I've got a 70' Sport Custom, 360, auto, that I just did a carb swap on. My factory carb had the fast idle cam out and a crack on the back of the throttle body, but lucky for me I had a Edelbrock 500cfm Performer laying around that I had bought for my 75' Bronco. Here is the bad part I bought a 2bbl to 4bbl adapter plate for it since I don't have the dough for a new intake right now, which may or may not be part of the problem. In the instructions for the carb it says if you have a timed ignition ( no vaccum at idle) to put the vaccum line on the passenger side of the carb, which I did. It says to block off or use another line with the drivers side, I put the vaccum line off the tranny on here( I also tried it blocked, it didn't help). I have the large port on the back of the carb blocked off. Here is the problem, it idles rough at low rpms, which I've got a better idle now but it idles up when you give it gas, and stays there for a minute and then slowly dies down backfires through the carb and dies, if you don't give it any gas. I've messed with the idle and air screws to no avail. I had trouble getting it to seal up to the intake, but that is fixed, there are no other air leaks, no linkage binding, no bad fuel. I have no idea what the problem is, I sure would appreciate any help at all. Thanks!

 

my guess are maybe bad fule filter, or something minor

does your carb have 2 idle screws, fast and curb idle?

 

Yeah its got a brand new site glass filter I put on with the carb, and so far it looks pretty good. I haven't seen any rust or any dirt come through. Yes it does have two idle screws, I've tinkered with them somewhat but it doesn't seem to want to idle. Do you know what the starting point for adjusting the idle is ( i.e. one and a half turns, two turns etc) I just got home from work and haven't had time to fire her up tonight, but I'm sure hoping the carb fairy stopped by while I was gone!!

 

are all vaccum lines connected and not leaking? im not too sure on the starting point.

 

Yeah I've got all the lines hooked up, but I'm not sure about one of them. The install book said if you had no vaccum off the distributor at idle to hook up the line on the passenger side of the carb, which I did. My question is the other vaccum nozzle on the drivers side which the book says is a constant vaccum line. I have the line from my power brakes hooked up there, but I'm not sure if its working right. I've switched and swapped lines but I've turned so many screws(probably the wrong way!!) that I've got the plugs pretty well fouled. I'm gonna go tinker a little bit right now, wish me luck!

 

[updated:LAST EDITED ON 24-Jul-02 AT 11:13 PM (EST)]Put your distributor vac line on the non-constant vac connection. I would block off the constant vac line. Run your power brake, etc on the manifold vac port. 1 1/2 to 2 turns out on the idle mixture should be close enough. I would take a real close look at the adapter for any leaks. Not sure if it will ever run right with the adapter.


Also, is your PCV valve hooked up and working?

 

Sounds like your initial timing is too far retarded....that's what happens on mine when I retard it too much.

 

Thanks for the info guys, I think I will try and block off the constant vac line and try that before I turn the distributor. But that is the second time today to turn the distributor so I'll probably try that too if blocking off the other line doesn't help. I'll let you know how it goes. Thanks again.

 

Great news I got her running I tried blocking off the brake vaccum line at first because I haven't been really sure about that line all along. I cleaned up the plugs and blocked off the line and fired her up, it was still missing and not wanting to idle so I loosened up the distributor and turned it clockwise and she smoothed right out. I could only turn it about half and inch because of my thermostat housing but man it made a world of difference. I cranked up the idle a little and started in on the air screws. I ended up about 2 or 3 turns on the air screws and about 7 and a half on the idle from all the way in. She's idling like a champ now, no miss, no carb backfire, and it opens up the secondaries pretty damn good for being an adapter plate. I was surprised! I've still got a little tweaking to do on the idle but all in all its running great, thanks for all the help!