[thefirebirdm@n]

Recently I was able to pickup a 17057253 carb for my wife's '68 Convertible with 350. I purchased it from SMI and they setup with their Stage 2 prep.

The first thing I'd like to do is just do a simple tune on it to try and tune it a bit for this car. I'd like to ensure the choke/fast idle is set properly, the curb idle is set properly and the A/F mixture screws are set properly. Some of these procedures may seem pretty simple but I've been doing a little research, have some questions and would like to ask some questions from the forum. I posted over on Cliff's forum but its not quite as active as this forum and sometimes it can take several days to get a response. And I am interested to hear from others here on this forum.

First: What is the proper method for setting idle? Should fast idle be set first or should curb idle be set first?

This is a 77 year model carb on a 1968 350. Looking at the 68 Service Manual from GM it specifies the following for fast idle:

Initial baseline setup: "With primary throttle valves completely closed and cam follower on the high step of the fast idle cam, turn fast idle screw in 2 turns after screw makes contact with the lever."

Actual adjustment during operation: "Adjust on the car with cam follower on high step and choke valve open to obtain RPM specified (2500 rpm)."

In the tune-up section of the Service Manual it also states the following for setting fast idle on a Firebird V8:
"To set fast idle speed, run engine in neutral, choke valve full open and fast idle lever on top step of fast idle cam and adjust fast idle speed screw for proper speed setting"

A couple of thoughts on this:

1 - Both procedures above state the choke valve has to be fully opened. To me that means the engine has to be fully warmed up and the choke has disengaged and the fast idle lever should be at the lowest step on the fast idle cam.

So is the correct procedure to wait till the engine has completely warmed up and the choke valve is fully open and then move the fast idle lever back to the top step on the fast idle cam (by hand) and then set to the appropriate RPM? In the past I have started the engine cold and set the fast idle by adjusting the fast idle screw while the choke was fully engaged but from what I am reading that's not the right way to do it.

2 - If the engine has to be fully warmed up and the choke completely disengaged before you set the Fast Idle, does it make sense to set the curb idle first, to the proper rpm, then set the fast idle?

3 - The fast idle setting specified in the service manual is 2500 rpm. The last time I tried to set a carb that high it sounded like it was racing. Is that really the setting everyone uses? I thought about 1500-1600 rpm was more than enough.

4 - The curb idle RPM specified is 650. Last time I set a carb at that RPM it seemed too low. I have also read you should set it at the lowest possible setting where you get a smooth idle so I usually set mine more like 750-800 RPM.

I've read Cliff's book cover to cover and I just looked through it again last night and I didn't see anything covering how to properly setup fast/curb idle. If I missed it let me know. Most savvy carb guys prob already have this down and I have tinkered over the years but I want to make sure I am doing it right so I can get the basic settings right before I start tuning the jets, rods etc... Thanks to those that reply.

[GregL]

Jimmy,
The curb idle speed of 650 rpm is probably listed for an automatic car in drive at time of adjustment. Agree, this is a little too low for most cars as a curb idle speed in neutral. Between 700 & 800 is more realistic. And you're correct 2,500 rpm on fast idle is just too high. Try it around 1,800 rpm and your car should start and run fine. Yes, make all adjustment with the car warmed up to normal operating temp.
Greg

[Goatracer1]

The fast idle is set HOT on the high step of the cam BUT when you start your car Cold the cam will drop to the second step after the engine starts. Remember the FAST IDLE RPM will be LOWER when the engine is COLD than it will be on the same cam step when the engine is HOT. You can set your HOT idle RPM either before or after because it has no effect on FAST COLD idle. These are only guide lines. Even when I worked for dealerships we set the adjustments on the older cars where they performed best. You can't do that legallly on emission controled cars.

[thefirebirdm@n]

Thanks for the replies guys. I appreciate it and if anyone else wants to chime in on the first post, please let me know. I do have a couple more thoughts/questions:

After a little more reading last night I found a few more references to setting Fast Idle when the engine is hot and you guys verified the same. Trying to understand the way the fast idle lever operates on the cam I think there is usually what 3 or 4 steps total? So if I'm understanding this right, the lever will be on the high position on the cam when the engine is cold but as soon as you start the engine it will drop to the 2nd position on the cam per what goatracer said. So if I set the fast idle rpm on the high step to say 1800 rpm the fast idle is actually going to drop down some while on the 2nd step and while the engine is warming up.

I'm also going to assume that as the lever moves to lower steps the rpms will drop with each step change. Should the lever move down a step on its own (as the engine warms) or does that happen when the owner blips the throttle? In my experience you have to blip the throttle a bit to get the engine to idle down? Also when the engine is fully warmed up I would assume the lever would be at the lowest position on the cam? Or does it come off the cam completely?

Another question I have is in relation to where the electric choke should be set. Right now I have it set dead on the middle tick mark. I know counterclockwise will make the fast idle mixture more rich and clockwise will make it more lean, basically controlling the angle of the choke valve and how much air is getting into the carb. But I haven't found ANY reference giving a baseline to use to start and what procedure should be used to fine tune that setting. From experience I've seen a lot of them moved over two ticks clockwise. Any recommendations or references on how to dial in the actual choke coil?

Also trying to better understand exactly what the 12v lead going to the coil actually does. I believe the coil starts off contracted which holds the choke valve closed and as the coil heats up it expands which releases the choke valve allowing it to open. Is the 12v just acting as a switch to "turn on" the coil meaning if the 12v is not hooked up the coil will not change regardless of the operating temp of the engine?

So far this is the method I think is correct given what I've found out so far:

1) Set idle mixture screws 6 turns out from lightly seated
2) warm up engine and verify choke is fully open
3) Set idle mixture screws for best lean idle (assume here turning them back in as far as you can without negatively affecting idle, also read that each mixture screw must be turned an equal amount)
4) Put car in Drive (automatic) and set curb idle
5) Move fast idle lever to high spot on the cam and set fast idle rpm

So later today I'm going to run through that process but I'm not going to do the mixture screws first, just the curb idle and fast idle. I want to ensure the car starts nicely every time and the choke operates properly. Then I'll repeat to fine tune the mixture screws.

[thefirebirdm@n]

No one else tinkered with these settings on the carb? Any feedback on the questions on the choke or fast idle lever on the cam? Still trying to determine how to set the choke coil properly.