General Category > Quadrajet Carb Talk and Tips
cam selection
Glutenfreecarbs:
I agree with Kenth. ALWAYS connect the distributor to full manifold vacuum; "ported" vacuum is just a primitive emissions aberration.
And I've seen either 23° or 30° idle timing for stock TBI 350s on various squarebody (and Camaro?) forums.
Cliff Ruggles:
It is NOT always possible to use manifold vacuum to the advance.
For some combinations you aren't going to have enough vacuum at idle to effectively use it. I run into that all the time here tuning engines that don't make enough vacuum to employ the vacuum advance.
In these scenarios you MUST locate a vacuum advance that is either adjustable for when it applies all the timing or one that has a really "low" rating on it.
There are also engines out there that are already making enough vacuum at idle with reasonable advance in them that adding more has the throttle plates almost or completely closed and they will not idle down to the desired idle speed. I run into that scenario on occasion as well.
The most common scenario for troubled engines brought here for custom tuning is that the engine builder or whoever chose all the parameters LOWERED the static compression ratio (for pump gas) then installed some whiz-bang modern lobe profile cam to try to bring all the power back.
Yes, companies selling short seat timing, fast ramp, tight LSA camshafts advertise that those camshafts will idle better, improved throttle response, better fuel economy, more power everyplace, walk on water, tuck you in at night and cook breakfast for you in the morning. It's marketing genius but in the real World lowering compression hurts power, vacuum, throttle response and fuel economy all else being equal, and closing the intake valve early and adding overlap isn't going to do much beyond killing off upper mid-range and top end power and put a little "attitude" in the exhaust note.
You'll also find yourself requiring a LOT of timing at idle and fuel to the mixture screws to make those engines happy. You will also need to speed up the mechanical advance curve as well. Here in lies the problem that I see. About 80-90 percent of the customer base installs some POS distributor advance weight/spring kit, unhooks the vacuum advance (most add too much timing now that we are running so much initial and bringing in all the mechanical right off idle).
This makes for a final product that doesn't idle well, lackluster power production, and all sort of tuning issues because the owner/tuner doesn't have the skill sets to improve the carburetors idle system to supply adequate idle fuel at the much lower than stock readings and some of the mechanical spark curve is in at idle so idle tuning is difficult or impossible right to start with.
I make a nice living sorting these things out and have for decades.
The FIRST thing I do with 99 percent of the vehicles brought here for tuning is to remove the distributor cap and replace the cheap Chinese weights and springs with OEM parts. Once I get the timing under control and NOTHING in below about 900-1000rpm's I move on to the carburetor an get the idle system up to par for the engine combo.
We put the carb back on, set the timing, hook the vacuum advance back up (in most cases it wasn't being used) and guess what? The owner comes back from their first test drive grinning from ear to ear, well, at least until I unload their wallet!
I started a thread on some of these tuning sessions and will be adding to it, time permitting. Right now I'm so busy I don't have time to reach around and scratch my backside........Cliff
tayto:
switched to manifold vac over a week ago. have about 12 or 13* @ idle now, from 7*. with my latest fillup looks like i picked up 1 or 2 mpg. definitely seems to like it at idle better, vac needle isn't vibrating like it use to and with the throttle plates closes vac slightly increased too. 16" in park, 14" in gear. still would like to try and close up the lower IAB but have not been able to source small brass set screws locally. i will probably be calling you soon Cliff there's some other stuff i would like to order as well.
Cliff Ruggles:
"switched to manifold vac over a week ago. have about 12 or 13* @ idle now, from 7*."
I'm confused? Timing at idle went from 7 degrees to 12 or 13 degrees?
Then below that you mention that vacuum increased to 16" in park and 14" in gear......
tayto:
Yes my timing at idle went from 7* to 13*.
My vacuum @ idle in gear went from 13" to 14". I assume this is because i was able to close the throttle plate more. Hopefully I'll have time this week to call.
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