[JohnIL]

I put Tune-up in quotes because it was more like troubleshooting than a actual tune-up. Lots of things were out of whack. When I first got the truck home, it ran rough. I pretty much ignored the way it ran because of the obnoxiously loud exhaust. I just concentrated on other things. After a few weeks, I thought maybe it might be nice to actually drive the truck, so I had to address the lousy idle and the horrible throttle response. What I found was a whole new list of sins by the previous owner.

I don't know exactly what led up to this problem, but the truck was only running on six cylinders. The back two cylinders (one on each side) were completely dead. There was zero spark on those cylinders. Obviously, the previous owner didn't know that. In order to make it somewhat drivable, he jacked with the timing and the carburetor. It was running WAY too rich and timing was advanced to the point that it would run for 5-10 seconds after the ignition was turned off. I traced the spark issue to plug wires. There was definitely spark at the distributor cap (I forgot just bad it hurts to get shocked by an HEI ignition). Anyway, a new set of plugs and wires got it running on all eight cylinders again. But, that made the run-on worse and it was running so rich that exhaust burned my eyes.

I attacked the timing next. The previous owner had the vacuum advance connected to ported vacuum. This added a ton of timing at idle. This explains why the engine refused to die when the ignition was turned off. So, I switched to manifold vacuum and set the timing back to factory spec. That improved things dramatically. Now, the engine would idle and turn off on demand. But, it was idling high (over 1200 RPM) and it was still incredibly rich.

The carburetor was next. I lowered the idle as low as possible without killing the engine. That was just below 1000 RPM. Then, I backed the idle mixture screws all the way in to verify that the power valve wasn't leaking. The engine died, right on cue. Good, I can check the power valve off the trouble shooting list. Next, I returned the idle mixture screws back to factory spec (1 1/2 turns out). Immediately, the idle got better. It smoothed out and the idle rose to about 1100 RPM. I was definitely on the right track. So, I connected a vacuum gauge and adjusted the idle mixture screws until I got to peak vacuum (almost 16 inches of vacuum). The idle rose another 100 RPM, back up to 1200 RPM. I attempted to lower the idle down to something in the 600-650 RPM range, but it refused to idle down below 1000 RPM. The engine was running smoothly, but the idle speed just wouldn't come down. That's when I noticed that the previous owner's homemade throttle linkage was binding up on the carb's throttle lever. Ugh, he really made a mess of this carburetor. The throttle linkage was nothing more than a long piece of all-thread and he had cut it just long enough that it was hanging up on the 90 degree bend of the carb's throttle lever. It was an easy fix. I lopped off about 3/8" from the end of the linkage rod. There's still plenty of material left to adjust the linkage, but now it clears the throttle lever. The engine settled right down to 650 RPM. All good? Not quite.

Now that the engine was idling smoothly and exhaust wasn't killing all the bugs in the neighborhood, I had a different problem. The carb now had a severe dead spot at partial throttle. Each time I shifted gears, the engine fell flat. Past experience told me that it was probably a bad (or misadjusted) accelerator pump. That theory was reinforced by the color of the accelerator pump cam. The factory installed green cam had been replaced by a red cam. More evidence that someone had had their way with this carburetor. I could see fuel squirting in the carb when the throttle was opened and there was no fuel leaking from the accelerator pump. That verified that the pump diaphragm was good. Next, I adjusted the accelerator pump linkage. It was adjusted so tight that the diaphragm was preloaded. That didn't hurt anything, but it effectively reduced the capacity of the pump. That definitely won't help a partial throttle dead spot. With the linkage adjusted correctly, the dead spot was better, but still there. The weird part was that it got worse as the engine warmed up. I incorrectly thought that it needed a stronger shot of fuel, so I moved to a more aggressive pump cam. The dead spot got worse. I went the other direction with a less aggressive cam. The dead spot got WAY worse. Hmm, that makes no sense. So, I put the red cam back in and parked the truck for the night. The next night, I talked it over with a friend of mine who has been working with Holley carbs longer than I've been alive. He said, "It's a Holley. The power valve is blown, of course." The observant reader will notice that I already verified that the power valve was not blown. So, I was skeptical. But, I hadn't actually laid eyes on the power valve, so it was worth a try. The next night, I pulled the front fuel bowl and metering block. The power valve passed visual inspection and I couldn't blow air through it. OK, it's not the power valve. But, since I was in that far, I thought I might as well replace it with a new valve, just to make sure. When I backed the power valve out of the metering block, I found the problem immediately. The previous owner (I'm really starting to dislike that guy) had attempted to increase the capacity of the power valve by stacking three fiber washers between the valve and the metering block. Wow, just wow. The middle of the three washers had split and was intermittently leaking air/fuel. I think the valve was OK, but the effect was the same, a dead spot at partial idle. I replaced the power valve and installed it with a fresh fiber washer (only one this time).

That seems to have fixed the problem. Now, the engine pulls hard through the gear shifts. It almost makes me want to keep the small block instead of installing an LS... Almost. Now that the carb is behaving itself, and the weather is turning cooler, I needed to turn my attention to the electric choke. The previous owner (are you starting to see a pattern here?) disabled the choke by backing the spring all the way off. He said that he couldn't get it to idle when the engine was cold. I wonder why? The choke turned out to be MUCH easier to fix than I expected. When I went out to the garage the following morning, it was about 45 degrees outside, so the engine was plenty cold. I loosened the choke housing and rotated it until the choke just snapped closed. Voila! It worked! No further adjustment needed. Now, with the engine cold, the choke closes and the high idle comes up to about 1300 RPM until the engine gets warm. Finally, something that the previous owner didn't mess up. Not too badly, at least.