250cid timing

[prodjay10]

I need to find out how to set timing.

1971 c10. 250cid

I checked timing with vacuum advance off and port plugged. Idle at 760rpm and it was 4 after.

What is idle to be set at?
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[prodjay10]

From what I gather, it should be 4* before tdc at idle. I Checked it again after driving it a while and the timing mark is way out of range. I would guess 40 before tdc. I try to adjust the distributor to get it back to 4 before but it looks like it'll die. Is it possible the harmonic balancer is spun?

Or am I looking at the wrong mark? It's a cutout or notch on the balanced correct?
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[Willy Prost]

Don't forget that the harmonic balancer may have shifted giving you a false reading as to what the timing really is.

[prodjay10]

That's what I was thinking. Would I have to replace it to set the timing?

[RichardJ]

You need to inspect see if that is the problem. If it is loose and spinning, it's only a matter of time before the outer ring flies off. I lost one at speed on the freeway and another one simply dropped to the ground when I started the engine.

If you are unable to see by looking, restart the engine a couple of times and check the timing. Each time you start it, the belts will pull the outer ring around and relocate the timing mark.

There are a couple of different timing gear covers with different timing tab locations.

[prodjay10]

Thanks. I will check it out.

[prodjay10]

Out of curiosity, does the crank pulley have a bolt holding it in?
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[Jason Banks]

No bolt. But, it can be tapped for one

[Jason Banks]

Let me expand on that answer. The crank pulley is actually the outer section of the balancer. Isolated from the inner with rubber.

The balancer can be bolted to the crank if the crank is tapped to accept a bolt. Otherwise it's a press fit.

[prodjay10]

So, I took it to a mechanic yesterday and he made a lineup mark on both the inner and outer sections of the balancer. I drove home then to work. I checked the marks this morning and they are 180 degrees apart!

Since this is supposed to balance the crank, maybe a new one will take care of some vibration I was feeling at higher speeds?

[prodjay10]

Thanks for your replies.

[GRX]

Vibration at higher road speeds or high engine speed (RPMs) while sitting still? Big difference.

This engine is internally balanced so that is technically a harmonic dampener. Plus inline six are very smooth harmonics by nature of their design so I doubt a bad dampener would cause significant vibration. In any event ... replace that loose thing ASAP.

[prodjay10]

It's been ordered. I'll probably have it done by tomorrow.

I feel vibration going over 55mph or over 2500rpm. I haven't checked sitting still. I'm not sure what it is.

[RichardJ]

Marking the hub and ring is a good idea, even on a good dampener for future inspection.
Chevy cut a notch on the Corvair dampener for that purpose.

Your mechanic should have just restarted the engine to check if the marks shifted.

I was lucky that I didn't loose my radiator when the one flew off on the freeway. Some guys have.
After my second one dropped off, I got a little smarter and added a single pulley on the outside to trap the ring from flying off.
That worked out well because I later added AC and made even better use of the new pulley.

[prodjay10]

He did restart the car but it didn't shift while I was there. I'll probably cut that mark into the new pulley just for future reference.

[70STOVEBOLT]

Don't quote me, but I believe the notch that is cut into the pulley should align with the keyway for the crankshaft. That is one way to tell if the outer ring has slipped.

[prodjay10]

I didn't check the notch on the new balancer.

Has anyone read and tried the timing suggested in this thread?

http://www.chevytalk.org/fusionbb/sh...hp?tid/239321/

It explains how a motor is more efficient with manifold pressure on the vacuum advance instead of ported vacuum. You have to adjust your distributor and it also increases emissions.

[factorystock]

Originally Posted by Jason Banks

No bolt. But, it can be tapped for one

In other words, they are pressed on. What was GM thinking

[GRX]

The early SBC like the 283 and 327 did not have a bolt either. Exception being the hi-perf solid lifter engines. Seem to remember the dampener bolt was introduced 2nd or 3rd year of the 350.

[prodjay10]

I had the balancer swapped out and I have it a 4° before, but I need to find out what total timing should be with vacuum advance and how to check it.

I heard 30° total, though I'm not sure. Does that mean 30° at 2500rpm?

[prodjay10]

Is the 30° total timing set by the points?

[geezer#99]

Originally Posted by prodjay10

Is the 30° total timing set by the points?

No!!

You have three parts in your timing set up.
1. Initial timing (base timing) which is set on the timing tab/balancer and is a fixed amount. You set it with a timing light. You rotate the distributor to your desired advance. Stock might be 4 degrees btdc but with today's fuels you can increase it to 12 degrees.
2. Mechanical timing is in your distributor. This is set by the weights and springs. It's a fixed total amount. Most distributors are close to 25 degrees.
3. Vac advance which adds more timing under light throttle high vacuum times. It can add up to 20 degrees or more.

Your total timing is your initial plus your mechanical which you want to be all in close to 3500 rpm.

Under light throttle cruise conditions you could see upwards of 55 degrees advance.

Here's a link to a lot more info. It's based on a higher performance motor than yours but the principles are the same.
http://www.badasscars.com/index.cfm/...d=76/prd76.htm

[prodjay10]

Thanks for the help!