I have a question for the 300 experts that I hope you can help me with. My girlfriends 82 bronco will idle fine and drop down to almost stalling and jump back up to normal idle. Sometimes it does stall out. The engine was just rebuilt and it did this before and after the rebuild. It has a remanufactured carb on it so that is fairly new also. all wires, plugs, cap and rotor seem to be good. We also replaced the brainbox less than a year ago. As far as I know there are no vaccum leaks, but it may be possible. I am thinking that the problem is carb related. Maybe I have the regular idle and fast idle settings screwed up? Thanks for any help.

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Well I just went out and set the fast idle down to 1400, which is what the sticker on the radiator support says. The regular idle is all goofy so it is hard to adjust with the problem I mentioned earlier. It will stay at about 500 to 600 rpm but will then drop dramatically and then will maybe go back up. I am thinking maybe a timing problem. I think it is set at 10 degrees btdc. I think that is what the sticker said. The manual I think says something more like 18 degrees btdc? Adjusting the fuel air mixture did not seem to do anything. The EGR valve is fairly new but we do not have the smog pump hooked up right now. Could this be a problem? Thanks, Derick.

People who say it can't be done should not interrupt the person doing it.

 

Big D, (I only say that because I'm DerEK) - anyway, when the rebuild was done were the timing gears replaced or checked? 18 BTDC sounds a little out seeing my '79 is at 6. If you have what's known as a back pressure induced EGR valve, the lack of thermactor could be causing your problem if it was also not hooked up before the rebuild. The air pump will increase the amount of air input into the exhaust system thereby increasing the amount of pressure or "backpressure" that these 300-6 EGR are calibrated to. You might have a vacuum leak in a component. I'd start with the air pump if it wasn't hooked up before the rebuild and the erratic idle was problem then. Then, I'd move on to isolating vacuum components to see if one of them is leaking. Don't forget any of the obvious such as plugs or wires.

 

Couple of Carburetor ideas.

Even tho the carburetor was rebuilt, doesn't mean it was done right in some cases.

The float could be set too low (not getting enough gas into float bowl(closeing too soon) causing a drop off in idle once the bowl empties and the float drops enough(opening too late) to let gas in). Make sure the float is set per specs.


Get some LOCKTITE and use this on all carb bowl to base screws. They have a tendency to loosen after a while and will screw up your idle. also check your base to manifold hold downs use locktite on these as well.

Gaskets. Even NEW gaskets leak. Base to manifold gasket use forma a gasket #2(non-hardening) lightly. make sure mating surfaces are ABSOLUTELY clean and free of any gasket material.


As for setting your idle. Find the mixture screw. (should be on the base behind a cap, remove the cap). Turn clockwise (counting turns) until it bottoms out (you'll feel it, be easy here you don't want to crease the seat). Write down number of turns.(this is so you can go back to original spec if needed)

back screw out 2-1/2 turns. start truck. back out in 1/4 turn increments waiting 15 seconds between turns. Your idle will smooth out and then start to stumble (too rich). Write down # of turns.

NOW turn screw back in, in 1/8 turn increments until idle drops again. (too lean). write down #. Back out 1/4 turn. write down # (final time). If your not sure of count at this point, shut down engine, screw in counting turns, then back out same #.

Adjust your idle speed to 700 or what ever is on the sticker.

This should get you close enough to run consistantly. You can tweak over time in 1/8 turn increments either way until your completely satisifed.

Larry

 

I am going to go out and throw all of that stuff back on and see if I have any luck, I'll post my findings. Thanks.

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Ok, hooked the smog pump up and nothing. I was hoping it would work but she still runs the same. It runs fine when it is on fast/cold idle but when I kick it down it will idle fine and then drop down. The idle will drop down and rise again own its own about 3 or 4 times and will then smooth out again. But if I give it gas while it is dropping down it will also smooth out for a while but will eventually start the process over again. Larry, I know how it goes with the base screws. My ranger used to have the 2.3 with that crappy Carter carb on it and they would come loose all of the time. I never bothered to use any loctite though. I am about to pull the carb off and check those screws. Do I have to take the top of the carb off to adjust the float?

People who say it can't be done should not interrupt the person doing it.

 

Sounds like you're getting warmer. You'll need pull the top cover off the bowl to adjust the float. Sorry to hear the air pump didn't help.

 

[font size="1" color="#FF0000"]LAST EDITED ON 10-Aug-01 AT 03:26 PM (EST)[/font][p]I pulled the carb off and checked the base screws and they were just a little loose. They tighened up about a quarter turn. Probably not enough to effect anything. The only thing I am worried about if I pull the top of the carb off is that if the gasket is going to be reusable? I don't even want to try and find another one around here since some of the stores around here don't even hire people who can speak english, besides not knowing anything about a car. Were moving soon so all of my manuals are packed up. How do you adjust the float? Is there a screw or a little tab that adjusts.

EDIT: Oh yeah, the timing does not seem to play any role no matter where it is set at.

People who say it can't be done should not interrupt the person doing it.

 

I'm sure you'll get different answers. If you get a rebuild kit it will have a new gasket and a needle valve replacement too. The float is adjusted by simply bending the arm of the float between the bulb and the swivel. The rebuild kit should have instructions on where and how much to bend it. When I bought my rebuilt Carter YFA, I had to remove the top cover (or bowl cover) in order to install a bracket (can't remember if it was the idle stop of dashpot) anyway, I put the top cover back on and have no problems - but the carb was rebuilt (fresh gasket). If you do try it -remember not to torque the screws too tight.

 

I already bought a rebuild kit a while ago and tried rebuilding the old carb. The throttle plate screws were seized in with some loctite so I could not get it apart. I rebuilt what I could and it worked like crap so we just got a reman. These carbs are not my favorite and I wish I could find a different kind. I think we are just going to hold off and maybe think about a 4 barrel intake and 390 cfm carb. It still runs good when cruising so we can move it to our new house where I will be able to work on it in my own garage.

People who say it can't be done should not interrupt the person doing it.

 

Now we're getting close.

1/4 turn not really beneficial but now you know they are tight.

The off idle circuit and the high speed circuits appear to be working fine (according to your posts), so you have to narrow it down to the idle circuit.

quick diagnostic here. This may or may not work. I've had a fair amount of luck with it.
Turn the idle mix screw in all the way and count turns. Turn out all way and remove (carb on car.). Liberally spray CARB CLEANER ) in screw hole (use tube). Screw mix screw back in all the way and back out the number of turns you wrote down. Crank up and see if she'll idle.

If it works it will work for a bit. In either event overhaul the carb (soak overnight in solvent, don't just spray).