fast idle cam issue?

[John P.]

Hello folks,

I am trying to make sure that my Holley 4 barrel ( 2818-1) on C2 327/350-1965 is functioning as intended. The carb has been professionally rebuilt, I adjusted dwell and timing, mixture per shop manual.

My question involves fast idle with "cold" start. Should any cold start at any ambient temp result in the fast idle being activated?

It seems that the initial fast idle, which I have set as per c2 shop manual, only activates intermittently. I know that an initial
accelerator pump should activate it, but not sure why it doesn't happen consistently. I have changed choke coil, lubed flapper
made sure choke rod functioning etc,

Any ideas?

[Edward J.]

Re: fast idle cam issue?

John, Yes, the choke generally will be activated on cold starts that a car has been sitting all night. unless your dealing with higher that normal temps in the morning. over 100+ degrees, even then there should be some activation only the choke will have a short warm up time. I would go into garage and check the choke on each start by pushing the accelerator once each morning and looking a the position of the choke butterfly and fast idle cam. make sure the butterfly is fully closed and fast idle cam is on the highest position.

Another thing I have seen is the aftermarket or reproduction choke coils are not always calibrated like GM. So if you still have a original or a NOS one I would chose either one of them.

[Steve G.]

Re: fast idle cam issue?

I don't know how to reference back to a specific post, so I copied and pasted my reply to a similar question in a previous thread.

"When the choke spring is cold it's spring tension tries to close the choke plate. However, if you lift the air cleaner off your cold engine and you haven't touched the throttle since it was last hot, the choke will be standing wide open. It won't close because the fast idle adjusting screw is being held against the fast idle cam by the throttle return spring. When you open the throttle you lift the stop screw off the cam, freeing the cam to move and the choke should snap shut. The choke is now "set". At room temp it should just close fully. If it isn't closing all the way, increase the choke spring tension until it does.

Once the choke is set and the throttle released the fast idle stop screw is now sitting on the high step on the fast idle cam. As soon as the engine starts there is a vacuum diaphram (or piston in your case) that pulls the choke open a predetermined amount. The engine has to get some air. This is called the choke break. With opening the choke this amount the fast idle cam is now allowed to drop off the highest step. But it won't drop until you open the throttle and lift the stop screw off the cam, just as the choke wouldn't close when cold until you opened the throttle.

This next part is often what is mistaken for the fast idle not coming on. If you start the car now without touching the throttle , when it starts it will run at the high step speed. Kick the throttle once and it will drop down to the lower step, the choke break having pulled the choke off a small amount. However, if you have your foot on the throttle when it starts as soon as the engine starts the fast idle cam will have dropped from the choke break action and when you let off the pedal you won't see the high step fast idle speed. The choke break has already allowed it to drop off.

After this has happened the heat warms up the coil spring and it loses it's tension. As it does the choke starts to come further off and the fast idle cam drops to a lower step then finally off altogether.

So, the first thing to check is to make sure the choke is closing fully every time you go to start it cold. Then do the start without touching the throttle after setting the choke. See if it consistently starts on the high step. Hint. The choke can not fully close without the fast idle cam being on the high step.

There can still be other problems. The choke break adjustment is a very critical measurement. If it is pulling off too far the engine will want to stall when you try to accelerate when cold and the fast idle cam will drop too far.

Even when the choke break is adjusted properly, there is another adjustment that determines how far the fast idle cam drops in relation to the choke break movement.
Steve"

[Timothy B.]

Re: fast idle cam issue?

John,

The general rule of thumb is at room temperature (70*) the choke valve should fully close. This should place the fast idle screw in the high step of the cam. Do like Ed mentioned and remove the air cleaner to see if the choke fully closes and sets on the high step. If not, the first place to start is the choke coil spring tension as it may need to be set a little tighter.

[Michael H.]

Re: fast idle cam issue?

Hello folks,

I am trying to make sure that my Holley 4 barrel ( 2818-1) on C2 327/350-1965 is functioning as intended. The carb has been professionally rebuilt, I adjusted dwell and timing, mixture per shop manual.

My question involves fast idle with "cold" start. Should any cold start at any ambient temp result in the fast idle being activated?

It seems that the initial fast idle, which I have set as per c2 shop manual, only activates intermittently. I know that an initial
accelerator pump should activate it, but not sure why it doesn't happen consistently. I have changed choke coil, lubed flapper
made sure choke rod functioning etc,

Any ideas?

You should not " lubed flapper" this should be free of any oil or lubracation.

Mike

[Stuart F.]

Re: fast idle cam issue?

Not a Holley fan, but I do know some models have a fast idle that consists of a hard plastic type cam in which a linkage tab sits inside. Adjustment of these is done by bending tab. Believe these are models having a divorced choke coil, and they also have a vacuum pull off. No doubt this being a 65 this is a moot point.

Stu Fox

[John P.]

Re: fast idle cam issue?

I appreciate the input. It appears that the choke doesnt close in AM with cold start after the single accelerator pump. I can lift the cam manually
and it will hold, but it doesnt seem to do it as designed. The choke coil is a new Holley part. The fast idle didnt work correctly with the original
choke coil either. Can the cam and cam shaft be rebuilt? I know a new cam is available but is it rather now new carb time?

[Steve G.]

Re: fast idle cam issue?

I appreciate the input. It appears that the choke doesnt close in AM with cold start after the single accelerator pump. I can lift the cam manually
and it will hold, but it doesnt seem to do it as designed. The choke coil is a new Holley part. The fast idle didnt work correctly with the original
choke coil either. Can the cam and cam shaft be rebuilt? I know a new cam is available but is it rather now new carb time?

There's really nothing to repair or rebuild in the fast idle cam. It is linked directly to the choke flap. If the choke flap is closing completely and the cam is not coming up to where it is supposed to, it is not adjusted correctly.

The first thing to do is ensure the choke is closing completely. With the engine cold, room temp, hold the throttle open part way with your right hand while you close the choke plate tightly with your left hand. Release the throttle while holding the choke closed tightly. Now release the choke plate.



1. Is the choke plate remaining closed when you took your hand off it?
If not, loosen the 3 choke coil screws and rotate the coil clockwise until it just closes the choke completely.

2. When the choke plate is completely closed is the fast idle screw sitting on top of the highest step of the cam?
If not, the cam link needs to be adjusted. You will need to refer to the service manual for the procedure. It will involve setting the fast idle screw on a different step and up against the rise to the next step. Then a measurement (specified) is taken between the choke plate and air horn wall.

If, with the choke closed tightly the fast idle screw is on the highest step, the cam link adjustment is probaby ok. It may still be off a small amount, but the problem won't show itself as what you describe. It would have to do with the speed it runs after the choke break has pulled the choke off part way.

Steve

[Stuart F.]

Re: fast idle cam issue?

These mechanical linkages seldom become out of adjustment unless someone has messed with them that don't have a clue what the're doing. 99.9 % of the problems come down to what has been changed, I.e. The choke coil was changed.

Over 60 years of rebuilding/cleaning carburetors has taught me that the adjustment dimensions shown in the "cover all models" instructions which are fun to read, but seldom lead to the discovery of something that needs changing or even "tweaking". The only ones I pay much attention to are the float level and drop, and sometimes the accelerator pump which comes down to being where new parts were installed. Same in this case with the choke coil.

Stu Fox

[Steve G.]

Re: fast idle cam issue?

Not been my experience Stu. I don't have 60 years, but I have a solid 40 as a journeyman. The holes the linkage rods fit into and the ends of the rods themselves wear and get sloppy. The slop doesn't warrant changing out the linkage parts, but it does throw the adjustment off. Choke rod adjustment is made with the slack in the linkage held to the plate open position. When the total of that slack changes, so does the adjustment. We are talking about a measurement that is held to 3 decimal places, so it's easy to see how just the wear can make a significant difference.

Then there's the cases where the previous overhaul wasn't done right and made these adjustments incorrectly, ie taking up the slack in the wrong direction, measuring on the wrong side of the airhorn.

Every carb number has a specific set of specs, there is no one size fits all. This is the biggest difference between between same model carbs used on the same car, one an auto the other manual.

Probably never resulted in a problem for you warm climate guys, but on a -30* Canadian prairie winter day, nobody wanted to restart his car 5 times before it would stay running then wait 15 minutes before he could move it without hesitating and stalling out. Cold engine driveability complaints were a big part of our world and those minute little adjustments were what made the difference between a car that you could hit the pedal twice, start it, put it in gear and drive away vs one that you nursed for it's first 20 minutes.

So it became habit. I check everyone of those adjustments on every carb I do. When you have them right they work correctly in all temps.

Steve

[John P.]

Re: fast idle cam issue?

good discussion and input. I did rotate choke coil to yield what I believe is the spec in the service manual -the flapper with the width of a 3/16"
drill bit from the full close position. Is this not correct?

It is certainly easy to put the choke plate I changed out back. This is a replacement carb but certainly has been on the vehicle for many years I believe. But cant be certain.

The specs also call for 1+ lean. I have it place there now, perhaps best to ignore for the moment.

So let me ask - when cold do I want the flapper fully closed with throttle released, or do I want 3/16" from the air horn edge?

[John P.]

Re: fast idle cam issue?

I reread the replies. Would be nice if the cam were easier to see! But I will try Steve's instructions.

[Steve G.]

Re: fast idle cam issue?

good discussion and input. I did rotate choke coil to yield what I believe is the spec in the service manual -the flapper with the width of a 3/16"
drill bit from the full close position. Is this not correct?

It is certainly easy to put the choke plate I changed out back. This is a replacement carb but certainly has been on the vehicle for many years I believe. But cant be certain.

The specs also call for 1+ lean. I have it place there now, perhaps best to ignore for the moment.

So let me ask - when cold do I want the flapper fully closed with throttle released, or do I want 3/16" from the air horn edge?

Just to be sure it's clear, fully closed. Ignore the index mark. It's a starting point.