Hi Guys,

I have been having problems with my 72 corvette. I have only had it for a couple of months. It has a 350 with a cam that has a noticable lope when it's warmed up. The previous owner didn't have any other information about the motor/cam combination. Please understand, I'm just starting to do some of my own work. I want to learn and to eventually be able to do most of my own mechanical work.

It has been terribly cold natured when starting, even when it's 90+ degrees outside. Once it gets warmed up (5 minutes or so) it runs like a champ. I figured that it's the choke on the carb. I try adjusting the various screws, to no avail. I have had a couple of mechanics try adjusting it, no improvement. I downloaded Lars' information and tried to see if I could figure it out.... no luck.

I have been reading lots of threads on the subject and it seems that lots of guys have been having good luck out of the box with their carb purchases. I figure, what the heck. I have tried everything else I can think of, so I order a new/rebuilt Q-jet with a electric choke from Sean Murphy Induction in California. It comes in, I bolt it on, everything is looking good, so I try firing it up. It fires up quickly, runs up to around 2000-2500 rpm's and dies. Over and over I try it, same results. I try tweaking the various idle adjustments, etc.....same result.

Does anyone have any ideas? Suggestions?

Thanks in advance for your suggestions.

 

When cold, an engine requires more fuel. The choke provides the needed fuel, by closing the plate at the top of the carb. The electric choke is adjustible, so that you can add or remove extra fuel for warm-up. You need to also lower the cold idle rpm's to about 1,500 to 1,600 rpm. Stalling just after starting usually means that the carb. is too lean. (Adjust the choke more rich.) Make sure you set the choke prior to start-up, by fully depressing the gas pedal 1 or 2 times. A shop manual will give better details than I can give you here...G/L

 

SMI makes good carbs.

Adjust the choke housing for the middle of the marks to start. You can go richer if the choke opens too quick - more spring tension for rich.

Adjust the vacuum pulloff when first started for about 1/4" open with the screw that has a spring on by the pulloff.

Adjust the fast idle screw (under the choke housing by the base) for about 1200 rpm's when cold.

Adjust the idle speed for about 750 when warm.

Adjust the idle mixture screws back to 1 1/2 turns out from bottomed out to start with. Then when the engine is fully warm, adjust for the highest vacuum reading with a guage using manifold vacuum. Follow Lars writup on this if you are unfamiliar with this method.

Leave the APT screw (brass plug with recessed allen) in the top front of the carb alone at this time. If you moved it - go back to what it was upon receipt.

Check for vacuum leaks. Sounds like you may have one as it idles high when cold and then like crap
as it warms up and the choke opens fully. Check all hoses on carb and follow lines to where they go. Put a gauge on the manifold and you should be pulling 15-18" at idle with a fairly stock engine.

Leave the jets and rods alone for now. Sean M. gets them usually right on the money for the specs you gave him when ordered.

If you still think the carb is messed up, call Sean M. and discuss. He may have you send it back and he will go over it and set it up. Good luck.

 

Thanks for the advice.

I found that the new carb had a 1/2 inch "port" in the back that was causing a vaccum leak and needed to be plugged. Don't laugh too hard. This is the first time I have tried to setup a carb. :o The old carb didn't have this in the back. Once I plugged to "leak" I was able to get the car to idle. I'm still having problems getting the idle smoothed out. When I try to lower the idle from around 1,500 rpms to 900-1000 rpms the engine dies. I figure that I need to get the lean/rich mixture set. I will use the above suggestions to try and get everything back to where SMI set them.

 

Is your timing set?

 

That port is the vacuum source for power brakes. You need to transfer the fitting from the old carb or get a new one at the auto store for the power brakes. No power brakes - plug it. If you still can not lower the idle, then there is something else going on.
- timing
- set carb idle mix with engine off at 1 1/2 - 2 turns from bottom out.

Did you install the gasket that came with the carb? Important it is correct and installed ok. Don't overtighten.

 

how about vac leaks???