1972 chev chevelle 350 mailibu misfire?

Question

I bought this 72 chevelle it starts up fine but is misfiring somewhere and trying to narrow it down to which cylinder the misfiring is coming from. the reaction is as the rpm climb over 1500 the carbeurator pops from the front left barrel,( the carb is a 4barrel high rise holley) ive had the timing adjusted and didnt work. the engine was recently built to drag but the owner didnt want to mess with it anymore i bought the car thinking i could fix it but ive never delt with an engine like this. the car doesnt back fire fromt the exhaust, anyone know if it could be a valve or cylinder or maybe the carburator itself problem? like i said, it will start fine, no problems at idle, i can move it but wont go very far, the poping will get louder as the rpm get higher. Anyone have some good troubleshooting advice?

Answer 1

I'm assuming that you have checked the valve timing? If the valve timing is OK, you've got a sticking intake valve.

Answer 2

make sure the plug wires are correct. then check the value adjustment. i two engine i build the same way . you have ensure everything is right.make sure the cam is in the right place,timingwise. do not take anything for granite.

Answer 3

it would be kind of hard to pinpoint the cylinder because of the fact that you are running a dual plane intake intake more than likely. this means that the two barrels on the left feed two cylinders on the left and two on the right. there is also a chance that you have damaged the power valve in that carburetor. i could not tell you which cylinder is at fault.  

Answer 4

sounds like you have an intake valve sticking staying open too long. You could have a bad lifter, bent push rod, or a screwed cam lobe

Answer 5

I would do the following: Remove both valve covers and have someone bump the starter as you watch the valve action taking note of a valve that may be sticking in the guide. Also watch each rocker arm for a full range of motion.

If you don't see anything out of the ordinary are the rocker arms  adjusted properly. If so go to the final test:

Remove each rocker arm and install a 0-1 dial indicator at the end of each push-rod (one at a time) bump the starter over two revolutions to check the intake and exhaust lobe lifts. It's quite possible you have a lifter that's stuck inside the lifter bore or a cam lobe that's wiped out. In that case the motor comes out and the block stripped down and a motor shop should check the clearance inside every lifter bore and or the cam and lifters replaced.

I personally have seen quite a few professionally built racing engines lately with lifters stuck in the bores. As it turns out the problem was traced to flat tappet and roller lifters that were slightly out of round.In one instance the valve stem was stuck inside of the valve guide.