C2: Hesitation above 3K rpm

[Mark F.]

My recently rebuilt '65 300hp is having difficulty above 3000 rpm. The original carburetor has been rebuilt with it's original pieces, and the engine runs great until 3K or 3.5K rpm; at this point it refuses to accelerate further. The distributor is original, but has a new (tested OK) vacuum canister.

The car is an L-75 with Powerglide.

Your advice, ladies and gentlemen, is appreciated.

Mark

[Mike Cobine]

Re: C2: Hesitation above 3K rpm

Wild guess. Do the secondaries open on the carb?

[Mike Cobine]

Re: C2: Hesitation above 3K rpm

Wild guess. Do the secondaries open on the carb?

[Duke W.]

Re: C2: Hesitation above 3K rpm

Yes, Check the throttle and carb. linkage to make sure the secondaries open. Tilt the auxillary air valve open so you can see the secondaries.

Duke

[Duke W.]

Re: C2: Hesitation above 3K rpm

Yes, Check the throttle and carb. linkage to make sure the secondaries open. Tilt the auxillary air valve open so you can see the secondaries.

Duke

[Brian Monticello]

Re: C2: Hesitation above 3K rpm

Mark,
In case you do not already know this, the 65 Corvette AFB had mechanical secondaries, they should not open until the primary butterflies are about 2/3 or 3/4 open. The secondaries will open at a rate much greater than the primaries and should reach the vertical point (all the way open) at the same exact time. If they do not, you have some adjusting to do.

Brian

[Brian Monticello]

Re: C2: Hesitation above 3K rpm

Mark,
In case you do not already know this, the 65 Corvette AFB had mechanical secondaries, they should not open until the primary butterflies are about 2/3 or 3/4 open. The secondaries will open at a rate much greater than the primaries and should reach the vertical point (all the way open) at the same exact time. If they do not, you have some adjusting to do.

Brian

[Duke W.]

Re: C2: Hesitation above 3K rpm

Yes, the AFB has mechanically actuated secondaries, but there is also an auxillary air valve above the secondaries that is counterweighted to remain closed until there is enough manifold vacuum to generate sufficient air flow to create a useable metering signal.

At WOT the secondary air valve should open all the way by about 2500 RPM.

All the various AFB adjustments are in the 1963 Corvette Shop Manual, which is the base manual for '64 and '65 models, and there is a supplement for each of those years to cover updates and new hardware.

When properly adjusted the AFB usually provides seemless and trouble free operation.

Duke

[Duke W.]

Re: C2: Hesitation above 3K rpm

Yes, the AFB has mechanically actuated secondaries, but there is also an auxillary air valve above the secondaries that is counterweighted to remain closed until there is enough manifold vacuum to generate sufficient air flow to create a useable metering signal.

At WOT the secondary air valve should open all the way by about 2500 RPM.

All the various AFB adjustments are in the 1963 Corvette Shop Manual, which is the base manual for '64 and '65 models, and there is a supplement for each of those years to cover updates and new hardware.

When properly adjusted the AFB usually provides seemless and trouble free operation.

Duke

[Pete Whelan]

Re: C2: Hesitation above 3K rpm

Mark,
Have you checked your fuel pump pressure and fuel lines for any restrictions? What is the condition of the fuel line inlet screen in the fuel tank?

Pete

[Pete Whelan]

Re: C2: Hesitation above 3K rpm

Mark,
Have you checked your fuel pump pressure and fuel lines for any restrictions? What is the condition of the fuel line inlet screen in the fuel tank?

Pete

[Mike Cobine]

Re: C2: Hesitation above 3K rpm

If he has fuel pump problems, then what will happen is the inertia of the car will run the engine far enough above the fuel supply that he will "run out of gas" and it will sound like it.

As soon as the engine slows enough for the fuel supply to catch up, it will run normal again.

Normal driving may not show this too much, but if he takes it out and floors it, it will be obvious.

[Mike Cobine]

Re: C2: Hesitation above 3K rpm

If he has fuel pump problems, then what will happen is the inertia of the car will run the engine far enough above the fuel supply that he will "run out of gas" and it will sound like it.

As soon as the engine slows enough for the fuel supply to catch up, it will run normal again.

Normal driving may not show this too much, but if he takes it out and floors it, it will be obvious.

[Mike Cobine]

I should have added

that the secondaries will act like he simply can't press the pedal down enough. You get the same feeling if you put a block under the gas pedal. The engien will run fine, even if you floor it from a stop, because it has enough fuel, but it has just hit the limits of air flow.

The two feel very different. One feels like you ran out of gas and you can her the engine starting to die. The other is simply like you have only pushed the pedal down part way.

[Mike Cobine]

I should have added

that the secondaries will act like he simply can't press the pedal down enough. You get the same feeling if you put a block under the gas pedal. The engien will run fine, even if you floor it from a stop, because it has enough fuel, but it has just hit the limits of air flow.

The two feel very different. One feels like you ran out of gas and you can her the engine starting to die. The other is simply like you have only pushed the pedal down part way.