Corvette Kiwi

I have a 1969 Corvette L-71 (427 V-8, 435 hp, Tri-Power 3x2 carbs, TI ignition, factory side exhaust, etc.). Currently, two carburation items exist:

1. If the car sits for a few weeks it takes a very long time to start. It's as if all the fuel has evaporated from the carbs and you have to crank quite a while to fill the bowls before the car will start. Is this typical with a stock Tri-Power setup?

With my other musclecars (with a 4-barrel carb), after a few months of non-operation all I do is pump the throttle a few times and it fires right up. Why is this not the case with the Tri-Power setup?

2. The jetting seems to be off as it runs rich at idle and big plumes of smoke pour out the exhaust when I floor it. Thus, for an "at sea level" car, what is the proper size power valve and what is the proper carb jetting? I would assume that the center "primary" carb would be jetted differently than the two outboard carbs? Or not??

1Fordman

1. Mine does the same thing when it sits. The fuel does evaporate and when you're cranking the fuel pump is filling 3 float bowls.

2. Has your car ever backfired when starting or accelerating? You may have a blown power valve from your description. Does your throttle plate on the center carb have a power valve backfire protector installed? When rebuilding the carbs I installed one.

6.5 is the standard power valve and (from memory) # 64 main jets. Mine is a 68 and I think the 69's possibly used # 62 main jets.

The outboard carbs do not have jets but use metering plates.

rponfick

Main jet would have little to do with idle richness. Usually throttle plate opening gap and idle mixture screw come into play here. L71 had pretty wild cam and 11.25 compression, are these still stock on your's?

Ralph

Corvette Kiwi

I think the cam and pistons are stock ... but I won't really know until I open up the engine to rebuild it.

Regardless, during the rebuild the engine will be put back to stock specs ... and that's why I was wanting to know the proper jetting and PV so that I can check everything while the engine is apart.

I guess my jetting/PV question is two-fold:

1. What was the original (stock) jetting and PV
2. What is the preferred setup that people use today (that runs better, makes more power, etc.)

MelWff

look at the Holley part number on the airhorn and go to their site. Under tech they have a PDF download that contains the jet and PV sizes. The center carb should be 4055-1 and both ends should be 3659.

rponfick

In anything you read from Holley or Chevrolet, it will say factory setting is 62, 63, or 64 on the center carb. Most say manual trans was 62 and 64 for auto, but I'm not sure if there were any auto L71's. Most new replacement carbs were 63 as a compromise. I took off off my original carbs. and put them on the shelf, and ordered replacements from Holley, and it came with a 63. Using an LM1 at Denver altitude, I settled on a 61 to get to 14 AFR at idle and cruise. Power valve is 6.5.

I've also hooked up vacuum advance can to manifold vacuum. You can tee off the vacuum line to the choke pull-off diaphragm hose on the center carb. This allows for cooler running at idle and in traffic.

If you are rebuilding anyway, you might want to consider a more modern roller hydraulic cam to lessen the oil issues. Slightly less compression doesn't hurt either.

Let the fun begin. Ralph.