A little background... purchased truck after it had sat for about 6 months, and not driven much at all in the years before that. The truck would stumble and bog down during acceleration when cold, and run fine once warmed up. I figured it was due for a carb rebuild and choke adjustment. I also replaced the plugs, wires, cap, rotor, fuel filter, and half of the vacuum lines.

I ordered the rebuild kit from Mike's Carburetor Parts (it arrived quickly and seems like good quality stuff). I tore down, cleaned, soaked, and sprayed the carb. I put it back together and adjusted according to the included instructions as well as the videos provided on Mike's website. There appears to be only three things that need adjusted on this carb; the metering rod, the float level, and the idle mixture screw. I set them all accordingly.

After reinstall the truck started right up, but still has the same stumble when cold. I moved to the choke, thinking that must be it. When cold and not running the choke is completely closed. When started it pops open about 1/4-1/3". When warm it is fully open. That appears to not be the issue.

TLDR: What way do you adjust the metering rod screw to fix the stumbling issue when cold? Is there something else that could be causing it altogether?

Some pics so you know what I am dealing with (taken with a warm engine)...

 

If the carb is adjusted correctly, there are a couple other things that could definitely be causing the issue.

First off, why did you only replace half of the vacuum lines? Vacuum leaks can cause stumbling issues, especially when cold. It's always been my experience that a cold engine is affected by vacuum leaks more than a warm one.

Another thing that could cause it is a leaking carb-to-manifold or manifolt-to-head gasket. When it's cold, the cold air is seeping in through the leak and causing it to stumble. Then, when it warms up, the metal expands and partially, or fully, seals off the leak.

Also, if your timing isn't set right, it can affect your acceleration off-idle.


Do you have a vacuum gauge? (Best engine diagnostics tool there is).
Hook up a vacuum gauge to direct manifold vacuum and get your reading at idle. A good, healthy engine should be steady and around 17 - 20 ft/lbs.
Then, to check for leaks, watch the vacuum gauge and spray all around the mating surfaces with a can of compressed carb cleaner. Watch for wiggling on the vacuum needle. If you find a spot that you can spray and continually make the needle falter, you've found a leak.

 

"...truck would stumble and bog down during acceleration when cold, and run fine once warmed up."

That sounds normal for a carbed engine. Cold running is just not their forte.

 

AbandonedBronco: I only replaced half of the lines because I took one piece into Napa, assuming of the lines were the same size. I got enough to do the whole truck, and realized that half of the lines were larger than what I bought. The rest are on the to-do list for this weekend.

I do not have one, but I have read on here enough to realize I need to get one soon. I will see about getting one this weekend as will and post any results (for this and the vacuum lines.

Harte3: My old 65 Fairlane never had this issue, it never stumbled like this. The truck will have this hesitation/stumbling/bogging down after letting it idle for 5 minutes and then driving it to town. At the stop sign 1.5 miles up the road (today, which is a 90 degree day) she will stumble when I accelerate. It just doesn't seem right. If it is like this when it is 90, I hate to see what it will be like when the Nebraska winter creeps in!

 

Nebraska winters are what turned me on to KATS tank heaters.

 

How is your fuel pressure going to the carb? are you running a mechanical pump or electrical vane type setup? have u pulled it apart and double checked the float setting? might wanna go thru and double check all the external fasteners as well. any minute vacuum leaks can surface around those areas. looks like you're rocking a yfa carb. the vacuum gauge and leak testing as previously posted will help out a lot. spend the extra 10 bucks. it'll be an asset to you for years to come. also check the throttle plate pivot area for radial and/or axial play. seems like a common problem area as well.

 

The heat "damper" in the exhaust manifold is super important on these, in that it needs to properly function. Sounds like yours might be stuck in the open position, or missing altogether. ~Bill

 

It turns out that it was a split vacuum line to the carb. It was split on the underside. I found it when I replaced the rest of the lines this weekend. It is running great now, and the stumbling has been reduced to almost zero. The odd thing is that it with the line split like that, wouldn't affect all the time? Not just when it was cold?

The Dude: I set the float level with the sweet paper ruler and the instructions that came with the kit. It was actually dead on, no adjustment needed. The vacuum gauge is definitely a purchase. I see where people are tuning their carb with just the vacuum gauge. I am sure that is a little better than the "by ear" method I use. I am going to stop by Harbor Freight and grab one. I assume their gauge will be good enough for a home shop.

Bill - I wasn't sure what you mean by heat damper? Are you talking about the hose that runs from the exhaust to the air cleaner that runs the flap in the "arm" off of it? I have the stock air cleaner and that appears to be working (closing).

Anyway, thanks for the help. I jumped the gun a bit. I should have replaced all those lines in the first place! Everything under the hood from the plugs, wires, cap, rotor, hoses, and belts have been OE so far. The vacuum lines were 35 years old, and to have only one broken isn't too shabby. These trucks, and the 300 are built to last!

 

Mine's missing all that and it doesn't seem to matter. I'm referring to the flap inside the exhaust manifold that works off a bimetallic spring, closes when trucks cold opens once the manifold warms up. Redirects exhaust gas under the carb apparently to heat it and prevent throttle plate icing. ~Bill

 

That's great to hear. Glad you got it sorted out and running well. I remember when I replaced all my vacuum lines. I had a new truck afterwards.
The simple answer to your question is that cold engines are affected more by vacuum leaks than one that's heated up. They seem to be able to "ignore" it better.