Could use some opinions

[BobT]

I recently picked up a grain truck and have been trying to get it ready for harvest. It's been sitting since 1999. So far costs have been minimal but I've been having a little trouble and I'm hoping someone can offer some ideas for me.

1968 Chevy C60 medium duty grain truck

427 V8

Holley four-barrel double pump carburetor

Here's what I've done so far.

• Replaced fuel filter and rubber fuel lines
• Took the carb in and had it cleaned and rebuilt
• Replaced spark plugs
• Replaced plug wires
• Replaced distributor cap
• Replaced points
• Replaced rotor
• Replaced alternator

Symptoms:

Runs rough especially at idle. I have to keep feathering the throttle to keep it running at idle. I am able to drive it but it lacks power. Exhaust smells rich while idling but I have to keep feathering the throttle to keep it running. If I close the choke or partially block the carb intake with my hand the engine revs up...quite a bit actually. This seems strange to me as I would expect it to choke down instead.

There are only a couple items that use vacuum, PCV valve and power brakes. I inspected the hoses for these and see no indication of cracks or leaks. The PVC valve does seem to work. When I stick my thumb over the end I can feel the ball move. I removed the power brake vacuum line and plugged the intake to see if there was a vacuum leak in the brake system but nothing changed so I am confident that is okay.

To me the symptoms suggest poor fuel flow. The fuel pump works and I am able to fill a quart jar in about 30 seconds of engine cranking.

I'm at a loss with this one.

Any suggestions are welcome.

[Macgyver55]

First off, was the truck running this way before all the items listed were done? Is it "hitting" on all the cylinders when it is running?

Was the carb done by a trusted source that knows how to properly clean and adjust them? If it was sitting that long it would be likely that there was considerable varnishing in the carb and it would need extended or repeated soaking. It sounds like an idle circuit is partially plugged, or the float level may be too low (they need to be adjusted while running) or there is a vacuum leak at the intake manifold or maybe even an open port on the carb.

If you are confident that the carb itself is ok, make sure the carburetor base gasket is correct,

[BobT]

Thanks, McGyver. I will be looking things over more closely to see if there are any unplugged vacuum ports. It doesn't run well enough to test the intake manifold gasket so the only solution there would be to replace it. This may become necessary to rule it out.

The fellow I brought it to was highly recommended by more than one source. My understanding is that he uses some kind of ultra-sonic bath?

I'm no Holley expert at all but something has caught my attention. I noticed it a bit odd that the idle mixture adjusting screws on my carburetor are located on the back of the carburetor near the secondary throttle plates. My first instinct was that if these screws are used to adjust the idle mixture ratio, shouldn't they be located near the primary throttle plates? So I decided to do some online research.

Without deviation, every video or document I looked at showed the idle mixture screws located on the front bowl of the carburetor near the primary throttle plates. One video I found suggested that this is always the case unless there are four mixture adjusting screws in which case two would be found on the secondary end.

I also find that adjusting these has little to no effect on my engine idle.

I'm beginning to wonder if maybe he mixed up the parts when he reassembled the carb and got the bowls on the wrong end. I will take it back to him tomorrow to talk about it.

[raymondk]

could be old fuel

[BobT]

All of the fuel that was in the tank and throughout the entire system, including the lines to the carb has been removed and replaced with fresh fuel.

I made sure of that before I started it the first time. Had to replace all rubber lines because they were collapsed and pretty much mush.

Thanks for the suggestion.

[Macgyver55]

I'm no Holley expert at all but something has caught my attention. I noticed it a bit odd that the idle mixture adjusting screws on my carburetor are located on the back of the carburetor near the secondary throttle plates. My first instinct was that if these screws are used to adjust the idle mixture ratio, shouldn't they be located near the primary throttle plates? So I decided to do some online research.

Without deviation, every video or document I looked at showed the idle mixture screws located on the front bowl of the carburetor near the primary throttle plates. One video I found suggested that this is always the case unless there are four mixture adjusting screws in which case two would be found on the secondary end.

I also find that adjusting these has little to no effect on my engine idle.

I'm beginning to wonder if maybe he mixed up the parts when he reassembled the carb and got the bowls on the wrong end. I will take it back to him tomorrow to talk about it.

You possibly just solved your own problem Bob....I dont claim to be an expert either, but I have worked on many, many Holleys over the years and have never seen them locate the idle mixture screws on the rear bowl side. Even though the bowls are similar, the idle mixture screws are often located on a "block" plate separate from the bowls and will fit on either end so its possible he inadvertently put the plate on the wrong end.

That would be your best case scenario to curing your problems! Let us know what you find.

[Eric Wettschreck]

Once you get the carb thing figured out (Swapped around) and stuff running, spray a bit of carb cleaner in different areas to find vacuum leaks. Intake, carb base, etc.

The motor will sound different when it sucks in a bit of carb cleaner. Good way to find leaks.

[BobT]

I figured it out.

I finally found a HSOforum where I can enter the list number from the carburetor and it would return the model number and provide links for information. My carburetor is a Holley 4150-G. From the results page I was able to follow a link to a complete exploded view of the carburetor and it clearly shows the idle mixture adjusting screws on the secondary metering block. This is completely different from almost every other view I found.

I finally concluded that I was sucking air somewhere so I went on that approach. I discovered that the secondary throttle plates are not closing completely and this is my air leak. My carb uses vacuum to control the secondary plate and at idle it doesn't allow it to close completely. When I manually pulled it shut, the engine purred like a kitten.

According to the drawing I found there is a spring inside the diaphragm assembly for the secondary throttle control. I suspect this spring is weak and not returning the throttle plates completely like it should.

[Macgyver55]

Good for you! Be careful with the spring though, they are often calibrated to determine when and how much they open by the spring tension, so its probably more likely that the throttle plate or linkage is binding rather than the spring being too weak.