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Discussion Starter · #1 ·
Finally had a dry weekend and got my new (replacement) carb installed (Holley 670 Street Avenger). It is new and clean, so hopefully it continues to work as it should. Happy to be able to drive the car again. :) Vehicle particulars in my sig line.

Last summer, when I was able to get the carb working enough to drive it, I was able to get the distributor set to not ping, or so I thought. It is a stock type distributor with a Pertronix II module. It doesn't ping around town, I can accelerate hard up a hill without any rattling noises. I went on the highway to test the kickdown setting on the new carb installation and even fairly light pressure on the accelerator set off a cacophony of rattling, like shaking a dozen spray cans.

I do not know the current timing setting. All I know for sure is that it runs well and pulls hard with no pinging under load in town, but pings when asked to accelerate at highway speeds. Would this have something to do with mechanical advance? Am I correct to think I'm getting good advance around town at lower RPM's and too much advance at highway speeds? What should I look for under the distributor cap, things binding, etc.? It starts fine and easy, both cold and hot starts. It is full with fresh fuel.

I appreciate any thoughts.
 

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I would suspect too much vacuum advance or a lean cruise mixture. What rpm is highway cruise at?
 

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If you have a timing light and a tach you can plot your distributor's advance curve.
 

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If you have a timing light and a tach you can plot your distributor's advance curve.
+1. And a hand vacuum pump and gauge. First go for a drive and note the rpms with corresponding vacuum levels you see around town, "hard" (not WOT) accel, highway cruise and highway accel. This will tell you what your engine sees when you drive it. Then you disconnect vac advance and use the timing light to plot your mechanical (rpm only) advance. Return it to idle and use the hand vac pump to see what additional vac advance you get at what vac levels.

Draw the curves on a sheet of graph paper, and then graph your rpms from idle to max, and the vac levels at each. This way you see what your engine does, what timing you get at each rpm and load, and whether it has comparatively high or low advance anywhere like highway cruise/accel. Then compare it against the spec's in the FSM, as many distributors are out of spec or replaced with a completely or partially wrong generic curve. Change whatever is wrong.

While it's counter-intuitive, sometimes cruise needs more advance to prevent detonation (pinging). The reason is that without proper advance (no or low vac advance at cruise) it's running relatively retarded. This causes a lot of heat to soak the valves and chamber area and can then cause det when you then push it. BTW - this added heat is also a primary cause of exhaust valve seat recession, burning, cutting or pitting - not the unleaded gas legend.

If you want to try an easy experiment first (without the timing plots) do this test:
Duplicate the throttle level (vacuum level) that you get det on the highway, but do it from a slow speed or dead stop, all the way through and past your cruise rpms. If you get det at the point it does when cruising, you have too much advance there. If you accelerate with no det, then you don't have enough cruise timing, but it didn't det because it never had the chance to heat-soak at retarded cruise.

I hope that makes sense. Just for background, we had that just recently with a Mitsu turbo car - and advancing the cruise timing was exactly the fix. I've also done a lot of experimenting on a small block with computer-controlled ignition (much easier to change settings and curves) and can duplicate this situation at-will. The factory prevents cruise det from over advance by adding EGR. If it has a knock sensor, it will retard timing only if EGR was not enough - but it usually is.

David
 

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Discussion Starter · #5 ·
I would suspect too much vacuum advance or a lean cruise mixture. What rpm is highway cruise at?
No tach, but I expect 2500 - 2700 RPM's for around 70 mph.
 

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Discussion Starter · #6 ·
+1. And a hand vacuum pump and gauge. First go for a drive and note the rpms with corresponding vacuum levels you see around town, "hard" (not WOT) accel, highway cruise and highway accel. This will tell you what your engine sees when you drive it. Then you disconnect vac advance and use the timing light to plot your mechanical (rpm only) advance. Return it to idle and use the hand vac pump to see what additional vac advance you get at what vac levels.

Draw the curves on a sheet of graph paper, and then graph your rpms from idle to max, and the vac levels at each. This way you see what your engine does, what timing you get at each rpm and load, and whether it has comparatively high or low advance anywhere like highway cruise/accel. Then compare it against the spec's in the FSM, as many distributors are out of spec or replaced with a completely or partially wrong generic curve. Change whatever is wrong.

While it's counter-intuitive, sometimes cruise needs more advance to prevent detonation (pinging). The reason is that without proper advance (no or low vac advance at cruise) it's running relatively retarded. This causes a lot of heat to soak the valves and chamber area and can then cause det when you then push it. BTW - this added heat is also a primary cause of exhaust valve seat recession, burning, cutting or pitting - not the unleaded gas legend.

If you want to try an easy experiment first (without the timing plots) do this test:
Duplicate the throttle level (vacuum level) that you get det on the highway, but do it from a slow speed or dead stop, all the way through and past your cruise rpms. If you get det at the point it does when cruising, you have too much advance there. If you accelerate with no det, then you don't have enough cruise timing, but it didn't det because it never had the chance to heat-soak at retarded cruise.

I hope that makes sense. Just for background, we had that just recently with a Mitsu turbo car - and advancing the cruise timing was exactly the fix. I've also done a lot of experimenting on a small block with computer-controlled ignition (much easier to change settings and curves) and can duplicate this situation at-will. The factory prevents cruise det from over advance by adding EGR. If it has a knock sensor, it will retard timing only if EGR was not enough - but it usually is.

David
Jeez, David, that is a whole bunch of homework! LOL.

I do have a timing light, vacuum gauge and a test meter that gives me tach readings, but I don't think I can make the meter work while driving. I do have trouble reading the balancer markings with the timing light, so I tend to fall back on the dreaded 'trial and error' method for tweaking the timing. I guess it will require some additional time to figure out how to get useful and reliable readings.

The mention of heat is interesting. This is the fourth summer on the rebuilt engine, but with very few miles on it because of the carb issues. I do find a lot more heat off the engine than before. This is exhaust heat not overheating the engine. My IR Temp gun shows around 190F at the waterneck, but most of the primaries, at the exhaust port are off the chart, +600F. I can feel the heat coming through the floor and the paint on the hood is burning off, the clear coat is peeling in lines over each valve cover. The plugs look a little on the rich side, so I don't think the carb jetting is lean. I've wondered if the heat condition is related to lean condition, retarded timing, or just the side effect of uncoated headers.
 

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Recently, I thought I had sorted out some timing/distributor issues until at highway speeds, I dropped the hammer to merge with traffic.

All of a sudden, there was pinging in 3rd (c6 automatic) up around 3200 rpm. Everything had been fine around town, though.

Having an old Mallory distributor, I opened it up and adjusted the mechanical advance limit down about 4 degrees and was happy with the results.

I could've just taken 4 degrees off 'initial' but I liked the grunt I was getting off the line and didn't want to fool with the low rpm curve. I hope this helps.

SGR
 

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... I do have trouble reading the balancer markings with the timing light, so I tend to fall back on the dreaded 'trial and error' method for tweaking the timing.
Make a better mark.
... This is exhaust heat not overheating the engine. My IR Temp gun shows around 190F at the waterneck, but most of the primaries, at the exhaust port are off the chart, +600F. I can feel the heat coming through the floor and the paint on the hood is burning off, the clear coat is peeling in lines over each valve cover. The plugs look a little on the rich side, so I don't think the carb jetting is lean. ...
That's a very strong sign of retarded timing. There was s small block with headers that he said would cook his feet and smell like burning in the car. We popped the hood in the evening dusk and you could see the headers dimly glowing. He had set the timing by 'ear' and was about 12° retarded. We reset his base timing, the headers cooled-off, his feet were cool, and the engine woke right up.

Do the acceleration test I described in my last post. It's easy and proves your detonation issue one way or the other. It won't tell you how much to change things, but at least you'll know what side of the fence you're on.

David
 

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I agree with PSIG on the amount of timing.Sounds like you have too little timing and too much fuel.The headers should not get that hot coated or not.It was 109 degrees here the other day and my headers with 2 1/2" exhaust under my feet didn't get that hot.A Craftsman timing light is not that expensive.Get one with the advance dial so you can check initial and at full advance.
 

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Discussion Starter · #10 ·
I didn't have time today to get into things, but I did remove the cap off the distributor to check if things were free to move. I pulled the vacuum line off to check if the vac motor would move the plate in the distributor and I noticed a kink in the line. When I installed the new carb it was getting dark out and I couldn't see well enough to catch that. The advance plate would move back and forth with me blowing and sucking on the line. I carefully installed the vac line and hit the highway and the pinging was much, much less. It did not go away completely, but is much better, so I did have a problem with vac advance not functioning.

I really need to get this whole timing thing figured out because I do think that it should not be making that much heat. It is great having the car drivable again, and aside from the heat, it is running real well. Thanks for the help.
 

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Discussion Starter · #11 ·
So I did spend a little time messing with timing on Sunday. I painted some marks on the damper to make things easier, even possible to see, with the timing light. There was a mark at 10*, I highlighted the markings at 0*, 14* and 30*. I bumped up the intial timing to 14*, vac advance disconnected. When increasing rpm's I watched the timing advance, so the mech. adv. is working, but it seemed to go past 30* pretty quickly, probably not much more than 2200 rpm. 30 was the last mark stamped on the damper, which is why I highlighted that number. Does that indicate the advance curve is off and more weight, or springs are needed to limit the mech. adv.? I was still getting some pinging at highway speeds, and even a bit under hard accel at low speeds.

Idle vacuum is around 15 - 16. 70 mph cruise around 18. Give it gas at cruise, passing without kicking down, vac. goes to 10.
 

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Full advance needs to go up to around 36-38 degrees at around 3000rpm or so.
If it goes over that, try backing off your initial advance until you get that reading.
Backing it off will also eliminate pinging.

If you end up with a good responsive engine, then you'll be fine.
If not, you might have to re-curve the distributor.
 

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Full advance needs to go up to around 36-38 degrees
:tup: It's better (at least at first) to have correct full advance than anything else. Your initial is mostly based on your cam timing and resulting dynamic compression. The same is true of your advance curve from initial to full mechanical - it's just a moving target. For now, rev the engine to where all the advance is in, set it at 36°BTC, and go drive. If you get any detonation pinging, note the approximate rpms and adjust your advance spring hooks to alter the curve slightly slower. The hooks are easily adjusted by tweaking them (bending in or out) through the access holes in the distributor breaker plate (if stock). Slightly bending the hook inward speeds the curve, and outward delays it.

One spring is usually stronger than the other. The lighter spring mostly affects the low-rpm curve, and the heavier spring does the higher-rpm. So, if you only ping at around 1200 rpm, bend the lighter spring's hook outward to slow the initial advance rate. Likewise, if it pings at maybe 2200 rpm, tweak the heavier spring's hook. If it pings across a larger range than a few hundred rpms, then either re-spring, or increase your mechanical advance so you can reduce your initial while maintaining your total.

Do a search for distributor re-curving concepts and articles like "Distributor Advance" and "Ignition Timing 101" for specifics and better explanations. Then you can move on to throwing the vacuum advance into the mix.

David

A bit blurry, but my pic of a DuraSpark distributor (same advance mechanism as a points version) shows a hole for spring hook adjustment:
 

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Discussion Starter · #14 ·
I have no idea what the advance gets to. I will have to extrapolate the damper markings and add my own marks beyond 30*. It seemed to go past the last mark pretty quick, with seemingly little RPM increase, but no idea where it is at 3000. I'll check out those articles, thanks for the links. My distributor is a points type with a Pertronix Ignitor II module.
 

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Maybe one of the springs popped off?
 

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Discussion Starter · #16 ·
OK, I added marks to my damper up to 40*. With initial 14*, no vac, at 2200 RPM it showed 34*, and at 3000 RPM it was 40*. With vac advance connected it showed 40* at 2000 RPM.

When I look at the difference between the mech only and w/vac adv. around 2000 - 2200, the vac adv seems to only add 6*. Should it be adding more? I seem to remember reading in one of Davids links that it said vac adv should add around 15*, but maybe I remember wrong.

Maybe my next step should be to set it up with mech adv reading 38* @ 3000 RPM and then see what it reads at idle, and see if it makes less exhaust heat. One other thing I did not do was drive it with the vac adv disconnected. Maybe I should drive it with the vac adv disconnected and see if the pinging goes away. Speaking of exhaust heat, the temp readings of the primaries does seem to be lower with more initial timing, but it seemed weird that drivers side was reading higher that the passenger side, by 100 and sometimes 200*F. 350F on the pass side and 450 - 550F on the driver's side. Huh?
 

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If you're trying to use the engine vacuum to check vacuum advance, that only works with the engine under load (driving). To get correct vacuum otherwise you need a hand-held vacuum pump.
 

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Discussion Starter · #18 ·
If you're trying to use the engine vacuum to check vacuum advance, that only works with the engine under load (driving). To get correct vacuum otherwise you need a hand-held vacuum pump.
That makes sense. That gets back to some of Davids original comments about tracking vacuum levels while driving.
 

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OK, I added marks to my damper up to 40*. With initial 14*, no vac, at 2200 RPM it showed 34*, and at 3000 RPM it was 40*. With vac advance connected it showed 40* at 2000 RPM.
That's all good, but total is too high. You are getting 26° mechanical, indicating you have a 13L plate. Every advance plate has two optional advance limiter slots, and one of yours is 13L (13 cam degrees = 26 crank degrees). If you're lucky, the second one is smaller, and you can flip the plate to use it. That would allow high initial timing, with ported vacuum, without exceeding 36-38° total initial and mechanical. If not, you have two choices - find one or mod yours to limit the advance more, OR keep the 13L with 10° initial and use manifold vacuum. Option 2 will give you 10 + 6 = 16° idle timing, will retard to 10+ on throttle tip-in when vacuum drops to avoid lugging det, and give 36 mechanical on heavy throttle, with 42 at cruise. Sounds pretty good to me for starters.
I seem to remember reading in one of Davids links that it said vac adv should add around 15*, but maybe I remember wrong.
I said could be 15°. Vacuum canisters can be anywhere from 6 to 16 degrees and at different rates. Most of them are 8-12° at various rates. The newer-style adjustable units change total, but only change rate a little. The old style with replaceable shims and springs can adjust both for a perfect match - but it's a royal PITA.
One other thing I did not do was drive it with the vac adv disconnected. Maybe I should drive it with the vac adv disconnected and see if the pinging goes away.
I would for testing only. Keep your mechanical and vacuum tuning separate. Get mechanical set and them add vac into the mix. That way you'll be dealing with known variables, instead of wondering which did what.
Speaking of exhaust heat, the temp readings of the primaries does seem to be lower with more initial timing, but it seemed weird that drivers side was reading higher that the passenger side, by 100 and sometimes 200*F. 350F on the pass side and 450 - 550F on the driver's side. Huh?
All the driver's side ports? That is a bit odd. Check each port to see if certain ones are hotter. Read the plugs to see if there is a relationship.

David
 

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Discussion Starter · #20 ·
I guess I'll have to disassemble the distributor a bit to check the plates. I did realize that I did have the vac adv. connected to manifold vacuum. I had the advance plugged into the port at the base of the carb right next to the pcv port. There is another line connected to the timed port, higher up on the crab, that goes to a little manifold on top of the waterneck and then line from there goes to the top of the vac adv. canister. I think it is a cold retard, if I remember correctly. I was thinking about swapping those lines around to see what happens.

I calculated my 70mph RPM to be around 2300 with my 28" diam tires and 2.75 rear gear. I'll start by setting mech adv. at 38* @ 3000 RPM and then see where the initial ends up, both without and with vac adv. If that doesn't work, I'll drop it to 36*. I imagine that the pinging would clear up if I was running premium, but I'd like to see if I could get it tuned satisfactorily on regular fuel.

Yes it was all the driver side ports. I was looking for it to be related to planes in the intake, but no, so it would seem to not be related to the carb and different fuel delivery side to side. 6 and 8 were hotter than 5 and 7. After my next tuning session, not until the weekend, I'll record temp readings and report back, and check the plugs again.
 
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