390 Strange Symptoms

[440sixpack]

Helping a friend try to figure out what's going on with his 390.

It's a fresh rebuild with about 500 miles on it.  everything was done right as far as we know, new everything and a pretty stock build. flat tappet RV cam.  new Holley 600.  new control module.

Ever since t was fired up it's had a bit of a vibration, under acceleration it almost feels like a miss to me.   other than that it starts and runs just fine.

We rechecked the firing order and pulled all the plugs.  # 1 was black,  soot not oil.  #7 was white.  the rest look normal. 

Put new plugs in it and  nothing changed.  all plugs are getting good spark.

Any ideas? 

[machoneman]

Pull the air/fuel mixture screws out. Use carb cleaner with one of those plastic nozzle extensions. Shoot into both holes. Re-install screws  backed out to the best idle speed.

Point is dirt/crap could be blocking the four bleed holes inside the primary side bores.

[440sixpack]

It's a brand new Holley.   I suppose it could be defective but I'm sure it's clean.

I'm saying it's a miss, it could be something out of balance.   it's much worse on acceleration , the rest of the time you can feel it but you wouldn't notice it if you weren't paying attention.

[machoneman]

Swap on a known good Holley. Goal is to eliminate the obvious.

[440sixpack]

Started out with an Edelbrock and it did the same thing.

[fastf67]

With number 1 plug black soot and number 7 white it will definitely cause a stumble. Number 1 (dry soot correct? not greasy at all?) sounds like a cylinder not lighting off completely and number 7 being white is the scary one with possible water entering cylinder or lack of fuel. If soot is greasy then you know the answer. I would start with the basics, check firing order then pull valve covers and distributor cap. Roll engine over watching valvetrain is moving correctly with firing order. If good, for #1 black soot, check for a weak spark. cap, rotor, wires, gap on plug, ect. Something is killing the spark, water or oil normally wont leave it a dry black soot. Hard to believe its getting more fuel then the other 3 cylinders assuming it is a dual plane intake if carb was way out of adjustment (balance). #7 white, water or lack of fuel? Ya, duel plane thing again. No white exhaust smoke? wipe finger in tailpipe? Still good? Bring piston to the top and use paper towel to check for possible water. if sure all is dry remove carb and check for obstruction (shop towel?) it's been done before. Would also be a good time to re-torque head and intake bolts also. Dual plane intake is cyl: 1,4,6 and 7 off one side of carb 2,3,5 and 8 on the other so thats why I dont think carb, one black one white on same side of intake?. Just what i would do before digging deep.

[Barry_R]

Got a valve hanging open or a bent pushrod?  That'll make them shake like a wet dog.
Pull the rockers off & put some air pressure into each hole.
You can listen for it to whistle out the exhaust pipe - or into the intake at the carb.
And it gives you a chance to eyeball all the pushrods.

[440sixpack]

The black one is on the opposite side of the intake of the white one.

They're all getting spark, new wires new everything.  when you pull the wire off you can hear the idle slow on all cylinders. the power is not at all impressive from this engine, but it's not a hot rod build so maybe I'm expecting too much.


 I suppose it could have a hanging valve.  but it sure doesn't pop through the carb or backfire. or make any odd noises. guides and valves are all new, which of course could be sticky.   we could  pull the covers and look though.

[Joey120373]

Did it get a new intake manifold?

I once had a costumer with a freshly built sbc 383, had a terrible miss. Went round and round trying to figure it out. It ended up being a chunk of the sand casting about 1 inch thick stuck in the intake manifold runner. It was well up in the port, couldn't see it from the carb side, and could barely see it from the head side if you held it just right. It was a brand new Edelbrock, and though its a long shot, or a no shot if he re-used a known good manifold, just goes to show what kind of weirdness an sometimes happen.

Have you done a compression check ?

[440sixpack]

No it's the same Edelbrock intake he had before.

The bent push rod idea has me going I think we'll pull the valve covers.  I assume the white plug would be most suspect ?

I bought a brand new '79 F-150 when I was 18 and  it bent a push rod when I test drove it.  they fixed it on warranty of course but about every 10,000 miles it would bend that push rod again.  I carried everything to fix it under the seat.  I never did figure out why it did it.  this rig is running a lot like it did with a bent push rod.

[440sixpack]

Well it's not on the top end.  everything looks perfect .  we cranked it and checked all the travels and valve action and everything is perfect.

A black plug maybe I can understand.  a white one I can't.    there are some deposits on the electrode so it's not totally  clean.  this thing has me stumped.

[1968galaxie]

Compression test and or leakdown test.
Both are easy to accomplish.

[Heo]

Vaccum leak?

[My427stang]

I had a PI that had some internal problems after a repaired exhaust crossiver that did some weird things.  Never figured it out, swapped the intake and all acted normal

Could be a vacuum leak at the gasket, could be a crack or porous area somewhere.

I agree compression test wouldn't hurt.  Also, Mopar truck trick was to block all the vents and PCV and see if the engine builds vacuum at a valve cover at idle.  Doesn't always work but late 90's Mopar trucks would have a failed plenum gasket and you could find it that way

[fryedaddy]

i had a similar problem,mine was a bad intake seal-gasket,only affected one or two cylinders

[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.