HELP!!!!!!! Engine stumbles at speed (Not a Z)

[Z-Dreamer]

With the wealth of knowledge on this site, I thought I'd share my problem with hope of receiving some suggestions and hopefully a solution to this problem. Anyway, I own a 70 Chevy C20 with a mild build 454, Comp Cams 4X4 cam, Edelbrock Perfomer 750 with vacuum advance, a Weiand dual plane, Accel HEI, headers. The problem is that at about 2800-3000 rpm under load the engine starts to lose power and stumble and when I add more pedal it falls on its face! I am at a loss as to why this is happening, I did have to replace my Holley RED pump with a new one and shortly after I get this problem! I can rev and hold the engine to 4000 rpm in park and it does not stumble.

Any and all suggestions would be greatly appreciated!

Thanks

Mark

[dj paul]

sounds like something with fuel or spark. not a hundred percent though. either that or timing. its kinda wierd that in park it doesnt. its not under load so it wouldnt be as noticeable. first thing to do is check the spark plugs, see if they fire, oil on them, etc. then check the rest. its prob not fuel unless your carb is in real bad need of tuning.

[MatMan]

When was the last time you changed the fuel filter?

[Some-Guy]

this happened to me once too the truck was fine in park you could redline it if you felt the need but as soon as it was under load it would seriously bog down or just flat out DIE turned out the fuel filter was clogged..changed it out and it was fine.. altho we had to get a new tank because there was so much rust and dirt in it that the filter clogged every 45 minutes lol (was a beater anyway)

[Tim240z]

Make sure the tank is still venting. The fuel cap may not be venting (clogged). Also, pull the distibutor cap and check for wear and pull the plug wires and check with an Ohm meter for resistance.