ive been having trouble getting my carb dialed in. i dont know much history on the truck, but the previous owner was running an electric pump to the carb. a couple weeks ago a friend and i were trying to dial it in and as soon as we would have it close and letting it idle and listen to it, it would rev up and not come down. i tried putting a pressure regulator on the line to the carb set at 4lbs, got it ti idle longer but eventually did the same thing. anyone ever have this problem before or have any suggestions? time for a rebuild or replace?

 

Jeremy - We need more info, like what the carb is, what it is on, what's been done to both the carb and the engine, etc. And, since we will have to go through that info time after time, when you get enough posts to have a signature it helps us all for you to put the info there.

 

that probly would help, its a carter carb, 4barrel unsure of the cfm, on a stock 302, and from what it looks like the carb has been on there for a while. i put a fuel pressure regulator in the line set to 4 lbs. we tried adjusting the carb and the idle and it did it no matter what we tried.

 

Ok. Let's play like it is actually an Edelbrock 1406, which is their version of a Carter AFB. That's a 600 CFM carb and may be more than a 302 needs so yours might be smaller. But, other than the bore size and jetting they are all the same.

You've done the right thing with the pressure regulator as those carbs don't like more that about 5.5 lbs. But that wouldn't normally cause the idle to speed up as that is usually caused by a vacuum leak or more ignition advance.

Let's take the latter first - where are you getting the vacuum for the vacuum advance? I would suggest you use ported vacuum, which is the driver's side port in front. And, make sure it goes directly to the dizzy and not through any vacuum switches or delay valves - at least not until you get this problem resolved.

Then, if that doesn't solve the problem, look for a vacuum leak - one that can change. Like the carb being loose on the intake, a hose that is cracked and changes with movement, a bad power brake booster, etc. One way to check the hoses is to disconnect them and plug the fittings. Another is to spray brake cleaner and when it finds its way into the mix the engine speeds up.

Anyway, see what you can find. And good luck.

 

I've been researching and from what I've seen so far is the carb is just too much for that motor. I'm currently looking for a carb somewhere around 400cfm and am going to do a lot more research before I buy another carb. But I've read so much about them that my head is spinning. I'm having trouble finding a carb the size I need with mechanical secondaries. Any leads on one would be greatly appreciated.

 

Also the truck has msd distributor, so like you said it could just be a vacuum leak or advance problem. Nothing to do but start troubleshooting and eliminating potential problems.

 

A too-big carburetor won't, by itself, cause the idle speed speed issues you're seeing....

 

You do not want mechanical secondaries. You want vacuum or velocity operated ones, like the Carter/Edelbrock has. But, as Chris says, a big carb won't cause this problem. So let's fix the problem and see if the carb will work for you. It might.

 

I believe I found the problem. Vacuum advance isn't working and the crank vent on the drivers side was plugged sending all the pressure through the carb. Un plugged it and it immediately settled down. Thank you guys for sending me in the right direction.

 

You are sure the vacuum advance isn't working? Or, is it connected to the "ported vacuum" connection which doesn't give vacuum at idle? That's where I think it should be connected.

As for the crank vent on the driver's side being plugged and sending pressure back through the carb, I don't understand. Unless you have extremely bad rings or a broken piston there should be no pressure. There should be a large vacuum hose connected to the back of the carb that goes to a PCV valve in the passenger's side valve cover. That draws the blow by out of the engine and burns it. But you can't pull from a closed system so they put a fresh air source in the driver's side valve cover to let clean air in and sweep the blow by out.

If that clean air port is plugged then there won't be much going through the PCV valve into the intake, but there shouldn't be pressure. However, that does mean you won't be getting the expected amount of air into the intake and that will cause the air/fuel mix to go fairly rich. A very rich mix can sometimes cause the RPM to vary as the engine loads up with too much gas and then burns it off and goes again.

In any event, if your air inlet is plugged you need to fix it. But don't just allow un-filtered air into your engine. There should be a hose connecting to a small filter in the air cleaner. Or, you can run a stand-alone filter in the valve cover.