273 super commando #4 and #6 not firing

I owned a 72 Ply Valiant back in high school, sold it and now that I'm older I wanted to get one like it. I ended up getting my exact car back, lol only now it has a 273 commando in it or at least thats what the man told me. I don"t know much about that kind of motor. The car starts right up and sound good but has a slight miss, so I took one plug out at a time to find out where. Turns out both #4 and #6 are not firing at all. Plugs look fine no oil. I moved plugs and wires around from cylinders that were working and still no fire on #4 or #6 Cap and rotor look new. I don"t know what else to do :banghead::banghead:

Run a compression test on those 2 cylinders (or all cylinders if you like). It's not uncommon for a head gasket to blow right between cylinders causing both of them to die.

plymouthvaliant said:

I owned a 72 Ply Valiant back in high school, sold it and now that I'm older I wanted to get one like it. I ended up getting my exact car back, lol only now it has a 273 Super Commando in it. The car starts right up and sound good but has a slight miss, so I took one plug out at a time to find out where. Turns out both #4 and #6 are not firing at all. Plugs look fine no oil. I moved plugs and wires around from cylinders that were working and still no fire on #4 or #6 Cap and rotor look new. I don"t know what else to do :banghead::banghead:

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What is a 273 Super Commando?

The Mopar 273 was Chrysler's first engine in its LA series of small block V8s. The 273 was introduced in 1964 wit a 2-barrel carburetor and made 180 HP. In 1965, a high performance version of the 273 called the Commando was introduced. The 273 Commando had a hotter cam, 4 barrel carb, and made 235 HP. In 1966, a limited edition racing version of the 273 called the Super Commando was offered. The Super Commando was equipped with steel tube headers, a 700 cfm Holley 4-barrel carburetor, a high lift mechanical camshaft, and made 275 HP. Production of the 273 spanned 6 years, lasting through the 1969 model year. The low end version was replaced by the Mopar 318, while high output versions were replaced by the Mopar 340. I wasn't sure what to do with it since I'm thinking of replacing it. The more I think about it, the more I want to fix my problem or rebuild it. In spite of the small size, the Mopar 273 is a good engine. As the first engine in the LA series, it's also an important piece of Mopar history.

What do you have for a distributor ? Worn out original maybe ?
If the points dont open and close the plug wont fire.

i agree with redfish, chk the dizzy

pull plug 4 and 6 and lay them on something grounded and turn the motor over - if it sparks, it's not electrical - if it doesn't, you have a bad dist., cap, rotor, or plug wire.

Or just pull all the plug wires off the cap and poke 4 and 6 in different holes. If the 2 cylinders fire you know the problem is from the cap down.

plymouthvaliant said:

The Mopar 273 was Chrysler's first engine in its LA series of small block V8s. The 273 was introduced in 1964 wit a 2-barrel carburetor and made 180 HP. In 1965, a high performance version of the 273 called the Commando was introduced. The 273 Commando had a hotter cam, 4 barrel carb, and made 235 HP. In 1966, a limited edition racing version of the 273 called the Super Commando was offered. The Super Commando was equipped with steel tube headers, a 700 cfm Holley 4-barrel carburetor, a high lift mechanical camshaft, and made 275 HP. Production of the 273 spanned 6 years, lasting through the 1969 model year. The low end version was replaced by the Mopar 318, while high output versions were replaced by the Mopar 340. I wasn't sure what to do with it since I'm thinking of replacing it. The more I think about it, the more I want to fix my problem or rebuild it. In spite of the small size, the Mopar 273 is a good engine. As the first engine in the LA series, it's also an important piece of Mopar history.

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The "Super Commando's" were RB engines in the mid 60's. Plymouth never had a 275 hp. 273. Those were in the D Darts and would be considered "Charger 273's". If the engine is a true Commando it will have a dual point dist., 4-barrel AFB, domed pistons for 10.5/1 compression and a solid lifter cam and was produced with a chrome air cleaner and wrinkle finish valve covers with chrome caps. I believe it was in 68' the 273's were all 8.5/1 motors with hydraulic cams. As far as your engine miss,, everybodys' suggestions could be a possibility. good luck! toolman

i never saw a 273 with hydrolic cam shaft even the later 2bbl versions but that dont mean much

Thanks everyone. When I pull the new wires one by one from the new plugs #4 and #6 do not make any difference in the way the motor sounds. When I pull any of the other wires the motor sound like $hit. I am waiting on a compression gauge to check the heads. I don't understand how the motor could sound pretty good without two cylinders firing.

You are "sorta" on the right track but you should NOT just "pull" plug wires. This is because (if there is spark at that cylinder, and there should be) The spark voltage goes WAY up when one is removed, and looks for the "easiest path to ground", just like lightning

What this means is, that when you pull a wire, it just might "crossfire" to another cylinder, and confuse your results

The better way to do this is one of a number of ways:

Pull the boots up off the distributor, and if you have a small probe, ground each one by slipping the probe down beside the wire in the cap. If not, you can slip small brads/ nails down in each one before you start the engine.

If you have "guts" you can pull the wires out of the distributor, and then GROUND the distributor tower with a probe, before you make a decision.

Redfish was getting at "checking your wires." You can check spark at "each hole" by pulling a plug wire with insulated pliers, and then checking the spark to a grounded screwdriver, or pull them out of the distributor and let them arc to a grounded probe, held near the top of the tower

To check the wires without an ohmeter, once you identify suspect cylinders, ASSUME the wires and plugs are bad for that/ those cylinders.

SWITCH those "dead" wires with two other cylinders, and repeat your test. If the same cylinder(s) show "bad," it's not the wires.

Next, inspect the plugs on those holes, and either switch them with good ones, or replace with new ones, and repeat your test.

Some things to cause this:

Points worn so bad, that they are opening on only SOME of the cylinders, made worse by a worn distributor shaft/ bushings

Bad/ carbon tracked/ wet/ cracked distributor cap and or rotor

Bad plug wires OR plugs

Solid lifters that are too tight PAY ATTENTION THIS ENGINE SHOULD BE SOLID

Bad valves-- run compression check

Leaky, blown head gasket

jimjimjimmy said:

i never saw a 273 with hydrolic cam shaft even the later 2bbl versions but that dont mean much

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Just trying to educate Jimmy.

plymouthvaliant said:

Thanks everyone. When I pull the new wires one by one from the new plugs #4 and #6 do not make any difference in the way the motor sounds. When I pull any of the other wires the motor sound like $hit. I am waiting on a compression gauge to check the heads. I don't understand how the motor could sound pretty good without two cylinders firing.

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Shiiii, I drove my 273 4bbl "S" home on 5 cylinders and That was on the freeway! Im guessing head gasket too, as you know: no compression means no fire in the hole. Blown gasket may show bubbles in the radiator or some serious blow by in the crankcase.

I bought a 67 Coronet with a teen years ago. It barely made it home. I tuned it up and it still ran like crap. I warmed up the engine and trickled a pop bottle full of water down the carb. It blasted the carbon from the valves and it ran great. It was my daily driver for a couple years. Apparently the previous owner drove it for too long with a couple cylinders misfiring and carboned the thing up.
Note: I trickled not poured water down the carb. I would use a carbon blaster product available today. You might want to try it. tmm

Yup cold water will cause hot carbon to shatter like glass. Cleans valves, piston tops, everything. It can screw up the firing end of the plugs too.
Today there is a chemical product that is applied through a vacuum port. Simply attach the container to a hose such as the one leading to the brake booster. Lots of smoke out the tail pipe though so use it outdoors.