70 Polara Pinging Issue

carfreak6970

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Hello Folks,

I am having a tuning issue on a 70 polara and I think the issue is carb related. My guess is either the springs for the rods are too light or I have a vacuum leak. Here are the details:



The car is a 70 polara convertible with a 440, 727, and 3.23 suregrip. The engine was rebuilt 15,000-20,000 miles ago and never really had a problem with it. It has a stock iron 4 barrel intake and stock iron heads. It was rebuilt to either 69-70 magnum status. It currently has a Carter AVS on it (I believe it is the 4640S) that I rebuilt but got from Chryslers at Carlisle last summer.



So the story goes is that the car initially had a 71 AVS (4966S) which is a 440 carb but it has smaller primaries. It ran good, no issue. The initial was set at 12 BTC. I decided to rebuild it and noticed that the secondary and primary jets were swapped. So I put the jets back where they should and put the carb back together. The car didn’t feel as spunky as before and there was a slight ping under heavy load. The initial was still at 12 BTC. I figured it was due to the carb being a little too small, so I searched out a 440 AVS with the 1-11/16 primaries. I found the 4640S that is currently on it last year and recently rebuilt it and installed it.

When I took it out for a drive I noticed the car had a noticeable amount of pick up and go and I was really happy with it, however it pinged now under any type of load. Load being pulling away from a stop sign at night. I could only “gingerly” accelerate to keep it from pinging. This is with it still at 12 BTC. So I figured I would decrease the initial and see what happens.

Well I was able to drop it to 8 BTC and took it out for a ride last night and pleased to report that there was no more ping. However, the engine temperature did not like the change. The temp gauge fluctuated like it never did before and after parking it for a couple minutes there was a puddle of coolant sitting under the over flow tube that was about 6 inches in diameter. Again, never did that before.

From what I am reading mopar big blocks can like 12-16 BTC with around 30-36 total timing. So it would make sense that when I back off the timing the temperature raises a bit (at least that is what I gather from searching mopar forums). But with me putting the timing in that 12-16 range I get pinging, which from what I read is too much timing. OR, a lean mixture.

Now I was able to verify that the 4640S carb did have the correct size jets and rods, but I had no way of checking the springs. So here is what I believe could be the top issues:



  • The amount of timing the distributor is putting in is in access of 20 degrees. Which I doubt. I don’t know what timing it is putting in and when, but considering this excessive pinging was not an issue with the other carb at this timing I am going to say it probably isn’t the distributor. But, I can easily check that, I have a distributor machine I can put this on to verify.
  • The springs in the carb are too weak preventing the rods from allowing more fuel into the mixture. I know with Holleys you can get power valves based on vacuum ratings and you can easily have a vacuum gauge to read where vacuum is at in certain situations and change your selection based on that. But I don’t know if the carter AVS has that ability with the springs. I could go through my spring stash to see if I have stronger springs that would allow more fuel into the mixture at a high vacuum rating. Only way to verify spring strength is by feel correct? Or was it based on height of the spring?
  • Lastly, there is some vacuum leak that is allowing air to come in at a different point and bypass the venturi which would not bring in the metered amount of fuel. I feel like this is probably my best bet. I reused the carb spacer and gasket. I have reused this carb spacer and gasket before and never had an issue, but that is not to say it is not an issue now. Or it could be that somewhere in the casting of the carb it is either warped or cracked. The reason I feel this is the best bet is that when trying to set initial timing and idle speed the rpm varies by +/- 50 rpm and the timing mark with the light does fluctuate +/- 3 degrees, which it did not do before this most recent carb swap.


So I can see if I have strong springs to allow more fuel in sooner. Is there a way I can check a vacuum leak at the carb? And I can limit the amount of timing the distributor puts in with an FBO limiter plate. Am I on the right track? Am I missing something? Is there anything else for me to check? I was going to get a small tach that I can have in the car so I can document when the pinging comes in to see if it is based on RPM, and where the pinging stops to help change the curve if need be.



Thank you
 

Big_John

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You got the pinging to stop, but it dumped a little coolant... Correct?

Simplest things are always the first on my list... So, I'd change it to 10*BTC and see what happens.

I kinda doubt the 4* change really affected engine temperature that much, so I'm thinking it's unrelated, but worth keeping an eye on and seeing what it does at 10*
 

'66 Fury I

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The problem started when you swapped the jets to the "proper" positions. Did this equate to smaller primary jets? If so, could larger jets solve the problem?
The "old school" test for vacuum leaks was to start the engine, run an unlit propane torch along suspect joints and watch/listen for a change in rpm when the leak drew in the extra air/fuel mix. LC
 

413

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Lean will ping.
Lean will run hot
ethanol fuel makes it lean

Your engine is lean

you had bigger jets on the primaries with the 4966 so it didn’t ping.

then you changed things.

rejet the 4640
 

MONC440

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Lean will ping.
Lean will run hot
ethanol fuel makes it lean

Your engine is lean

you had bigger jets on the primaries with the 4966 so it didn’t ping.

then you changed things.

rejet the 4640
I agree it sounds like you need larger jets or smaller metering rods.

You can see if the metering rods and springs are working by removing the air cleaner and loosen the covers so you can turn them. Start the engine and rotate the covers above the rods then blip the throttle and see if the rod assembly pop up. If not you need a stiffer spring. If they do the spring is working. You can change the rod to the next size smaller which is the same increasing the jet.

If you can get an O2 sensor in the exhaust you can tune your fuel jets based on that.
 

carfreak6970

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The problem started when you swapped the jets to the "proper" positions. Did this equate to smaller primary jets? If so, could larger jets solve the problem?
The "old school" test for vacuum leaks was to start the engine, run an unlit propane torch along suspect joints and watch/listen for a change in rpm when the leak drew in the extra air/fuel mix. LC

Yes on the 4966S I put the .089 in the primary side and the .095 back in the secondary side. So that means when the car didnt mind 12 BTC the primary jets were .006 in diameter bigger. So go that size or half that with the change in jets?

Thank you for the propane torch idea. Ill give that a go. Would starter fluid do the same thing?

Lean will ping.
Lean will run hot
ethanol fuel makes it lean

Your engine is lean

you had bigger jets on the primaries with the 4966 so it didn’t ping.

then you changed things.

rejet the 4640

I agree it sounds like you need larger jets or smaller metering rods.

You can see if the metering rods and springs are working by removing the air cleaner and loosen the covers so you can turn them. Start the engine and rotate the covers above the rods then blip the throttle and see if the rod assembly pop up. If not you need a stiffer spring. If they do the spring is working. You can change the rod to the next size smaller which is the same increasing the jet.

If you can get an O2 sensor in the exhaust you can tune your fuel jets based on that.
Thank you for that rod test!! that makes sense. I am thinking this could be a lean issue.

When it comes to O2 sensors I really do not want to drill into the exhaust... I have seen these clamps:

Amazon product

Could I just get a simple AFR gauge with an O2 sensor and use those clamps for tuning purposes?
 

Justin Plant

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When most folks start jetting down a carb, they forget the air that pulled through it is still the same CFM. If you go too far, well you get what you got.
 

MONC440

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You can use an O2 sensor that goes in the tail pipe and is used just for testing and tuning
 

Big_John

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You could just look at the spark plugs and see if it's running lean.

Look for a YouTube video... there's probably a dozen that will tell you what to look at.
 

1970FuryConv

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Sounds like the 4640S carb gave larger primaries, but with smaller primary jets. That seems like a recipe for a lean mixture that would require you back off timing to avoid ping.

I also think the radiator overflow is unrelated. Could be too high a coolant level for a radiator with no reservoir.
 

CBODY67

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First thing to do is to verify, via the FSM, what carb should be there and what the jets/rods are that go in it. If the carb is of a different known model year, look in that year's FSM for information. Do NOT guess about what is supposed to be where!

You can get a Strip Kit for the Edelbrock AFBs and AVS carbs. A selection of primary metering rods, pri and sec jets, etc. NO adjustment to the rod heights, just different springs to phase the rods to particular vacuum levels. I got one of those (383 4bbl) and swapped a few around, seeking better fuel economy, but NO appreciable difference in the way it ran, so I put it back like it came.

Also, in the FSM, note the tune-up specs for your engine. You could well discover that initial base timing is in the 7.5 degree BTDC range, with the stock distributor. IF the distributor in your engine is not original to it, you need to determine just how much advance is in the distributor (mechanical) and in the vac advance unit (vac advance). You can do this with a dial-back (analog or digital) timing light and a dwell tach (to measure engine rpm). I've found that a quality analog version (Craftsman) has given me good results, but others in here like the later digital models, FWIW.

SO, put thing back like they came. Do some baseline tests and go from there. I believe the real issue is that you are running a 9.7CR motor on fuel which is closer to what leaded regular used to be, as far as octane goes. For best results, you need a pump octane of about 94+, which is not really "there" with modern fuels, even before ethanol was added. Especially unleaded. Most unleadeds at the lower altitudes are 93 pump octane., which is close and useable.

Now, with everything back to stock Chrysler FSM specs in the carb you're using, if the pinging you are hearing is just under part-throttle acceleration, not WOT acceleration, you might try slipping an Allen wrench into the nipple on the vac advance can and see if it might index with a nut inside the can. IF the Allen wrench definitely drops into a nut and will turn it, turn it 1/4 turn clockwise and recheck for the ping. If the ping is diminished or disappears, you have slowed down the vac advance at that vac level (really, across the board) so that you can still run the spec initial advance and things will be quieter.

As for total timing advance (mechanical + initial), 34-38 degrees BTDC is usually the total advance that Chrysler B/RB engines like. Sometimes, up to 40 degrees, if the fuel will work with it that high. By observation, most of the 383 2bbls had a pretty much optimum advance curve from the factory, but the 4bbl engines usually had less total, more like 31 degrees total BTDC, from the factory. "Performance/race" engines supposedly like that 38 degrees advance to be all in by 2000rpm, whereas the factory OEM specs are closer to 4300rpm or so.

DO make sure that the vac advance is hooked up to the correct ported vac source on the carb, too. And that all base timing readings are taken with that line unhooked from the vac advance unit and the line plugged.

NOW, if you want to do some vintage tuning, go to the local drag strip and purchase some 100 Research Octane race gas from a vendor. With the vehicle set to factory specs, it should run like it was supposed to. Then modulate the timing back so that you get no clatter at WOT and see where things end up with your available 93 Pump Octane super unleaded.

As for "jetting up" for E10 fuels, increase the jet orifice diameters by a factor of about 1.05, to end up with the 14.2 to 1 "stoich" AFR for E10, rather than the 14.7 AFR of E0 fuels. This is where the Strip Kit will come in handy.

Enjoy!
CBODY67
 

carfreak6970

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First thing to do is to verify, via the FSM, what carb should be there and what the jets/rods are that go in it. If the carb is of a different known model year, look in that year's FSM for information. Do NOT guess about what is supposed to be where!

You can get a Strip Kit for the Edelbrock AFBs and AVS carbs. A selection of primary metering rods, pri and sec jets, etc. NO adjustment to the rod heights, just different springs to phase the rods to particular vacuum levels. I got one of those (383 4bbl) and swapped a few around, seeking better fuel economy, but NO appreciable difference in the way it ran, so I put it back like it came.

Also, in the FSM, note the tune-up specs for your engine. You could well discover that initial base timing is in the 7.5 degree BTDC range, with the stock distributor. IF the distributor in your engine is not original to it, you need to determine just how much advance is in the distributor (mechanical) and in the vac advance unit (vac advance). You can do this with a dial-back (analog or digital) timing light and a dwell tach (to measure engine rpm). I've found that a quality analog version (Craftsman) has given me good results, but others in here like the later digital models, FWIW.

SO, put thing back like they came. Do some baseline tests and go from there. I believe the real issue is that you are running a 9.7CR motor on fuel which is closer to what leaded regular used to be, as far as octane goes. For best results, you need a pump octane of about 94+, which is not really "there" with modern fuels, even before ethanol was added. Especially unleaded. Most unleadeds at the lower altitudes are 93 pump octane., which is close and useable.

Now, with everything back to stock Chrysler FSM specs in the carb you're using, if the pinging you are hearing is just under part-throttle acceleration, not WOT acceleration, you might try slipping an Allen wrench into the nipple on the vac advance can and see if it might index with a nut inside the can. IF the Allen wrench definitely drops into a nut and will turn it, turn it 1/4 turn clockwise and recheck for the ping. If the ping is diminished or disappears, you have slowed down the vac advance at that vac level (really, across the board) so that you can still run the spec initial advance and things will be quieter.

As for total timing advance (mechanical + initial), 34-38 degrees BTDC is usually the total advance that Chrysler B/RB engines like. Sometimes, up to 40 degrees, if the fuel will work with it that high. By observation, most of the 383 2bbls had a pretty much optimum advance curve from the factory, but the 4bbl engines usually had less total, more like 31 degrees total BTDC, from the factory. "Performance/race" engines supposedly like that 38 degrees advance to be all in by 2000rpm, whereas the factory OEM specs are closer to 4300rpm or so.

DO make sure that the vac advance is hooked up to the correct ported vac source on the carb, too. And that all base timing readings are taken with that line unhooked from the vac advance unit and the line plugged.

NOW, if you want to do some vintage tuning, go to the local drag strip and purchase some 100 Research Octane race gas from a vendor. With the vehicle set to factory specs, it should run like it was supposed to. Then modulate the timing back so that you get no clatter at WOT and see where things end up with your available 93 Pump Octane super unleaded.

As for "jetting up" for E10 fuels, increase the jet orifice diameters by a factor of about 1.05, to end up with the 14.2 to 1 "stoich" AFR for E10, rather than the 14.7 AFR of E0 fuels. This is where the Strip Kit will come in handy.

Enjoy!
CBODY67
According to the 69 & 70 FSM I do have the correct size jets (.101 primary, .095 secondary) and the correct rods. I do not know about the springs.

According to the FSM the timing can be set at 5 deg BTC on auto 440 cars or for 70 either 5 of 2.5 deg BTC depending on standard or high performance.

Depending on manual or auto, std or performance timing changed. Ill only list the HP auto stuff since that is what the engine was model after:
69
- 2 to 7 at 550 (1100 engine rpm)
-9 to 11 at 850 (1700 engine rpm)
-14 to 16 at 2400 (4800 engine rpm)

70
- 0 to 4.6 at 600 (1200 engine rpm)
- 5.7 to 7.7 at 800 (1600 engine rpm)
- 10 to 12 at 2300 (4600 engine rpm)

I was able to verify my distributor degrees this morning on my Sun Machine at the following:
- 5 deg at 800 (1600 engine rpm)
- 8 deg at 1000 ( 2000 engine rpm)
- 10 deg at 2200 (4400 engine rpm)

Vacuum advance is as follows:
5 deg at 13 inHg
10 deg at 16 inHg

So my distributor is very close to the 70 version, but doesnt add as much as the FSM says.

Everything is hooked up the way it should be. The vacuum pod is not receiving vacuum at idle. The car is only fed premium fuel. Tuning the car is done with a craftsman professional timing light that has the adjustable advance, and a vintage analog snap on dwell tach meter.

Thank you for the descriptions on how to tune the vacuum advance and jet for E10 fuels. I did not run across that before. I guess with all my tests I only did part throttle acceleration and not WOT which would make sense that the vacuum can would still be adding in advance at that point.

However I do have some cause for concern for using an Edelbrock strip kit. I was under the impression the Edelbrock kits were only the 2 step rods (AFB version) and the avs had the 3 step rods. To accommodate the three steps the rods were longer so the rod caps were domed, where as the AFB were flat. I read that if you wanted to use the strip kits from Edlebrock you needed to go with the flat rod caps. Are you saying that is not the case?
 

cbarge

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I get it that you want the car to be stock as per specs.
But if you want to drive it you have to tune it to rhe gas we have today-- not like the 1970 gas.

In my 3 cars, all had to have bigger jets, different metering rods and the accelerator pump adjusted for an extra shot of fuel.

Next you need to burn that fuel correctly by setting the timing.
I had to recurve all my distributors.
More initial timing with a shorter mechanical advance.
Hotter spark with one step cooler spark plug.
Yes big block like 10 to 12 degrees initial. But when all in at 2800 is more than 36 degrees. And pings...yes get the advance plate from FBO.
Keep in mind every engine is different.
It will tell you what it needs- more fuel, more timing,etc. Just listen to it!

Always tune with a vacuum gauge!!
With stock cam, vacuum should be a steady 18 inches or close to it. Bigger the cam the lower the vacuum.

As mentioned, todays corn fed gas pumps runs leaner and can cause engines to run hotter.
Retarding the timing will make an engine run hot.
Make sure the cooling system is up to snuff
Hope this helps.
 
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