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Hi guys, I’ve got a 72 base 350. I had the engine rebuilt and got it back in and running. When I put the timing light on it, I was thinking it should hit at about 8 Degrees Before TDC. It runs best at about 16 degrees before TDC. But still a little rough. What’s the next step on getting closer to correct timing and getting it to smooth out? Any thoughts would be helpful. Thanks
8 deg is for (proper?) emissions; 14 deg is for better performance. It will run better set greater than 8 deg, mine is set at 14 I believe. If it is still running rough, could be the idle mixture screws are starving the engine. What rpm is it idling at? If you have set timing at 16 deg, what is the timing when you hook the vacuum advance back up on the distributor?
You need a dial back or digital timing light. Unplug the Vacuum advance and set your light for 36 degrees.....rev the engine to 3000 + RPM and set it for 0 on the tab. Disregard the initial, you don't drive there, you drive at part throttle to full throttle......your 36 may be higher or lower than 3000.....just wherever it stop climbing/moving. This is now your total timing. Now check the initial and record that number.....you will need it in a minute. Plug the Vac advance in and check again......the difference between unplugged and plugged is how much your Vac advance can is pulling.....record that number too and post it here. Now you are done, proper timing set......forget what GM and your buddies tell you, the procedure I just gave you is the correct way.
From there, grab a vacuum gauge and hook it to full manifold vacuum. Adjust your idle screws for the highest reading on this gauge. Done. Post your results
My tach isn’t working. I must have installed the cable incorrectly. I’m guessing I was idling at about 650. After plugging the vacuum advance back in , there wasn’t much difference at all in the timing.
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
Originally Posted by Jebbysan
You need a dial back or digital timing light. Unplug the Vacuum advance and set your light for 36 degrees.....rev the engine to 3000 + RPM and set it for 0 on the tab. Disregard the initial, you don't drive there, you drive at part throttle to full throttle......your 36 may be higher or lower than 3000.....just wherever it stop climbing/moving. This is now your total timing. Now check the initial and record that number.....you will need it in a minute. Plug the Vac advance in and check again......the difference between unplugged and plugged is how much your Vac advance can is pulling.....record that number too and post it here. Now you are done, proper timing set......forget what GM and your buddies tell you, the procedure I just gave you is the correct way.
From there, grab a vacuum gauge and hook it to full manifold vacuum. Adjust your idle screws for the highest reading on this gauge. Done. Post your results
Jebby
I disagree. What you've done is half the job (higher RPM@WOT), and disregarded the lower end/lmid-load points where an engine spends most of its life.
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
Originally Posted by Rpconst
Jebby, I don’t have a dial back or digital timing light but I’m sure I can get my hands on one. I’ll round one up and then post the results. Thanks.
Get a timing tape that fits your balancer diameter, or put a piece of tape or magic marker mark on your balancer at 36* from the stripe, and a regular timing light will work just fine.
Simplest approach is to have engine running with advance can disconnected and vacuum line plugged to prevent leakage. Adjust distributor position to find best idle between 12 degrees BTDC and 16 degrees BTDC and lock it down. That will NOT get you best performance; but it WILL get you 'better' performance and better idle quality. Your advance can should then be reconnected to MANIFOLD vacuum fitting (not a ported/timed vacuum fitting).
If you wish to maximize engine performance, get a recent copy of Lars' paper on setting timing for performance. It is a more complicated process, but it will teach you what the engine needs for proper ignition timing setup and how to accomplish that.
The distributor has been converted to electronic. From what I’ve read, dwell is no longer relevant.
Should've pointed out that conversion in p#1 and disclosed which system. Dwell remains quite relevant but no longer adjustable w/ electronic.
Perhaps there exists a wiring / ballast / resistance issue ?
Thanks for all of the tips so far. The information has all been valuable and gives me a better overall understanding of setting the timing. I have a small setback in that the tach cable is broken due to being pinched off on the firewall. It looks like I have the distributor in the wrong location. See the pic of the orientation in the aim manual. Is this what everyone is using for 1972 base? If so I’m 2 positions off.
Honestly I’m not sure about my electronic conversion. A friend did it. I tried to look up the number on line but had no luck. I am using the original distributor and was planning on using the original mechanical tach. Here is a pic of the conversion. Will the mechanical tach work now?
The picture is too close to tell anything and no cap.
Do you have your wires set up the way the diagram looks, #1 cylinder is to the left of cap window when looking from above, and does the vacuum advance point to the back of the passenger side valve cover?
If everything is set that way you should have no problem with the tach cable.
you put the distributor body wherever the tach cable fits best. then you decide where you want #1 and set the rotor up in that position as you install the distributor, having the engine at #1firing TDC. NOT #1 other TDC which is 6 firing. generally #1 and 2 are up front to shorten the distance the longest plug wires have to go.
Last edited by derekderek; Jul 6, 2020 at 04:53 PM.
OP Rpc
Looking at 23U-V4 ignition module & the way the rotor is attached w/studs & nuts (instead of OE screws).
Maybe we're looking at a Pertronix Ignitor; perhaps Ignitor II. Perhaps a superseded Pertronix component ?
Suggest you contact Pertronix tech line directly: inquire for ID confirmation and install instructions-wiring diagram.
909.599.5955
Timing off
