I have a 300 Straight 6. I recently Rebuilt my Vacuum system and that got the truck running a little better. But im still only getting like 10 MPG.

Im running so rich at a constant speed its bogging and missing. It has no power at constant speed and throttle.

It im accelerating or running at highway speeds it runs much better but still has a noticable miss.

Any idea what could cause this?

Does the one bbl Carb have an A/F adjustment screw on it?

Would it be worth it to convert to a 4 bbl carb with more avaliable adjustment?

 

You can usually adjust the idle mixture(unless the carb has never been rebuilt and the adjustment is blocked off). There is no adjustment for higher speed operation.

Usually when it runs very rich, it's spilling fuel over into the engine because the fuel level is too high in the fuel bowel. This can be caused by passing dirt through the carb making the float needle stick open, or a problem with the float such as a hole in it or it has abosrbed fuel making it sink.

I would get it idling or running the best you can, and take a flashlight and shine down the carb throat while it's running. If you see fuel dripping into the engine, then your fuel level in the carb is too high.

 

It idles great.

I will take a look down the carb later today.


But if that isnt the problem i guess the only way to do fix it would be to switch to another adjustable carb?

 

Idles good, but running rich when driving, is generally caused by an improperly functioning choke.

 

If it idles good, I would suspect a timing problem. If it was spilling fuel over into the engine it would idle rough. What year is this engine? Does it have a vacuum line going to the distributor?

 

That's a good idea also.

 

Its an 82.

And no i dont believe it has Vacuum on the Distributor.

 

This was a concern of mine as well. My choke plate is kind of sticky when moved by hand.

What can i do to check, clean, or adjust it?

 

Well the first thing I would do is go for a drive to warm it up then stop somewhere and take the air cleaner off to see if the choke plate is opening, if it isn't opening it will be sucking in more fuel and not letting in as much air. So you know if that is actually your problem or not.

 

- USE CAUTION - fire hazard...

Get a can of carb spray and go at it with the engine running. Spray the linkage on the outside of the carb as well. Choke adjustment info can be found in a shop manual or rebuild kit. If it hasn't been screwed with, it shouldn't need adjusting. Make sure the heat stove plumbing (Tubes to/from the exhaust manifold) and electric choke heater connection (if equipped) are in tact as well.

I've wrote about it too many times to write it again but these carbs have a rubber diaphragm (accelerator pump) in the bottom of the float bowl that can rupture and cause the engine to run rich...

 

After getting out and driving it, it is also bucking when letting off the clutch or gas. Also running so rich if i let off the gas it will quietly backfire a time or two.

And my heat stove plumbing is long gone. It originally had it but only have the tube is there now.

 

Just a thought... is your truck from California? The reason I ask is that on 49 state 4.9L engine equipped vehicles, the feedback carburetor system came into being in 1984, but California often is ahead of other states regarding emmission systems. In your post above, you stated that you did not believe the distributor to have a vacuum [advance]. The feedback system does not have vacuum advance... everything is controlled by a computor.

If your truck does have the feedback system, there are quite a few reasons why it might be running rich. Look at your distributor for a vacuum line or does it have an electrical module on the side of the distributor with wires running to it? Did your carburetor have electrical wires connected to it? Does your truck have an O2 sensor in the exhaust manifold? Is there a gang [3] of solenoids on the back of the valve cover [driver's side]?

By the way, welcome to the forum!

 

According to the title it's been in TN it's whole life. I will go take a look at the engine to check all that and to check my choke in a bit.

But I'm certian there are no wires or solenoids around the engine or carb. I do have a single broken vacuum line right behind the air pump(?) that is red or pink. I couldn't figure out where it went.

There is also a black connector on the card that is not hooked up. I'm not sure what it's for either.

 

The black connector might be the wire which should be attached to the choke housing. It provides the current to the heating element, which is what causes the choke to open up. Without it working, your engine would be running rich!

Take the air filter housing off, as suggested by Sw1tchfoot, and see if the choke plate is opening?

In one of your posts, you mentioned that the heat tube for the choke is gone. The choke is supposed to get heat from this tube as well as the electrical heating element, so it is possible that you are operating without any heat! By the way, if the heat tube is gone, there will be a vacuum leak, as the carburetor pulls a vacuum through the choke housing to draw heated air from the tube as it passes through the exhaust manifold. This system is normally a loop which gets its "make-up" air from above the throttle plate via a port at the top of the carburetor, which normally has a red vacuum line.

 

Alright, i will take some pictures of the connector and the vacuum line that isnt hooked up.

See if we can get this figured out.