I have a 460 stock bottom end, mild comp rv cam, stealth intake, edelbrock 600 carb, stock exhaust manifolds with dual 2.5" straight pipes. Duraspark ignition. It starts fine and runs smooth at idle and lower rpms but at higher rpm under a load it gets really screwy. It usually has a small backfire just off idle at 1000 rpm, but then is fine above that untill around 4000 rpm. I drove it last night and in HI range 2nd gear it ran fine till about 6500 rpm. I shifted to third and it almost acted like it was either flooding or running out of gas by 5000 rpm. LO and 2nd and it was hiccupping and cutting out bad by 4500 rpm. I have tuned the carb according to the book. Timing is set at 14*. I have replaced the ignition module and it still does the same thing. Cap, rotor, plugs and wires are all fairly new. This engine was just overhauled 2 years ago and it ran fine until the last couple months. It is in my mud truck so it does see Higher rpms. Sorry for the long post but does anyone have any ideas what might be causing this?

 

I think I would start by removing the sparkplugs and look for discoloration like too white or too dark. They should all be a fawn color.

 

my capri used to have a similar problem, turned out it didn't have the fuel pressure it needed at higher rpms even though it was fine at idle.

 

Spark plugs wont tell me anything. It burns too much oil. (royal purple was a bad idea) Before it started burning oil the plugs always looked good. Driving it to town today it would run fine in hi range but crappy in lo. Ran hi range at the mud bog today and it ran fine. There wasnt enuf mud in it to worry about running lo range.

 

Have you done a psi test on all the cylinders ? Had a vacuum gage on it ? Checked the timing curve & total timing with a timing light ? Checked the fuel psi with a gage ?

 

We checked vacuum when we were tuning the carb. Checked initial and total timing. Have not done compression test recently. But if it was a compression problem wouldnt it be more consistent? Why would that make a difference between high and low range. Have not checked fuel pressure or volume, but filter is clear and It is a mechanical fuel pump feeding an edelbrock 600 cfm. How much does it need?

 

I would look in that direction also, fuel flow is very different when reving 5000 RPM and under load at 5000 RPM.

Also loaded running at 5000+ RPM is really hard on the old Flat tappet cam'd 460's. Your are pushing the max piston velocity in that range on a stock bottom end IMO.

 

Thank you everyone for your suggestions. I will look into the fuel situation.

 

Its been a while but i got my problem figured out. I added an electric fuel pump and cleaned what looked to be the beginnings of a mouse nest out of my fuel tank and it runs much better now. Thank you all for pointing me in the right direction.

 

Good to hear your getting things sorted out.