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Discussion Starter #1 (Edited)
Last fall, I picked up a 1964 Chevy II 300 with 194ci-6cyl and 3 speed manual column shift. I've been driving this ol' beauty as a sunny weather daily driver. (Here in NC, that's most days.) When I first got it, it ran pretty rough and was only getting about 14 MPG. Since then, I replaced pretty much all of the normal tune-up / maintenance parts:

  • spark plugs and wires, cap, rotor, condenser
  • PCV valve
  • fuel filter
  • air filter
  • battery wires, ground straps
  • carburetor rebuild (Rochester B4C 1 bbl)
  • heater hoses
  • thermostat
  • fan belt
Also, a previous owner had replaced the fuel pump quite recently.

I've been running high octane gas, and I've been including lead substitute in every other fill-up. The car has improved from 14 MPG to 20 MPG (highway+city driving), so the tune-up work definitely seems to have helped.

I initially adjusted everything to factory specs, including the valves, dwell, timing (8 deg BTDC), plug gap, idle mixture, idle RPM (500), choke/high idle, etc. At these specs, the car tends to stall at idle -- especially when the engine's cold. I've had to turn the idle speed up a bit to help. It seems to be fine at about 570 RPM low idle.

The problem is, there is almost always a slight hesitation when accelerating in first gear. This is especially annoying when I'm stopped going up a hill. I have to feather the clutch a lot to keep from stalling. Once the car is moving, the "power curve" kicks in, and it's fine. There is plenty of power, and I don't have any hesitation at all once the car is moving. It's only an issue starting out from idle.

The timing seems to advance as it should. Vacuum levels seem about right, and there don't seem to be any vacuum leaks anywhere. When I disconnect the vacuum advance and accelerate, the dwell angle remains pretty constant -- only varying about one degree. So I figure the distributor is still in good shape.

I'm inclined to believe the problem is with the acceleration pump, since that seems like an obvious culprit -- even though I've replaced the spring and plunger. My brother told me that he remembered hearing something years ago that this is a common problem with this type of carburetor, and that there is a special kit (heavier spring?) to jazz up the accelerator pump a bit.

I was wondering if any of you all might have some ideas about this. I'm not looking to burn rubber necessarily, but I do want it to perform a little better out of first. If this problem is common for the Rochester B4C, I'd be willing to try modifications or a different carburetor if need be.

I'm otherwise very happy with this vehicle. It's a pleasure to drive -- especially on the open road.
 

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I've had the same problem in the past on other vehicles and found it was in the accelerator pump linkage. It's usually worn and sloppy. You may want to double check the carburetor specs. found in your kit and make sure your accelerator pump linkage is properly adjusted. I've found that little or no slack in the linkage will probably cure it. Another issue to consider would be the float level. If the float level is set too low the fuel level in the accelerator pump reservoir may be too low to sustain a good shot of fuel.

MTNNOVA
 

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67 4 door, 65 wagon (in pieces)
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Read this:

http://www.thecarburetorshop.com/Troubleshooting.htm#Acceleratorpumps

Bog from a stop is virtually always (and generally erroneously) diagnosed as a faulty accelerator pump (see the section on “accelerator pumps” for testing). Most modern carburetors are designed to function with roughly 0.020 (20 thousanths) clearance between the center of the throttle plate edge, and the throttle body at a point equidistant from the throttle shaft bearing areas. This clearance allows for maximum velocity of idle air past the idle ports. Exceptions to this are GM carburetors with the idle speed air screw, and end carburetors on tripower. Setting the idle for the highest vacuum idle reading will result in too little clearance of the throttle plate; forcing too much of the idle mixture through the lower idle port and too little through the idle transfer slot. This will cause a phenomena called “puddling” where little droplets of gasoline adhere to the intake manifold runners. When the throttle is opened, there is now sufficient velocity of air to sweep all these droplets into the cylinders, creating a mixture which is too rich to burn, hence the bog. As soon as the overrich mixture is pumped out the tailpipe, and a normal mixture is ingested by the cylinders, the bog disappears. A defective advance mechanism can also cause bog; as can a defective accelerator pump. If bog exists only from an idle, not when accelerating from a constant speed, the idle adjustment is probably the culprit.
 

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Discussion Starter #4
Thanks. This is exactly how I set the mixture. Both Chilton's and the Chev factory manuals called for adjusting this way. I'll try leaning up the mix to see if it helps.

Setting the idle for the highest vacuum idle reading will result in too little clearance of the throttle plate; forcing too much of the idle mixture through the lower idle port and too little through the idle transfer slot. This will cause a phenomena called “puddling” where little droplets of gasoline adhere to the intake manifold runners.
 

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Discussion Starter #5
Thanks. I was pretty careful setting the float level, so I don't think that's the issue. But I'm not so sure about the pump linkage. I'll check that out.

I've had the same problem in the past on other vehicles and found it was in the accelerator pump linkage. It's usually worn and sloppy. You may want to double check the carburetor specs. found in your kit and make sure your accelerator pump linkage is properly adjusted. I've found that little or no slack in the linkage will probably cure it. Another issue to consider would be the float level. If the float level is set too low the fuel level in the accelerator pump reservoir may be too low to sustain a good shot of fuel.

MTNNOVA
 

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67 4 door, 65 wagon (in pieces)
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Thanks. This is exactly how I set the mixture. Both Chilton's and the Chev factory manuals called for adjusting this way. I'll try leaning up the mix to see if it helps.
I had the same problem with mine. I ended up rebuilding it anyway just because I already had the kit but the adjustment was the key.
 

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Discussion Starter #7
Timing

I wasn't able to turn down the idle mixture much without it wanting to stall.

As an experiment, I decided to see what I could do with the timing. Problem is, the vacuum advance was bumping against the fuel line, so I couldn't advance much more than 8 degrees BTDC, which is the factory spec and what I already had set. With some finagling, I was able to bend the fuel line a bit to get it out of the way, and I advanced the timing to about 11-12 degrees BTDC.

The hesitation is gone!

Can this be right? This seems like a significant difference from the factory settings.
 

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I wasn't able to turn down the idle mixture much without it wanting to stall.

As an experiment, I decided to see what I could do with the timing. Problem is, the vacuum advance was bumping against the fuel line, so I couldn't advance much more than 8 degrees BTDC, which is the factory spec and what I already had set. With some finagling, I was able to bend the fuel line a bit to get it out of the way, and I advanced the timing to about 11-12 degrees BTDC.

The hesitation is gone!

Can this be right? This seems like a significant difference from the factory settings.
sounds right...good job!!bm
 

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Sending this up for other 6 cyl guys that may be having issues today.
 

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Most hesitation can be cured by advancing timing. 11-12 degrees initial will probably make your car run cooler as well. Congrats on figuring it out before screwing up your carb. Most people don't.
 
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