Another question on timing-

[enaberif]

if you look at a fast idle cam you can see how it's curved and has a couple of steps. as the engine warms up the choke is warmed up and the fast idle cam rotates and falls down the steps and decreases the idle.

This will only work properly if ALL of the choke components are hooked up properly and set correctly which includes the air flap in the air cleaner which needs a vac source and the heat riser tube.

problem is most of our trucks are missing most of this stuff so we get used or think that our truck is normal when it's not.

[VileZambonie]

Obviously you can manually kick it off of fast idle but it is designed to work automatically. The only manual input should be at first startup by depressing the accelerator to set the choke. If it's not coming off of fast idle you need to either manually "deal" with it or fix it.

[philo_beddoe]

Can i stop by one of these saturdays, and we'll have a look see? I'm 10 min. up the road.

[Greybeard]

From my experience the steps of the fast idle cam are also to provide various choke settings due the whatever warmth stage the engine is in. No engine needs ckoke when the temperature is 105 degrees, yet as the temps cool there are varying needs for choke. Below freezing and full choke is definitely required required. But what about when the engine has already been driven for many miles and is then shut off for twenty minutes at 30degrees? The engine cools but not completely, full (high idle) choke would not be necessary. The mi-metal spring and stepped cam deal with it.
 I tend to agree with Blazer>

My original point from your question in post 20 is the electric choke is not totally automatic. It still requires driver input.

If you start up cold with the choke set to highest idle and don't touch anything, come back 30 minutes later and it will be screaming, it will not go to normal idle by itself.

Every carbed vehicle I have ever owned new or not was the same way. However, when the high idle is adjusted properly the engine should be 'screaming', but running at about 1200-1400 rpm.

Since the sixties the rule of thumb to driving a cold engine is that it fine to take off immediately if the temp is at freezing or above if driven gently with no stomping on the foot feed. Below freezing to about 15-20, warmup for a minute or so, and below that warm up until the a small blip of the throttle lets the ckoke come off. If the choke is working correctly it will NOT drop off of high idle until the spring is warm enough. The colder it is the longer this takes. Having the heat stove and moving hot air up to the snorkel aids in warming the engine faster but is not necessary. Mostly it's for preventing carb icing at around fifty degrees ambient temp.

YMMV

[Greybeard]

The original question....

I know my setting is 8 degrees btdc, the manual states that as well as the decal on top of the rad support. But that is for a stock factory application.

How do you determine your timing when you make changes or modifications? Such as a different carb (different barrels) or the intake or even a different engine? What do you use as a reference?

First and foremost to setting timing is to be absolutely certain there are NO vacuum leaks anywhere. The PVC valve is clean, the brake booster is in good working order, and no lines are pinched or age hardened.
 
Then you best bet is to buy a book on timing and read it. SuperChevy online has some good articles as do other HiPo sites on Chevy's. Timing can get complicated, the vacuum can, the advance weights and the initial all need to be balanced for the setup engine. No two engines are the same. What works for me at 900' elevation likely will not be optimum for someone else at 3000' elevation.  A very good vacuum gauge, a fairly accurate digital advance timing light, a host of springs and weights for the distributor, and an adjustable vacuum advance canister will make whatever you have scream to the best of it's ability. Don't forget that stock plug wires will get you from point a to point b, but good after market wires will get you there quicker.

Now which comes first the carb or the timing? Ask that of ten mechanics and you will get very different answers. The fuel ration has to be right to get efficient combustion BUT without good spark igniting it .....?

[blazer74]

You guys both have good valid points, i only tend to agree with blazer because i have never seen a GM factory v-8 (from the 70's) kick down entirely on its own.

However, my question STILL stands, what and how do i make an adjustment to make this fast idle drop down simply, quicker? Regardless whether the choke does it on its own or i have to tap the pedal.

Assuming the linkage is adjusted correctly, when you first start and you blip the throttle hi idle should drop to a lower hi idle.
reduce initial spring tension on the choke coil. Tighter on longer, looser on shorter.    Make a mark so you can go back to your starting point if    needed.

If you have rivets on the choke coil you will need to carefully drill them out and replace with correct screws.

[blazer74]

With the choke fully closed and on the high idle step of cam.

The green lever connects to the  choke Coil and the choke flap.

The silver half butterfly looking deal to the right of the green lever  is the top of stepped cam and is also top heavy and operates with gravity.
It operates independently from the green lever and strictly by gravity.

The green lever only  holds the silver lever from rotating and dropping down by resting underneath it.  The other end of the silver lever is the stepped cam which is hard to see under the bottom of the choke housing.

When the engine starts the pull off retracts via vacuum and opens the choke flap slightly and pushes the green lever fwd and down away from the silver lever which does not move because the throttle linkage and return spring are exerting pressure on the the stepped portion of the silver lever at the bottom where it's hard to see.

This is why you have to blip the throttle to release the throttle return spring pressure from the stepped portion  of the silver lever to allow the silver lever to drop down by gravity ( remember it's top heavy) to the second step again at the bottom where you can't see it.

Ok the silver lever only goes far enough to the second step which is your lower hi idle setting because the green lever which was moved by the pull off is still underneath the silver lever just lower.

Now as the choke element heats up and rotates the green lever down out of the way completely and also opens the choke flap.
With the green lever out of the way again we blip the throttle to release the throttle linkage and return spring pressure on the silver levers cam lower step so gravity can pull the top heavy side of the lever down and the Lower part of the cam moves out of the way of the throttle linkage ( the part under the choke housing that's hard to see) allowing the linkage to drop to normal curb idle which is controlled by the screw on the drivers side.

Remember the silver lever controls the hi idle speeds via the steps and the screw on the linkage and operates completely by gravity and being top heavy. It will not move if the throttle return spring pressure is being exerted on it even if the green lever is out of the way.


That's why we blip and that's why its not totally automatic.

Hope that helps understand how it works. 

[Greybeard]

WOOHOOO! Good! Good! Good! Exactly!

Was that too enthusiastic? Good explanation anyway.

I was starting to think I was losing my memory about chokes not being totally automatic. I didn't really want to drag out the extra Q-jets or Holley's I have sitting in the basement. 

[blazer74]

Thanks, glad it wasn't confusing.

Had a extra assembled quad handy.

[Greybeard]

Doesn't everyone that owns a quad have a few extra sitting around? Notoriously temperamental carbs. When they work they work awesome, touch them however and all bets are off from then on. Holley's are more idiot-proof, I like Holley's...DUH!

[philo_beddoe]

After reading post 52 about 35 times i was able to follow it. I'll try a slight less tension on the coil.  Thanks for the input.

Also, refresh my memory. Timing adv connects to manifold vac right? And how about the sensor on top of snorkel?

[Greybeard]

No,  timing advance goes to ported above the throttle plates, otherwise, your timing would always be advanced at idle (high vacuum).

I don't really know about the snorkel thing, never ran an engine with a stock air cleaner, I have always used a 6" tall K&N on everything with a splash guard around the outside.

[philo_beddoe]

Ok, got it. Oh and which way do i turn to lighten up on the choke coil spring? Wait, ported vs. manifold....now that's a BIG discussion!

[blazer74]

Clockwise,  mark before you start so u can go back.
Small adjustments make a big difference.

[blazer74]

Some have a little tab that won't let you move it. (Later  Models) All depends on what you have and where you got it.