carfreak6970
Member
So my issues resides in a 70 Polara. It has a stock rebuild 440 running an AVS 4640SA carb that was rebuilt. Timing settings are 8 deg BTDC with an idle speed of 800-850 rpm pulling about 13 inHg at idle. I initially had the initial set at 12 BDTC but would ping at moderate to heavy acceleration so I backed it off to where it is now.
So my current issue is the idle speed increases once the engine has been run up for awhile. I noticed this last month when I took it for a drive an hour away. The idle was high enough to the point that the brakes were struggling to slow it down when I got to my destination. Once I fired it up a couple hours later for the return trip the idle speed was back to normal at the beginning and was high, but not as high once I got it back home. I tried to replicate it when I was in for Carlisle. I didnt drive anywhere near as far but when I got the car back into the garage I did some digging. The idle speed was about 950-1000 and the timing was hovering around 12-14 ( hard to tell with all the stuff in the way). Everything appeared to be in working order on the carb and the idle speed screw was against the stops but when I disconnected the vacuum advance the idle speed dropped to where I set it initially and the timing was back down to 8 deg BTDC.
The port I have the vacuum advance attached too is this one:
The port just to the left of the passenger side idle mixture screw. Which I think is Ported Vacuum (not seeing vacuum at idle).
This is the vacuum reading from that port at idle:
Which is 13 inHg at idle.
Now if that is suppose to be ported vacuum that port should not be seeing any amount of vacuum at idle. So first off, am I plugged in at the wrong port? I know the idle speed screw is turned in 5/8 of a turn from the .040" transfer slot opening. So could that idle speed screw be opening the throttle blades enough that the ported vacuum is seeing vacuum at idle? Or if this ported vacuum port is seeing vacuum when its not suppose to could something be wrong with the carb?
My other issue, that I think is the problem, is my vacuum advance is sticking. I know when I had this distributor on my machine at 13 inHg the canister would add in about 3-4 degrees of timing (which would be 6-8 at the crank) so it makes sense that if the port is getting 13 inHg the canister is adding in more timing. But if I disconnect the line and the timing decreases to my initial setting, plugging it in back in should make it go back up to that higher timing but it doesnt. And since this higher timing thing happens when its warm and driven and not initially (as in when its cold or warmed up finally and sitting in the driveway) would lead me to believe the vacuum canister is faulty. Am I correct in my line of thinking?
I know my distributor puts in a total of 9 deg (18 deg at crank) at 2200 RPMS. So even with the 12 deg BTDC that is only a total of 30 degrees of timing which is well below the suggested of 34-38 degrees of timing (depending on sources) of the amount of timing an RB motor can take. But if the canister is sticking that could be adding anywhere from 6-18 degrees of timing (max timing this canister is adding 9 deg at distributor) which would put me at the upper edge or even over the amount of timing an RB would take and hence the pinging.
I will add that even at 8 deg BTDC I am getting some pinging at moderate acceleration.
So my current issue is the idle speed increases once the engine has been run up for awhile. I noticed this last month when I took it for a drive an hour away. The idle was high enough to the point that the brakes were struggling to slow it down when I got to my destination. Once I fired it up a couple hours later for the return trip the idle speed was back to normal at the beginning and was high, but not as high once I got it back home. I tried to replicate it when I was in for Carlisle. I didnt drive anywhere near as far but when I got the car back into the garage I did some digging. The idle speed was about 950-1000 and the timing was hovering around 12-14 ( hard to tell with all the stuff in the way). Everything appeared to be in working order on the carb and the idle speed screw was against the stops but when I disconnected the vacuum advance the idle speed dropped to where I set it initially and the timing was back down to 8 deg BTDC.
The port I have the vacuum advance attached too is this one:
The port just to the left of the passenger side idle mixture screw. Which I think is Ported Vacuum (not seeing vacuum at idle).
This is the vacuum reading from that port at idle:
Which is 13 inHg at idle.
Now if that is suppose to be ported vacuum that port should not be seeing any amount of vacuum at idle. So first off, am I plugged in at the wrong port? I know the idle speed screw is turned in 5/8 of a turn from the .040" transfer slot opening. So could that idle speed screw be opening the throttle blades enough that the ported vacuum is seeing vacuum at idle? Or if this ported vacuum port is seeing vacuum when its not suppose to could something be wrong with the carb?
My other issue, that I think is the problem, is my vacuum advance is sticking. I know when I had this distributor on my machine at 13 inHg the canister would add in about 3-4 degrees of timing (which would be 6-8 at the crank) so it makes sense that if the port is getting 13 inHg the canister is adding in more timing. But if I disconnect the line and the timing decreases to my initial setting, plugging it in back in should make it go back up to that higher timing but it doesnt. And since this higher timing thing happens when its warm and driven and not initially (as in when its cold or warmed up finally and sitting in the driveway) would lead me to believe the vacuum canister is faulty. Am I correct in my line of thinking?
I know my distributor puts in a total of 9 deg (18 deg at crank) at 2200 RPMS. So even with the 12 deg BTDC that is only a total of 30 degrees of timing which is well below the suggested of 34-38 degrees of timing (depending on sources) of the amount of timing an RB motor can take. But if the canister is sticking that could be adding anywhere from 6-18 degrees of timing (max timing this canister is adding 9 deg at distributor) which would put me at the upper edge or even over the amount of timing an RB would take and hence the pinging.
I will add that even at 8 deg BTDC I am getting some pinging at moderate acceleration.
