Technical Timing Trouble with a SBC 350

Are you using the mark on the damper to time? You cannot use it if the tab is from another engine. You must use a new mark on the damper that corresponds with the zero mark on your existing tab. The diagram from Gimpy explains this.

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WOW! Thanks one and all for taking the time to help me figure this out! It's like having a bunch of cool uncles over hangin' in the garage, telling stories, drinking a beverage... I love it! So I appreciate you saying I give good info. I pride myself on that.

So I got to the shoebox yesterday and here's the latest. You guys already figured it out and I'll tell the story of how I got it going through your quotes. In order of posting:

I thought about this. How 'loose' is are the throttle/trans cables and the linkage on the carb. I found that the throttle cable was tight. Fixed it. I also learned that on my carb there's an idle, and a Fast idle. That was the other part of the problem. When the choke was on, the fast idle controls the idle until the choke closes, and the main idle is supposed to take over (or something like that). My carb wouldn't come off the fast idle for some reason so I lowered it to 750 RPM that way.

Yep. After what I found above, this became clear.

I did not do this but I thought this was a super cool strategy that was worth applauding!

Just to address your question, I had to add my own pointer to my block (to the timing chain cover). After I added this, I went about finding my TDC mark using the stop method I think someone called it. Then I added new timing tape to my damper at that new zero mark, with zero on the newly acquired TDC mark.

In fairness, bonzo-1 's post came first, but I'm putting it last because it sums up what was wrong.

Now on to where I am. With the vacuum advance plugged, and all other open ports plugged throughout the vehicle, the engine idles well at 700-800 RPM. With this my idle timing is at 10 degrees BTDC. From what I have read, SBC's take from 8 - 14 degrees well at idle. So next I revved the engine until the timing stopped advancing. This was at 30 degrees. So this is where I need my uncles to advise: A SBC is supposed to have 35 degrees total timing. How do I make this so? Do I need to? The engine idles well and rev's well. Do I need to do anything?
Am I looking at a bushing change in the HEI?
Again everyone, thank you for your insights and advice. It's really cool to be a part of this community.

 

Plug the vacuum advance back in, drive.

 

Outer hub of the balancer isn,t slipping is it?

 

You seem to have the TDC thing figured. But No, 30 degrees total advance is not enough. I assume this engine is pretty much stock. No big cam. For a stock cam and HEI, this timing method will get you by

Loosen distributor hold down bolt enough to rotate dist.
Unhook and plug vac adv.
Rev motor past the all in point. 3500 or so.
Put timing light on balancer, turn the distributor counter clockwise about 5-6 degrees and set to 35-36 degrees at the balancer.
Don't worry too much about where the initial advance ends up.
Tighten hold down bolt.
Reconnect vac adv.
Done.
There are a few more things you can do to the HEI, such as limiting the vac adv and the mechanical advance to achieve the correct amount of each.
initial advance at idle + mechanical advance = 35-36 degrees.
initial advance + mechanical advance + vac advance = 48-52 degrees.
Adjust mech. adv. and vac adv. to achieve these numbers.
For example.
14 ini. + 22 mech. = 36 total or 12 ini. + 24 mech. = 36 total
My SBC is 18 ini. + 18 mech = 36 total.
Now add vacuum advance.
14 ini. + 22 mech. = 36 + 12 vac. = 48

 

Do you have a vacuum gauge that you can hook up ? It will tell you where the motor is happiest with the timing.

 

Well I have my Saturday planned.

 

Yes! I have one. I'd love to know how to do this.

 

Just adjust to the point of the maximum vacuum on your guage.

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Make sure you hook guage to a manifold vacuum source.

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The important thing to keep in mind, 34° or 36° or 38° or in the ballpark, but don't get TOO wrapped up in the exact numbers, you could take 5 identical engines, everything, and each might respond best to slightly different total timing numbers. Find out what works best in YOUR engine.

By limiting the amount of advance inside an OEM type distributor, there is a lot more "room" to play with in terms of the initial or crank timing degrees. Everything is set initially without vacuum advance connected, this is a cause for confusion with some.

Then they hook up the vacuum advance, and run into engine knock, and back off on the crank timing to compensate. Don't do this.

Vacuum cans are adjustable, but they often bring in a lot of additional advance - 10 to 15 degrees or even more. They are usually adjustable to "when" not "how much". You might have to limit the amount that can be brought in by the vacuum can, or get a different one depending on what you're working with. This is important, because on a street motor you want vacuum advance, it will be much better behaved and responsive and run cooler, and improve economy by maybe 20%.

When you're cruising along on level ground in high gear, the RPM level will typically be lower than the balls to wall wide open throttle right? Consequently the mechanical advance will be something less than 34 or 36 or 38. The timing will be retarded, relatively speaking. Generally any engine should be run at maximum advance at all times under all conditions, short of knock or predetonation. How to do this?

This is where vacuum advance comes in. On level ground the engine load is very low, maybe 50 horsepower only is needed. Engine vacuum will be very high. But RPM is low...Fuel mixture is very lean... and needs lots of advance... as much as 50 degrees btdc timing is not unusual. Just under that specific condition. Just remember the vacuum advance is a totally separate and distinct system based on engine load that complements the mechanical, which is solely RPM based.

Don't make adjustments to either in order to compensate for a mis-adjustment on the other. Take the time to get it right, and you'll thank yourself every time you drive it.

 

When the vehicle is road-worthy again, I'll be able to tell what I need to do with the vacuum advance. You gave a ton of other great info in here as well, all of which I will make sure to take into account while I'm doing this Saturday. I agree with you 100%, I want this to be right, not close enough. That's why I value all of you so much for your help in allowing me to learn this stuff for the first time, and to learn it right, so I don't have problems when I try to help someone else, or for the other cars I see in my future. I really can't express my gratitude to all of you for your insights.

 

Advancing initial timing will result in more total timing. As long as it starts good when warm. Changing from 10 BTDC to 14 BTDC will change your full advance 4 degrees to 34 degrees. This is assuming that the timing tape you have is designed for the diameter of the damper you are using.

It depends on what you are going to use the vehicle for how much you really need to tune it. There are kits for tuning the advance curve but that degree of tuning usually is done after testing on a dyno so the best advance numbers can be determined. That might be as little as 1 to 2 horsepower at high rpm. For street use you should be fine where you are.

 

Yes it it. MSD sells a timing tape set that has several different tapes for you to choose from based on your damper diameter. For my SBC, it used the 8" diameter tape. Here is the product information if you are interested: https://www.summitracing.com/parts/msd-8985

 

For a vacuum leak ,Cover the carb best you take a manifold vaccum sorce,usually the power brake booster fitting on the intake manifold light a cigar and blow the smoke into the intake,it will come out the vacuum leak.