Well....let's see I just rebuilt the carb on my '73 302 2bbl Mav. The car has only 56,000 original miles on it. I replaced the gaskets from the manifold to spacer and from spacer to carb. I then put the carb back on and fired it up and it idles rough at about 800-900 rpm. The car ran fine before rebuilding the carb and I only touched the carb. The timming looks good. I replaced the fuel filter. The fuel pump pumps good. I crimpped off and closed up the line that goes from the choke to the exhaust manifold. The idle mixture screws are at about 4, 4 and a half turns out. They were at 5 when it was running before. The float level looks good. I switched the carb spacer to one without EGR and plugged the necessary vacuum line. I'm stumped, I don't know what else to do, please help me out!!
A couple of things to do. Look down into the carb with the engine running to see if there is fuel dribbling out anywhere. If so the float level is to high or the float is sticking and not closing. Next, move one of the idle screws inward and observe the idle quality. If it improves, do the other one the same. Go back and forth to get the best idle, favoring the most outward setting or you will get a flat spot off idle. Let us know how you make out.
Okay, It sounded like there was a vacuum leak. So the hunt began. I had the air cleaner junk off and was listening around for a leak. I then put my hand and covered the air going into the carb and the dang thing smoothed right out. I then put the air cleaner and everything back on and fired it again. It was still running rough and had a faint sound of a vacuum leak. So I got the top half of my jack handle and used it as a listening tool. I thought I heard something around the back of the carb so I leaned in and tried to get a closer listen and all of a sudden the dang thing smoothed on out and ran fine. It was at an idle for quite a while because after I shut her down the radiator began to overflow. My guess is that there was some sort of vacuum leak around the carb/spacer to manifold and that it sealed up when it got hot enough. I started it up from dead cold tonight and it took a few more cranks that normal and a little gas to help it. I think I have a little more adjustment to do. Now for the questions. For the startup I adjust the auto choke right? Clockwise lean, counter rich? I think its 2 lines lean right now from center. Also on the mixture screws the further you turn the screw out the richer it gets with gas and the further in you turn it the leaner it gets with gas and gets more air or uh umm....hmm I've never actually been quite sure what does what I just screw it in the direction it wants to go. Right now the screws are at about 5 turns out. I'm guessing that's too much. Am I right? and explain to me richer/leaner, gas/air, in/out? Thanks for your help. So far I have learned a lot and have refreshed my memory a little. Now if I could just get rid of this light headness from the carb cleaner and gas fumes.
I would go over it again to find what happened or it may come back. On these carbs, the idle needle function is reverse of the older carbs. The air passage is fixed and the needle controls the fuel. Not all other carbs work like this. It is an emmisions control thing to keep the public from misadjusting the carb to far away from (their) optium or poor starting etc is the result. Its kinda poor anyway. Many later cars have plastic caps over the needle heads to limit how far they can be turned unless someone has taken them off. You can see from this that if the needles are set 'in' to far to cut the fuel, an off idle hesitation will result before the mid fuel circuit begins to meter fuel.
a general rule of thumb is to turn both of those idle screws all the way in, then each one out 2 1/2 turns, then adjust by ear from there. If you have a vaccuum gauge, hook it up to vaccuum and adjust the screws to where the gauge reads its highest. On my old autolight 2 bbl on my mav, in the back a metal pipe goes into the spacer(might have been pcv) if that is loose, it will leak. One that always gets me is that one vaccuum port off the passenger side towards the middle bottom, you might have overlooked that. Lastly, this sounds dumb, but once I put my spacer on upside down, having the flat side of the spacer down. It somehow worked for a while, then eventually a major vaccuum leak occurred. Also, aluminum tends to warp, check the surface of the bottom of the carb and spacer with a flat edge to make sure they are not warped.
My carb is a Motorcraft. There is a plug on that vac port on the passenger side. I did find a nice leak through the PCV valve coming out the back of the carb. I fixed that and it still runs rough. I turned the mixture screws out 2 1/2 turns. To get it to run there I had to crank the idle screw in a bunch. I had my choke linkage hooked up wrong, fixed that and nothing changed. When I squirt carb cleaner at the back of the manifold/spacer on the passenger side the idle changes and I hear a hissing sound. I have all new gaskets and I checked the spacer and it seems square. Also that part of the manifold/spacer gets damn HOT!!! What can I do?? Also, when I drive and keep it at a constant speed I can faintly feel the car chugging along. What's that all about?? This was going on before I messed with everything, but it does it a little worst now. Hopefully I can fix this soon. Keep sending the feedback, it helps!! Thanks
if you are hearing a hissing sound come from the back, can spray carb cleaner and hear the idle go up, then that is definately 100% the source of the problem. Try sticking your finger along cracks around where you think the leak might be, It could smooth out, or the hissing might stop. On my mav, I have almost a little tree coming off the very back of the manifold, check those vaccuum lines to make sure they are all there and not cracked. There could be a chance that the actual intake manifold is leaking. I have had vaccuum leaks more times than I could say, and after much cussing, and sticking my finger everywhere and especially using a section of hosing to your ear you WILL find the leak, let us know what happens. good luck PS, it might be a good time to pull out a vaccum drawing and see where everything goes, to make sure it is right, there are only a few lines.
Okay I found the leak. The gasket from the manifold to the spacer was the wrong one. I got the new one today and it's all sealed up. I can't hear any more vacuum leaks and the car is still running rough. I put my hand over the carb and it smoothes out like before but now if I block too much air it wants to die. I messed with the screws and that doesn't do anything. I've checked everything over and over and over again and I'm stumped. I'm all out of ideas. One other thing is that when the car ran before it would kind of chugg along. Could this be the fuel pump?
at this point i would suggest getting a vaccuum/fuel pressure gauge. The thing is great and handy, and can be bought real cheap. the mav book tells you how your vaccuum should be, and it also serves to tell you how much fuel pressure is coming from the pump. If you don't hear any vaccuum leaks, there is a good chance that prob is takin care of. Another way to check fuel pressure is to pull the line off at the carb and crank it into a cup(not styrophome) and watch the fuel come out, should be a decent amount coming out. Does it have points? often they will do that. Use a standard match book to adjust them, works real good, make sure they are in good condit. Shot in the dark, but are you runnin on all 8? have you done a compression check? One other way to tell is to pull each spark plug wire off while running and the one that pulls off that does NOT make a difference in the idle is your dead cylinder. Lastly, maybe someone on this board understands how the egr works, maybe that is causing the prob? Don't know much about emissions. Is your vaccuum advance hooked to the right port? on my 72 w/out egr, the advance is hooked into that port on the carb you said you had plugged.
Sometimes the spacer plate between the manifold and carb will burn through. This is a thin wall between the inlet and the throttle bore, and if it burns through, it creates a vacuum leak that is a bear to find. Pull the spacer off, and scrape the soot off of it, and check it out. Clean it thoroughly and check for a small hole or crack. Beyond that, check the tune. Hope this helps, Earl
Well I had the carb flow tested. That's good! I've changed the wires, plugs, and fuel pump. The points, cap, and rotor look good. There are no more vacuum leaks. The sounds like it's not running on all 8 cylinders. So I did a compression test and the highest is about 150psi and lowest is around 130psi. The car doesn't smoke at all. When the car gets really hot it smooths out. My car is getting hot now cause my radiator is pissing on my altenator. Geez, now I have to replace the radiator. I'm guessing the distributor is bad. Could this be my problem? A local shop said they'll hook my car up to a scope machine and find the problem for $35. To bad I ran out of money.
130 and 150 lbs is good, though they say a diff of 20lbs between cylinders might not be good. You are probably alright in that aspect though. Trying to find out if you are running on all 8 is easy. You know how to roll your r's with your toungue, well, but a constant low moaning while rolling your r's, that is kind of what running on 7 cylinders sounds like, all 8 will be smooth. Go ahead and pull each spark plug wire off, one at a time while the car is running. If you are indeed not completely running on 8, you will tell real easy. When you pull a wire off, and the idle does NOT go to SH*t, that is that bad one. Since you have pretty much ruled out compression, it can only be gas or electric, and I am guessing electric. Of course, you could just pay the 35$, but they will pressure you to let the fix the prob, either way, let us know what the prob is, I am curious.
Well I changed the distributor and the car still runs a little rough. My next guess is the valves?? Maybe they just need an adjustment or maybe the heads need to be rebuilt. Well I'm going to find out what the problem is before I put on the new heads, 4 barrel carb, manifold, cam, electronic distributor, water pump, balancer, and pulleys I have sitting on the shelf just waiting to get put on this car.
if memory serves right, the lifters on the 302 are non-adjustable hydraulics. Would strongly suggest getting the engine scoped for 35$ to see what is going on sometimes you run straight into a problem that just cannot be fixed without the proper diagnostic equipment. Did you ever find out if the roughness was caused by a bad cylinder or is it just a miss?
I haven't found out yet, but I will have it scoped. What I want to know is how the scope works. How can they find out if its a bad lifter or a bad valve or can they? I'd like to have some knowledge of how the scope works before I take to someone.
