'71 Chevy C/20 bogs and falls on it's face under a load. Idles fine. HELP!!!!?

sound,s like a vacuum leak or bad exhaust gas recirculation valve [ E.G.R.] ...remove the valve & check for carbon deposit,s as well as diaphragm integrity. check all vacuum line,s,hose,s including brake booster hose & valve. a vacuum gage will help. check timing as well as mechanical fly weight timing advance in your distributor. check all gasket,s that seal off vacuum..especially the intake manifold gasket..squirt some oil around the mating surface while running..if it suck,s it in you have a faulty gasket....also double check valve rocker adjustment,s...if your running stock ignition...replace cap,rotor , point,s,condenser...this will give you Access to the fly weight,s for the mechanical advance so you can be sure their free & there are no loose spring,s...best of luck!..edit..to all that answered a possible cam shaft because there was a problem with soft metal shaft,s...that was back a long time ago & that would not cause a backfire...the lobe,s would wear down preventing the cylinder from recieving a proper air/fuel charge...then close the valve early...this would account for lack of power but not a backfire situation.

Small block Chevrolet engines have some camshaft issues. It is a combination of cams not being real hard, tight valve guides, and or a valve spring with too much pressure. An easy way to tell if you have a bad cam is to remove the valve covers, start or have someone crank the engine and watch the rocker arms, if one or more are not moving as much as the others, than the camshaft has badly worn lobes and needs to be replaced along with all of the tappets (lifters) and both timing gears and the timing chain. The reason it pops back through the carb under a load is, the exhaust valve or valves are not opening enough to let out all of the exhaust, when the intake valve opens the left over exhaust will blow past the open intake valve and cause the pop or backfire through the carb. At low speed or idle the exhaust valve will open enough to let out the smaller amount of exhaust gas.

It is not a hard repair, just time consuming, about 10 hours.

If your engine idles smooth chances are the valves are ok, but you should do a compression test before you take it apart just to make sure.

TOM

It sounds like you might have several small problems.

You say it's a crate engine,so will assume the engine is sound.

Check the ignition timing-this is important!

Change the PCV valve to eliminate the possibilty of a vacuum leak there.

Is this the 1971 carburetor?

Popping through the carb indicates a leal condition.

Check the accelerator pump.Remove the air cleaner,hold the choke plates open,and open the throttle.

You should see a steady stream of gas squirting in to the primarys.

If not then I would replace the carb if it's original.

Is the fuel pump new?

If the new engine never ran right then probably some old part was bad.

Hope this helps.

Sounds like it will be an easy fix.

Popping is not a cause of the carb. it is caused by open valves when they shouldn't be, if there is a slightly radical cam in it this will happen, if not check the timing and I see you have checked the advance..

Your next stop would be to ceck the compression. I am looking for low pressure that would be caused by anything in the head from worn valve guides to poorly seated valves. since the timing can be manually adjusted I bet you set it slightly BTDC try as a test moving it towards TDC a couple of degrees if the popping lessens then you may have a worn timing chain. That's an easy fix. I am looking at intake valves either stuck partially open or closing late. Hope this helps and leave the carb alone it's clean as a whistle

its could be starving for gas. check the rubber lines at the tank to frame and from the frame to the fuel pump for cracks, sucking air. also the strainer in the tank could be stopped up killing your fuel flow. check the vacuum advance on the distributor. take off the line at the carb and see if you can suck the advance back and hold it with you tongue to make sure it is working. don't be squeamish, this is shade tree stuff. a stopped up exhaust can cause the same problem. a loose worn out distributor, wobbling bad, will also cause this.

verify the rigidity practice-with the engine out of ingredients (stopped)are you able to change the shaft coupling by way of hand? If no longer possibly the grease contained in the strict tube has solidified or it is over tightened-whether that could be a stern rigidity attempt turning the prop by way of hand (engine out of ingredients and stopped) if there is extreme resistance the project is contained in the leg.

Carb backfires is the #1 sign of ignition timing off or worn cam lobes. Other things as well lick broke rocker arm and what not, but a broke rocker or spring would cause a pretty bad rough idle cond.

Accelerator pump?