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timing a big block chevy

32K views 9 replies 4 participants last post by  Big Dave 
#1 ·
I need to check the timing on my big block and need help! do I unhook the vacum advance before I start? what should my timing be set at I am thinking 6 to 8 degrees advance at idle. the engine has a small cam, headers, high rise intake with a 750 holley carb thanks for the help
 
#2 ·
If just a basic BBC, yes, 6 to 8 degrees at idle with vac. adv. unhooked and plugged should run decent all around.

A better check is total advance, with vac. adv. plugged a basic BBC is comfortable at about 38 degrees all in at around 3,000 rpm's. That would also tell you your distributor is working well.

Good luck. :thumbsup:
 
#3 ·
I just checked it at idle and it is right around 12 degrees advanced, that seams like alot. I started having a slight backfire as I let of the gas to down shift after I changed to the holley carb. That is what led me to look at the timing as I never had that problem with the edelbrock carb I replaced and I suspected timing as the issue.
 
#5 ·
Your back fire has nothing to do with timing. 12 degrees advanced at idle with vacuum advanced disconected is about what I would expect from your set up. "Mild" is a relative term (about as informative as 3/4 race and full race was back in the fifties when describing a cam). Your cam's duration and overlap will dicate your air fuel mixture at idle which has a direct bearing upon your initial timing numbers.

Your back fire is because the engine running lean as you closed the throtle bores at high RPM on the down shift. Well I am assuming your foot went off the gas and onto the brake pedal while down shifting as that is generally why some one down shifts (to slow down suddenly using engine compression toi assist rear braking). You need to stick a vacuum gauge on your intake and measure engine vacuum with your cam as you may have to change your power valve to compensate for a lack of fuel. (Holley tech site has an right up on how to pick your powerr valve based upon engine vacuum).

Big Dave
 
#9 ·
Still working on this problem, I checked the vacum again and have right around 14 on the gauge what size of power vavle should I put in this thing? I pulled the plugs and they look good not fouled at all.The car starts and runs great just getting that backfire when I let off the gas or down shift.
 
#10 ·
You divide your 14 by two to get 7 which is a whole number. They only sell fractional numbered power valves for some reason so the size you want is reduced by a half size to a 6.5 that will be marked 65 on the valve.

http://www.holley.com/data/TechService/Technical/power_valves.pdf

You may be running lean at idle (which promotes high EGT and overheating) due to your reversion from your cam. An examination of the throttle bores will indicate how much of the transistion slot is uncovered to get your car to idle. The first photograh shows where the throttle plates should be at idle:



the second picture shows where they often end up if your cam has a reversion problem.



The solution is to bore two small holes in the throttle plates of the primary bores to allow more air to flow when the throttle is closed in the hopes of drawing in more gasoline to feed the engine. If you where to fatten the mixture by increasing the main jet size at idle it runs pig rich at high RPM. You start ot slowly with a small hole and keep increasing the size until you get a positive responce or the motor runs worse. Holley sells the throttyle plates with holes of various sizes already stamped in them an a set of two with a small hole is a good place to start.

Big Dave
 
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