remember that at a high manifold vacuum your 9.5 cr motor is only a 5.0 cr motor and the rate of fuel burn is slow (flame travel speed) vs full throttle allowing higher compression ratio on same said motor.

.....=o&o>.....

 

befor you go nuts "ping" timing it or recurving you distributor put a timing light on it and see whats what why throw money at a problem you don't know if you have. You need to know intial timing and total timing with just the mechanical and with both mechanical and vacum advance. Get a timing tape for your damper and a timing light.

 

Then let me ask this, what is the purpose of the vacuum advance?

 

Gas mileage improvement when cruising down the highway.

 

The use of a timing light to find initial and maximum total advance is a small fraction of information needed before you can recurve your ignition, you need to plot out the mechanical advance at 500 rpm increases and the vacuum advance as you apply vacuum until it also stops at maximum advance with notes on degrees vs inches vacuum applied.
When advance begins, the amount between initial advance and maximum advance with both the mechanical and vacuum units working together besides your fuel / air ratio with a wide band meter while road testing with the fuel grade, average altitude, humidity and temperature you'll use the truck plus road readings on the engine vacuum gauge.
This is where the old style vacuum advance unit is handy with washers and plunger stop limits adjusted vs the newer units that only have the spring tension adjustment. I still run the old style vacuum units, have located and used a vacuum advance rebuilder company in Oz for the 4.6L Rover motor and they can rebuild the FE units also. Rover units fail on the average of 19 months each at $155 USD stealers price, had one rebuilt shipped no core for $75 USD.
Or you can be happy and slap in a reman dizzy from AutoZone and believe they have the advance set perfectly for your motor, how you drive and the loads the motor is subjected to like Mr. Average truck owner.

.....=o&o>.....

 

Mel, the first thing you need to do is find a competant shop. Those guys who thought you 'didn't need' your vacuum advance don't know what they are doing, they might screw something else up as well. DinosaurFan

 

Again at reply #22, "setting timing at max.rev'd up,you established initial". WTF is that?
Please don't give bad advice for others who don't know and screw them up.

Doesn't anyone listen to Bear 45/70, he said everything but a hands on set up?
As far as a DuraSpark dizzy with the componet failures they have I would run one only after installing GM components in it. Not brand bashing, just run what works.
.....=o&o>.....[/QUOTE]

WTF was that? THAT was a simple way to set the max, for sure, at the same time the initial is established at idle. If the max is 28*, and the initial falls to 0 at idle, you have way to much advance in the distributor and should address that problem. On the other hand, if it falls within the recomended range, you have a base line to start from. As far a giving BAD ADVISE to others, maybe you should direct that remark to the ones that said, and the others that agreed, about 35+* of timing in a "leaded fuel" era FE engine. I stand by what I said about the limits of advance these old engines will and won't tolerate, I stand by the idea of setting max at a specific RPM to gain information about where the initial will fall, as others have also mentioned. And if you go back to the start, I also recomended checking all the above things, initial, max, and at what RPM changes happened. My responses were right on to the asked questions concerning the vehicle they were asked about. I don't recall questions about race cars with dual staggered points, and switches on the shifter, nor non FE unleaded engines........And I'm still waiting for an answer about how that timing was monitored going down the road, in a real world, real load, situation...........Another question, If none of the vacuum advances are suppose to be working at an idle, why does every tune up guide dictate that the vacuum advance be disconnected and plugged prior to setting the timing?...........aMP

 

I feel as tho I ruined mine while trying to nurse a bad battery along. Right about that same time the module acted like "a by-pass resistor" was bad.

Everything was perfect as long as I had the starter engaged but "let the key go to run" and it'd sometimes crap out. Most of the time it'd run fine, sometimes run crappy, it never broke down and left me stranded anywhere, just acted up.

Got that new BlueStreak Brand module in '79 and never ever had a problem since.

Just in case it -is- making a difference, which it "shouldn't" be.

My vacuum advance started effecting my idle ignition timing after I got my engine rebuilt.

If the vacuum advance is getting invloved at idle and you set it at "6 degrees BTDC" then what have you got?

Alvin in AZ

 

Moneypits reply number 22, "set the full, and let inital fall where it will",
as he digs a deeper hole back pedaling and changing words.

.....=o&o>.....

 

Deeper hole? WTF, can't you read? Set the full and let the initial fall where it will. What is it about that statement you don't understand? It works. aMP