exhaust popping advice needed 3

[rymar]

I have a new 350 chevy engine in a 84 chevy pickup. I HAVE A POP OUT OF EXHAUST WHEN I RAP ENGINE UP. WITH A TIMING LIGHT ON IT THE MARK ON THE BAL. WONT COME CLOSE TO TIMING POINTER. I DROPPED DIST. IN AND OUT ABOUT A DOZEN TIMES IT DOES NOT HELP.I EVEN INSTALLED A NEW TIMING SET IN THINKING THAT TIMING CHAIN WAS INSTALLED WRONG IT WASNT.TRUCK IDLES OK AND HAS GOOD POWER BUT AT BASE TIMING IT SHOWS ABOUT 50 DEGREES OR BETTER. TIMING MARK ON BALANCER IS BEHIND WATER PUMP.DO YOU THINK I HAVE A DIST. PROBLEM OR CAM ISSUES OR VALVES. WITH A VACUUM GAUGE HOOKED UP IT SHOWS AROUND 15 WITH SOME BOUNCE IN NEEDLE.

 

[MotionMetal]

Generally after you install a reman. set the timing and break in the cam. Change the oil, pull the valve covers and check the preload on the hydraulic lifters. This may be causing your low and unsteady gauge reading. As for your timing mark, with the #1 plug out bring your piston up to TDC using a tool that you wont lose in the hole. Check the mark on your balancer and If its not within 2-3 degrees from TDC than either the balancer is damaged or your on the wrong bank. Hope this helps

 

[Fabrico]

Yes, check that balancer mark against real TDC.
Are you disconnecting any vacuum advance during tests?
Did you check the plug wire order one last time?
Are you sure you are on the No. 1 plug wire?

 

[franzh]

You did not state if the pop is during acell or decell.

Popping in the exhaust during decel is caused by air and fuel burning. If your engine has an air-pump, it could be discharging into the exhaust during decel, a guaranteed bang.

If it is a light popping and not a backfire, your fuel mixture is too rich and bleeding fuel during decel. Also, if the exhaust manifold has a large crack or gasket leak, it could ignite a rich unburned mixture in the manifolds.

Popping during acell is typically caused by ignition failure, usually plugs or plug wires, or a VERY lean fuel mixture.

Franz

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[patdaly]

In either 84 or 85, Chevy changed the Balancer and cover location of TDC. If you mix and match generations, you get exactly what you describe.

Check actual TDC against the balancer.

 

[TheBlacksmith]

A friend of mine did this once by mixing parts on a small block Ford, so you wouldn't be the first.

 

[larrycoyle]

Another thing you could be hearing is exhaust noise coming through the valve cover. This would be caused by the exhaust valve guide being worn pretty bad but I have seen this on many occasions.

Larry Coyle
Cylinder Head Engineering

Larry Coyle
Cylinder Head Engineering, LLC

http://www.cylinderheadengineering.com

 

[turbocohen]

HMM,, sounds like a rotor pickup wire doing an Al Gore.
Check the toothed pickup outside of the rotor shaft under the weights for a fractured wire. The two wires that go into the pickup break over time due to the back and forth movement of the vacuum advance. An 84 HEI distributor pickup coil wire should defintely be shot by now.

It is easy to change.. drive the pin out of the cam/oil pump gear and soak the shaft with liquid wrench to soften the buildup inside. spin the shaft as you remove it. It can be a hassle to remove due to deposits.

Good luck with your inconvenient mission.