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Troubleshoot Chronology

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Posted September 8th, 2010 at 08:48 PM by MacDee

Let me say first that the BASIC PROBLEM I’m having with my engine is that it won’t idle. It seems as though as soon as the engine speed drops below a certain speed, say about 1200 rpm, the engine will just quit, as if the fuel has simply been shut off. Driving it home after the engine was installed was very anxious. I had to keep my foot in the gas even as I came to stops to keep it running. This is the approximate sequence of actions taken so far to try to find and resolve whatever the problem is:

With the engine running, I tried pinching off each hose leading to the manifold vacuum:
Vacuum advance (to carburetor);
PCV (to carburetor);
Transmission modulator (to intake manifold).
No effect.

With the engine running, I sprayed carburetor cleaner at any areas where I thought there could be a vacuum leak:
Around the base of the carburetor;
Around the base of the carburetor spacer;
Along the top of the manifold where it meets to the head (could not get at the bottom);
At transmission modulator fitting on manifold.
No effect.

Re-adjusted the valves.
No effect.

Bought a new timing light and vacuum gauge (already had tach/dwell meter) and attempted adjusting timing and idle mixture. Settled with timing at 10 BTDC and idle screws turned out about FOUR TURNS to get it to run at about 1200 rpm.

Removed vacuum advance and capped the port on the carburetor, just to remove a “variable” from the investigation. Attempted timing and idle mixture adjustments again. After getting it as good as I could get it, I took it for a short drive. Though it would run, it was running really bad. It “surged” continuously at low to moderate speed, and “idled” very unsteadily.

Roger offered to come over and have a look. Just before he arrived, I did some more spraying with the carb cleaner and discovered a change in engine speed when I sprayed at the seam between the carburetor main body and the carburetor throttle plate! I removed the carburetor and when I turned it over, Roger noticed two holes where it APPEARED that screws were missing! (There were six screws attaching the throttle plate to the main body, but there two more counter-bored holes in the throttle plate where screws could go, but the adjacent blind holes in the main body were not tapped. I subsequently learned this is normal, but at the time it seemed fishy….)
Roger thought he might have a 12-24 tap, but we’d need two more 12-24 screws (very rare size), so we set off for Tacoma Screw in Everett only to discover the Everett Tacoma Screw is closed on Saturdays! I called Carter on the off-chance he might have some 12-24 screws laying around his shop. He invited us over to have a look.

At Carter’s place, we removed the throttle plate and discovered that the top of it was machined in such a way that it wasn’t really all that flat (two parallel mill passed that were not quite co-planar, leaving the middle slightly depressed relative to the ends). Carter and I hand-lapped the plate with emery cloth on a piece of flat bar stock while Roger tapped the two additional holes.

We scavenged two additional screws from an old Holley 4-bbl Carter had acquired at a swap meet many years ago (it was also “missing” the same two screws!), and re-assembled it with eight screws instead of six.

We took the carb home, installed it, and ran the engine. It was no better. Maybe even worse. Not only that, but after messing with it for a while, the engine started missing! We popped the distributor cap off to discover one of the magnets in the Pertronix rotor had fallen out. I had improperly installed the module and the wires caught the tape wrapped around the rotor that hold the magnets in. The tape was all wrapped up around the rotor and the magnet was stuck to side of the distributor body. Roger told me he could probably fix it and explained the proper method of routing the wires around to the module. For the time being, we installed points instead.

Carter offered to go through my carburetor thoroughly, to see if he could see anything wrong with it. He also suggested trying his old swap meet carburetor to see how it worked on my engine.

While Carter was going through my carburetor:

I removed the intake manifold and removed the coolant line fittings to it. The forward fitting had been leaking (it was actually so close to the adjacent shock tower that the fitting had probably been bumping it. Might explains why it came loose!). I spliced the coolant hoses together and re-installed the intake manifold.

I installed Carter’s “swap meet” Holley (similar to mine, except a 600 cfm model). It was missing the choke mechanism and the linkage fitting on the throttle lever. I plugged the vacuum port on the carb that goes into the back of the choke housing (some models draw heat through the housing with vacuum), as well as capping all the other vacuum ports, and manipulated the throttle from under the hood. Low and behold, it ran pretty well with the [dirty, old] “swap meet” carb… and it idled, too! I became convinced the problem was with my carburetor.

Later, Carter reported that he’d gone through my carb and found some more surfaces that were “not as flat as they should be” (which he fixed), but nothing else that seemed obviously wrong. I re-installed my reworked carb and fired it up. It was no better.

In desperation, I called the Holley Tech Hotline. I explained the problem I was having and everything we’d done to try to fix it. They asked me to send it to them, so I did.

When I got it back, they’d said they only needed to make some adjustments, and declared it “OK”.
I installed it. It was no better.

At his suggestion, I arranged with Roger to try my carburetor on his Ford pickup (352 V8 with a four-barrel carb). Low and behold… the old truck ran just fine with my carb on it!

Okay, so maybe it’s not my carb’s fault, but…

I went and borrowed Carter’s “swap-meet” Holley again, with the intent of installing it complete, replacing the missing parts with parts scavenged off of my carb. I attempted to document everything I did that day:
  • I installed the “swap meet” carb in exactly the same configuration it was in when I tested it before. I hooked up the gas pedal, but everything else was exactly as it was on the first test: all vacuum ports capped, no vacuum advance, no PCV, no choke mechanism, vacuum port to choke housing plugged.
    It ran pretty much the way I remembered. It ran, it idled. I was able to get it to idle down to about 1000 rpm in neutral, put it in gear and then not have it die.
  • I installed the choke, left all the ports capped.
    It ran the same.
  • By this time, I’d gotten my repaired Pertronix part back from Roger, so I re-installed the electronic ignition and tried it again, all ports still capped.
    It ran even better!
  • I re-installed the vacuum advance.
    It didn’t run any worse, but it didn’t run any better either. It seems to have no effect on timing, at least at idle. Interestingly, removing the vacuum line while it’s running has no effect, even with an open vacuum port!
  • Hooked up the PCV valve.
    Seemed to run perhaps a little worse. I took it for a drive and realized that even though it will “idle”, it still does not run right. It still “surges” continuously at low to moderate speed, and “idles” very unsteadily.
  • I removed the PCV again and capped the port. I ran the engine again, but did not drive it. It ran not much better.
  • On a whim, I double-checked that the distributor gear was engaged on the right tooth. It was.
It should be noted here that Carter’s “swap meet” carb is a “reverse idle” carb. I’m not sure exactly what that means, but I know that whatever I was experiencing with my carb’s mixture screws would not relate directly to this carb. As it turned out, it was a moot point anyway; throughout the whole process, turning the “mixture” screws on Carter’s carb had ABSOLUTELY NO EFFECT. I could turn them all the way to their stops and it had no effect on the vacuum reading, or on the way the engine ran!

Also throughout the whole process, the vacuum reading stayed right around 10 in Hg at “idle”. Nothing I did had much effect on that. The only exception was that it went up to about 12 in Hg right after I re-installed the electronic ignition.

Another interesting note is that when I remove the vacuum gauge while the engine is running (exposing the vacuum port), the engine actually speeds up!

Go figure.

I wish I could.
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  1. Old Comment
    pbrown's Avatar
    I re-installed the vacuum advance.
    It didn’t run any worse, but it didn’t run any better either. It seems to have no effect on timing, at least at idle. Interestingly, removing the vacuum line while it’s running has no effect, even with an open vacuum port!
    Note that the vacuum advance port is taking its signal from above the throttle plates. There should be no vacuum at idle on this port. You would be able to uncap it and not effect idle while running. It will register a vacuum signal when the throttle plates start to open.
    Posted September 8th, 2010 at 10:38 PM by pbrown pbrown is offline

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