Carter AVS Part 2 on the car tuning

[Geoff 2]

Remcharger & SCG. You need to brush up on your reading skills..& your ignorance.

[1] Where did I recommend total WOT of 50*??????????????
[2] At cruise, some engines will need up to 60* of cruise timing. [ And in rare cases over 60* ]
[3] In this thread, I have not recommended any change to initial timing or the centri curve in the dist.
[4] What I am recommending is changing the idle timing...maybe you pair don't know the difference.

 

[Geoff 2]

Martin,
Back to the job at hand.
Your VA unit adds 16 crank degrees. The factory initial timing for that engine is 5*. I would advance that to 10*. If you get pinging, back it back down to 5*. So with 10* init, you need 20* from the VA unit to give you the ideal 30* at idle. The VA unit gives 16*, so you are short 4*. Note that 4 is 1/4 of 16.
You need to remove the VA unit from the dist [ dist itself can be left in place ].
With the VA unit on the bench, you can see how the arm pulls in until it hits the main body. You need to measure that travel distance. And increase it by 1/4. Example: arm moves 0.160" to the stop. That distance needs to be increased to 0.200". You do that by filing/grinding the stops on the arm to get the reqd amount.
While you have the VA unit out, attach a hose & suck on it to make sure the arm moves until it hits the stop.
Turn the Allen head fully CW until it stops or clicks. VA unit can now be refitted.

Connect VA unit to manifold vacuum, start & warm up engine. Check/set initial timing once engine is warmed up. Then re-adjust idle speed/mixture screws. Do all this in gear if auto trans.

Ready for a test drive.

 

[HS 69 Charger]

@Geoff 2 . So the biggest change is moving the Vac Adv from port vac to manifold vacuum. After our previous adjustments (especially low rpm screw) we have the throttle plates mostly closed (020-040) transfer slot so we have very little port vacuum at idle hence no Vac Adv at idle and only the base initial timing 5* (even if I moved to 10* its still way below what we found as desired. Now we move over to Manifold and we basically get all the vac adv at idle. Even with no modifications if I set initial to 10 and add vac adv I would be at 26 which is much closer to the ideal number. So even without any modifications I would be substantially closer to ideal simply by moving to manifold vacuum and moving initial from 5-10*
So my first question at this point is does manifold vacuum also step down during aggressive acceleration. Its easy to see how port vacuum drops down but does Manifold vacuum also drop down. I'm guessing its less but still serves the whole purpose vacuum advance is targeted for?

 

[AR67GTX]

Martin, when the throttle is wide open the manifold vacuum (and ported vacuum) drops essentially to zero. It’s not really zero but depending on the engine it’s down so low in single digits that the advance canister can’t activate. So under hard acceleration there is no vacuum advance (with either ported or manifold vacuum) and all the engine is seeing is your initial or base timing and the centrifugal timing - somewhere typically between 34 to 36 degrees on the street when centrifugal is all spun in, depending on the engine and fuel. Cars running racing fuel can often run 40 degrees, maybe even a little more.

 

[Don Frelier]

The 383 in my charger is set to 35 degrees BTDC at idle.

 

[sc440]

@Geoff 2 . So the biggest change is moving the Vac Adv from port vac to manifold vacuum. After our previous adjustments (especially low rpm screw) we have the throttle plates mostly closed (020-040) transfer slot so we have very little port vacuum at idle hence no Vac Adv at idle and only the base initial timing 5* (even if I moved to 10* its still way below what we found as desired. Now we move over to Manifold and we basically get all the vac adv at idle. Even with no modifications if I set initial to 10 and add vac adv I would be at 26 which is much closer to the ideal number. So even without any modifications I would be substantially closer to ideal simply by moving to manifold vacuum and moving initial from 5-10*
So my first question at this point is does manifold vacuum also step down during aggressive acceleration. Its easy to see how port vacuum drops down but does Manifold vacuum also drop down. I'm guessing its less but still serves the whole purpose vacuum advance is targeted for?

With carburetor blades properly adjusted there should be no vacuum at idle on the ported vacuum port, as soon as the throttle blades open up to a certain point there is a hole in the throttle bore that is uncovered and allows vacuum to go to the ported nipple. Once that hole is exposed at I certain position of the throttle blade the vacuum is the same at both ports. So no vacuum on ported until throttle opens a little then same vacuum on both ports.

 

[Geoff 2]

Martin.
At 26* for idle, you are verry close to the ' happy' spot. So I would try it there, do a test drive, watch for surging & flat spots. Do you have the factory air cleaner?

 

[Ray70Chrg]

@Geoff 2, I've been following the thread closely and I assume this procedure would be the same for a carter afb carb. Also, why do you want the original air cleaner to be used?

Thanks, Ray

 

[Geoff 2]

Ray.

Yes, same procedure for AFB carbs.

The reason I was asking about the air cleaner the OP is using:
- OEM units can be quite restrictive at higher rpms, act as a choke, & richen the mixture
- the new cam will extend the rpm range slightly
- these #46XX carbs are jetted very lean. I was going to recommend that Martin increase the pri & sec jets by one jet size if he is using a low restriction aftermarket air cleaner [ worth trying ]
- hence the question about air cleaners.