Black Widow Build (1960 1/2 ton)

[60ShortStpeGMC]

Although my last entry said "running driving truck", that's not quite right. Before the clutch went out I was tuning a Rochester 2jet, trying to get it to run on my single barrel manifold.

I was using a jinky adapter. I say jinky because it was a $14 item that, well, looked like it was produced as an after thought.


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Too its credit, the adapter did it's job. But I was experiencing hesitation during acceleration from a stop. Which, my research said, was probably a vacuum leak. Sure enough, the vacuum can on my distributor was shot. Replaced it.

Adjusted the points. Adjusted engine timing. Still hesitation during acceleration from a stop. Good 'ol hose-to-the-ear-trick revealed a vacuum leak at the base of the adapter.
Well, bummer, but not too surprising. The gasket there was very thin. In fact I had a second gasket on there that I'd cut out of thin cardboard.

Figured the ultimate test was to replace the gaskets with a gasket sealer. Yep, I did that. Very carefully. And only as a test - wanted to know if that would cure the leak. But, alas. Still sucking air - less than before, but still a leak. Still hesitating on acceleration. Uh-oh, looks like more than a vacuum leak...

Started thinking that maybe the adapter isn't a good match with the intake manifold. Figured I'd need a new gasket, a good one, and maybe clean each surface real good. Possibly check the base of the adapter, make sure it was "level" or flat or whatever.

That means removing the intake manifold.

Dang, I only wanted to remove the intake in order to replace it with the new-to-me 2 barrel manifold I picked up a while back. But, that manifold doesn't directly fit my 2jet either. Bummer. But heck, gaskets are cheap, right? Keep the two-barrel on hold a while, clean and adjust the surfaces. Fix the vacuum leak, then proceed to checking the carb.

But that means removing the intake manifold.

Browsing around the net I found some quality adapters that might actually work with my 2jet and two-barrel manifold. After much reading, the manifold fits a "small Rochester" carb as-is. Good to know. But my 2jet is the "large" variety. After more reading, turns out that

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sells an adapter for a "small Rochester" manifold to a "large Rochester" carb. Nice stuff too.

Luckily, I had the money. Bought a Dashman's adapter and ... the two barrel manifold is in action.


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Man, I had to fight my way through 40-odd years of gunk!


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Still, hope I'm not shaving a yak here. Even if the new adapter/manifold swap cures the vacuum leak, the hesitation on acceleration hasn't been totally addressed. Likely something with the carb. Since I rebuilt it, I wouldn't be surprised something's amiss. I guess it's time to break open the 2jet again.

[60ShortStpeGMC]

First thing I did was adjust the float.

Second thing, it seems like the original vacuum pump spring was a li'l bit softer than the new one. Very good condition still so I swapped 'em out. The neck of the new one seemed to hang up on the linkage it attaches to whereas the old one swiveled smoothly.
After all is said I done, I suspect this was unnecessary.

Third, I shortened the throttle rod.

Put it back together and stuck it on the engine.


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Had to play with the idle a lot. Ended up lengthening the throttle rod a tad. After that, I managed to get a decent idle and air/fuel mixture.

And no hesitation during acceleration.

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I did a happy dance right there in the garage.

Had to fart around with the throttle return spring. Actually might have an issue with the throttle body blades. They seem to want to stick open that last 1/16" of an inch instead of returning to the proper idle speed. Changed the bracket location and changed the springs and the issue is almost gone. I'm letting that issue sit a while. After a few more miles I'll do more about it. Yes, that's the wait-and-see technique.

Meanwhile, a running driving truck. woot!

[60ShortStpeGMC]

So the hesitation during acceleration showed up again. Dadgum.

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Decided to gasket-ize the new adapter. Right in the middle of the two adapter pieces. Just in case. While I had the carb off I inspected the throttle blades. They were sticky before I rebuilt it. Certainly not sticky. Double guessing myself is all. But, found that the throttle cam moved a wee bit before the accelerator pump moved.

Hmm.

Sure enough, the throttle blades would open about 1/16th before the accelerator pump would engage. "Gee" I said, all by myself in the garage. "Wouldn't that mess with the air/fuel mixture?" I'm sure the old-timers reading this are saying 'saw it right off'.

Long story short, adjusted the "throttle rod" to remove all play in the throttle cam. Hesitation at acceleration gone.

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[azlamm65GMC]

Nice

[60ShortStpeGMC]

Ran through the old posts. Dang! Got rid of that big-base rochester a long while back. The aluminum adapter was really very nice but didn't work out. Yes, carb icing is a real thing.

Have a small-base rochester on there now. Hey, no adapter! New tires. She's running good.


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Recently had a notion to get rid of the points. Bought an ignition module, cap, rotor, wires and plugs. Everything worked great. Couldn't spin the distributor enough to get it tuned, vacuum advance pot hit the coil bracket.


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Time for a new bracket. I guess.


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Got timing between 5 and 10. Seems right. Runs well. Drove well. Quickly developed fuel leak during test run.

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Must be a rochester thing.

The original coil bracket, if you're curious, can be seen here.


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[60ShortStpeGMC]

Fuel leak wasn't a Rochester thing after all. Turns out that I replaced the needle, seat, and float - leak went away.

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Hesitation at acceleration came back. Messed with the accelerator pump rod more. Upped the curb idle speed. A fast idle, but almost no hesitation. Test drive was ok but engine would "diesel".


Took another look at the timing. Dialed it in, again and adjusted curb idle speed to 800 rpm. I think that's right. Sounded good, better than before. And no "dieseling".



Test drive was good. Even had accelerator response. I probably need to adjust air/fuel mixture. But, until then, time to drive it a while.

[60ShortStpeGMC]

So after poking around this site, I found that the idle speed is spec'd at 400-450 rpm. My 800 rpm was pretty far off.

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Turned down the curb idle speed, but just couldn't reach 400-450 rpm. Even with a touch up on the timing. Settled at 600 rpm.


So, it drove well still. No hesitation and no leaking. Can't be all bad, right? I'll be convinced after I try a cold start tomorrow.


Learning to love my nerd kit...


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