Running rough on start up

[clinem03]

I have been having issues with my rig running rough during start up. It was idling low on start up and even after it was warm, and the engine was rocking a bit under the hood. I figured it was the carb, as the PO put on a cheap one from Autozone. I tried to tune it a bit but the only way to get it running consistently was to increase the fast idle screw, but then it ran too fast. I have been on the hunt for an original Motorcraft 2150 2bbl carb and pick up a rebuilt one from Nick over at Northeast Classics. I put on the new carb last night, got the engine warmed up and tuned it a bit to the point where it was idling well but I did notice a bit of a "pop" coming from the exhaust on a pretty consistent cadence. Then this morning I tried another cold start to see how she does and she ran rough again with a low idle on start up.

I am new to working on carbs and tuning them but have done lots of research, but wonder if I am not tuning it right. where should I start? There are so many screws to turn, and is there an easier way to tune it right without just messing with each screw and trying to get it to idle correctly? Also, with the 'pop' coming from the Exhaust, could I have a bad spark plug that could be causing the issue...or could my timing be off?

 

[spap]

Curious when was the last time you did a tune up, plugs cap rotor maybe wires , fuel filter. I always like starting from fresh.
ans what year is your bronco, do you have points ?

 

[clinem03]

76, manual original 302. I rebuilt the front end (timing chain out) about a year ago and added a new distro. I did not replace the plugs or the fuel filter, but have replaced the fuel pump a few times so far as I had some issues with it a few months back.

 

[m_m70]

Then this morning I tried another cold start to see how she does and she ran rough again with a low idle on start up.

Is your choke hooked up and operating?? Assuming it's an electric choke. This is a good thread for choke adjustment and explanation.
https://classicbroncos.com/forums/threads/adjusting-choke.109348/

Have you checked for vacuum leaks?

What's your timing set at?

 

[DirtDonk]

Yep, check the choke. If it's closed too far it will choke off the engine and it will run rough until it starts to open up.
Newer carbs had a vacuum operated choke pull-off that was adjustable so that as soon as the engine started spinning and developing vacuum, the fully closed choke would open up to a set amount that allowed it to work, but the engine still to breath.

As m_m said too, if it's not closing at all it might run poorly for awhile. But "runs rough" kind of sounds like it's too far closed too.

Paul

 

[bronco italiano]

Could it be the start of coolant leak into a cylinder? Check your coolant level

 

[nvrstuk]

Like a couple have mentioned. Is it running "perfect" after it warms up? Hard to get the engine running properly in all conditions if you aren't fixing all the potential problems that might be causing it to not run properly in all conditions... meaning- there might be more to it than just the carb.
My .02 before you go nuts tuning and re-tuning the carbis to make sure your engine is tuned up properly. TIme it, check to make sure each and every plug is firing at idle by pulling plug wires (carefully) and read the plugs. Your '74 has the DuraSpark correct? This is the "ABC's" of tuning THEN go back to your carb since this takes a whopping 20 minutes.

I agree that from the symptoms you gave us that the choke isn't set properly. Those little 2 bbls are the Model T's of carbs so do what m70 and Paul said and you should be good to go!

 

[clinem03]

Ended up adjusting the choke plate to be more open on starting up and it did the trick.

 

[nvrstuk]

Great!!!

Now take that $2K you just saved and...

 

[MS73HD302]

Nice fix little trick I used to read all the plugs collectively was just a 2x4 drilled with 8 holes and marked each plug’s porcelain with a sharpie the cylinder number. Made it much easier moving them all to the good light!

 

[nvrstuk]

I'm so old (lol) that I used to use yardsticks for exactly that!! Now that you can't get free yardsticks from the local hardware store any more so I have to rip plywood strips.

 

[clinem03]

Great!!!

Now take that $2K you just saved and...

Spend it all on gas to drive the Bronco!