Expert: Brad Sears - 5/7/2010

Question
QUESTION: On a 66 Corvette 327/350hp (L79) when you pull the vacuum hose and plug it to set the timing, does it normally drop the RPM of the engine to the point that it will stall? Thanks!

ANSWER: Unfortunately I am on the road this week and my reference books are back in the office. If you can wait unit the week end I will have had a chance to check the vacuum line routing book and see if that line is supposed to have ported or manifold vacuum. for your engine.
Brad

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QUESTION: I believe it's manifold vacuum. The distributor vacuum connection is on the base plate of the carb, which pulls 13 inches at 800 RPM of idle. I have done a total vacuum on the car and it advances at a little more than 2900. The book which calls for 30 @ 3000.

ANSWER: You are correct that if it is on the base plate it is manifold vacuum and if the engine stalls when the vacuum line is disconnected and the carburetor idle is correctly set I would suspect a worn timing chain. My standard test for timing chain slop is to remove the distributor cap. With a wrench on the lower pulley nut turn the engine in one direction until the rotor moves. Now mark the timing damper to a reference on the block and turn the engine in the other direction until the rotor has just moved. This takes the slop out of the timing chain. If the vibration damper has moved more than 1/2 inch, the chain is loose and should be replaced. If the chain is OK then let me know and we will go one from there.
Brad

---------- FOLLOW-UP ----------

QUESTION: Thanks Brad. The timing chain, sprockets, lifters, push rods and cam are new (stock application). I also rebuilt the distributor, installed a new coil and resister, and had the alternator rebuilt and installed a new voltage regulator. The motor is completely stock to 350hp specs, right down to compression (11:1). I did a compression check on #1 cylinder while I had the plug out to set compression stroke and it was up around 210-215. The stock cam has a 222 duration and a 447 lift. Thanks for all you help Brad.

Answer
I'll write more to this answer when we get home tomorrow.
Brad
A question. Which type of automatic choke do you have on that car? The one with the hot air choke housing on the carburetor itself or the one with the choke in a well in the intake manifold?