carb help please

mcditalia · 10-15-04, 08:06 PM

When starting my car cold, the choke doesn't seem to work anymore as the car dies right after it starts and doesn't idle at 1,200-1,500 rpm's any more as it used to until warmed up. I have to crank it several times and keep my foot in the gas to maintain the fast idle. Once warmed up, the car fires right up again if tuned off. Never used to do this, but started shortly after I changed the filter, pump and plumbing to the carb. Is the carb the culprit and where do I start? (Holley 4bbl replacement for 350hp application).

67HEAVEN · 10-15-04, 09:15 PM

What kind of choke? Mechanical? Electric? Can you post a photo?

Tom Bryant · 10-16-04, 11:03 AM

If you are using the original type divorced choke with the thermostat coil mounted on the intake manifold you may need a new thermostat coil. Any Chevy that I ever used for a daily driver from this period needed the thermostat coil replaced every other year to keep the choke working properly in the cold weather. Of course it should last much longer in a seasonal car. If you can work the choke freely without sticking with the carb held open I would suspect the choke thermostat coil to be the culprit. The bi-metal spring just looses it's ability to return to the closed position after several thousand heat cycles.

It's part # 1709K in the Paragon catalog www.corvette-paragon.com . $14. in my 3 year old catalog.

Tom

allcoupedup · 10-16-04, 02:42 PM

A mechanical choke uses a bi-metallic spring and engine vacuum to open and close the choke. There is a small piston that keeps the choke open under vacuum - at startup the spring keeps the choke closed against the vacuum. As that coil spring heats up, it gradually gives in to the force from the engine vacuum and opens up.

A common problem on this type of choke is a dirty piston. Open it up and clean it out with some carburetor cleaner and work it back and forth. It should free up. The fast idle is controlled by the position of the choke. If your little choke poston is not moving any more you will not have fast idle.

Good luck.
Brian

mcditalia · 10-16-04, 05:03 PM

Uhm, I think its an electric choke. I must apologize but never had a carburated car before to maintain, always learning something new. Downloaded a pic of the carb, am I right? Will try looking at the above suggestions.

67HEAVEN · 10-16-04, 05:20 PM

Quote:

Originally Posted by mcditalia

Uhm, I think its an electric choke. I must apologize but never had a carburated car before to maintain, always learning something new. Downloaded a pic of the carb, am I right? Will try looking at the above suggestions.

The black circular item, to which two black wires are connected, is the choke. Yes....electric.

Check that you're getting 12 volts at the choke connection with the key on.

JohnZ · 10-16-04, 07:34 PM

If the hot feed wire to the choke housing is connected to the coil (+) terminal, it isn't getting the full 12 volts it needs; it should be connected to the ignition switch side of the ballast resistor. The electric choke is adjusted by loosening the three little screws on the choke housing retainer and turning the housing in small increments - there's an index mark on the top, and it will say "Rich" in one direction and "Lean" in the other direction. Turning it toward "Rich" will give you more choke action at cold start and it will take longer to come off the choke, turning it the other direction will do the opposite. When the ignition is "off", the carb goes to full choke; with the ignition "on", current heats the coil spring in the housing, expanding it, which pulls the choke off (which is why it needs a full 12 volts to adjust it properly and ensure it pulls off fully).

When the choke is on (blade closed), a rod link from the blade shaft operates the plastic fast idle cam below it which is "stepped" to provide a fast idle until the choke opens fully; when the choke opens fully, the fast idle cam drops out of the way and idle rpm is controlled by the idle speed screw on the driver's side of the carb, at the throttle arm.

You won't find any of this in the Shop Manual, as original carburetors didn't use electric chokes; they used a remote thermostatic coil in a well in the intake manifold with a rod up to the choke lever starting in '66, and used a hot-air choke prior to that.

mcditalia · 10-17-04, 11:14 AM

Thanks for everyone's input. Checked the choke connections and the lead + wire is spliced in a pink wire that connects to the + terminal of the coil. If this is wrong I'm puzzled as to why it worked fine for 2 years? I borrowed a voltometer, and set the scale to 15v DC current. Tested the battery and got more or less a 12v reading. Tested the choke leads with the ignition to the "on" position and got approximately 2/3 less volts than what I got with the battery, I guess about 4 volts by subtracting the bars. Real technical right?

With choke on at cold start up, the carb plate should be closed right? If I try to start the car will I be able to see immediately if the plate closes assuming the choke works? Plate always stays completely opened once the car is shut off hot and stays that way. I'm a real amateur I know.

myonnone · 10-17-04, 12:00 PM

Sounds like your on the right track. You should get the same voltage (not current) reading on the choke as you did on the readings from your battery. The choke should be closee when you starting the engine closed and open as the engine heats up.

If you need to make any adjustments follow JohnZ's directions above.

mcditalia · 10-18-04, 03:43 PM

I went down to my local speed shop and got a replacement electic choke for my carb. The guy said it's pretty straight forward to replace, just unscrew the 3 screws and it should come out.

Anything I should keep in mind during installation? Do I have to adjust it after installation or is it set up already?

Sky65 · 10-18-04, 04:05 PM

Disconnect the wires and remove the 3 screws, and the retaining ring if equiped. Remove the choke cover with spring. Note the small "lever" inside the choke housing. Look inside the new choke cover and note the spring and the "loop" on the end of the spring. Install the new choke slipping the "loop" over the "lever" and hold the cover flat against the carb. Install the 3 screws and retaining ring loosely. Rotate the choke cover and clockwise and watch the choke plate in the carb throat. As you rotate the choke the plate will close. I would set the plate so it just closes and secure the screws. Install the wires. As John said you should have full battery voltage, not dropped voltage after the ballast resistor. Then see how it works. Too much ckoke, back off on the tension. Not enough choke, dial a little more in. Many chokes have indexing marks on them. If so, try starting in the middle.

mcditalia · 10-21-04, 11:48 AM

I'm still waiting for the choke to arrive. In John's post the + lead of the choke should be connected to the ignition switch side of the ballast resistor. Does that mean the + choke wire should be spliced to the wire coming out of the driver's side firewall before it connects to the ballast resistor?

Right now the + choke wire is connected after the ballast resistor going into the + wire of the coil as shown in pic. If I uderstood correctly it should be spliced in the bottom pink wire of the ballast pic?

Ron Miller · 10-21-04, 11:58 AM

Quote:

Originally Posted by mcditalia

I'm still waiting for the choke to arrive. In John's post the + lead of the choke should be connected to the ignition switch side of the ballast resistor. Does that mean the + choke wire should be spliced to the wire coming out of the driver's side firewall before it connects to the ballast resistor?

Right now the + choke wire is connected after the ballast resistor going into the + wire of the coil as shown in pic. If I uderstood correctly it should be spliced in the bottom pink wire of the ballast pic?

Correct, connect it to the ballast resistor side not going into the coil. You can connect it directly to the screw terminal of the ballast resistor if you like, that's a good place to connect. Just make sure it's not on the side with the wire going to the coil.

hatdragracer · 10-21-04, 12:39 PM

Not related to choke, but something looks funny with your vacuum lines routing. The 'T' right in front of the black choke housing looks like it is connecting a port vacuum source (just behind the bronze float level inspection plug) and an intake vacuum source (below the float bowl)

Was this done intentionally? Does it 'T' to the vacuum advance?

I thought Vacuum advance should use only port vacuum, if you 'T' port and vacuum together they will just cancel each other out right?

I don't think this would have a big impact on performance as I don't run any vacuum advance, but I was just curious why you chose to route this way.

JohnZ · 10-21-04, 07:33 PM

Good catch - the "T" is not necessary; the angled port below the front float bowl is always manifold vacuum on a Holley, and the one on the side of the metering block can be either ported or manifold vacuum, depending on the carburetor model. You can remove the "T" and connect the hose to the port under the float bowl, or to the one on the side of the metering block (if you verify first with a vacuum gauge that it has full manifold vacuum at idle). Having the "T" and two hoses won't bother anything, it's just an extra hose and "T" that don't need to be there.