C2 RPM problem: part 2

[Mark F.]

Recently I asked for suggestions as to why my '65 300hp Powerglide car refuses to accelerate past 4K RPM. As you fine folks suggested, I:

1. Checked the AFB (recently rebuilt by a professional) to ensure that the secondaries were opening fully. They were not; I readjusted the linkage and solved that problem. The vacuum-operated blades above the secondaries opened between 2500 and 3000 RPM.

2. Changed the points and condensor. Dwell adjusted to 30 +/- 2 degrees.

3. Checked (visually) the centrifugal advance. Weights free, springs appear OK (sorry for the non-scientific terminology).

4. Plugs are burning properly (AC45).

5. Hydraulic lifters are adjusted as suggested, zero + 1/2 turn.

6. I tried a spare (identical) carburetor. Same result: 4K RPM max.

One modificatiion to this all-stock L75 was the addition of an L79 cam. Could the cam be demanding too much air/gas mixture from my original AFB? The jets and the metering rods are original for the L75 PG engine.

In neutral, the engine seems to be willing to approach redline, but I'm not sure. The mechanical tach bounces too much for me to be positive about this.

Sorry for this long post. I'm really stuck on this one.

Thank you all for your kind assistance.

Mark

[Duke W.]

Re: C2 RPM problem: part 2

The L-79 cam extends the useable power/operating range to 6000 RPM. The original AFB should perform okay with nothing but idle mixture adjustment, but the distributor needs the proper vacuum can first. Does the car still have Powerglide?

Rev the engine to 5000 no load with a dwell meter attached. The dwell should not vary more than 2 degrees. If it does the distributor needs to be overhauled/blueprinted - bushing clearance inspection and replacement if required, a new wobble free breaker plate, and shim up the end play to two to seven thou.

For engines that rev over 5500 I highly recommend the high breaker arm tension points.

Also, with the L-79 cam the vacuum can should be a Delco "236", the equivalent of which is readily available at NAPA - VC1810.

What happens at 4000 under load? Does it start missing? Completely cutout? What?

Duke

[Mark F.]

Re: C2 RPM problem: part 2

Duke:

The car has its original Powerglide. Under load, it starts missing at 3500, and completely cuts out at 4000. I'll check the no-load RPM with the dwell meter today.

Thanks for your help,

Mark

[Duke W.]

Re: C2 RPM problem: part 2

Check the dwell stability at high revs, but it could also be fuel starvation due to a clogged fuel filter or other restriction in the fuel supply system. A clogged fuel filter can cause the engine to rev and then just quit - like turning off the ignition, but a short period of misfire can occur due to leanness just before it cuts out completely.

If the dwell is stable at high revs and the fuel filter is not plugged, it's time to pull the tank for an internal inspection, and blow out the metal fuel line with air.

IMO the fuel tanks should be pulled at least every 20 years for inspection and replacement of the inlet filter sock - even on a low mileage car.

Varnish formation can be worse on a low mileage car due to old fuel from lack of use.

You can also use a lead extensions and hook up the dwell meter in the cockpit while you drive the car. You just need one long lead to connect to the negative coil terminal and use any available ground in the cockpit. If the dwell is steady when the engine begins to miss, it's probably fuel starvation. The coil is suspect too, and it can usually be eliminated or indicted by substituting a known good coil.

Fuel pump is also a suspect, but my experience is that when the diaphragm tears they leak, so if there is no fuel leaking at the pump, it's probably okay and is the last thing I would check.

Duke

[Mark F.]

Thanks (again) for your help, Duke. *NM*