Runs very rough until truck warms up

[85Nis720]

My Nissan had been sitting for to long and I decided I wanted to get it running again, so I bought a new rebuilt carb and installed it on the truck. However when you first start the truck it runs VERY rough and won't get much above 800 rpm. If you hold the gas pedal down a little bit and get the truck up to about 2000 rpms and hold it there for 10-15 seconds. You can let off the gas, truck stays about 1800 rpm and gradually comes down and the choke shuts off. From that point on the truck runs great until you let it sit and cool off and it will do the same thing all over again. It is a 6 wire feedback carburetor.

 

Things I have done:

Fast idle is about 1800-2000 rpms

Warm idle is 800rpm

Timing is set to 3Deg BTDC

Complete tune up

[nl320what]

Sounds very much like there is low compression and warms up and expands parts which seal better.

[85Nis720]

I'll check compression again, last time I did all 4 cylinders were around 150. Can't rember if I did that on a warm or cold engine though. Acts like it is being flooded out with too much fuel to me.

[datzenmike]

Everything and every adjustment on a replaced carburetor is suspect.

 

If you can adjust the choke a little leaner try that. It's electrically heated so this will have to be adjusted and tried and then wait until tomorrow to try again when cold.  It's an '85 so I expect the adjustment is sealed and can't be moved.

 

The normal procedure for starting a carb with an automatic choke is to pump the gas pedal at least once. (two or more can be done when extremely cold) One pump will release the interlock on a cold choke and set it to on. At the same time the fast idle cam is pulled up into place and the throttle closes against it and not the warmed up idle position. As the choke warms up and begins to open, the fast idle cam slowly releases and the throttle moved down the steps reducing the idle until fully warmed up.

[85Nis720]

Datsenmike, if you step on the pedal once before starting the butterfly closes and the fast idle cam rotates into place like it's suppose to. The choke looks all sealed up so I'm not sure how to adjust it to make it leaner. Only runs rough for first 15-30 seconds after startup then it maintains it's fast idle and rpms slowly drop down to around 800 when the truck is warm.

 

For adjustments on the carb all I see are the fast idle screw, idle screw and what looks like a air/fuel mixture screw on the passenger side base of the carburetor. Am I missing another adjustment somewhere?

[datzenmike]

I think that sounds right. The idle mix and the idle speed are only used when the engine is fully warmed up. The choke manages the cold warm up period by forcing a rich mixture. The fast idle simply accelerates the warm up time.

[85Nis720]

Yes like I said truck runs great after about 30 seconds. I guess I'm thinking it's something to do with the "feedback" part of the carb. Since it has a throttle position sensor, o2 sensor. What is the truck able to adjust or change by looking at these values? Also there is one more small screw on the linkage of the carb (to the rear of idle screw and almost directly above fast idle screw) that I'm unsure of what exactly it does, does anyone know what this is?