Carb Vacuum and timing - Team Camaro Tech
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post #1 of 8 (permalink) Old Nov 12th, 14, 04:46 PM Thread Starter
Rick
 
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Carb Vacuum and timing

Put a rebuilt Rochester Quadjet on my 67 with the 327 275HP to try to help solve some stumble problems. Seemed to work better but not perfect. Wondering if it is a vacuum issue. The Carb sent to me has three vacuum ports and the original on the car only had one. Two of the ports read about 15 psi at idle and the other one reads 0 but goes up with engine speed. If I'm in front of the car one is one the left side by the choke (reads 15 at idle), one is on the front left side(read 15 at idle) and the other is lower front right side(reads 0 at idle). Which should I hook up the hose to. It is a HEI system.

Also I set the timing at idle to 8 degrees advanced. Is that about right for this motor. It is stock.
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post #2 of 8 (permalink) Old Nov 13th, 14, 06:20 AM
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Re: Carb Vacuum and timing

Here's a good link on vacuum tuning. Hopefully it will answer some of your questions.

https://www.centuryperformance.com/f...a-Vacuum-Gauge

'69 SS396 X66 L34 08D M21 BS
460ci Gen VI 9.75:1 AED 950 carb
241/249 @ .050" .625/.625 110 LSA

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post #3 of 8 (permalink) Old Nov 13th, 14, 07:05 AM
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The 15 psi port by the choke is manifold vacuum. Hook up your vacuum advance to that and make sure all unused ports are capped. I would suggest you find and read John Hinckley's Timing 101 articles and all your questions will be answered.
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post #4 of 8 (permalink) Old Nov 13th, 14, 07:54 AM
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Re: Carb Vacuum and timing

That would be 15 inhg...

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post #5 of 8 (permalink) Old Nov 13th, 14, 02:03 PM Thread Starter
Rick
 
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Re: Carb Vacuum and timing

Thanks everyone. I give it a go and see how it runs.
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post #6 of 8 (permalink) Old Nov 13th, 14, 04:20 PM
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Mike
 
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Quote:
Originally Posted by onovakind67 View Post
That would be 15 inhg...
Oops. I wasn't paying attention. Thanks for the catch.
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post #7 of 8 (permalink) Old Nov 14th, 14, 02:07 PM
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Steps
 
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Re: Carb Vacuum and timing

Quote:
try to help solve some stumble problems.
Please note tuning by vaccuum ONLY tunes in the idle circuits and the idle advance
A poorly tuned idle can cause issue with off idle stumble, and so can a poorly adjusted accel pump, as can a poorly designed cent advance curve , and a VA unit that has not been adjusted to the vaccuum requirements of the engine.
Getting the idle ciruits of the carb and the idle advance, tuning with a vacc gauge, is just the start of removing a stumble

Keep in mind as u adjust the dizzy initial , u are also adjusting how fast/ slow the cent comes in under load , and adjusting full WoT power above when the cent is all in.

So it really becomes a matter of having the idle u want , or having the power when the wheels are turning

Dialling in an engine is NOT simply looking at 1 part of the engine specs... its establishing correct mixtures and timing for WoT , idle AND cruise, each individually, then moding the carb cruise idle and WoT circuits.... plus the idle, initial , rate of curve, total and the VA in the timing.

And that CAN NOT be done with a vacuum gauge..
At minuim a measured distance and stop watch and vaccuum gauge...which is very hit in miss when compared to using Data logger with min of rpms and O2... preferably O2 IMAP TP sensor and knock sensor

Regardless do not have the initial +cent+ VA exceed 40/ 42 degs without using a knock sensor.... and check that after tuning the idle in...

My Spelling is not incorrect...it is creative

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post #8 of 8 (permalink) Old Nov 14th, 14, 11:40 PM
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Re: Carb Vacuum and timing

I would advance the timing a little and see how it runs.

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