Tuning / Tweaking a 17057253

[thefirebirdman]

Some of you may have seen my posts on the 1904 Q-Jet I was working on, on my wife's '68 Firebird 350 convertible. I've removed the 1904 and will be calling Cliff to order a full rebuild kit and doing my first rebuild on that carburetor. In the meantime I was able to pickup a 17057253 carb. 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.

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.

[Cliff Ruggles]

2500rpm's on the high step of the fast idle cam is WAY too high, IMHO.

I'd shoot for 1500-1800rpms for first start up on the highest step of the cam, then fine tune the pull-off for the most ideal open angle for the choke flap.

No manual needed, it's just trial and error, and set the rpms for your liking, not what the service manual states....Cliff

[thefirebirdman]

Cliff, thanks for the response. I'll be calling you for a kit on the 1904. That'll be a fun project. Soon as funds are released.

After doing a lot more research on the ole interweb I figured out how to set the choke coil in the housing for initial startup. Will use your recommendation for fast idle. I would like to confirm, should that be set at startup when the engine is cold or should that be set after the engine has been warmed up and after I have moved the fast idle lever back to the top step of the fast idle cam?

Also, just wanted to verify with you. To fine tune the choke-pulloff, I have to increase/decrease the obstruction in the diaphragm tube per the instructions in your book correct? Or is that controlled by the adjustment screw attached to the rear of the choke pull-off?

I'm also going to reset the idle mixture screws. Two questions:

Where do you like to set them for startup? I've heard anywhere from 2-3 turns out from seated to 6 turns out.

With the vacuum gauge attached do you first set one side for max vacuum and then the other or do you make equal incremental changes to the idle screws i.e. half turn each side, until you get max vacuum. Pretty sure its the latter but would like to verify.

[Cliff Ruggles]

The screw/spring on the primary pull-off adjusts the open angle of the choke flap on cold starts.

Set the idle mixture screws about 3.5 to 4.5 turns on the 253 carburetor to get it running, then fine tune for maximum vacuum at the leanest settings once fully warmed up.  This is done by turning them out for best vacuum, then in until the engine slows slightly, then back about 1/2-3/4 turns each.  Balance them when finished if an open gasket or cut down divider is used in the intake (or single plane intake).

What we are looking for with idle fuel is the best idle quality at the leanest settings......Cliff

[thefirebirdman]

Thanks Cliff, what I have done to date is set the fast idle with the car cold. I set it to about 1600 RPMs. Have to check that on next cold start. I also discovered I had the choke set completely wrong. I reset it, with everything completely cold so that the coil is set so that the choke flap is just slightly cracked open. I'll fine tune it with the spring adjustment.

I also set the curb idle to about 750 RPM. One thing I noticed is that my RPM gauge and the Hood Tach don't match up. When I set it to 750 RPM per the gauge the Tach is reading about 900 RPM. I went by the gauge.

Next steps are to fine tune the choke flap and verify fast idle and then do the adjustment with the mixture screws. Next after that would be to set the timing. Right now I have dead on 9deg. Most of the reading I have done in several books suggest most Pontiacs benefit from 10-12deg initial timing so I may bump it up to 10 or 11 deg. Next step after that is to check mechanical advance.