I am changing out the original Motorcraft four barrel carburetor on my 1978 2WD F-150 Super Cab Trailer Special. It has a 106000 miles on it with a stock 460 with MSD ignition. What size carburetor can I run on this engine? I have a Holley 750 or a Holley 870 Street Avenger available. I primarily use this to haul large boats and trailers. Any suggestions would be appreciated.

 

A 750 would be a great choice. an 870 is overkill and I don't care for that series anyway myself.

dial in the 750 and you'll be happy.

 

I concur. I have a 750cfm summit carb (basically a rebadged Holley 4010) and love it.

 

Thanks, that's what I thought. As a matter of fact I installed the 750 today.. started up runs fine just high idle. I can't get it below 850. I checked for vacuum leaks and screw settings. I got the carb from a friend of mine mine who's a hot rod guy, so I may have to check the jetting on it.

 

Jetting is irrelevant while idling. You either have binding linkage, a weak throttle spring, a vacuum leak or a combo of all of them.

 

I'm fairly certain it's a vacuum issue. I had the idle down to about 550, but after I ran up and down the road to test it out it came back up. I'm going to go through and change all the hoses for that are vacuum lines and see if that makes a difference. There aren't many but some of them are looking pretty rough.

 

First I'd get a vacuum gauge. Ideally you want to see about 17-22" of vacuum at idle with a steady needle.

Get a gauge and report back.

In the mean time you can spray brake cleaner along the intake, at all your vacuum accessories (hoses, booster inlet, canister, etc). Listen for the engine to stumble. If it does then there's your leak.

Replacing all rubber hoses is always a good idea as well, so you are on the right track.

 

Already checked vacuum. It was 16 max, but not steady. The needle would fluctuate between 15 and 16. I had already sprayed the base with carb cleaner, no stumble. Now I will spray all hoses.

 

Did you hook your vacuum advance to the ported or manifold source? if you're set for ported then hooked it to manifold it's going to idle a lot faster with the advance.

 

I went with the small fitting that exited the carb on the lower front passenger side under the float bowl. It comes out at an angle. The threaded port on the front center of the aluminum base spacer I plugged with the plug sent with the base spacer.

 

Sounds like you have a fast idle cam hanging up on passenger side that the choke works.

 

I will check that again. Did I mention that I have a manual choke? I pulled the carb off the truck yesterday, and took everything apart on that side and reassembled it. I couldn't find anything that was out of place or sticking. And yes I did put the little gasket underneath the choke mounting plate back in place.

 

In addition, the set screw for the fast idle cam is not in contact with the cam

 

you're hooked to manifold vacuum. nothing wrong with that if your distributor is set up for it but very few since the early 70's are. try the ported vacuum fitting on the right side in front of the choke housing and plugging the manifold port you're on and see if that doesn't cure it.

 

So, I am back with some more issues on the 3310-2 Holley carb I was diagnosing in my earlier posts. I have run the truck for several weeks and it was running very rich. I checked and reset the air fuel mixture several times, but have not had any success there. Several times when I have been driving and accelerated I've had a backfire in the carburetor. Yesterday, Memorial Day, I decided to check everything again. I back down the air mixture screws and screwed them out 1 turn each side. I then started the truck up and checked my timing. It was retarded by about six degrees. I reset it to 12 which is the OEM suggested timing. I then started it up and checked my float level. The front primary float was low so I adjusted it to the proper setting. However, while I was adjusting the float fuel was running out of the float adjustment screw.I then turned out the air mixture screws equally until the engine idled smoothly, which was only about 1/4 turn each. I then drove it down the road. As soon as I accelerated it backfired. One other thing is that I have a manual choke, but have never had to use since I installed the carb. Have I missed some important adjustment? Also, on any video I have watched, fuel is never running out of the float adjustment screws while the floats are being set.