I have a newly rebuilt Mc2100 carb on my 304, V8 and everything is fine except when I start it for the first time of the day.
It starts fine but then idles rough for 1 to 2 minutes before the fast idle kicks in, after that it is fine and runs beautifully all day, it is just that first cold start that I have this annoying rough idle, so where should my choke be?
Yes, pushing throttle to the floor once, and yes choke flap closes completely, she starts right up runs fine for a few seconds and then starts to idle rough for a couple minutes like she is not getting enough gas.
Brand new fuel pump, fuel filter and hoses, also replaced all my vac hoses with new and made sure all was nice and tight, re-torqued the carb to manifold bolts, no vac leaks that I can find.
She runs awesome after this initial couple minutes.
My distributor vac hose is hooked up to a cto switch on the thermostat housing where it has always been.
I did not have this issue with my old mc2100 carb, only after the rebuild.
When its first started and starts to run rough crack open the choke plate a little farther and see if it smooths out. It most likely will. Which means choke is too aggressive.
If it gets worse opening choke a little more then its not tight enough.
Most MC carbs have a choke pull off, either internal or external. these are adjustable. Most carbs are set in the 3/16ths range with the engine running and cold. You can check by using a drill bit (don't drop it!) to measure the opening.
The choke plate opening (or duration) is adjusted by rotating the black cap on the side of the carb. You will need to loosen the three screws (or drill out the rivets on later models) in ordeer to rotate it. The easiest way to set the choke is to run a garden hose on the choke housing, using the water to cool it. I like to set mine so the plate lightly snaps shut when I tap it with my finger in the closed position.
The choke pull off is a piston inside the housing(early models), a set screw on the top of the airhorn (intermediate), or an external vacuum motor on the side of the carb (late), usually behind the choke housing.
Early models weren't adjustable. The intermediates are adjusted by rotating the screw in or out to get the desired gap. Late models are adjusted by bending the linkage.
Start the engine and measure the gap between the plate and the side of the carb. Distances vary, but 3/16ths is a good place to start.
Once these adjustments are done, you can adjust your high idle. This is done using a screw that sets on a cam. High idle settings vary by model, altitude, and transmission type. I usually run mine in the 700RPM zone (manual trans), with it dropping off to a preset 500RPM's at idle with the cam disengaged. Colder weather may require a higher idle.
The steps on the cam are adjustable by bending the linkage rod.
Make sure that when you are done, the choke is completely open and the high idle is disengages with the engine warm.
All of these adjujstments can be modified by you at any time. If you go out in the dead of an Alaskan winter and find the idle is too low (screw), choke comes off too quickly (low idle before the engine is warm enough) or won't completely open (black cover) when warm, or it runs lean/rich on initial startup (choke pull off), you can make the adjustment on the spot.