1973 Chevrolet TruckDear Ole Mechanic;

I recently purchased an old ranch truck.  It is a 1973 Chevy with an inline 250 cubic-inch six-cylinder engine with a four-speed manual transmission.  It seemed to run OK when I got it--considering that it had sat for several years.  The odometer only shows 48,000 miles, which the fellow that I bought it from said was accurate, because his Dad bought it new.  He also told me that the head had been “reworked.”  After I put about 500 miles on the truck, it started missing badly.  I pulled the spark plugs and number six was fouled out with black, oily-looking stuff.  I put in a new set of plugs and it ran OK for about another 400 miles.  Number six had fouled out again, so I cleaned all of the plugs even though the other five had only a light-tan color.  I also put anti-fouling adapters on all six plugs and reinstalled them.  Now, as I try to pull up a hill, the engine wants to buck and snort.  Out on a level highway, it runs fine and it idles OK too.  What the heck is going on?

Don’t Run Right
 

Dear Don’t Run;

I am not sure that I can give you a clear-cut answer to this, but I can give you a number of suggestions to chase down, isolate and fix the problem.

First, check for vacuum leaks.  This applies not only to vacuum hoses, but carburetor and manifold gaskets as well.  A vacuum leak allows more air into the engine without adding the needed fuel, causing a lean air to fuel mixture.  A lean mixture could cause the engine to misfire, resulting in "bucking and snorting."  Usually, a vacuum leak causes poor idling in addition to "bucking and snorting;" but, you say that the engine idles OK. However, I have seen stranger things happen with a vacuum leak.

No vacuum leak?  Next, I suggest that you remove the anti-fouling adapters to see if they are causing the poor hill-climbing problem.  If they are, then replace them only on the plugs that are fouling out.  At least, replace them until you find out what is causing spark-plug fouling and correct that problem.  Due to the age of the truck, I suspect that the valve-guide oil-seals are shot or they may have been incorrectly installed (or left out) when the head was “reworked”.

Still won’t pull a hill?  Now it is time to check the ignition timing with a timing light.  First, check the base timing, with the engine idling and the vacuum-advance hose disconnected and plugged at the distributor.  It should be somewhere between six to ten degrees BTDC (Before Top Dead Center) for your engine.  If that checks out OK, then rev up the engine while watching the timing-mark on the crankshaft pulley.  The RPM's should cause the centrifugal or mechanical advance to advance the mark on the pulley.  If it doesn’t, then you need to check and free up the advance-weights and springs in the distributor.  If that checks out OK, then it is time to check the vacuum-advance.  With the engine still at idle, watch the crankshaft pull mark as you unplug and put the vacuum hose back on the vacuum advance.  The timing mark should advance.  If it doesn’t, then check the advance-chamber for a vacuum leak and the hose for blockage and make sure that the hose is connected to the correct vacuum connection on the carburetor.1973 Chevrolet Truck

If all of the above check out OK and the truck still don’t pull satisfactorilly, it is time for a compression test.  Pull all of the spark plugs out and run a compression test on all of the cylinders.  Don’t forget to ground the coil wire or disconnect the distributor and hold the throttle open while running the test.  The compression of all cylinders should be within ten percent of each other and be around 100 PSI.  If they are not, then you are probably in for some serious engine work, such as pulling the head and grinding or replacing valves or tearing the engine down to do a ring job.

If everything else checks out, then it is time to disassemble, clean and overhaul the carburetor.

Covering the repairs needed for low-compression or overhauling the carburetor will require a textbook, not a newspaper article.  Good luck.

Herr Professor Nuzanbolts