283 engine performance

[Jack F.]

My 57 has no power under load. Engine rebuilt with 500 miles on it. Carbs just rebuilt and while engine no longer sends "smoke" out the exhaust it still has no power under load. Clearly I am using the wrong Corvette restorer. But, that aside, I need your help in figuring out what is wrong.
Rebuild of engine seems fine, I had it inspected by NCRS judge before purchasing a year ago. My local Corvette restorer in New Milford, CT rebuilt the carbs to solve the lack of power. He thought car was perfect, not so. Still will not climb even the slightest uphill without down shifting more than necessary and eventually having to let the clutch slip in and out to inch your way up even a modest incline. The carb rebuilt eliminated the smoke. Plugs with 500 miles on them after rebuild were full of carbon and were replaced.
Curiously when driving at modest speed (35mph) on level road in 4th gear it will accelerate beautifully. But, if you down shift it hesitates and NEVER gets full power in any of the lower gears. Only 4th gear seems to act normally.
Can it be something in the spark/electical system? The spark may not be advancing properly under certain conditions?
Aside from finding someone else to work on my car, it is best if I at least know what needs to be done even if someone else does the work for me. So, any help you can give me in diagnosing the problem would be much appreciated.
Jack Farley
jufarley@optonline.net

[Duke W.]

Re: 283 engine performance

More information would be helpful, like:

What is the original engine configuration - HP rating?

Was it rebuilt with OE replacement parts or hot rod parts? What non OE equivalent parts, if any, are installed?

Does the engine hesistate, misfire, or does it appear to be hitting on all eight?

Basic diagnostics should begin with a compresssion check. Then test the ignition map and compare it to OE specs. The carburetor(s) should be checked for proper function including opening of the secondary throttle valves when the accelerator is pressed to the floor and proper operation of the auxillary air valve.

All of the above is doable by any competent DIYer with basic engine diagnostic tools like a compression gage and timing light.

Duke

[Jack F.]

Re: 283 engine performance

Thanks Duke for your quick response.
It is a 283/270 (solid lifters, Duntov cam)
All OE parts used in rebuild.
Compression is perfect!
It does NOT misfire when under load.
Does "Ignition Map" mean setting the timing? I did do it myself but timing at least while not under load seems fine. While sitting with hood up and working the accellerator all seems perfect including the kick in of the send carb past medium throttle.
Am I correct that "secondary throttle valves" is referring to 2nd carb? If so, that works perfectly when testing at a stand still.
Not knowledgeable about auxiliary air valves???
What I did notice is that the accelerator plunger does work. i.e. no gas squirts into carbs when pumping the accelerator. We had to manually close the choke to start the engine when cold.

[Duke W.]

Re: 283 engine performance

By "ignition map" I mean that initial timing and the distributor centrifugal curve are to spec. Since your dual point dist. has no vacuum advance, you don't need to worry about that.

A dial back timing light is easiest to record the igntion map, but you can use a conventional light with marks on the balancer, too.

I'm not totally familiar with the 2 x 4 setup, but as the accelerator pedal is depressed the primary throttle valves on the rear carb should begin to open and at some point the external linkage will begin to open the front primaries. Then as the accelerator pedal approaches the floor the secondary throttle valves of both carbs should begin to open via the linkage on each carb and reach full open with the pedal on the floor.

The secondary side of each WCFB has an auxillary air valve, which is a counterweighted offset shaft valve above the secondary throttle valves. Its purpose is to prevent airflow through the secondaries until sufficient flow demand is achieved to produce a useable venturi signal. This prevents the "lean bog" that happens if all eight throttle valves are opened at low revs. You should be able to easily push this valve open with light finger force. It should open and close smoothly with no binding, and you need to manually open the aux. air valve to verify that the secondary throttle valves reach full open.

It sounds like the accelerator pump does NOT work.

There are several on this board who have experience with the 2x4 setup and can probably provide more help.

I would first check the ignition map, then systematically go through all the various carb and carb linkage adjustment specs in the service manual.

Duke

[Bill Lynch]

Re: 283 engine performance

If you haven't already, I'd be checking the coil and ballast resistor (if the '57 has one..?)

A dead coil can act like your description. You can try a cheap coil from Autozone and see. Sometimes repro and correct NOS parts just don't perform like cheap OTC replacements.

[Jack F.]

Re: 283 engine performance

Thanks Bill - that brings back memories (I'm 68) and I just could not think of it. I have not done any serious work on an engine for 40+ years, this is going to be fun to get back into the swing not that I am "retired"... a misnomer if there ever was one. As soon as you mention "coil" I'm virtually certain that is the problem. My 57 is 50 miles from me in storage but will get to it early next week and let you know. What is your email? Mine is jufarley@optonline.net.
Jack

[Bill Lynch]

Re: 283 engine performance

Hello Jack, my email is wblynch@hotmail.com

I'm certainly not any kind of authority on old engines but I do remember 3 or 4 cars in my past that did this same thing!

Congratulations on having such a nice car for your retirement hobby!