66 SB Erratic Engine Performance

[Dave K.]

[Timothy B.]

Re: 66 SB Erratic Engine Performance

Dave,

Sounds to me like that carburetor needs to go back to restorer to have surfaces checked for flat. I think it's a carburetor problem.

[Gerard F.]

Re: 66 SB Erratic Engine Performance

Dave,

I think your first issue, that is the increase in idle after a normal run, is entirely normal. My 67 S/B with a Holley 3810 does this all the time.

I think it has to do with the temperature of the fuel in the fuel bowls. As soon as you get cooler fuel from the tank into the bowls, the problem alleviates itself. I do not think there is anything you can do with the linkage to resolve this annoyance, it is just that present day fuels have a lower boiling point.

On your second problem, with erractic idle after a hot run, I think is related to the above, and it is due to fuel percolating into the air horn from the bowls, from boiling of the fuel, or a hung up float. Sounds like an overrich situation, with fuel dripping into the air horn, and probably from the secondary.

Next time this happens, pull your air cleaner cover, and see if you have fuel dripping into the air horn. Try tapping on the fuel bowls with a screw driver handle to see if it alleviates the problem of a stuck float.

You might try setting your float position down below the site hole, that's easy on yours, tough on mine with no site hole, and non-externally adjustable floats. If you find your floats are hanging, pull the errant bowl (probably the secondary), and bend the float to a central position in the bowl, making sure the pivot, fore and aft, has no slop in it. Braking tends to move the float forward, and it may hang up on the metering plate.

You might also try leaving the idle a little higher. I find it very hard to get these old engines to idle to original spec with the present day fuels.

Jerry Fuccillo
#42179

[GL Anderson]

Re: 66 SB Erratic Engine Performance

You should set the float levels if you haven't yet. Also did you check the used VAC to see if it was good and at what vacuum does it come in at? I have a 4160 on my car and it works great. As far as timing goes here is how I do it, I know there are others that will have a different method but this works for me. I set the points at 30 degrees dwell first. I have a dial back timing light so I set it at 36 degrees, unhook the vacuum to the VAC and plug the port, rev the engine till it no longer advances and turn the dist till the timing mark on the balancer reads at 0. Let the idle static timing be what ever it is. Tighten the dist hold down bolt. Then reconnect the VAC making sure it is hooked to a manifold vac source and rev the engine until it no longer advances again. Dial the timing light until the balancer mark is at 0 again. Chevys like about 52 to 54 total timing under these circumstances. Now set your idle speed to 6-700 and take the car for a ride to see if you have the issues solved. If your VAC doesn't work correctly search the archives there are long discussions about this. I used a B-26 can for mine as recommended by Duke and it was just the right thing.

[Joe T.]

Re: 66 SB Erratic Engine Performance

Hi Dave: One sure give-away on fuel perculation is, with the engine experiencing the problem, turn it off and remove the air filter. Fuel overflowing into the warm intake will evaporate and then condense into a visible vapor cloud. If its there, you will see it..no problem. I don't know if this particular problem is yours, but this might lead to a diagnosis...Good-luck...Joe

PS: You can also see this with a warm engine turned off if you force the accelerator pump to pump some raw gas into the venturi,,,the condensed vapor (contradiction in terms?)becomes apparent...jt

[Phil P.]

witch hunt for cheap fuel problems *NM*

[Joe T.]

Re: witch hunt for cheap fuel problems

sorry for just trying to help ID some symptoms of fuel perc. I'll just keep my mouth shut and fingers off the keyboard

[Jim T.]

Re: 66 SB Erratic Engine Performance

Dave you mentioned that your distributor was rebuilt with new advance weights. I am not a distributor rebuilder and don't know who did yours, but I do not believe that new distributor weights are the same quality as what originally came on your 66. I do not believe that the original quality distributor weights have been available from GM parts for some time now. Distributor sliding weights were cut different for different horsepower Corvettes. The tack drive distributor shafts for Corvettes have different top pieces brazed on for different horsepower Corvettes. Try installing your original removed weights and lightly lubricate the pivot pins and under the advance weights where it slides. My 64 Corvette had a idle issue that was not present when the engine was cold. Idle was different when driven. After paying to have the problem checked, he rebuilt the carb, problem still present. I discovered the problem by accident one day when I removed the distributor rotor and saw the restiction of the operation of the centrifical advance system because it needed a good cleaning and light lubrication. Solved the problem of the centrifical staying advanced at idle. From them on I learned to service the centrifical advance system on my cars.

[Duke W.]

Re: 66 SB Erratic Engine Performance

The first issue may be due to throttle shaft bushing wear. I know it's common on old AFBs.

At idle on the 300 HP engine air flow past the throttle plate is "critical". The pressure ratio is less than 0.528 and air flow is "choked" at sonic velocity past the opening. More manifold vacuum will not increase air flow.

This is one reason why 300 HP engines idle smooth and SHP/FI engines that idle at a pressure ratio greater than 0.528 have lope.

So if the throttle comes back with the shaft slightly cocked due to excess clearance from wear and the opening area is larger, the idle speed will increase. Kicking the throttle often returns idle speed to normal.

Another thing to check is that the centrifugal advance operates freely and returns to zero at idle. If it doesn't come back to zero at idle, idle speed can increase.

The second problem - idle instability - could be caused by either and overly rich mixture due to fuel percolation or a loss of total idle timing. The second issue could be caused by a leaking VAC and being as how yours is used, I would check it both hot and cold with a vacuum pump to be sure it holds vacuum.

IIRC the OE '66 300 HP VAC is either "355 16" or "360 16" and the specs are 0@6" and 12@16".

I often hear that the "distributor was rebuilt by a reputable guy to OE specs", but rarely is actual test data quoted. I'm always suspicious of that. Engineers live on test data as only test data verifies that a system meets specs.

It's easy to determine the spark advance map on the car with a vacuum pump and dial back light or timing tape on the balancer.

Duke