carfreak6970
Well-Known Member
Hello,
I am not to sure if this is a fuel issue, or a timing issue so I figured I would post in this section:
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:
1. 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.
2. 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?
3. 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
I am not to sure if this is a fuel issue, or a timing issue so I figured I would post in this section:
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:
1. 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.
2. 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?
3. 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
