Timing issue 70 Polara

[carfreak6970]

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.

 

[GOLDMYN]

Hi, I'm not a carb guy, the only comment I want to pass on. I had my 4640 rebuilt by "Woodruff" and one of their comments was that the 4640 runs lean so they made the necessary changes.

 

[Justin Plant]

A lean condition will idle fast. Turn your screws out a quarter turn and see what happens.

 

[Darter6]

The ported nipple is always above the throttle plates. Yes you are on the proper port.
Sounds like you have to adjust the air mixture screws.
You know your timing is at 8*before with the vacuum hose disconnected.You verified that.
Now you have to adjust the mixture screws along with the idle screw. Look for the highest vacuum reading with the mixture screws.
Then adjust the idle screw to desired RPM. Try again with the mixture screws looking for the highest vacuum reading.Then again adjust your idle screw for the desired RPM. Doing this will set the throttle blades in the correct position and you should no longer get a vacuum reading at the advance port.

You will have to open the distributor, remove the points plate and vacuum advance canister. That will get you to the mechanical weights.Check to see that they are moving free clean and lube the bushings, also remove and oil the felt pad under the rotor.
Check that all is moving freely.
Now some advance canisters are adjustable and inserting a 3/32 allen key you can increase or decrease by turning 1/2 turn to see if it helps with the pinging. I don't remember right turn or left to decrease vacuum advance.
Sounds like you may have done this already and sounds like you have a distributor machine.Man that would be nice.
This may or may not help you but maybe someone else that reads this.
Are you using 93 octane ?

 

[CBODY67]

I concur that the vac advance is and should be hooked to ported vacuum. The one thing I see out of what is a hot base idle speed that starts 100-150rpm faster than specs. Should be more like 700rpm. With the throttle plates open that much more, more possible vac at the ported vac port and vac advance action.

Another thing is that 13.4" Hg vac at the manifold vac port seems a good bit low to me. Something wrong there, it seems.

Pinging wiht the recommended octane fuel MIGHT happen if the vac advance is too active. So some adjustments there might be needed, as mentioned.

If the ign system is still points, the point dwell must be in-spec FIRST before the base timing is set.

You can check the mechanical advance without taking anything apart. Just grab the dist rotor and twist it in the advance rotation direction. This will manually advance the weights (against spring pressure) to the max allowed by the slot in the mechanism. Release the rotor and it should spring back to "no advance" quickly and resoundingly.

IF you suspect the vac advance of "sticking", just unplug and cap the ported advance port hose and drive the car. Off-idle throttle response will become a bit less, but the car is still very drivable and such. Just like if the vac advance diaphram had failed. Fuel economy will drop too, BUT if the hot base idle speed is now stabilized, that can pinpoint your problem.

A SIDE ISSUE can be a hairline crack somewhere in the vacuum supply piping that is closed when the engine cool and then opens slightly when the engine/under hood heats it up. Might be a fitting on the power brake vac connection, the hvac "tree" on an intake manifold runner, or the cruise control vac diaphram. Several possibilities that mght be easy to track.

For general principles, ensure that the intake manifold bolts are tight and the carb base nuts are similar. If you are using an aftermarket thin base gasket or a Mr. Gasket style "stack" insulator gasket, just "snug", not "tight", as that can break the baseplate area. As I discovered back in about 1975 in chasing the famous "hot soak" situation on my '70 Monaco 383 4bbl car.

Fuel octane? When the car was designed and sold, it was rated at "Premium" fuel, which was 97-100 Research Octane level. To get to the current "Pump Octane" numbers, subtract 3-5 Research Octane numbers, which means Super Unleaded 93-94 Pump Octane (so you can run near to the desired 36-38 total mechanical advance + base timing levels.

When you rebuilt the motor, what thickness head gasket was used? A thicker gasket might lower the compression ratio a bit, but if the heads were cut and the block decked, each of those can raise the compression more to "blueprint specs", which means more than suspected, within the stock range.

Just some thoughts,
CBODY67

 

[65sporty]

I would also check that 0 on the balancer is TDC, if the balancer has slipped, your 8 degrees may be more at idle

 

[halifaxhops]

I bet the throttle plates are not closing. only way you will get vac on the ported nipple. Semi common.

 

[carfreak6970]

The ported nipple is always above the throttle plates. Yes you are on the proper port.
Sounds like you have to adjust the air mixture screws.
You know your timing is at 8*before with the vacuum hose disconnected.You verified that.
Now you have to adjust the mixture screws along with the idle screw. Look for the highest vacuum reading with the mixture screws.
Then adjust the idle screw to desired RPM. Try again with the mixture screws looking for the highest vacuum reading.Then again adjust your idle screw for the desired RPM. Doing this will set the throttle blades in the correct position and you should no longer get a vacuum reading at the advance port.

You will have to open the distributor, remove the points plate and vacuum advance canister. That will get you to the mechanical weights.Check to see that they are moving free clean and lube the bushings, also remove and oil the felt pad under the rotor.
Check that all is moving freely.
Now some advance canisters are adjustable and inserting a 3/32 allen key you can increase or decrease by turning 1/2 turn to see if it helps with the pinging. I don't remember right turn or left to decrease vacuum advance.
Sounds like you may have done this already and sounds like you have a distributor machine.Man that would be nice.
This may or may not help you but maybe someone else that reads this.
Are you using 93 octane ?

I only run premium in anything with a carb. Which around here is 92-93. I guess I should have specified above that all my vacuum readings were done at a ported vacuum port, not manifold vacuum.

I concur that the vac advance is and should be hooked to ported vacuum. The one thing I see out of what is a hot base idle speed that starts 100-150rpm faster than specs. Should be more like 700rpm. With the throttle plates open that much more, more possible vac at the ported vac port and vac advance action.

Another thing is that 13.4" Hg vac at the manifold vac port seems a good bit low to me. Something wrong there, it seems.

Pinging wiht the recommended octane fuel MIGHT happen if the vac advance is too active. So some adjustments there might be needed, as mentioned.

If the ign system is still points, the point dwell must be in-spec FIRST before the base timing is set.

You can check the mechanical advance without taking anything apart. Just grab the dist rotor and twist it in the advance rotation direction. This will manually advance the weights (against spring pressure) to the max allowed by the slot in the mechanism. Release the rotor and it should spring back to "no advance" quickly and resoundingly.

IF you suspect the vac advance of "sticking", just unplug and cap the ported advance port hose and drive the car. Off-idle throttle response will become a bit less, but the car is still very drivable and such. Just like if the vac advance diaphram had failed. Fuel economy will drop too, BUT if the hot base idle speed is now stabilized, that can pinpoint your problem.

A SIDE ISSUE can be a hairline crack somewhere in the vacuum supply piping that is closed when the engine cool and then opens slightly when the engine/under hood heats it up. Might be a fitting on the power brake vac connection, the hvac "tree" on an intake manifold runner, or the cruise control vac diaphram. Several possibilities that mght be easy to track.

For general principles, ensure that the intake manifold bolts are tight and the carb base nuts are similar. If you are using an aftermarket thin base gasket or a Mr. Gasket style "stack" insulator gasket, just "snug", not "tight", as that can break the baseplate area. As I discovered back in about 1975 in chasing the famous "hot soak" situation on my '70 Monaco 383 4bbl car.

Fuel octane? When the car was designed and sold, it was rated at "Premium" fuel, which was 97-100 Research Octane level. To get to the current "Pump Octane" numbers, subtract 3-5 Research Octane numbers, which means Super Unleaded 93-94 Pump Octane (so you can run near to the desired 36-38 total mechanical advance + base timing levels.

When you rebuilt the motor, what thickness head gasket was used? A thicker gasket might lower the compression ratio a bit, but if the heads were cut and the block decked, each of those can raise the compression more to "blueprint specs", which means more than suspected, within the stock range.

Just some thoughts,
CBODY67

I do not believe the block was decked or the heads cuts, and not to sure what thickness head gasket. That was all done by my father before I knew how an engine worked, so anything is possible. I am running an insulator and though I havent checked the intake or carb bolts lately, probably wouldnt be a bad thing to look into. I believe you are correct in that I should just unplug the vac canister and drive it to see what happens. if everything stabilizes and no changes due to temperature I know my issue is either with that port, the rubber line, or that vac canister. If not there are other things I need to look into.

I forget why I set that idle above 800 when I dropped the initial from 12 to 8. I guess I could try an close the throttle plates (closer to the .040 opening of the transfer slot) and see if that takes vacuum away at the ported vacuum port.

I bet the throttle plates are not closing. only way you will get vac on the ported nipple. Semi common.

If the throttle plates are not closing, why would the idle speed screw be against its stop? Or are you saying once the carb is warm and the secondaries open they are not closing fully and therefore moving the idle speed screw stop lever (i think thats its name)? because isnt that screw stop movable?

I would also check that 0 on the balancer is TDC, if the balancer has slipped, your 8 degrees may be more at idle

I verified that two years ago when I had it out here over winter.

 

[CBODY67]

Thanks for the reply and additional information.

In a normal rebuild, the cyl heads might be "surfaced", but not by a great degree. Just enough to get a consistent flat surface, which is usually not very much. A composite head gasket was probably used, so no issues with that, although the OEM gasket would have been .018" steel shim in nature.

An "out-lyer" in the carb could be throttle shaft wear, where the carb body/base plate wears enough so that the throttle plates do not always return to the same location when closed. There is supposed to be some clearance there, so things don't bind, but I have not seen any real specs of how to measure that clearance and what is "out of spec", either.

Secondaries not fully closing can have an affect on idle speed, too, but some carb cleaner can normally take care of that. Usually, they stick closed, it seems, from non-use.

Another even more "far out" "out-lyer" would be throttle return springs which get weaker with heat, relying only upon the return spring on the carb itself. Or a throttle cable that might need some lube on its inner cable area? Although the only areas which can accept lube are at the very ends, where then inner cable is exposed.

Please let us know how things progress,
CBODY67

 

[carfreak6970]

This past weekend I got sometime to work on the polara. So before initial start up I took off the vacuum line from the carb to the vacuum canister to see if it would hold vacuum without collapsing. Which it did. So I installed it back on the canister and plugged the port on the carb. Started it, everything appeared to be fine, so I went for a drive. Upon return the car sounded normal so I hooked the timing light and tach to check everything. everything turned out fine, no issues or changes. I then took my vacuum pump to the line on the vacuum canister and was playing around adding vacuum and releasing it at different rates. No matter how much vacuum I added or how quickly I released the vacuum the vacuum canister always acted as it should. So that checked out.

Next up was connecting a vacuum gauge to the manifold vacuum port at the back of the intake and connected another gauge to the ported vacuum on the carb. Manifold vacuum was at 17.5 inHg, where as ported vacuum was 13 inHg at idle. I decreased the idle to 650 and the ported vacuum dropped to 0 and after adjusting the idle air speed screws I got the manifold vacuum up to 18.5. Connected the vacuum canister to ported vacuum reved the engine a bit and everything went back to where it should be. Disconnected all the gauges and went for a drive. The car drove great, no pinging and no changes to the idle speed. Once I got back to the garage I got tach and timing light out again to verify nothing changed.

So I believe the issue is the carb I have on the car is from 69 and I was using the 70 service manual. According to the 70 service manual a carter AVS should have an idle speed of 800-900 rpm. However, the 69 service manual that covers the carb that I have on the car says the idle speed should be 650 rpm. So it appears that the location height of the ported vacuum port must have changed between 69 and 70 in order to get a higher idle speed without that port seeing a vacuum signal.

I appreciate everyone's help on this one!

 

[CBODY67]

My '70 Polara/Monaco FSM, for Federal Emissions, indicates what your '69 FSM indicates as to hot base idle speeds. Back then, as I recall, the ONLY engines which needed a hot base idle speed over 700rpm were some California CAS-spec cars with manual transmissions. Like the 340s with manual transmission in E-body cars or 440 6bbls cars.

Glad you got things figured out!

Take care,
CBODY67

-------------------------
Update -- After re-reading the original post, noting the 440 engine and such, I went back into my '70 Polara/Monaco FSM and looked at hot base idle speeds. I need to state that my main concerns in this manual related to my '70 Monaco Brougham 383 "N" application, so I figures the 440s were different, which they were NOT, at least in 1970, but the '69 FSM's information was different AND what I had suspected. Interestingly, the 383s with a Holley 4160 had a spec idle speed of 650rpm rather than the 700rpm of the AVS. Which might mean the Holley venturi is a bit more efficient than the similar Carter AVS venturi?

So, my apologies for my lack of specific knowledge of the 440s vs 383s in the 1970 model year, relating to carb hot base idle speeds.