Hot Rods Timing & Advance question SBC w/HEI

All of you who don't die first will get old and forgetful.....so how about some help jogging my memory. I'm dealing with a mildly modified 350 and want to check my timing using a adjustable timing light. The advance and timing numbers that I'm using were given me by the guy who built my oddball intake manifold and until just recently the engine ran great with those settings.
While messing with the car a couple of nights ago I discovered that the distributor hold-down had loosened and the distributor could be rotated. I want to recheck the timing to make sure it's back where it was.
The settings are as follows: In the distributor's mechanical advance---28degrees, In the vacuum advance can---15degrees, initial timing setting---12-15degrees, all advance in by 3000rpm.
To accomplish this I disconnect the vacuum line to the vacuum advance, run the engine at idle speed, dial in 12-15 degrees advance on my timing light and rotate the distributor to the point that the timing mark on the balancer is being lit at the 0 mark on the timing tab. Yes?? At that point I should be able to reconnect the vacuum advance, take the engine up to 3000rpm, dial in 28 degrees on the light and should see the timing mark at 0 again, Yes?? Button it up and call it a day.
Have I overlooked anything? or made a mistake?

 

On the surface, yes...as you stated.
Depending on how the vacuum is, ported or full. If it's ported, you do not have to disconnect it, as there is little to no low rpm vacuum in the "ported" nipple.
On one hand, I don't use a dial light, but, yes, I believe that's the way they work, zero on the damper 12 to 15 in the light. Or...use an old fashioned light (or keep yours at zero) and read the 12 to 15 on the damper.

You can check the max. timing by just putting the light to zero, rev the engine to 3000+ rpm and the full timing should be in by then, and depending how your vacuum is setup, you should see around 39/40 degrees of timing.

Nice to see someone that doesn't set the timing...by the book..!

Mike

 

41woodie,
I'm going to disagree with you little. Your method should put you in the ballpark in step 1. If you check it at 3000 without the vacuum line attached it should read 0 at 40 degrees if you set the initial to 12. More than plenty with today's gas in my opinion. Nothing should have changed except the initial by rotating the distributor. Some dial timing lights do not work as well as others.
Pete

 

yes What Pete said. The workings of the the Distributor wouldn't have changed because the clamp came loose.Set the Initial timing,and you're done.

 

yeah, but....HEI advance mechanisms seem to wear out over time, I'd set the initial timing, then rev it up to 3000 or so, and check what the total timing gets up to. I would not really want to run more than about 36 in a small block chevy. (all this would be done with the vacuum advance hose disconnected).

 

From what I understand (squirrel has it all over me on electrical, I am a dumbass at this stuff) The cap mounted coil sets up a magnetic field under it, which travels down through the steel rotor screws, and erodes the holes in the advance weights over time? Squirrel will probably explain it way better, and I will totally defer to his superior knowledge on electrical matters, but this was how it was explained to me, or what I understood anyway...

 

For those that answered how would you compare the method I described to using a vacuum gauge and locking down the clamp at the highest vaccum reading obtained while rotating the distributor while the engine is idling. It seems like the vacuum gauge method would come closer to matching a particular engine rather than being a generic method.

 

Maximizing vacuum at idle won't tell you a damn thing about when spark happens at WOT...which is what's important...


(WOT = wide open throttle)

 

Agree 100%, and this method would probably end up over-advanced in most cases, which will not be pretty...

 

So you're saying that the timing light method is safer? Because maximum vacuum would occur at more than 12-15 degrees of advance and that added to the advance in the distributor would bump the timing to a dangerous level?

 

that is right.

 

Very possible. Highest vacuum at idle is fine for setting idle mixture screws but I wouldn't even think of using it to set timing. Idle is not where you are going to have problems.
Pete

 

Thanks to all for great information

 

Setting the crank timing with a vacuum gauge is a good method if there is some question about the accuracy of the timing mark, say possibly a slipped damper, distributor or wires installed out of phase etc. It doesn't replace a timing light though and as always higher initial timing requires limiting the mechanical advance so the total never exceeds 36 or so.

 

Well, I don't know about you all, but I spend a lot more time at idle than I do at WOT.

I just went through a similar issue as the OP with the 283 in my 30 pickup. I acquired the car with the engine/trans already installed. As far as I know it's an original engine pulled from a 66 Impala and all that was done according to the previous owner is it was cleaned up and painted and some dress up items installed, but the pulley does not match the front cover and the timing marks are all off. I located TDC using a positive stop on #1 cyl. and made new tdc mark on the cover, the pulley has timing marks on it. The initial timing was set way advanced (according to the marks) so I backed the initial timing down to 12 degrees and the car ran like crap. Low on power, the car would not accelerate well at all, (had a damn foriegn POS dust me off) would not idle well and would sometimes stall, the vacuum dropped a few numbers, and the even the exhaust note had a real blatty tone to it. Screw that! I've ALWAYS set the timing by the book before, at low idle with the vacuum advance disconnected and plugged. For decades I've always set the timing like that. Having read on the net that some people advised setting the timing using a vacuum gauge for the highest vacuum reading. So I gave that a try, and I wasn't happy with those results either, too much advance. Now I had hard starting with detonation when first firing and detonation at low idle, it would even detonate when slowing down just before the trans would downshift. Again, it didn't idle well, the idle was rough and like I said, I would sit there at a stop light, hoping it wouldn't stall and listen to it rattling away, which was embarrassing as hell and I knew it wasn't doing that old engine any good either. So I went at it again just the other day, and this time I just went by a tachometer and my ears. I slowly retarded the timing watching the tach and listening to the engine at the same time, and I could clearly see when I reached a point where the rpms dropped a lot, the idle got rough and the engine sounded like it was laboring. Then I reversed direction and started advancing it watching the tach and listening to it until it rached max idle speed and then past that the idle again started to roughen and then I could hear pre-ignition starting. I went back and forth repeatedly really getting tuned in to how the engine responded at the different timing settings, and finally settled on a point just as the tach reached the highest idle speed, while the idle was still nice and smooth, no pre-ignition, and I locked it down there. Then I set the idle speed and did a quick adjustment on the air idle screws and took it out for a test drive. Ahhhhhh, much, much better. It starts well, idles great, smooth and steady, and accelerates very well, great response to the throttle. I've put about 60 - 70 miles on it like this, and I like it.

This may not be setting the timing by the book, and I've never done it this way before, but screw it, for this engine I don't see how it can get any better.

 

now that you've done all that, how about you get out the timing light, and see what the timing is at different rpm, with and without vaccum. Plot the advance curve (without vacuum advance), and see what you have. Might be interesting.

 

Now you unplug the vacuum advance, put a light and a tape on it, see how much initial and total you have, then go in the distributor and restrict the mechanical advance until you are around 36-38 degrees total.

 

Those vacuum advance cans are different. The last 2 numbers of the part # is how much advance the can will allow.

 

Question. You talk about having 36 degrees of advance with out the vacuum connected, wouldn't the vacuum advance add even more? Or since the throttle blades are open at 3000 rpm vacuum is diminishing? Would 36 degrees be the max for any engine or just a SBC?
Sorry if that doesn't make sence. I'm fighting a partial throttle stumble and my brain is fried.

 

I don't trust the "dial back" timing lights very much and they definitely won't work right with an MSD unit. I like to use the positive stop method to find TDC and mark it on pulley and pointer, and then apply timing tape aligned with the new marks.Now disconnect the vacuum hose and plug.
Rev engine to full advance, watching the timing on the tape, and adjust for 36-38 degrees advance @ full advance. Reconnect vacuum line. I have had best "feel" and response using manifold vacuum rather than ported.
What you use for initial timing, and "all in" rpm is open to experimentation according to your compression ratio and cam overlap. Just do/n't get too much, if any, above the 36-38 total.
My theory about the effect of vacuum advance on WOT performance is that if you are ar WOT, there should be very little or any vacuum to pull that additional advance in to the picture. So far this has worked for me since mt first SBC build back in the late 59s.

 

Believe it or not a V8 can have 50 degrees (or more) ignition timing BTDC lightly loaded steady cruising down the highway on level ground. Low compression motors can stand more, and earlier, mechanical advance. Maybe 36 to 38 degrees, while higher compressions engines less. It's a real interesting subject. Think of vacuum advance as just as much as a vacuum retard unit, any time the throttle is mashed the vacuum (temporarily) goes away, till the speed levels off. It adds to better mileage and all around driveability.

Keep in mind that these mechanical advance numbers may not work with lower octane fuel. A common routine is run it up (no vacuum advance) to "all in" RPM and adjust the initial to wherever it lands to a total 36 degree mechanical. Most V8s seem to like more than 10 degrees initial, that's why usually some kind of limiting is required inside the distributor, the stock curve has too much mechanical.

 

How does one go about limiting the travel of the mechanical advance. Is it something that can be done to a stock distributor or is something aftermarket required?

 

Depends on the manufacturer. Can be kind of a PITA. On many Fords, the vacuum advance and pivot or breaker plate must be removed before access. To remove the reluctor and swap the slot, a small wire clip, etc. This is one advantage of the aftermarket distributors from the usual suspects. However it's accomplished it's worth looking into. Even a pre-curved distributor will need a little final tuning so it's important to understand. Somebody pointed out a lot of folks will spend serious coin on their engine but leave a lot of performance on the table by ignoring this approach.

 

You don't have 12 degrees on your timing marker on the front of the motor? If you do just use a common timing light and set it. The distributer rotating won't have changed anything but your initial timing the rest of the BS is already set.

 

Unfortunately this is me after completely rebuilding my 1966 poly I just plopped a 1980s parts store reman distributor on top and called it good. Now I have a stumbly truck just like sixtyford.

I feel like I need remedial classes just to understand most of this timing talk. If a fellow was really interested in tuning his distributor what would he need? Just a timing light timing tape and patience? Anybody got a nice step by step for dummys? I am going to try Blues4U's method today after lunch, but that doesn't do much for my total advance.

 

I have read and reread the initial post. There is no setup to be done his distributer was already set up and his car was running fine. All that happened was that the hold down clamp loosened up and his timing got retarded. All that needs to be done is reset the distributer initial timing. he does not need to build a now house just wiggle the one he is in.

 

It's a little complicated at first. There's more to it than the timing most of us understand - the part where the distributor is rotated clockwise or counterclockwise, and using a timing light. A lot more. It's a rewarding part of engine tuning.

If you search "distributor curving" there will be more than enough to keep you busy. Alternatively you can send your distributor off for curving, this is where those old behemoth Sun distributor machines come in. They spin them up and check for proper operation, advance and retard at different RPM etc. A replacement distributor is often pretty cheap as well. The originals get worn out like everything else and makes for erratic performance. It's very important when running points especially that everything is tight and operating smoothly, but there's usually a noticeable improvement even when running a module. The mechanical distributor is the "brains" of the ignition system. If you get a fresh distributor curved for your car, he will determine beforehand what engine, transmission, weight, gearing, carb, intended use etc and dial in a suitable advance curve. The only thing left for the operator might be to adjust the vacuum advance after some highway runs.

 

You are absolutely right. I think we are into a much more theoretical discussion now.

 

I got a friend that actually does his distributer curving with a drill and a clamp in his bench vise. he has rigged up tach to his drill uses an analog dwell meter and that is all he has ever needed. He did this setup for his high school science project and has used the same setup for 30+ years. We set up a mopar distribute with his setup then we took it to a local distributer shop to be checked because I told him that it couldn't work that way. it was spot on.

I like a sun bench and if I had one I would use it. I would really like a big old expensive distributer machine, but I have learned my way around all that. I don't do distributes for a living and one thing that I learned a long time ago, the guys that invented that equipment are no more intelligent than any of us and anything can be done if you just use this simple concept, thiMk.

 

I'll go with that, still want one in my garage, even if there's not much call for it. This is a hobby for me after all, but it's more than that too.

I'd have been money (and time) ahead just sending off a distributor to Bubba, but this was the best way to learn. Could probably disassemble, re-assemble, R&R the distributor and then time an engine blindfolded by ear, by now. It's a good skill to have.