flat spot

[roastbeef]

Hi,
So done a few jobs on the vair last few months.
Wheel bearings, diff seals, brakes. I rebuilt both carbs with new choke and throttle
Spindles, basic gasket kit. Now runs 10 times better than before as carbs are not sucking so
Much air!
I just seem to have a flat spot when I apply throttle gently from about 45-50 mph.
Apply throttle hard and seems OK. ????
Any ideas? Cheers.
Oh its a 62 700 sedan with power glide.

[66CorsaConv]

It may be a Power Enrichment issue but I can't remember from 40 years ago if my EM had a PE circuit.

Depending on just how far you are from closed throttle idel, maybe ab idle transfer slot issue or Vacuum Advance not working.

[Scott V]

no power enrichment in 62 carbs.

check if your accelerator pumps are working. check your timing & your carb sync/balance. if all that is ok maybe its time to step up a jet size.

-Scott V.

[bbodie52]

I would also suggest checking the function of the vacuum advance mechanism. It effectively fills a gap during low RPM transitional periods when engine speed is low, so the centrifugal advance mechanism has not engaged, but the engine needs additional timing advance. If your vacuum advance is not working for some reason, this might explain a performance lag at low engine speeds.

The attached article describes the need for ignition timing advance, and how the engine's needs change with different operating conditions. The centrifugal advance system within the distributor only functions at higher RPM ranges. The vacuum advance responds to engine timing needs at lower RPM ranges, and the two systems work together to optimize engine performance. This article was written to describe the distributor and timing functions in Corvette V8 engines, but the same principles apply to the Corvair engine. The following was extracted from the longer article, which is attached and makes for very informative reading.

VACUUM ADVANCE: The vacuum advance system consists of a vacuum diaphragm mounted on the distributor body. The diaphragm is spring-loaded in the zero-advance position, and has a rod that connects to a hole in the breaker plate, which is the movable plate the points are mounted on. When vacuum is applied to the diaphragm, it pulls on the rod, which in turn pulls on the breaker plate, rotating it with respect to the eight sided cam on the distributor shaft which opens and closes the points. When viewed from the top, the distributor shaft (and the eight-sided cam for the points) turns clockwise. When the vacuum advance rod pulls on the breaker plate, it rotates the breaker plate (and the points) counter-clockwise, which “advances” the opening of the points (triggering the coil to fire the spark plugs). A typical vacuum advance unit, when fully deployed, will add about 15 (crankshaft) degrees of spark advance over and above what the distributor’s centrifugal advance system is providing at the moment, which depends on engine rpm. They are two independent systems, but they work together to provide the correct amount of spark advance.

...this is what “spark timing” is all about – managing the point at which the spark plug fires under different operating conditions. This point is expressed as “spark advance” in degrees of crankshaft rotation before the piston reaches top dead center...

Timing, Vacuum Advance, and Centrifugal Advance.pdf

Timing, Vacuum Advance, and Centrifugal Advance

(2.09 MiB) Downloaded 15 times

The carburetor accelerator pumps have a similar function to the vacuum advance mechanism, in that they provide a momentary fuel enrichening when the throttle is first opened. At that time the air flow through the venturi is just starting to function, but there is a time delay before vacuum from the venturi effect in the carburetor throat can actually draw fuel from the float bowl. The mechanically injected shot of fuel from the accelerator pump mechanism helps to fill the time gap to keep fuel in the air flowing through the carburetor throat until the fuel from the float bowl can begin to flow properly and catch up with the engine's needs.

A faulty carburetor accelerator pump or a faulty distributor vacuum advance can result in a performance power gap at some stages of engine operation. Both systems need to be functioning properly to help the engine to maintain and produce a steady flow of horsepower at all stages of vehicle operation. Both of these systems operate in the area between idle and high-RPM operation. Without them, there will be a lag in power at lower engine speeds or when the throttle is first opened.

[64powerglide]

Bump the timing to 15 or 16 & try it. I had a new 62 102 hp 4 speed & ran it at 18. That's where the mechanic at the Chevy dealer set it. It was an amazing improvement from 13.

[roastbeef]

Cheers all. Forgot to say If has a Petronix
Ignition instead of points.
I should be able to check the advance with the
Timing light?
Cheers

[bbodie52]

Both systems I mentioned can actually be observed functioning when you open the throttle.

The vacuum advance lever on the side of the distributor should move back and forth as the engine speed is increased and decreased while observing the vacuum advance mechanism.

The function of the accelerator pump in each carburetor can be watched with the engine off, the air cleaner removed, and the choke butterfly held open so that you can look down the throat of each carburetor. When you open the throttle rapidly, you should see a jet of fuel injected into the carburetor throat as the throttle linkage on the side of the carburetor pushes an internal plunger mechanism down... forcing fuel from the float bowl to squirt into the carburetor throat.

These videos explain the distributor advance mechanisms...

[flat6_musik]

A lean stumble or flat spot is generally could also be that your vacuum advance canister is shot and is a source of a vacuum leak, causing some leanness and not allowing full advance. A float level set too low could also cause some leanness. I DO agree with bumping the initial timing to add more "pep to your step" but it's something that has to be done carefully, due to that dirty low-down enemy, detonation.