timing a big block chevy - Nova Tech
Nova Tech  
Find Nova Parts at SS396.com     
GROUND UP & SS396.com         
Official Sponsor of Team Nova
   

Auto Insurance



Engine General Engine Discussion.

Novas.net is the premier Chevrolet Nova Forum on the internet. Registered Users do not see the above ads.
Reply
 
Thread Tools Search this Thread Display Modes
  #1  
Old May 21st, 10, 10:13 AM
seitzman seitzman is offline
Tech Team
Ken
 
Join Date: Oct 2007
Location: wisconsin
Posts: 68
seitzman has a positive reputation
Default timing a big block chevy

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
Reply With Quote
Sponsored Links
Advertisement
 
  #2  
Old May 21st, 10, 11:40 AM
DRC68SS DRC68SS is offline
Tech Team
Doug
 
Join Date: Jan 2010
Location: Stupid *** tax ripoff Jersey
Posts: 187
DRC68SS has a positive reputation
Default Re: timing a big block chevy

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.
__________________
68 SS 350 Nova (now 383 stroker)
Reply With Quote
  #3  
Old May 21st, 10, 11:53 AM
seitzman seitzman is offline
Tech Team
Ken
 
Join Date: Oct 2007
Location: wisconsin
Posts: 68
seitzman has a positive reputation
Default Re: timing a big block chevy

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.
Reply With Quote
  #4  
Old May 24th, 10, 07:36 AM
seitzman seitzman is offline
Tech Team
Ken
 
Join Date: Oct 2007
Location: wisconsin
Posts: 68
seitzman has a positive reputation
Default Re: timing a big block chevy

What do you guys think is it timing or do I need to look at the jetting on the carb?
Reply With Quote
  #5  
Old May 24th, 10, 12:52 PM
Big Dave Big Dave is online now
Site Moderator
Dave
 
Join Date: Mar 2006
Location: Tampa FL
Posts: 14,915
Big Dave has a positive reputation
Default Re: timing a big block chevy

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
Reply With Quote
  #6  
Old May 24th, 10, 10:15 PM
seitzman seitzman is offline
Tech Team
Ken
 
Join Date: Oct 2007
Location: wisconsin
Posts: 68
seitzman has a positive reputation
Default Re: timing a big block chevy

Thanks for the info, I will give that a try, sounds like what is happening
Reply With Quote
  #7  
Old Jun 6th, 10, 04:52 PM
seitzman seitzman is offline
Tech Team
Ken
 
Join Date: Oct 2007
Location: wisconsin
Posts: 68
seitzman has a positive reputation
Default Re: timing a big block chevy

OK I checked the vacum on the motor and it has about 14 inches of vacum but the gauge has a tremedous amount of rapid fluctuation, not a lot of movement in vacum range just a lot of flutter what do you think the problem is
Reply With Quote
  #8  
Old Jun 6th, 10, 07:39 PM
71BBNOVA4sp 71BBNOVA4sp is offline
Senior Tech Team
Richard
 
Join Date: Nov 2007
Location: Gainesville Raceway, FL
Posts: 540
71BBNOVA4sp has a positive reputation
Default Re: timing a big block chevy

If the gauge holds with some needle shake with an aftermarket cam this can be normal with Hp cams- high lift long duration because of valve overlap
__________________
if it aint a classic it's crap
Reply With Quote
  #9  
Old Jun 27th, 10, 09:53 AM
seitzman seitzman is offline
Tech Team
Ken
 
Join Date: Oct 2007
Location: wisconsin
Posts: 68
seitzman has a positive reputation
Default Re: timing a big block chevy

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.
Reply With Quote
  #10  
Old Jun 27th, 10, 01:32 PM
Big Dave Big Dave is online now
Site Moderator
Dave
 
Join Date: Mar 2006
Location: Tampa FL
Posts: 14,915
Big Dave has a positive reputation
Default Re: timing a big block chevy

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/TechServi...wer_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
Reply With Quote
Sponsored Links
Advertisement
 
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Big block chevy block number seitzman Performance 10 Mar 21st, 10 07:58 PM
big block chevy heads Logan Engine 17 Jan 14th, 10 10:26 PM
71 small block to big block frame mounts 5525scuffy Engine 1 Nov 25th, 09 07:57 PM
Big block timing question blk74nova Performance 20 Jun 20th, 09 05:03 PM
big block heater box vs small block rustysrustyduc Heating & Cooling 7 Jun 3rd, 09 08:32 PM


All times are GMT -4. The time now is 09:44 AM.


2009 AutoForums