5.2 magnum options

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vitamindart

vitamindart

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ok, I put a 5.2 magnum in my 66 valiant. it has a 904 w/a small stall 22-2400.
I have a miss that wont go away. swapped carbs, and the entire ignition system with zero change. it was rebuilt before I got it ( about 8,000 miles ago ). I have a feeling this is why it was pulled. im thinking it has a sticky valve or two.
do I just upgrade to the EQ heads? will porting help the stock lower end?
any suggestions for a cam? it currently has a larger than stock comp computer cam ( not verified )
I am running a 600 holley on a air gap knock off intake and have a set of shorty headers to install when it goes back in.
really just looking for a cruiser with some grunt that we can enjoy. car barely was used last year because of the miss.
 
vitamindart said:
ok, I put a 5.2 magnum in my 66 valiant. it has a 904 w/a small stall 22-2400.
I have a miss that wont go away. swapped carbs, and the entire ignition system with zero change. it was rebuilt before I got it ( about 8,000 miles ago ). I have a feeling this is why it was pulled. im thinking it has a sticky valve or two.
do I just upgrade to the EQ heads? will porting help the stock lower end?
any suggestions for a cam? it currently has a larger than stock comp computer cam ( not verified )
I am running a 600 holley on a air gap knock off intake and have a set of shorty headers to install when it goes back in.
really just looking for a cruiser with some grunt that we can enjoy. car barely was used last year because of the miss.
Click to expand...

Try a compression check, to rule out a bad valve job. EQ heads sound great, as is. I would always get a good valve job, worth every penny. I used to rebuild a lot of engines. "Rebuilt" engines, I never trusted and with good reason.
 
compression check was ok 150+ on all cylinders dry and cold even.
just pisses me off I had the pan, intake and valve covers off and peeked around all looked good. should have suspected something when even the guy who rebuilt the heads said this was the first set of magnum heads he didn't find a crack in at least one head. I have a feeling the 8,000 miles were very Hard miles.
 
What do you mean, "a miss?"

What RPM, one cylinder, intermittent, stutter, etc etc etc Does engine load affect it?

have you looked over to make sure all the valve gear is intact, and that you don't have a flat lobe, etc?

You say you changed ignition, but WHAT ABOUT the supply VOLTAGE? Maybe you have voltage drop in the wiring TO the ignition.

Fuel supply? Pressure?

Way way WWWWAAAAAYYYYYYyyyyyyyyyyyyyyyyyyyyyyyy too many things left lying about here, before diving right into a major engine rebuild.
 
Comp test, leak down test, cylinder balance test.

When/where does it miss? Idle, under load, accel, etc?

Swapping cam and heads is a great idea, we can help you with that but first, solve miss.

You can call the shop if you need help with diagnosis, my name plus dot com.
 
Comp test, leak down test, cylinder balance test.

When/where does it miss? Idle, under load, accel, etc?

Swapping cam and heads is a great idea, we can help you with that but first, solve miss.

You can call the shop if you need help with diagnosis, my name plus dot com.
 
miss is worse cold, under 2500 rpm, load or no load. seems like its more than one cylinder not a steady rhythm. I am not sure I don't have a lobe or lifter failing ( roller cam ) that is part of the reason for the tear down.
it can be hear, felt & seen by the shaking of the car. in gear out of gear, cruising all the same.
it may still miss above 2500 but at that rpm its hard to hear/feel.
I haven't looked at the voltage while driving but while in the garage there is 12+ at the module/distributor or MSD box when it was in the car.
I have built, played with and tuned several cars. this one is driving me nuts! I seriously suspect a valve or cam issue. but the fact that it bounces around seems odd.
 
One bad valve can disrupt other cylinders by screwing with the flow.
I had one bad one once that you would swear only half of the cylinders were firing.
 
bent pushrod. probably more than 1. They do not take much abuse if stock magnum ones.
 
Pushrod is an easy check. Just look at them/pull them and roll them on a piece of glass. Lobes are pretty easy to see too, just pull the covers and rotate the motor over, looking at every rocker. Hydro rollers will colapse with no oil pressure after a few minutes, but it should still be evident. You got a computer cam in a carbed motor, not the most efficient but its what you got. You sure your plugs are 18436572 CW and no arcing? A transposed cylinder will be worst at idle. You could always put the timing light on each individual plug lead and watch for any misfires at idle. That and a compression check, thats pretty easy to do with no disassembly.
 
If you dont have a way to do a cylinder balance test, which drops one at a time, and compares RPM drop between each cylinder, an old track trick when your cylinder balance tester is back at the shop is put water in a spray bottle and with engine idling, spray water on plug wires, coil wire, etc. If you have a bad wire or cylinder, you will get a nice lightening show.

You can also pull wires off cap and loosely install them so with a well insulated pair of long pliers and engine running, you can remove each wire to see if it gets worse or doesnt change. Then you will know which cylinder to look at.

Get some carb spray or something that burns well and spray around intake mating surface, carb base, vac ports etc. Look for change in RPM as you spray. That will find any vac leak you may have.

The factory cams are not known for going flat as they are a roller. I would think more like broken valve spring or bent push rod.

Running worse when cold makes me think intake not quite sealing but expands as engine heats up. Look at intake gaskets to see if evenly crushed.
 
What cylinder is it missing on?
 
i cant tell which cylinders are missing, I have tried the carb cleaner on the intake ( no change ) I also tried the water mist on all the wires ( no change ) the cam was in the engine when I got it ( so I ran it, its a Comp computer cam ) I have tried pulling a wire at a time but can never identify any one cylinder worse than any other.
I tried moving the timing light from wire to wire, it is almost like they all miss on occasion.
that is part of the reason I switched plugs, wires, distributors and even the carb chasing
this miss.
 
ok, stupid question is it possible I have an internal vacuum leak ( under the intake ).
could that cause the erratic misfire?
 
vitamindart said:
ok, stupid question is it possible I have an internal vacuum leak ( under the intake ).
could that cause the erratic misfire?
Click to expand...

It is possible to get an intake leak on bottom of intake mating surface, which will cause high idle, poor acceleretion and typically, will suck in oil causing engine to smoke and use oil. This, if minimal, could possibly get better when warmed up as intake swells with heat but kind of a long shot. The intake gasket compression would tell you if that was happening or not.

You have never mentioned using a vacuum gauge on your engine, have you done that? What do plugs look like? Rich, lean, oil spots?
I am going back to old school tuning/diagnosis stuff here which needs to be done to find problem.

Another question but needs to be addressed, which pieces have worked previously, carb, dist, coil, intake, etc? Did this engine ever run right?
 
I have put a vacuum gauge on it. it was surprisingly steady. plugs look ok, a little rich but not bad. no oil spots, doesn't use a drop. ( what little I have gotten to drive it )

the carbs were both new to me, same with all the ignition pieces.

honestly, this engine has had a miss since I installed it in my car. I was unable to hear it run when the previous owner had it.

the gasket looks to be compressed evenly ( intake )
 
i've used the spray bottle trick reccomended by magnum mopar , on my car before and it works great , also try it when its dark out ,helps to see sparks, mine was two part problem spark plug wires that were grounding on headers,and the main plug on the motor that has the coil and ballast wires in ,a lot of the connections were corroded my misfires went away after repairs, good luck rule out wiring before mechanical.
 
Sounds like intake is not problem, nor broken valve springs as they make vac gauge jump around. You need a known good carb and ignition. Even if you swap dist out for points, its easy to bypass the MSD. If you are running factory oragne box, swap it for know good, very common failure. Newer msd go bad often too. If you remove all unknown variables then the problem will present itself to you easily. Got a buddy with some parts you know are good, try them out. Sounds like you wont even have to drive it to figure out if its fixed so dont bother with throttle linkage or kick down, bolt it on and start it up. Take you timing light and check for constant spark on each wire, if not swap out box.
 
i dumped the msd box, I have a re-man dist. and the GM 4 pin module ( petronix version ). never thought about it being bad, I have a stock 4 pin module and different dist. I can swap in. I will give all the wires a good look over as well.
maybe I will even just run a temp. hot wire from the battery to the ignition just to rule that out.
Magnum, do you have a suggestion for a cam upgrade in the 318?
 
magnummopar said:
If you dont have a way to do a cylinder balance test, ...........................

You can also pull wires off cap and loosely install them so with a well insulated pair of long pliers and engine running, you can remove each wire to see if it gets worse
Click to expand...

NEVER EVER pull wires off like this, for a couple of reasons. One is the reflected high voltage pulse can destroy ignition systems.

The second reason is that the fire can "crossfire" and lead you down the wrong path as to what cylinder might be missing

Rather, use a shorting probe to SHORT the voltage on each cylinder. You need two hands, one with the probe and one with insulated pliers. Pull the wire up out of the dist, and as it comes up, insert your grounding probe

vitamindart said:
I tried moving the timing light from wire to wire, it is almost like they all miss on occasion.
that is part of the reason I switched plugs, wires, distributors and even the carb chasing
this miss.
Click to expand...

A timing light won't tell you squat, especially with MS systems

HAVE YOU checked the wires and the plugs, or changed them?

Also........Google and check ROTOR PHASING
 
67Dart273 said:
NEVER EVER pull wires off like this, for a couple of reasons. One is the reflected high voltage pulse can destroy ignition systems.

The second reason is that the fire can "crossfire" and lead you down the wrong path as to what cylinder might be missing

Rather, use a shorting probe to SHORT the voltage on each cylinder. You need two hands, one with the probe and one with insulated pliers. Pull the wire up out of the dist, and as it comes up, insert your grounding probe



A timing light won't tell you squat, especially with MS systems

HAVE YOU checked the wires and the plugs, or changed them?

Also........Google and check ROTOR PHASING
Click to expand...

x's2 The spark needs to ground somewhere other than back to the ignition box or coil. I clip a test probe to a ground and go around the cap, you can always ohm the wires and get some indication what shape they are in.
 
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