jotto

In an effort to get my Vette running at her best I'm going back to basics.

Timing, lots of posts and good info on here but what is the best way to limit advance on the curve. I'm aiming for a max of 52 degrees to start with and tune from there.
In my case, the mechanical advance on my distributor is 14 degrees and the vacuum is 24 degrees. If I set my timing to 36 degrees all in by 3000 my initial is around 22 degrees. At this setting, my total is 60 degrees so what should I limit? I'm thinking vacuum would be easiest.
If I were to plot the timing curve, should it be pretty linear?

Once I have the timing set, I will move on to the idle mixture screws. Again, lots of info and I think I know the answer to the question but I will ask it anyway just to be sure.
Do I set my idle speed to the desired setting and then adjust idle mixture screws for greatest vacuum reading ensuring both screws are equally adjusted? ( will gently seat them first and then back out equally starting at around 1 1/2 turns ) I may need to keep adjusting idle speed as I adjust mixture screws. Also should I leave the vacuum can connected when setting mixture screws so the mixture will be correct for my idle conditions?

TIA.

toobroketoretire

The 36 degree @ 2000 rpm figure is recommended for drag racing when high octane gasoline is used but does not always pertain to street engines. For street engines you usually want around 29-30 degrees total in by 3500-4000 rpm to limit pinging. Yes, you can increase it but the more you do the more likely you'll end up with pinging problems under a heavy throttle when using 87 octane.

The 52 degrees is the desired total advance at cruising speeds so select a vacuum advance that will give you that when combined with the initial and mechanical advance. I recommend no more than 10-12 degrees initial advance to prevent hard starting so with say 12 degree initial and 14 degree mechanical (26 degrees) you would want a vacuum advance of 26 degrees to arrive at that 52 degrees.

Good luck...................

jotto

Whilst I appreciate your reply, the research I have done and information posted over the years, I still believe a SBC likes a total of around 36 degrees all in by around 3000 rpm. I know fuel mixtures are changing and the use of more ethanol etc changes timing required but here in the UK our decent pump fuel is still pretty good at 95 RON which I think is on a par with your 91 Octane? Not 100% sure about that though!

jnb5101

The 36* number is fine, if the engine doesn't detonate. The 22* advance number seems a bit high, but if the starter can spin the engine it can be used-although I'd reduce it about 6*and increase the centripetal the same amount. The vacuum advance is too much. You might feel "trailer hitching" or jerking at low speed cruise. A total of 52* is max for modern fuels. I limited the total on my SBC and BBC to 42*, and the BB is right on the edge of "trailering", but I think it's more a matter of it's cam.

jotto

So what mechanical advance is generally accepted as being a good place to start?

I have the Mr Gasket spring set somewhere so will experiment with that.
If I set it so all in at 36, trying to attain 18 initial and 18 mechanical and then limit vacuum to 16 that would get me to the desired 52.

What curves are you guys running?

toobroketoretire

It would take a dyno to find out what it "likes" so without a dyno you're just guessing. If 100 octane gasoline would allow 36 degrees of advance reason tells me 91 octane would allow about 32 degrees of advance because of it's 9 points lower octane rating.

pltmgr

I would limit the vacuum advance first. There are examples of home made limiters on the forum. Usually the mechanical advance is limited by the slot under the weights and is more difficult to alter. From what I read the "experts" (not me) say start with you desired total and let the initial be determined by your total minus your mechanical advance then work on your vacuum advance. If your car runs well with 22 degrees initial I'd leave it there.

Mixture screws- I use highest vacuum.

Big2Bird

16-18* initial with a 20* stop is good. All in by 3000rpm is good. If it pings, back off in 2* increments till it does not.

Big2Bird

He said 3000rpm, not 2000 rpm.

diehrd

32 degrees is ok for a GM fast burn head and many aluminum aftermarket heads like 36 degrees full advance @ 3000 rpm

Idle air , make sure the transition slots are set right and your motor idles at that setting , in many cases you will need to adjust the rear barrels to maintain proper idle speed while maintaining the correct transition slot setting.

Once your set on transition and idle use a vac gauge and set idle a/f to highest vac reading ..

Big2Bird

Read the Chevy Powerbook sometime.

diehrd

How about you just say what you mean ?

jotto

Ok, dumb question now but I have seen how to adjust the transition slots on a Holley but nothing about the transfer slots on the Quadrajet. Is this adjustable?

toobroketoretire

As the transition slots are machined into the base plate and therefore not adjustable the idea is to adjust the position of the throttle blades so the fuel is only being delivered only thru the slots and not the main discharge nozzles. For street engines that idle at less than 1000 rpm more than enough air/fuel can be delivered thru those small openings but for track racing engines that often idle well above 1000 rpm additional air/fuel is needed and that's where the mechanical secondary (aka double pumper) carburetors come into use with their 4-corner idle mixture screws.

diehrd

I assumed holley , I can not answer on a Q-jet ..

jotto

So once again I ask, how this is done on the quadrajet? I guess it is done by the idle screw as that seems to be the only way of setting the butterflies in any way. I have never seen a measurement for this on a qjet nor any mention of using the idle speed screw to adjust anything but the idle speed.

Shark Racer

If you have the carb off, you can see the transition slots in the base plate. Basically if you have the idle screw backed out all the way or just slightly engaged to hit your base, warm idle you should be good.

After that, you can set up idle bypass air if you need more idle speed with less throttle angle. Since you're running a stock engine IIRC, you shouldn't need to play with bypass air. Bypass air is controlled by some metered holes in the baseplate - many 70's carbs have them from the factory. Many Holley tuners do idle bypass air by drilling small holes in the throttle plates; Most Q-Jets don't need this.