After my car gets warmed up (10 minutes or so) and I give it some gas, it starts to knock real bad. If I slowly accelerate, it is fine, anything more and it will knock real bad. It is has slowly been getting worse over the last few days. I have an L-48. I put in new gas (92) which I always use, checked all the plug connections and I have a feeling my timing is bad.

I have a timing light and vacuum gauge but can never really understand what everyone is talking about on here about all in, total timing....

Can someone give me the down and dirty on how to set timing. What should I set my timing light dial to and how far should I turn my dist? The dist has never been rebuilt, the spark plugs are about 1 year old and I drive the car about 150 miles per week, some of that is normal driving and some is really fast. The car idles at about 600-700 rpms in park.

With the timing off, it is starting to run hot again (over 200) which is not normal, usually I am below 180.

Thanks
Dan

 

As far as the timing goes you could start at 12 BTDC and see how that works for you or 10 if needed. Could be your timing is too high now causing it to ping or knock. My automatic idles 600-700 in drive and IMO too much timing can make it run hotter. I'm sure better minds can further expand on this, hope this helps. Ed

 

ED79, I don't understand the BTDC....what should I set the timing light at and what should I set the timing to on the harmonic balancer? I always just "wing it" and I want to finally figure out the correct way to do it. Thanks

 

Just set the initial timimg at 8 degrees before top dead centre and start from there .

Disconnect the rubber hose at the distributor and plug it with a golf tee or something similar . Hook up your timing light to # 1 plug wire , preferably as close to the spark plug as you can . Start it up and let it idol ( you may have to adjust the idol speed screw to keep it running at 700 or so RPM with the vaccume hose off the dist. )
Check to see what your timing is and if you need to change it you have to loosen the hold down bolt at the base of the dist . , just move the dist a little bit and snug the bolt down so it wont move . Clockwise will retard the timing and counter clockwise will advance it .
When you get it set at 8 put the hose back on the vacc advance. If the vac advance is working the RPM should jump up when you re- connect the hose. You may need to lower the idol speed back down .
Take it for a ride and see how it runs , if it stumbles when u accelerate it may need a little more advance ( go up a couple of degrees at a time and test drive it untill the stumble is gone )
Once you have done this a few times it is easy .

Bill

 

It sounds like you have a dial back timing light. When you point the light at the balancer, turn the dial until the mark on the balancer lines up with the 0 (zero is TDC 'Tap Dead Center' - BTDC means advance - ie. Before Tap Dead Center) on the timing mark. Wherever the dial is at, is what the timing advance is at. I added a little white paint on the balancer mark, to make it easier to see.

At idle, with the vacuum advance disconnected and plugged, you set the base timing. I set this to the stock value, which for my 72 base engine is 8 deg. BTDC.

With the vacuum advance disconnected and plugged, your timing will be the base timing, plus whatever mechanical (centrifugal) advance you get inside the distributor.

At idle you shouldn't be getting any mechanical advance, but as you increase the RPM's you should see it come in, and the timing become more advanced. Eventually there will come a point where there is no more mechanical advance, the weights are 'all in', and that is your 'total timing'. This should be around 36 degrees, and you should see it around 2800-3000 rpm. So when you rev above that amount you should not see the advance go beyond that 36 degrees.

Lars has stated that it's this total timing that is the most important, and you should get that set correctly, and let the base/initial timing fall where it may.

After you get the total timing set, hook the vacuum advance back up. You should see the timing at idle jump from whatever the base timing is plus about 16 degrees.

 

Since this started all of a sudden, check to make sure the vacuum advance is working. A spring may have broken or fallen off and it's stuck in the advanced position. The timing doesn't usually change all by itself.

 

Mashaman,
ZZ71S sent me Lars paper and it did help out. I was able to get the total timing set to 36 but the idle timing was at 25?????? I had the vacuum advance disconnected and plugged. I did not see the big jump. I am going to try everything again this evening when it cools down. I did take it for a test drive and it was still knocking real bad.

As I was setting the timing, I noticed the the electric choke was moving. I have a eldebrock carb 600 cfm. The electric choke thing I am talking about is black, has the 2 choke wires connected to it, has about 6 hash marks on it that line up with 6 hash marks on the carb and is held on there with 3 screws. Is that supposed to move? I tried to tighten all 3 screws but it still rotates.

Bashcraft, I will pull the dist cap tomorrow and see if anything has fallen off.

 

If your mechanical advance is only pulling 11 degrees, something is wrong somewhere. It should pull somewhere around 20 degrees or so.

You can't just set the timing at 36*@3000rpm, you have to make sure that it stops advancing at 3000rpm. The stock advance springs will allow advance up until 4-5000rpm sometimes. Ya gotta play with advance springs untill the advance stops advancing at 3000rpm. Then set timing at 36*@3000rpm. Your idle timing should be around 12-18 with vacuum advance unplugged, advancing to 36 as you rev the engine up to 3000rpm. That's all there is to it. Now plug your vacuum advance back into full manifold vacuum, and if it gives you more than 16 degrees on top of the idle timing, it's too much, gonna have to do something about that.


Scott

 

Could be a bad diaphragm as well.
Are you sure its a spark knock and not something more serious?
Usually don't hear a spark knock with no load on the engine.

 

Ooooh! That's an excellent point. This guy doesn't sound like he has any experience with engines, he could be mistaking a rod knock or something.


Scott

 

It only knocks when i give it a lot of gas while driving. I reved it up to about 4500 rpm in my garage (no knock) but it did advance up to about 40 before stopping. Maybe it is time to overhaul my distributor? I pulled the intake off last year and the bottom part looked fine, maybe the internals are going bad. When you say bad diaphragm, do you mean in the vacuum advance. I remember when I installed the new one a few years ago, there were adjustment procedures on the vacuum advance itself. Could that be a problem also?

Thanks for the help. Scotty, you are correct, I do not have much experience with engines but it is definitely not a rod knock, I know what that sound is. I know that this is not a major repair, so I don't want to support the local repair shop yet.

 

Did you try setting it on 12 degrees? That's 6 hash marks above O on your harmonic balancer gauge. Be sure to unhook advance and plug. Ed

 

You may wish to replace that adjustable vacuum advance if you do not know how to use it (I for one do not!). Go to NAPA and get a stock-equivalent replacement. Not expensive. And Lars has a paper somewhere with recommendations as per year and application but generally the "book" unit will work fine.

 

Sorry dgood, I misread your post.

If you rebuild your distributor the B25 vacuum advance can (NAPA part # VC1807) is the unit recommended by tuning guru Lars Grimsrud.

 

If it were on my vehicle I'd replace it no matter what.

From what I've learned from Lars the adjustable cans are not the way to go.

 

Off to the Napa and advance. Going to rebuild it and see what I come up with. I will try to get a non-adjustable can also.

 

But leave the vac advance unplugged and drive the car,if it knocks its not an advance issue.

You could have a carbon buildup issue,get some GM top engine cleaner or Sea Foam and run through the de-carbon procedure and see if that helps.

Oh and stay away from 4500 rpm blasts in neutral.
You might have already spun a rod bearing and that would just finish it.

 

Well, I got a new dist and all the new goodies to put in it. Set the timing again and I was able to achieve 12 at idle (probably a little more) and 36 all in, so it was not a total waste of money. The old one did look pretty rough inside and all the parts looked very old. Took it out for a spin and as soon as i gave tried to give it some gas, it was knocking real bad. Kinda like a hail storm on a tin roof.

Roger, I am going to run up to the store and get some sea foam and try it with the vacuum advance disconnected and I will go from there.

Dan

 

Here we go again. I disconnected the vacuum advance and it still knocks real bad. Tried the sea foam and it did make the engine run better but it still knocks real bad.

Sitting in the driveway contemplating putting a for sale sign on it, I heard a vacuum leak in the drivers side dash. Pulled the head light switch, lights did not come up. Pulled the headlight bypass switch and the right (passenger side) light came up. It appears I have a vacuum leak on top of all this.

I am going to let it cool off and check the wires and plugs and other connections around the engine. Something must be disconnected vacuum wise and electrical. Shazaaaaaam

 

You seem to be having trouble grasping the principles at work here. Here are some things to keep in mind:

The sum of the timing at idle + mechanical advance + vacuum advance should not exeed 52*.

The sum of the timing at idle + mechanical advance should not exeed 36*.

The mechanical advance should be in the neighborhood of 20*.

The timing at idle with vacuum advance disconnected should be less than 18*

OK, now, here's where we get into trouble with vacuum advance. Lets say the factory ignition timing is 8* BTDC. Let's say the mechanical advance pulls 20*, so when you are up to speed it's at 28* (this may not happen until 4-5000rpm, but let's keep it simple and assume you have installed the proper advance weight springs to allow full mechanical advance at 3000rpm.) The factory want total timing at highway cruise to be 52* for optimal fuel mileage, so they install a vacuum advance canister that pulls 24* ( 8 + 20 + 24 = 52 )

Now, you say to yourself " I'm going to advance my timing so I have 36* at full mechanical advance." So, you get the timing light out and you set the timing at idle, with vacuum advance disconnected, at 16*. Now your 20* of mechanical advance gives you 36*. Everything is cool until you cruise on the highway, 'cuz now the vacuum advance is throwing another 24* on top of that, for a total of 60*. Way too much.

OK, none of this is necessarily what is causing your problem, just trying to lay a little groundwork. Now, you have told us that your timing curve is 40* all in @4500rpm. Your timing is way advanced, back it off.

This is your problem right now: YOU NEED TO ADJUST YOUR IGNITION CURVE SO IT'S ALL IN AT 3000 RPM BEFORE YOU DO ANYTHING ELSE!

Set your timing back at the stock spec to get the knocking to go away, and install lighter advance weight springs to get advance maxed out at 3000 rpm, instead of 4500 rpm, then advance timing at 3000 rpm to 36*. Then set your vacuum advance to pull 16* of advance at around 12" of vacuum. Done and done.

Google search for, and read several times, these articles: (I am assuming you have an HEI distributor.)

An HEI Ignition & Timing Primer--Lars Grimsrud

How to Set Your Timing for Peak performance--Lars Grimsrud

Distributor Vacuum Advance Control units Specs and Facts for GM Distributors--Lars Grimsrud

If you follow my advice, and download and read and re-read these papers several times, and absorb the knowlege and wisdom that they possess, and you are still having trouble understanding timing, then you should probably be institutionalized, they are that simple and easily understandable. Good luck!

On another note, the part number that Bob Onit recommends is for a pre-HEI distributor. If you mentioned that you have a points distributor, I must have missed it. If you did, sorry, I'll shut up now.


OK, get those articles and study them, and all will become clear! And if it doesn't become clear, get an average intelligence chimpanzee to read the articles and set your timing for you!

Hey, c'mon, I'm just kidding about the chimp, have a sense of humor, will ya?


Scott