[78T/A462]

I just installed a rebuilt 400 in my dads trans am. I ran it for 20 minutes and had the rpms bewteen 1500-2200 to break the cam in for those 20minutes. At that time it was missing and had exhaust leaks coming from the heat crossover bewteen the heads and the intake. After the cam break in I changed the oil, filter and plugs and attempted to find the misfire. I set the motor at tdc, checked the rotor position. checked firing order and even re adjusted the valves to be on the safe side and the same misfire. Beings the heat risers weren't filled with aluminum I cut a very thin piece of sheetmetal and placed some gasket sealer on it, slid it right where the heat riser is on the head and it stopped the exh leak. Does anyone have any suggestions of what the misfire could be? Flat cam? Could I have burnt a valve with the exh leak during the cam break in?? Been on it for a week and im pretty frustrated already...Thanks for the help.

[prostreet64]

Is all of this going on at idle/low speed rpm's? By 'misfire' do you mean that the engine is running rough like maybe a cylinder is out? What condition are the plugs in and what type of ignition system do you have? Pretty easy to check and see if you wiped a lobe by measuring the lift at each pushrod. What cam and springs? Have you driven the car yet or does this misfire occur while the engine is not under any load? Just thinking out loud to get the diagnostics going.

[78T/A462]

This goes on while the car idles or runs. It runs rough like a cylinder is out. The plugs are brand new ac delco. It has a hei system. Cam is a comp cam hydraulic, [email protected] and .560 lift. I have pulled it out of the driveway and than back in the garage and it does the same thing whether it's at idle or in gear.

[Malky]

A compression check will tell you if you have an internal problem like a burnt or stuck valve. In my experience, good results remove the nagging doubt that it is something serious and allow you to relax and concentrate on ignition and fuel.
Either a timing light or one of the various inductive gadgets that are held against each wire will confirm the ignition system is working.
Double check the wires are hooked up right.
That's a big cam. What carb are you running?

[goquick]

What kind of distributor do you have? Some msd's will burn the feed wire inside, but not always to the point the engine won't start.

[lust4speed]

Wouldn't be the first time a bad plug got through the inspectors. With the engine idling, pull a sparkplug wire from the distributor and go around pulling wires until the engine rpm and sound doesn't change. That should be your bad plug. Pull the wire from the sparkplug and it will knock you across the room, but pull it from the distributor and it shouldn't bite you - and if it does then that wire is probably bad.

[prostreet64]

Did the breakin period seem 'normal'? Any indication of a problem or was it first noticed when you idled down to set timing and so forth? Hopefully it is just one plug or wire that is bad but I think you need to check the lift on each lobe to be sure you haven't wiped the cam. When you first began the breakin did the motor fire immediately and I assume all the proper precautions were taken to breakin a flat tappet hyd. cam?

[lust4speed]

Yea, but the tapping should be heard long before it gets bad enough to misfire. Basically the valve is still opening and doing its job, but just not opening as far because of the excess clearance. But that does bring up another point that he might have a valve adjusted way too tight and hanging open, and this could also cause a misfire.

[78T/A462]

The distr is a factory HEI system. The car took a little time to fire at first but fired up. I will try the spark plug wire thing and a compression check. The carb is a edelbrock 750, not my choice, was my dads but seems to work good. When I try to idle the car lower than 1500rpm it starts to slowly die. I did try adjusting the valves again to zero but the same thing. There was no real tapping real loud except for the exh leak out of the heat riser. BEsides that the rockers were decently quiet.

[78 GHOST]

I agree with the compression check, spark plug check, and the other good ideas mentioned. In fact, using a leakdown tester would probably be better if you have one handy.

Have you verified that you aren't suffering a vacuum leak?

Anytime an engine has trouble idling at a reasonably low speed, it's possible you're suffering a vacuum leak.

Using your hand, partially cover the primaries choking off some of the airflow. Ordinarily, the engine should stumble and begin to die. If you've got a vacuum leak, the engine will increase RPMs a bit. This happens because you're enrichening the mixture by reducing the airflow with your hand.. the vacuum leak is providing too much air and a lean condition. I had a car which did this on one specific cylinder.

Good luck and keep us posted.

[Geoff]

A 256@050 hyd cam in a 400 engine? Not surprised it runs rough, especially if it is low on compression ratio. All manner of problems I can think of with a cam of that size: PCV valve open from insufficient vacuum. Engine blubbery rich at idle because the metering rods in the carb are in the rich position. You will probably need the softest springs in the Edelbrock catalog for that cam, may be even softer. The carb will also need the idle circuit to be recalibrated. Plugs are probably loading up because of the rich idle, one or two may not be firing causing the roughness that you are describing. If they are covered in black soot, they are probably misfiring. You want NGK plugs in a 5 heat range, projected nose core. To get the car to idle without overly loading up, remove the carb metering piston springs & drop the piston/rods assys all the way into the holes. Block the PCV hose. With clean spark plugs, you should at least get a stable enough idle to check for a cylinder out, but the engine is probably ok, just needs the above to be sorted.