I recently did a detailing on the engine compartment, which included upgrading the distributor to an electronic unit, replaced the plugged, and had the carb rebuilt. Now that it is back together, I get serious engine knock at more than approx 2/3 throttle.

Before I pulled the distributor, I made sure the timing mark on the balancer pointed to "0" degrees on the timing tab. The rotor was pointing to the #1 plug lead in the cap (pretty sure, anyways).

I have set the timing to 4 degrees before TDC.

So if I start from a stop at light to moderate throttle, it seems fine, bit when i give it about 2/3 throttle or more, I get major engine knock. Another symptom i have is that there seems to be a bog or stutter when I blip the throttle from idle. Not sure if that is related or not.

8s it possible that I put the distributor in180 degrees off? The car wouldn't start would it? Woild incorrectly gapped plugs do this? What? What can I look for to troubleshoot this?

Thanks for your help,

Rob

 

Your distributor is at least close to the proper position or else it wouldn't start or run well at all; 4 degrees sounds pretty spot on or even conservative. What about your vacuum advance on the new distributor? After the carb rebuild, did you adjust the lean/rich mix screws?

 

To me it sounds like the distributor could be 1 degree off.......but also did the distributor have a gasket between the intake and the dist. housing. If not, I'm thinking it could knock for some reason, possibly.

 

I'd forget about 4 degrees at initial... it's total timing you should be concerned with. And don't worry about vacuum advance, it drops out when you open the throttle.
How much advance are you getting at 2500 rpm? That's when detonation occurs.

 

Check that all vacuum hoses were put back on and in the proper spots. Also check carb base gasket is sealed and the correct one.


Sounds like it's leaning out.

 

Exactly what distributor did you install?
Who rebuilt the carburetor?
What octane gasoline are you using?

 

What is the MECHANICAL ADVANCE CURVE on the new distributor, how does that compare to the one that CHEVY gave you?
What is the VACUUM advance curve? Ditto.
Is the vacuum advance adjustable?

How many miles on the engine?
What is the condition of coked up hot spots in your cylinders.
Is the carb running too lean?
Who set up the accelerator pump?
How is your choke working?
Do you have any vacuum leaks (lean condition)
What is the idle vacuum?

This is a START.

Unkahal

 

Are you sure you have the firing order correct?

 

The 4 degrees is the factory setting. Yes, 8 did adjust the mixture screws for the best idle/highest vacuum

 

Does have that gasket. Definitely piston knocking.

 

I can only check the idle timing with my lightly, and the spec sheet for the disti says that it should be at 30 degrees at 2500 rpm.

 

yes...

 

the engine has about 70k on it. Seemed to be fine with the stock points type distributor. Carb was rebuilt be a local well reputed shop. Quite sure there are no vacuum leaks - sprayed starting fluid around the base and other areas.

I'm running Esso premium 94 octane.

 

I'll repeat: Detonation dosen't occur at idle. If you can't check your advance at 2500 - 3000 rpm, you're wasting your time. Seriously man, learn how to use a timing light, It's a must and you'll know where you stand.

 

So perhaps I should clarify - my timing light does not have a dial to adjust for higher RPM levels. If it can still be used as-is to check the timing, with timing tape, perhaps you can teach me. Appreciate the constructive feedback Old Man.

 

The only other thing I can think of, it sounds like your timing chain jumped. That did happen to me with an old beater in the 70's. It sounded a lot like that.
I sure hope it's something simple for you.

 

I may be misunderstanding the problem. By 'knock' do you mean detonation (preignition or 'ping')? Or do you mean a mechanical sound, like a 'rod knock'?

 

Hi DRWET - it is definitely detonation.

Rob

 

I just read this thread this morning, and understand you replaced the distributor.
I'm thinking that the mechanical advance springs are weak, and combine that with some vacuum advance your getting into detonation territory.

As you may know, the factory timing specs are for emissions, not best power. To get best power, you need to plot your advance curve on some paper starting at idle and working up to 3000 RPM in 200 RPM intervals. At each RPM segment use your timing light to see where the timing is at on the crank damper.

You will have to get a correct timing tape for your size balancer, clean the balancer really well with some brake clean or lacquer thinner, and stick it on with the zero mark aligned with the mark on your damper. I always spray these with a clear lacquer spray from a rattle can to seal it on. Now you can use your regular timing light and read directly off the damper.

You want to have you mechanical advance all in by 3000 RPM, but it should be a slow progression from 1500 up to 3000. You don't want it all in at 2200 RPM.....that will cause the issues your having now. Changing the springs on the flyweights (under the rotor cap) control the rate at which mechanical advance comes in.....it's also OK to use a combination of light and heavy springs to get the curve just right. You can get a spring kit from most speed shops. Check all this with the vacuum advance disconnected and the line to the carb plugged.

The next thing to look at is your vacuum advance. It's not uncommon for these to pull 20 degrees plus advance into the timing. If you have initial timing set to 15 degrees, then add 15 degrees from mechanical, then add 20 from vacuum.....your up to 50 degrees of timing. You can buy a limiter that attaches to the vacuum advance arm that limits maximum advance, and it's adjustable....about 5 or 6 bucks. I think about 46-48 is the sweet spot for these engines, but look up Lars on this site and he has a paper for setting up timing....he's the go to guy for correct information.

I hope this helps

 

What the poster is missing here....It DID NOT have detonation BEFORE he made the 'changes'...now HE DOES....so obviously, it's something he did.