47-55.1 51 Chevy 3100 w/ 235 - MIssing/running rough

[duramaxguy]

My '51 has always been the smoothest running vehicle I have. Fires right up and runs smooth at all RPMs.

Coming up the hill the other day, it started surging and coughing (not backfiring) and lost most of its power under a load.

My initial thought was a fuel blockage of some type. I replaced the fuel filter, fuel pump, and have rebuilt the Rochester B carb. No effect.

Now I am suspecting an ignition or mechanical problem of some type. All plug wires are secured and the cap/rotor are in good condition.

Suggestions of troubleshooting steps?

Thanks in advance.

[Foot Stomper]

Start with the basics, not fuel related because most times rough run is electrical and not fuel...most times. If you can rule out bad fuel then look to ignition. Working carbs never just go sideways!

Start by confirming timing, timing advance, points and condenser next, then coil. Coils can (and do) fail under load. Then wires and cap/rotor and then plugs.

I had a condenser fail on me and chased that pr!ck for awhile!

Don't worry about muffler bearings, flux capacitor or blinker fluid, stick with a solid electrical diagnostic and I'm sure you will find and solve your problem.

[dsraven]

check the engine compression, check distributor vacuum (including the actual vac hose/tube) and centrifugal advance mechanisms for binding and proper operation. rotor and cap for wear, corrosion, carbon tracking and integrity of the center electrode. plug wires on both ends for poor insulators and/or wire crimps etc, plus the actual wire for chaff marks or carbon tracking or excessive stiffness or a soft spot indicating a partial break in the wire at that point. spark plugs for proper gap, wear on electrodes, deposit build up and carbon tracking. ign coil for any oily residue indicating a leak, spark output for type, color of spark with a spark checker and acceptable max length of spark. valves for lash and any possible valve train issues. timing for correct base and max advance. intake manifold for leaks or missing vacuum plugs or cracked vacuum fittings. fuel pump for output and pressure. engine vacuum to be in acceptable range. exhaust system for restrictions. carb for securement to intake manifold, possible contamination in bowl, proper float adjustment and choke operation. fuel filter for sediment. gas tank cap for proper venting. heat riser operation to ensure a full sweep of the plate with no sticking and also proper operation from cold to hot engine. throttle linkage to ensure full throttle is available. air filter for restriction.
a lean mixture is harder to fire so the ignition system can break down simply because it is working harder to fire the plugs due to an intake manifold or vacuum leak. when the engine works harder it can also show problems otherwise hidden from a partially plugged exhaust system. also the heat riser valve operation can act like a restriction in the exhaust if it is stuck partially closed. rattle the muffler and listen to ensure the baffles aren't loose inside and moving to close the outlet under high flow conditions. sometimes a dark area will show a bad ignition system is shorting a plug wire when you see the spark arcing (that you couldn't see in the daylight) to another ground source. ensure to pull the wires out of the cap and coil and check the hole and the end of the wire for issues. a ballast resistor can also cause issues if it is compromised. a poor contact in the ignition system somewhere can also leave you scratching your head. a few voltage tests at key places can reveal low voltage conditions. check the fuse panel for corrosion or bad connections, check the battery connections and body/engine grounds for integrity. check the contacts on the ign points by running a clean business card through and looking to see what is stuck to the paper on the card. like said, a simple thing like an ignition condenser could throw you off if it happened to give up the ghost and sometimes they will; break down under heat. check the connection/insulator in the distributor where the wire connects to the points, sometimes the insulator is cracked or the wire is broken inside the insulation. a tug on that wire should not equal a longer or stretchy wire.
sometimes things run so reliably on everyday business that we tend to neglect or forget a few basic things. then when we make them work hard they show issues. upon checking we find out we hadn't serviced them properly for a long time and are shocked at what we find. I know it has happened to me before and I look back and think wow, I didn't realize it was that long ago since I did that whatever.
hopefully you find your little issue and these hints helped somebody