390 Strange Symptoms

[gregaba]

Had this problem a long time ago. Turned out to be a stuck heat riser valve in the left side exhaust pipe.
Greg

[Ranch]

Plug wires ok? Don't run them to close together, maybe change a couple around see if it effect different plugs....JMO  Hey it's free

[FElony]

Long time ago I had a WOT misfire that I could not pin down for a while. Turned out water somehow got down into one distributor cap hole, and the contact area between plug wire end and cap was jammed with green corrosion. Why only one, I'll never know. Changed cap and fixed.

Other problem I'm guessing is a breach on intake gasket. Last week I mentioned something about mismatching gaskets by port type/size, or not getting enough clamp from using end gaskets under the intake that may not play nice with shaved heads and/or deck.

[wowens]

1st post said rebuild. Also said 2 different carbs had same issue. Also said plug wires and cap are known to be good. Also said all valves appear to be operating correctly with covers off.
Are the heads the same parts from pre rebuild  ?
Non adjustable valvetrain ? We're heads milled ?
If heads were milled, was intake side also milled ?
Could valve preload be wrong now ?
Could be a vacuum leak or valve not closing properly,
A valve not closing completely can make other cylinders do strange things.

[440sixpack]

Valves are all working great.  preload was set properly.  spark is hot and getting to every plug.   no heat riser in the exhaust system anymore.

I thought about a vacuum leak, I'll check it again but after we broke the cam in we set the timing and the idle mixture and the vacuum was good and steady.


I put a rag slowly over the air horn and it dies.  won't it speed up if there is a vacuum leak ?

[gdaddy01]

if it is power brake car , maybe the booster . I had the problem with a 292 in a 56 t bird , pcv and brake booster hooked to the same intake runner .

[wowens]

Remove carb , look down intake runners with a bore scope.  Maybe rag in intake runner. In the mid 70's I worked at a Mercury dealer. We had a Cougar 302 that had a stumble like this and weird plug readings. After 2 valve jobs and a long block assy we put a water hose to each intake port. One port trickled water out carb pad. Cut it apart & that port had a casting flaw. Was a thin cast iron wall with a pencil sized hole in it. New intake fixed it.

[440sixpack]

I don't see how something could have got in there, but I won't rule anything out.  it's worth a look.

[440sixpack]

Pulled the carb and can run a vacuum hose down the runners a long ways, I think it's fine but I'm going to borrow a high qualitybore scope and check it out to be 100% sure.

when we pulled the carb I noticed something I don't recall before,  is it normal when you tip the carb, even 5 degrees for fuel to pour out the throttle plates ?  maybe it is but I don't remember that.

We're going to try a known good carb just for grins ,  I don't think that's the problem but we're grasping at this point.

[machoneman]

Pulled the carb and can run a vacuum hose down the runners a long ways, I think it's fine but I'm going to borrow a high qualitybore scope and check it out to be 100% sure.

when we pulled the carb I noticed something I don't recall before,  is it normal when you tip the carb, even 5 degrees for fuel to pour out the throttle plates ?  maybe it is but I don't remember that.

We're going to try a known good carb just for grins ,  I don't think that's the problem but we're grasping at this point.

No, it's not normal pretty much for any carb. Was that the Holley?

I ask as my bro' and I had a real hair-tearing out session about 5 years ago with his gennie '70 Boss 302 and a 780 vac. Holley.

On a hunch, the wet throttle plates and wet venturi areas had me pull both the primary and secondary needle and seat assemblies. I immediately knew something was wrong as both pulled straight up with ease and no resistance. Come to find that our gasohol here in Illinois affect the o-ring (shrank them) on outside of the needle/seat and allowed way too much gas into the venturis. New Holley branded o-rings and voila! Engine ran great. 

[shady]

Are you sure it is actually a miss & not a balance problem? Didn't use a 428 - 410 flywheel-flexplate?

[440sixpack]

Yes, a new Holley.


Same flex plate.   but it could be a balance issue I can't rule it out.   but that wouldn't account for the weird plugs.


I wonder if you can lift and slip the engine far enough ahead to change the flex plate if all else fails ?  this is a 4x4  so it's a lot of work to pull the trans and transfer case.