1970 Chevelle 307 rebuild BTDC question(s) - Chevelle Tech
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post #1 of 11 (permalink) Old Jul 7th, 20, 9:39 AM Thread Starter
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1970 Chevelle 307 rebuild BTDC question(s)

Hello all – first – thank you in advance.

So, I am rebuilding my 307. I have reached the point at which it is time to install the distributor. I have talked to a great deal of people, but I am CONFUSED. I understand that with my cam, a bit hotter, I need to install the distributor at 12 degrees BTDC. I have my balance set and I will drop in the unit. Here is the questions(s)
Once it is in, and initial startup and break-in of new cam has occurred – how do I set the timing at 3KRPMs? Won’t my timing reading be off because of installing it 12 degrees btdc? Do, I just set my digital timing light at the desired timing (not sure what it should be at 3k) and make the marks match on 0 TDC?
Any insight is most helpful – I am trying to learn 😊
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post #2 of 11 (permalink) Old Jul 7th, 20, 10:04 AM
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Heres how I do it...

Bring the #1 piston to top dead center on the compression stroke (both valves closed) and drop the distributor in so the rotor is physically pointing towards the #1 cylinder. Might take a try or two and you will have to align the oil pump shaft so it maates up.

Put a small mark in the distributor that you can see with the cap installed so you know which terminal will be #1

Install cap and wires and rotate slightly CCW so the #1 terminal is just before the mark.

This should get it running. Once running, then use timing light to set base timing around 12 degrees. Is this a flat tappet cam?

Before all this, prime the oil system and also install a timing tape on the balancer. Dont use the dial back feature.
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-Kevin
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post #3 of 11 (permalink) Old Jul 7th, 20, 11:00 AM Thread Starter
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Re: 1970 Chevelle 307 rebuild BTDC question(s)

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Originally Posted by 65cayne View Post
Heres how I do it...

Bring the #1 piston to top dead center on the compression stroke (both valves closed) and drop the distributor in so the rotor is physically pointing towards the #1 cylinder. Might take a try or two and you will have to align the oil pump shaft so it maates up.

Put a small mark in the distributor that you can see with the cap installed so you know which terminal will be #1

Install cap and wires and rotate slightly CCW so the #1 terminal is just before the mark.

This should get it running. Once running, then use timing light to set base timing around 12 degrees. Is this a flat tappet cam?

Before all this, prime the oil system and also install a timing tape on the balancer. Dont use the dial back feature.
That all sounds good. The guy I worked with said to drop mine in 12 degrees BTDC - I am just a little confused on how to time it like that. I assume it would time like any other? Or do you think I would need to add or subtract timing because of this?
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post #4 of 11 (permalink) Old Jul 7th, 20, 1:10 PM
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Re: 1970 Chevelle 307 rebuild BTDC question(s)

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Originally Posted by s1nemesis1s View Post
That all sounds good. The guy I worked with said to drop mine in 12 degrees BTDC - I am just a little confused on how to time it like that. I assume it would time like any other? Or do you think I would need to add or subtract timing because of this?
12 degrees with the vacuum to the distributor disconnected. If it doesn't ping while driving leave it there. My L79 stock spec is 4 BTDC but I run it at 12 and it likes it there. Again thats with vacuum disconnected but reconnect to full manifold vacuum before driving it.
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post #5 of 11 (permalink) Old Jul 7th, 20, 9:48 PM Thread Starter
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Re: 1970 Chevelle 307 rebuild BTDC question(s)

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Originally Posted by Billdini View Post
12 degrees with the vacuum to the distributor disconnected. If it doesn't ping while driving leave it there. My L79 stock spec is 4 BTDC but I run it at 12 and it likes it there. Again thats with vacuum disconnected but reconnect to full manifold vacuum before driving it.
So full manifold vacuum on the advance - or should I use ported from my carb? Thx!
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post #6 of 11 (permalink) Old Jul 7th, 20, 9:54 PM
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Re: 1970 Chevelle 307 rebuild BTDC question(s)

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So full manifold vacuum on the advance - or should I use ported from my carb? Thx!
Most carburetors have both metered and full. Although my car is supposed to be hooked to metered it runs much better on full vacuum and I am sure most people here will agree. It doesn’t matter if you take it off the manifold or an unmetered port on the carb.
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post #7 of 11 (permalink) Old Jul 8th, 20, 11:30 PM
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Re: 1970 Chevelle 307 rebuild BTDC question(s)

for break-in at 2000 to 3000 RPM you need to have timing at 45 or 50 degrees to avoid over heating. ignore the initial, when the engine starts immediately raise the RPM to 2200-2500 and check the timing. it needs to be advanced, like 45-50 degrees. the advance mechanism built into the distributor will be in play. hook up the vacuum advance to manifold vacuum, that will also help.

if the engine spins over freely but won't quite start, the timing is probably retarded. advance it. pull the vacuum advance toward the front of the engine.
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post #8 of 11 (permalink) Old Jul 9th, 20, 12:09 AM
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Quote:
Originally Posted by s1nemesis1s View Post
That all sounds good. The guy I worked with said to drop mine in 12 degrees BTDC - I am just a little confused on how to time it like that. I assume it would time like any other? Or do you think I would need to add or subtract timing because of this?
What he was implying was to set the timing at 12 degrees after you start it. You cant just drop it in at a certain degree. It's just a guess until you get it started and get the light on it. As others have stated, you want lots of initial (idle) and total timing during break the cam break in. Then you can work on final adjustments to dial in the initial, vacuum advance, and mechanical advance (revved up). Make sense?

-Kevin
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post #9 of 11 (permalink) Old Jul 9th, 20, 3:54 AM
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Re: 1970 Chevelle 307 rebuild BTDC question(s)

Good point, as you said you cant "drop it in 12 deg advanced"

Put it in as you typically would and when your revs are up just check total timing.. finish your break in

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post #10 of 11 (permalink) Old Jul 9th, 20, 3:20 PM Thread Starter
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Re: 1970 Chevelle 307 rebuild BTDC question(s)

Hmm, so not 12 degrees BTDC? I have been advised by a few to do this. hmm. Or am I misunderstanding?
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post #11 of 11 (permalink) Old Jul 9th, 20, 5:36 PM
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Re: 1970 Chevelle 307 rebuild BTDC question(s)

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Originally Posted by s1nemesis1s View Post
Hmm, so not 12 degrees BTDC? I have been advised by a few to do this. hmm. Or am I misunderstanding?
Yes 12 degrees is a good starting point but as has been said before you can't just drop the dist.in and end up at 12 degrees unless you get lucky and it ends up right at that point !
To put it another way there is no way to tell what the timing will be until the engine is started.You can try to get it to that point but it's still a roll of the dice.
Follow the instructions in post # 2 and gofrom there.After it starts use your light to give it about 45-50* timing per what Tom said so it doesn't overheat during the cam break-in.
Unless your timing light is a dial back or you installed a timing tape on the balancer you will be estimating this.

John
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