I have just rebuilt my 460. The problem is it runs VERY rough. It will not idle at all. If you do not keep your foot on the gas it will die.

After reading the other posts, I plugged all intake hoses. No change. I replaced the lifters, no change.

The fuel pressure with vacuum unplugged is 40psi.

Even when I rev it up to 2000rpm it still runs rough,

Compression tests show 110 to 130 per cylinder with 15 minutes of breakin time.

The plug insulators are brownish. All look about the same. Not really light tan in color.

The interesting bit is the vacuum test. The vacuum is very low, 3- 9 inches. It also fluxuates wildly. Rapid oscillations from 4 to 9 inches at 1000rpm. It does not really change much as it revs to 2000rpm. My vacuum diagnostic charts don't really show this case.

It it hard to maintain steady engine speed. You need to adjust the gas constantly.

The rings and pistons are new. The valves, bronze guides, springs, and rockers are new. New cap, rotor, wires, and plugs.

The cam is a Crane "RV" low end power cam.

The engine ECM is for a 302. I plan to change to larger injectors and a calibrated 90mm MAF as soon as these problems are fixed. My thinking is as long as I keep the 460 under about 3600 rpm, the 302 ECM will work well enough.

I need this truck to get to work. Any thoughts or suggections are very welcome.

John

 

You most certainly have a vacuum leak. Check around the intake. You can use starting fluid to find the leak if you spray it in small squirts around the sealing areas. When the engine speed increases then you found your leak. Don't get carried away with the stuff. It is very explosive.

 

Check the valve lifter clearance. You may have gotten the valves down into the head far enough that the valves are held open a smidge. Loosen the rocker arm nuts about 1/2 a turn each, almost to where they clack. Do this before you fire it up. If you note a marked improvement, you may have to get different length pushrods... or, do as I did years ago on my Dad's 429... put some washers under the rocker studs. He (engineer) couldn't figure out the geometry, and why the washer worked.. Brain fart, I guess. That car may still be running somewhere...
'Smoker is right, check for intake leak also...
tom

 

The problem was very advanced timing. I could not time it myself as it would not stay running without me at the throttle pedal. When I finially got a buddy to help I found I had rotated the distributer gear a bit on reassembly. Having the distributer housing lined up with my alignment mark on the block didn't work.

Thanks for the help.

John

 

NEVER use starting fluid on any engine. You might as well let the water out and rev it to hell and back. It will kill the rings.

 

Doesn't the 302 have a different firing order than a 460???

JJ

 

Glad to see you got it running. Using small amounts of starting fluid to find a leak will not hurt anything. Its when you dump a ton of it into your engine to get it started that it starts hurting things. The vaccum leak can be hurting the rings just as much as a small squirt of starting fluid.

 

Is your ECM from a Mustang? If so, 302 HO's have the same firing order as a 351, which is different than a 460.

302 HO/351 13726548
460 15426378

JJ

 

By the way, Most Starting fluids have a lubricant in them to prevent damage to the rings.