timing question

[john1977]

I recently had my 77/350 rebuilt, speed pro hp kit,flat top pistons,mild cam,edelbrock performer intake,true dual exhaust,magnaflow mufflers,rebuilt stk HEI dist. rebuilt stk Q-jet carb.,magnaflow headers. 9.5 : 1 comp. 350hp. 3.36 gears. This car won't turn the tires over!!! Although it is very quick and responsive. Now the question, when I set the timing , 12degrees it has a nice lope but as soon as the vaccum line is hooked up I loose the nice sounding lope. Am I doing something wrong here or what?

 

[JohnZ]

Do you want it to run right, or have a "lope" at idle? Base idle timing (vacuum disconnected) doesn't mean much for tuning unless you know how much centrifugal advance your distributor is providing and when it starts and stops. You want about 36-38 degrees total timing (base plus centrifugal, no vacuum) at high rpm, and you want the centrifugal advance to start 100-200 rpm above normal idle speed and be "all in" by 2800-3000; that will determine what your base timing setting should be, not a "book" number. Then when you connect your vacuum, it will add 12-15 degrees to your base setting, which will increase your idle speed, so you then re-adjust your idle speed screw to get your desired idle speed and re-adjust the idle mixture screws for highest steady vacuum at that rpm.

Most distributors provide around 24 degrees of centrifugal advance, so with 36-38 degrees of total timing, your base (no vacuum) timing setting would be around 12-14 degrees, and around 25-30 degrees when you connect the vacuum advance.

For a complete technical article I wrote that explains the theory and operation of timing and vacuum advance in understandable terms, check out the October issue of "Corvette Enthusiast" magazine - should be out in a week or so.

 

[Robin7TFour]

john, thamk you for the detailed answer. that will help me dial in my 74. :duh

 

[clar2001]

your base idle vacum sould not change your timming on Q-jet until your throtle body starts to open, then this allowes vacum maby it is connected in the wrong spot.

 

[john1977]

my dist. vacum is connected to ported vacum, is this not where it should be ?

 

[clar2001]

If you remove the vacum line you should not fell any suction on that spot on the carb. until you are off idle, if there is a vacum there at idle there is a problem .

 

[john1977]

Ok there is vacuum at idle where my dist. is plugged to, you say that I should find a port that only pulls vaccum when the carb starts to accelerate is this right?????

 

[john1977]

Just checked the vaccum where dist. is pluged to, 17in of vaccum at idle.

 

[JohnZ]

I won't go into all the gory detail (it's in the "Corvette Enthusiast" magazine article), but you will get much better performance, a stable idle, and improved idle cooling with the advance connected to full manifold vacuum. "Ported" vacuum was an interim band-aid emissions strategy before the days of catalytic converters, intended solely to retard timing at idle to help the A.I.R. pump system more efficient, and nothing more.

Most people don't understand the theory and operation of timing and vacuum advance, including many "mechanics", especially since most modern cars don't even have distributors any more; the article explains it so you can deal with it.

 

[clar2001]

On a stock HEI distrubtor if you have a ported vacum/direct vacum, that will fully advance your dist timming . You should only have vacum at off idle.
Most Q-Jet the dist hookes up in front drivers side .
John when you plug your dist. vacum in at idle does your rpm increase . thuse losing your lope from the cam.
Do you have a chiltons manual to look up the dist hook up .I tryed posting a pic but it dident work ,I do have a pic if you want me to e-mail it to you,
Weren't you also haveing a problem with a dead spot from idle when hitting the gas . this could also cause that problem.

 

[john1977]

Yes my rpm's increase when I plug the dist. hose back on. and I have to adjust back down. No I don't have a current chilton manual. I never had a stumble,it was more like a small delay before the secondaries came in and have adjusted to come in quicker. My dist. hose was pluged into the front of carb on the drivers side, but I do have 17 in. of vacuum at idle at this port. I have a unported vacuum tube to the rear center of carb. just below the secoundaries butterfly. If you want to e-mail picture jlwood@ee.net Thanks for all your help; jw