Timing confusion

[Mikes73]

As already said, back down the initial timing a bit to say 12 to 14 degrees and hook the distributor to ported vacuum. That should solve most if not all of your problems

 

[Don C]

Mike and John, after thinking about it some more and verifying the location of the ported vacuum opening in the carburetor throat I tend to agree with you about how soon the ported vacuum receives more manifold vacuum. However, I believe that the throttle blade has to be open further than off-idle and that the throttle blade has to fully expose the port for the vacuum to equalize.

Who says an old dog can't learn? Thanks for convincing me.

I like the idea that you can start the car without any vacuum advance being applied. But, when I really believe it would be beneficial is when it is cold, and when the choke is set and the throttle is on fast idle the distributor will still get some vacuum advance, depending on how much manifold vacuum is generated during cranking.

It'll be a while before I have mine running, again, so it'll be a while before I can connect a pair of vacuum gauges to see where the transfer takes place, at least on my Holley.

 

[Bill73Ragtop]

My idle is around 900rpm in park at operating temp, and I can't turn my curb idle down any more.

That you cannot drop the idle is significant. Usually this is due to a vacuum leak. If possible, it should be looked into.

 

[Don C]

I agree with Bill, something is not right, with vacuum leak at the top of the list. Other possibilities include damaged primary throttle blades or secondary throttle open too far.

 

[c9zx]

Do the throttle blades have holes drilled in them? Chuck

 

[basstrix]

Do the throttle blades have holes drilled in them? Chuck

I was wondering the same thing.

 

[Stanglover]

In my case, I NEEDED to drill a 3/32" hole in each blade to regain control of the idle mixture screws. I didn't think the carb specialist at the local performance shop knew what he was talking about until I saw the results. The reason for the holes was to allow setting the primary blades to close off the transfer slot to minimize fuel and maximize air flow, as mentioned in an earlier post. Basically my carb was too rich at idle which meant that I had to open the curb idle screw just to get it to run (more air) The cam I have produces 18" hg. Now I can get max vacuum at idle on the idle mixture screws, which I could not do before. This is not a new idea. I'm not sure why some seem to be questioning holes drilled in the primary blades.

 

[basstrix]

In my case, I NEEDED to drill a 3/32" hole in each blade to regain control of the idle mixture screws. I didn't think the carb specialist at the local performance shop knew what he was talking about until I saw the results. The reason for the holes was to allow setting the primary blades to close off the transfer slot to minimize fuel and maximize air flow, as mentioned in an earlier post. Basically my carb was too rich at idle which meant that I had to open the curb idle screw just to get it to run (more air) The cam I have produces 18" hg. Now I can get max vacuum at idle on the idle mixture screws, which I could not do before. This is not a new idea. I'm not sure why some seem to be questioning holes drilled in the primary blades.

Idle speed is controlled to a large extent by the amount of air allowed to enter the engine. If one is unable to close off that air flow, one could end up with a minimum-achieveable idle speed that is greater than desired. Surely you can agree that if someone was to have drilled the blades with TOO LARGE of a hole, it could result in a situation of excessive idle speed. The question is pertinent to the situation and should not provoke a defensive response.

 

[jbojo]

In my case, I NEEDED to drill a 3/32" hole in each blade to regain control of the idle mixture screws. I didn't think the carb specialist at the local performance shop knew what he was talking about until I saw the results. The reason for the holes was to allow setting the primary blades to close off the transfer slot to minimize fuel and maximize air flow, as mentioned in an earlier post. Basically my carb was too rich at idle which meant that I had to open the curb idle screw just to get it to run (more air) The cam I have produces 18" hg. Now I can get max vacuum at idle on the idle mixture screws, which I could not do before. This is not a new idea. I'm not sure why some seem to be questioning holes drilled in the primary blades.

Idle speed is controlled to a large extent by the amount of air allowed to enter the engine. If one is unable to close off that air flow, one could end up with a minimum-achieveable idle speed that is greater than desired. Surely you can agree that if someone was to have drilled the blades with TOO LARGE of a hole, it could result in a situation of excessive idle speed. The question is pertinent to the situation and should not provoke a defensive response.

I agree, airflow at idle will have a lot to do with achieving desired idle. My system first uses throttle blade position (IAC) to get close to the desired idle rpm and then various the timing to hit the target. It's not a carb but the principles are the same. Assuming that the throttle blades have not been drilled a vacuum leak as already mentioned should also be checked.

 

[Don C]

I'm guessing that if those holes were plugged you would be able to achieve both desired idle speed and idle mixture. 18 inches of idle vacuum shouldn't require that additional air source to get the throttle blades closed enough to cover the transfer slots.

 

[Stanglover]

In my case, I NEEDED to drill a 3/32" hole in each blade to regain control of the idle mixture screws. I didn't think the carb specialist at the local performance shop knew what he was talking about until I saw the results. The reason for the holes was to allow setting the primary blades to close off the transfer slot to minimize fuel and maximize air flow, as mentioned in an earlier post. Basically my carb was too rich at idle which meant that I had to open the curb idle screw just to get it to run (more air) The cam I have produces 18" hg. Now I can get max vacuum at idle on the idle mixture screws, which I could not do before. This is not a new idea. I'm not sure why some seem to be questioning holes drilled in the primary blades.

Idle speed is controlled to a large extent by the amount of air allowed to enter the engine. If one is unable to close off that air flow, one could end up with a minimum-achieveable idle speed that is greater than desired. Surely you can agree that if someone was to have drilled the blades with TOO LARGE of a hole, it could result in a situation of excessive idle speed. The question is pertinent to the situation and should not provoke a defensive response.

No defensive response at all. While we're getting off the timing issue that was first raised, it is all relevant to achieving the best running engine possible. Every engine is different and different approaches are sometimes needed that may be "out of the box".

I absolutely agree that If the holes were drilled too large, then the fine control would be lost and one would have the opposite effect.

This is what worked for my engine and may not work on others. Drilling holes in the primary blades was done by an expert tuner with years of experience setting up race cars. I would not have done this on my own.

I may post separately on this as in the past there has been much discussion on the forum with regards to MY engine and timing woos.

 

[Stanglover]

I'm guessing that if those holes were plugged you would be able to achieve both desired idle speed and idle mixture. 18 inches of idle vacuum shouldn't require that additional air source to get the throttle blades closed enough to cover the transfer slots.

Don, The hole were drilled because the carb was too rich at idle with the transfer slots set where they should be and there was no control with the idle mixture screws. I relied on the experience of the tuner. This engine is running near perfect now, so I have no doubt the "out of the box" mods were the correct approach for my '71 351C 4V.

 

[Don C]

My fault for confusing posts, I thought that I was still reading Omie01's post about not being able to lower his idle speed. Yes, sometimes modifications are necessary to get our cars running correctly.

 

[Stanglover]

My fault for confusing posts, I thought that I was still reading Omie01's post about not being able to lower his idle speed. Yes, sometimes modifications are necessary to get our cars running correctly.

Yes Don, this is Omie's post, but as usual we sometimes get off track a bit although all related.

To be clear as to why I added my problem and results was because I too had idle speed issues not caused by timing. While I did get my timing close to where it needed to be, it was not until my Holley Street Avenger 670 was stripped, adjusted and modified, was I able to get it spot on. I now have full control of the carb and distributor.

I intended that my experiences may help Omie solve his issues as they could be somewhat the same. We have not heard back from Omie with an update.

I had no intention of taking over his post, I hope that is clear.

 

[Don C]

No, I didn't think you were a thread pirate, it was obvious you were relating your experience to help.

I just need to pay attention to whoever is posting and not "assume".

 

[Stanglover]

No, I didn't think you were a thread pirate, it was obvious you were relating your experience to help.

I just need to pay attention to whoever is posting and not "assume".

Don, thanks for your support of my intentions. However, it is all too easy for any of us to get off track and leave the original poster out of the picture. I think we all could be more careful in that regard.

Geoff.

 

[Omie01]

Finally got back to it, set my initial at 14 deg, then plugged my vac adv into manifold vacuum, now the pinging is gone, but still feel it could be much better. Since I have now run out of time up here in Minnesota, I probably won't get back to it until spring. I thank everybody for the wisdom on this journey!!

 

[Stanglover]

Finally got back to it, set my initial at 14 deg, then plugged my vac adv into manifold vacuum, now the pinging is gone, but still feel it could be much better. Since I have now run out of time up here in Minnesota, I probably won't get back to it until spring. I thank everybody for the wisdom on this journey!!

Omie, glad to learn you have it running better. 14 deg. is where I'm at too on initial, but I use ported vac. not manifold. That could change also, still playing with it. Like you, I'm about done till spring, but a rare warm, sunny spell (55 F) has meant a bit more driving time. With the cooler air, the engine is running stronger than ever, no pinging at all. I say this as encouragement that you will achieve your desired timing and loose the frustration that I know all too well.

Have a good winter and stay in touch.

Geoff.

 

[barnett468]

Finally got back to it, set my initial at 14 deg, then plugged my vac adv into manifold vacuum, now the pinging is gone, but still feel it could be much better. Since I have now run out of time up here in Minnesota, I probably won't get back to it until spring. I thank everybody for the wisdom on this journey!!

switching to manifold vac will in crease the potential for pinging unless your initial timing was too low causing the heads to get hot . vacuum advance is disabled under moderate to hard acceleration.