Just got back from the break-in drive. Everything seems fine except I have worse pinging than I ever had before. It's always been there when the engine is under load, but now happens under any acceleration or as soon as I start up even the gentlest of hills. I do have the timing advanced more than before (I think it was at 0 before the rebuild), but I thought I'd be able to get away with that. Tomorrow I'll check the vacuum advance timing, but I doubt that is the problem because the ping starts even with just the slightest throttle.

Previously I assumed it was caused by carbon buildup in the combustion chambers, but I can obviously rule that out now.

I assume this is ping. It sounds like someone vigorously shaking a penny in a soup can under the hood, only when the engine is under load.

If the distributor is the same as it was before the rebuild then you'll have to put the timing back to what it used to be. The early electronic distributors came into use the same time as emission controls were getting serious. This means the initial/base timing is pretty late, but there is a lot of mechanical advance so that the timing under load and at speed is close to normal. If you've advanced the initial timing then there's too much advance everywhere else.

Also, how old is the gas in the car? And how's the rocker oiling?

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Joshua

The factory timing was 0 (TDC). I'll try resetting it to that, but that's where I had it before the rebuild and I had the ping then too.
Good question. The gas came out of the pump in February, and the tank is still near full unfortunately.
The rocker assembly is oiling well.

You may find that the carburetor is calibrated too lean. The whole fuel/emissions balance is quite delicate and it was not meant for E10 fuel which will lean the mixture compared to non-ethanol fuel. One jet size larger would probably do unless you've made other engine changes such as improved exhaust or a different camshaft and I know the latter is true.

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Joshua

Not intending to hi-jack this thread, yet wanting to ask a question relative to engine tuning, and carb calibration. Where does setting the float level fall into this subject? I casually observed the float level listed on the last carb rebuild kit, and it seems to me that the float level was revised as time when on from the 1960's into the late 1970's. The last carb kit I looked at was for a Mono-jet, where I noticed this evolution.

Just went to the shop and looked up the spec I was thinking of: Mono-jet's float level was revised between '73 and '74 depending upon Heavy duty or light duty use. Prior to 73, all were set at 1/4 inch. after they evolved to 11/32's My Carter BBS kit is for the mid 60's so it provides 1/4 inch as the float level. No other information available to add any depth to this question. AND the whole question may not be relevant to the pinging Rich006 is having with his rebuild, depending upon where his float is set.

Mono jet. Carter bbs. Apples and oranges. Can't set one up compared to specs for another brand of carb
Even within the same carb, different years, different emissions regions, hi altitude etc can make a huge difference.

My point was regarding the change in calibration due to the onset of emission standards and the effect it had on how the same carb was calibrated by the factory.

The float level is part of the carburetor's calibration. Even within a single model year, different variants of the same kind of carburetor (different BBSs, for example) sometimes had different float height specifications.

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Too many people who were born on third base actually believe they've hit a triple.

Last night I reset the timing to 0. I also noted that the timing advances pretty quickly to 30 degrees when the throttle is opened (that's with initial timing at 0). Thanks to Doug Dutra for putting the note in his book to add timing marks at 20, 25, and 30 degrees.

The ping is much reduced now, but still constant when under load (it just takes greater load than it did before). If better gas doesn't solve the problem, I might try a larger jet. I'm thinking I have to solve the detonation problem before it will be worth trying to improve my ignition system.

Regarding float level, I rebuilt the carburetor a few years ago and set the float according to the instructions that came with the carb. It was a NOS carb, I suspect from mid-70's, newer than the car but not by much. And don't worry about hijacking the thread; it has already been down several tangents.

Try adjusting vacuum adv can? Vac Adv hose plugged into ported vacuum and not direct manifold vac?

If it were me, I'd get a recurve kit from Charles Brooks and shorten the mech adv slots and use diff mech adv springs.
Lou

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Home of Slant6-powered fun machines since 1988

It is time to plot out the existing ignition advance curve.
Disconnect the vacuum line to the advance can and plug it on the engine side.
Determine the ignition timing at idle, then turn in the idle screw and record ignition timing at 500 rpm increments.
This assumes you have a tach. If you don’t it is easy to wire in an under hood tach for this trouble shooting episode.
This will get you the timing curve with the base and mechanical advance.

Getting the vacuum advance curve is a bit more involved but just as important.
You need to have the car - engine under load to get the operating vacuum. When the car is throttled in the driveway while not moving the throttle plate in the carb is not necessarily where it is while motoring down the highway at the same RPM.

Attach a vacuum line - gauge to the port on the engine that the distributor advance can is attached to. Have the hose long enough that you can wire the vacuum gauge to the driver side windshield wiper, or in the car where it can be viewed. Then go for a drive, record the vacuum readings at idle, crusing steady state on a flat highway, accelerating up a hill at part and full throttle, note the conditions where the vacuum is highest and where it is lowest and where it is at a mid point and transitions. The engine vacuum at flat level ground cruse speed is most important as that is where you want to set the advance can to be fully activated
Record that info and keep it available for future reference.

Then pull the distributor, get an inexpensive hand vacuum pump from Harbor Freight, one with a gauge, they are sold as vacuum brake line bleeders.
Attach the vacuum pump to the can and observing the mechanism inside of the distributor apply vacuum to the can, Note the amount of vacuum needed to start moving ( adding advance )
the mechanism inside the distributor, and the amount of vacuum needed to fully move the mechanism.
If the advance can has what looks like a large hex on the end, it is adjustable. The proper sized allen wrench will fit into a socket inside of the vacume line attachment point and allow the tension on the control spring to be adjusted.
IIRC they are about 10 turns from full in to full out. You can experiment and see how adjusting the vacuum can changes the point at which vacuum starts to move the mechanism. The total movement is built into the can.

One final test will tell you how much vacuum advance you are getting. Re install the distributor, start the engine and attach a timing light and the vacuum pump to the advance can.
With the engine running apply vacuum to the can in maybe 2 lb increments from zero to the highest vacuum reading recorded in the road tests, and record the changes in ignition timing.

With the information from the engine vacuum and the advance can tests, you can make an accurate determination of the existing mechanical advance curve and know where to set the vacuum advance to activate at steady cruse conditions.

To use ported or manifold vacuum to activate the advance can is another deal. Just remember that manifold vacuum = ported vacuum at all throttle positions except idle. At idle ported has zero vacuum while manifold vacuum is highest at idle. There are engine combinations that like ignition advance at idle to idle cleanly and those do well with manifold vacuum operating the advance can.

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Doo Ron Ron and the Duke of Earl are friends of mine.....
https://www.youtube.com/watch?v=uX8Nj8ABEI8

I was thinking that. Thanks so much for posting the detailed instructions. I have all the equipment and I'll plan to do that this weekend. My vacuum can does have the hex, but I wouldn't have known it's adjustable.

I also found some good explanations and pictures in this two-part Mopar Connection article: Part I, Part II.

Those articles looked at a higher-compression modified /6 as well as a completely stock one. One takeaway for me is that my 8.9:1 engine will probably want the timing to advance more slowly and to a lower total, compared to the stock engine. Of course the first thing to do is find out what I've got.

Wait, are you running 8.9:1 with the 2106R cam? Have you calculated the dynamic compression ratio or done a cranking compression test?

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Joshua

I have not. I do have a compression tester.

Here are measurements I took today:
1) Mechanical advance vs. RPM
2) Vacuum while driving, in different conditions
3) Timing advance with vacuum manually applied to distributor
4) Effective compression ratio, computed 2 different ways

1) Mechanical Advance: (vacuum line to distributor disconnected and plugged)
RPM - Deg BTDC
750 - 0
1000 - 1.9
1250 - 8.2
1500 - 15.1
1750 - 20.9
2000 - 21.5
2250 - 26 (idle was starting to wander a bit, making precise measurements difficult)
2500 - 27
2750 - 28


2) Vacuum readings while driving (gauge connected to ported vacuum, distributor not connected to vacuum):
Idle 0
Cruise 12-14 (best guess 13; surprisingly hard to find a level stretch of road!)
Uphill full throttle 1
Uphill part throttle 6-8
*Note: ping occurs when vacuum is 1-3 in.

3) Timing advance with vacuum applied manually to distributor
In. - Deg.
0 - 0
2 - 0
4 - 0
6 - 0
8 - 2
10 - 6
12 - 12
14 - 17
16 - 17.5
18 - 17.5

4) Effective Compression ratio. For Joshie225, I calculated the effective compression ratio two different ways. From the instant the intake valve fully closes (55 ABDC) until TDC the "effective stroke" is 3.46" for a compression ratio of 7.7:1. From 0.050" of lift (31 ABDC) until TDC the stroke is 3.92" and the ratio is 8.5:1. So for those who want to think about such things, I welcome your opinions. To get those numbers I had to bust out the Law of Cosines and the Quadratic Formula. My diagram is below; if you want the algebra I can share that too.