Hope you don't mind my asking about something not directly related to a truck.

I just had a 1968 302 engine rebuilt which I installed into my 1969 Mach I Mustang. I'm told it came out of a 1968 truck, and came from Ford with a cast iron 4 bbl intake manifold, and a matching Ford 4 bbl carburetor. The carburetor and intake were pretty badly corroded, so I replaced them with a Holley 0-1850 600 cfm vacuum secondary carburetor and an Edelbrock F4B intake manifold, from another source. I had to rebuild the Holley, as it had sat in someone's garage for about 20 years, and there were a lot of gummed up parts inside. After much soaking with Berryman's B12 and scrubbing, I cleaned up every part pretty well and made sure all the passages were clear. I think I assembled it properly with new diaphragms for the accelerator pump and secondary actuator, but I left their springs alone. I replaced the old power valve with a newer part of the same rating (12.5" opening, I think).

So after a difficult initial start up, I find the engine running extremely rich by the smell of the exhaust. I've adjusted the idle mixture to get about 15" Hg vacuum, but it's very unsteady. In fact, the idle is very rough, until I open a little bit of vacuum leak into the manifold. Then the idle speeds up, smooths out, and the vacuum jumps up to about 18". I will check the plugs tomorrow, but I'm sure it's idling way too rich.

Another problem is stumbling whenever the throttle opening is increased. Tapping the throttle from idle causes a stumble, and if I slowly bring it up to about 2000 rpm and goose it, it also stumbles and causes a popping out the exhaust. I'm pretty sure I adjusted the pump linkage correctly; I shortened the spring loaded bolt on the end of the throttle lever so it just touches the pump lever with the throttle completely closed, then turned the idle screw 1.5 turns in to set the idle speed. (I think that's low enough that it's not exposing the transfer slots too much.) I can see the fuel shoot out of the squirters pretty much instantly when I actuate the throttle by hand, so I think it's working.

So I studied a book on Holley carburetors hoping to learn something, but I am still trying to figure out a couple of things:

The idle mixture is obviously too rich, but I can't figure out what's causing it.

The throttle opening stumbles would indicate that it's actually too lean, and maybe I need to change the pump profile.

I installed a Comp Cams FS268H-10 cam (straight up) in place of the stock cam:

268/268 @ .006", 219/219 @ .050", 110 degree lobe separation
.456/.456 valve lift I/E with 1.6 Comp Cams roller tip lifters

a little bigger than stock, but nothing too radical.

So is there something I'm overlooking? Am I correct in assuming that the idle is too rich if I can get it to run much better by introducing an air leak into the intake? Can the accelerator pump be doing too much, and that the stumbling and popping I'm seeing are actually due to too much fuel being shot into the intake? Is there any easy way to tell if the secondary idle circuit is working properly? It has a metering plate. Since the problems are all in the carburetor, I can just take it apart again, but what should I look for besides the obvious?

Thanks in advance to any carb gurus who can help.

 

you'd rather run it rich that lean, lean will burn valves and pistons

 

I had same problem with an old rebuilt Holly. Solved problem by swapping to Edlebrock. An auto restorer buddy told me he's never gotten good idle, low speed perf. from vacuum secondary Hollys. Have you gone to a smaller low speed jet? Just trying to help. I am sure that my post will be laughed at by everyone smarter than me.

 

that's one of the reasons i switched to edelbrock. also i can change jets on the truck.

 

Thanks for all the replies; I just saw them now, for some reason, my browser did not highlight the thread as having new replies.

I found the problem after rebuilding the carburetor again. I found that it was indeed the power valve due to a small collection of liquid fuel in the chamber behind it, though it was not due to the typical blown diaphragm. It turned out that the new gasket I installed with the new power valve actually had two gaskets sort of pressed together. I didn't notice it when installing it, and the two were misaligned when I torqued down the valve. So fuel was leaking right into the intake manifold, especially at idle, when the vacuum was high. I removed one of the gaskets, reinstalled, and it's running a lot better now. I also had to clean up the spark plugs because they had been so badly fouled by being run so rich before.

It's still not idling very smoothly though, and blipping the throttle still gets a sluggish response, including a popping out the exhaust when doing so from a high speed. I haven't gotten as far as getting it onto the road to check performance yet.