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Old 07-19-2008, 09:54 AM
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Timing/carb idle mixture adjustment

Ok, I wish to understand completely the relationship between timing and the low speed air mixture screw adjustment on my 350 sbc. This is what I am doing, and what I have always done. I have a bog I would like to workout. Is this correct?

By the numbers;

1. Lock centrifugal advance weights in full advance.

2. Plug vacuum advance vacuum line.

3. With warm motor running at 2000 rpm, use Advance timing light to adjust all in timing @ 32 degrees.

4. Adjust idle linkage screw to allow motor to now idle at 850 rpm and set air mixture screws for highest vacuum.

5. If idle after air mixture screw adjustment climbs, close linkage idle screw to recover 850 rpm and continue to adjust air mixture screws for max vacuum.

6. unlock centrifugal weights & return vacuum advance to operation.

7. Use advance timing light to check maximum total advance and make note of what rpm range this occurs. Get all the timing in by 2000 rpm using necessary springs & weights.

Is this the way it is done?

What if my motor is difficult to turn over when starting because of too much initial advance timing?

I hate breaking the nose off of starters!

Thanks guys.

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Old 07-19-2008, 10:26 AM
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Your motor is one of those border line motors as to wether locked out timing is best or not.
With the timing advance locked out, it won't move with rpm so you don;t need an advance timing light for that.

If the locked out timing is just too much:
You'll want around 24deg initial at idle and somewhere around 30 to 36deg at max rpm advance 3000 ish. often vortec motors with proper jetting and healthy compression ratio don't need much max timing for best power. (fastburn head)
Leaner jetting or richer jetting than optimum will tend to want more timing.
Don't be fooled by this. The right jets will want the least amount of max power timing. The only way to find that is testing. 29 to 36deg is normal for this motor.

(with the Isky cam) The more idle timing the better. (best idle quality, throttle response and manifold vacuum) If it runs around fine with the mech timing locked out go with it. If not, dial in the required curve with lots of base idle timing. 24deg or so. but 30 to 36deg max timing.
That means a very short but smooth mech advance curve. say 10-12deg.
You find the max timing point by testing on the track (best MPH) or on a dyno. start at 30deg. the motor will tell you what is best. (best mph)


Then you want around 12 to 15deg of vacuum advance for cruising (ported vacuum) the rate in and out will need to be custom dialed in.

If the motor is hard to start when hot, you can install a ignition cutoff switch on the power lead to a HEI distributor or on the power switch on wire on a MSD box to disable the spark during hot cranking with lots of spark advance at idle.
You can also add a cranking spark retard box (MSD) "starter saver" that knocks the spark lead back during starter cranking.
All Fords have this built into the OEM spark box, GM is just stupid.

I suggest you start with slightly rich (safe) main jetting in the carb.
On 600's like the 1850 I like 66 pri jet and a #21 sec metering plate as a starting point for tuning. You can rob the sec metering plate from a holley 3310 carb (#21)
equals a #75 main jet size. This will probabily be your cold/fall season weather best power jetting, but probabily a little fat for a hot humid summer day. But a good safe staring point.
Adjust the idle mix screws for best idle and manifold vacuum. May need to remove the carb and flip it over and preset the throttle opening at idle on both the primary and secondary throttles so the transfer slot exposure is right at idle. A yellow or purple sec diaphram spring will usually be right on.
may need a larger accel shooter and fine tune the accel shooter cam.

What is the max day temp and altitude there?

I sugggest a Champion RS10YC spark plug.

Don't forget to hang on !!!!

The only way you'll ever break the nose off a GM starter is if you use the wrong starter bolts and leave the end support brace off. If you don't have the end brace, get one. (GM dealer or wrecking yard)

Last edited by F-BIRD'88; 07-19-2008 at 11:11 AM.
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Old 07-20-2008, 08:35 AM
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Quote:
Originally Posted by DENCOUCH
Ok, I wish to understand completely the relationship between timing and the low speed air mixture screw adjustment on my 350 sbc. This is what I am doing, and what I have always done. I have a bog I would like to workout. Is this correct?
NOT completely.

Quote:
Originally Posted by DENCOUCH
By the numbers;

1. Lock centrifugal advance weights in full advance.
Do not do this!

Quote:
Originally Posted by DENCOUCH
2. Plug vacuum advance vacuum line.
This is correct. Remove it from the vacuum cannister and plug the hose at that end or directly from the vacuum port being used.

Quote:
Originally Posted by DENCOUCH
3. With warm motor running at 2000 rpm, use Advance timing light to adjust all in timing @ 32 degrees.
Watching the advance timing light, increase the RPM until the timing mark no longer continues to move. Note that RPM. It should be in the 2500-3000 RPM range. If not swap the advance weight return springs until the timing stops in the RPM range I listed above. Usually for a stock GM HEI using one medium spring and one light spring will work OK.

After you have gotten the timing to stop in the above RPM range, adjust the timing in the 32-38 degree range. This is your Total Mechanical Timing. Vortec heads work best around 34 degrees. All other heads work best in the 36-38 degree range.

Quote:
Originally Posted by DENCOUCH
4. Adjust idle linkage screw to allow motor to now idle at 850 rpm and set air mixture screws for highest vacuum.
If using an automatic trans, set the idle to 650-750 RPM in DRIVE. The idle RPM in PARK will be higher by a couple of hundred RPM. Be sure to have the parking brake applied and a helper with the brakes applied as well, whenever setting/checking the engine RPM while in gear. Any higher than that and you will begin to get some mechanical advance as it will begin around 800 RPM. If you do have to idle above the 800 RPM then the reading you get for your initial timing will be incorrect.

Quote:
Originally Posted by DENCOUCH
5. If idle after air mixture screw adjustment climbs, close linkage idle screw to recover 850 rpm and continue to adjust air mixture screws for max vacuum.
This is correct except for the idle RPM. See answer above.

Quote:
Originally Posted by DENCOUCH
6. unlock centrifugal weights & return vacuum advance to operation.
Centrifugal weights never should have been locked. Yes as to returning the vacuum advance to operation. If using FULL manifold for your vacuum source the RPM will increase and should be lowered back to the correct idle speed. I recommend using the FULL manifold vacuum source.

Quote:
Originally Posted by DENCOUCH
7. Use advance timing light to check maximum total advance and make note of what rpm range this occurs. Get all the timing in by 2000 rpm using necessary springs & weights.
Checking the timing with the vacuum advance hooked up will yield the Total Timing and will freak most out because the figure will be the initial + the mechanical + whatever the vacuum cannister added. Could be in the 50 degree range. This is normal and OK as under WOT the vacuum is not here. The figures could be that high under cruise conditions only.

The total mechanical timing (without the addition of the vacuum advance) should occur in the 2500-3000 RPM range for most street applications.

This is accomplished by changing the centrifugal advance weight return springs and/or the weights and sometimes by limiting the total travel the weights can move. This is usually done before actually setting the position of the distributor for the timing in degrees that you wish to use.

Quote:
Originally Posted by DENCOUCH
Is this the way it is done?
See my comments above.

Quote:
Originally Posted by DENCOUCH
What if my motor is difficult to turn over when starting because of too much initial advance timing?
Lower the initial advance. There are other methods that are used with locked out timing, but are usually for race applications rather than street.

Quote:
Originally Posted by DENCOUCH
I hate breaking the nose off of starters!

Thanks guys.
Breaking off the nose of the starter could be caused by improper timing and severe engine 'kickback'. It can also be caused by using the incorrect starter mounting bolts, loose mounting bolts, and the lack of the end support brace.

I'd look at the timing first.
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Old 07-20-2008, 09:08 AM
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Quote:
Originally Posted by DENCOUCH
I have a bog I would like to workout.
Your 'bog' more than likely is due to not enough fuel when going to WOT either from a stop or a low speed roll. Try increasing the squirters by two sizes and then re-test.

Do this after setting your timing as I suggested above.
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Old 07-20-2008, 09:56 AM
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Thanks Fellas

Thanks for the help with this. I am on the timing & tuning today as yesterday was spent replacing a timing chain cover & gasket. The motor is back together again & ready to run.

I added a HEI cut-out switch to help with the starting.

I need a starter front brace... my starter is missing one, so off to the GM parts dealer tomorrow.

Thanks again.
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Old 07-20-2008, 01:43 PM
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Okay, I have been doin' stuff this morning.

I have the Vacuum advance out of the loop, and dialed in the distributor @ 32 degrees BTDC when the advance is all in @ about 2200 RPM.

With the Vacuum advance still out of the loop, when the idle is adjusted at 800 RPM (in PARK), the advance is 10 degrees.

Question; Is it now time to limit the total advance on the centrifugal advance from 22 Degrees to something closer to 10 degrees?

I am fishing for 16 Degrees at idle, and as I see it, I am going to have to shave off some advance someplace!

Thanks.
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Old 07-21-2008, 08:28 AM
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Quote:
Originally Posted by DENCOUCH

I added a HEI cut-out switch to help with the starting.
Is this a street or race vehicle?

The above switch is usually used for race only applications where the initial timing is very high or the centrifugal advance has been locked out.

With the initial timing you are running (10 degrees) or what you want (16 degrees) you should not have any problems turning the engine over to start.


Quote:
Originally Posted by DENCOUCH
I have the Vacuum advance out of the loop, and dialed in the distributor @ 32 degrees BTDC when the advance is all in @ about 2200 RPM.

With the Vacuum advance still out of the loop, when the idle is adjusted at 800 RPM (in PARK), the advance is 10 degrees.

Question; Is it now time to limit the total advance on the centrifugal advance from 22 Degrees to something closer to 10 degrees?

I am fishing for 16 Degrees at idle, and as I see it, I am going to have to shave off some advance someplace!
Since you want to keep the Total Mechanical Timing at 32 degrees and you want the initial to be 16 degrees, you will have to limit the total movement of the centrifugal advance plate. You appear to have a total movement of 22 degrees at the present. To get the initial to 16 degrees and the total to 32 degrees you will have to limit the movement to 16 degrees. This can be accomplished by blocking off a small portion of the advance plate slot by adding a small amount of weld (this is a trial and fit method). Some distributors have restrictors available to accomplish this. Check MSD.
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Old 07-21-2008, 09:07 AM
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Thanks!

I yanked out the distributor & broke it down last nite. After transcribing the total advance travel swing on the base plate, I drilled the plate to accept a pressed in roll pin and bushing. This serves as a stop for the advance. I am going to have to use the timing lite to determine how close my machine work is, and I shall try her out today to see where my timing lies.

Thanks
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Old 07-21-2008, 09:37 AM
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Quote:
Originally Posted by Frisco
Is this a street or race vehicle?

The above switch is usually used for race only applications where the initial timing is very high or the centrifugal advance has been locked out.

With the initial timing you are running (10 degrees) or what you want (16 degrees) you should not have any problems turning the engine over to start.
This is a street car with a racey 106 LSA cam in it. if you want best idle and throttle response, you want more than 16 deg of intial timing.
This cam has a lot of overlap which causes a lot of EGR effect at idle which slows the fuel burn speed at idle and low speed, requireing more initial timing at idle.

If you set the distributor curve up as I recomended, (24 at idle, 30-36deg at max advance and 12 to 15deg of ported vacuum for cruising, using the fuel and spark plugs I recomended, the idle and throttle response and spark plug life will be better and much more stable in and out of gear.

The switch on the ignition allows you to run this agressive timing curve without starter kick back.
A "starter saver" cranking retard box is the more expensive fix.
Both work.
If you only use 10- 16deg if initial spark timing at idle, like you would use with a milder cam with a wider LSA (street cam) and then depend on manifold vacuum advance, Guess what happens when you put it in gear? Manifold vacuum drops and you loose the spark timing. Now the motor loads up and heats up and right when you need the spark advance for good throttle response, it drops further as you punch the throttle. (manifold vacuum drops= timing drops= poor throttle response.)
So you readjust the idle speed screw to get the idle speed back. What happens now when you go to neutral? the manifold vacuum rises and increases the spark timing back and the rpm (idle speed rises) and so does the mechnical advance cause you used very light advance springs, flairing the idle even more severly.
Your method of employing manifold vacuum controled advance to get the idle quality works (sort of) on mild, moderate cammed motors with wide LSA but not with these types of high overlap cams.

With this high overlap cam, my set up is a lot more cleaner and stabler.
The mechanical advanc curve is much smoother and does not bounce from too light a spring tension.
The throttle response is much better and does not degrade after idleing in gear even all day as yours will.
The idle and timing do not flair up when going from idling in gear to neutral.
The carbs throttles are in the right position at idle.

When you employ straight manifold vacuum: The ignition timing will also flair way up when ever you get off the throttle at high rpm speed cause the vacuum advance always follows intake manifold vacuum.
If/when you suddenly go back full throttle like during a shift. You have way to much timing ( full mechanical and full vacuum advance) right when you only want full mechnical advance) This is how you bust piston ring lands on a shift.
This why drag racers want to eliminate the vacuum advance.
When set up like this, it busts motors.
The problem is not the vacuum advance, its how its controled.
My way, the timing does not flash during a shift and does not bust pistons.

If you use ported vacuum advance: when you suddenly close the throttle at high rpm (like during a shift) , vacuum advance stays at 0 cause the the throttle is closed.
Now you jump back on the throttle and do not have excessive spark advance and do not bust pistons.

Put a timing light on the motor and try it your way.
10deg base timing and 24 deg of mechanical advance travel and light mechanical advance springs gets you full advance at 2200-2500rpm and about 5 to 6 more deg of mechanical advance at 900rpm (your idle speed in neutral) = 16deg at idle (900 rpm) + some vacuum advance say 8deg.
you have 10+6+8 deg of combined timing 24deg in neutral at 900rpm.
Now put the trans in gear.
Idle speed drops, mechanical advance drops, idle speed drops further, available manifold vacuum drops and vacuum advance drops, power valve fluctuates open and closed, idle goes up and down further fluctuation timing and manifold vacuum and power valve causing a over rich idle.
idle quality deteriorates as spark plugs load up from over rich mixture and incomplete idle combustion. you have about 10 to 16deg at idle best case when the motor wants 24++deg at idle.
Put the motor in gear and the idle flairs back up over 1200rpm.
repeat as nesssessary. Don't forget to buy stock in Spark plug company as you'll be replacing fouled plugs often.
You soon grow to hate your car. cause it idles inconsistantly eventually loads up in gear, flairs up when going to neutral from in gear and runs hotter than it needs to.

My way gives the proper amount of idle timing in gear and in neutral. (24deg)
The motor idles in gear just as clean as in neutral and does not flair when going from in gear to neutral cause the spark timing doesn't not change.
mechanical timing is smooth and stable from right off idle to full advance (around 3000rpm) and does not bounce, cause the spring tension is heavier.

Spark plugs last years, not weeks. The engine has the right amount of spark advance at all times, not just some of the time. You do not bust pistons from way over-advanced timing during a shift.
My method does require a simple $10 spark interupt switch and/or crank retard function to help the dumb cheap GM starter motor work when hot.

Ford has had this simple function in their Duraspark spark boxes for over 25 years.
Chryslers have a gear reduction starter that can spin over a diesel motor.
You never see a Ford guy with a hot start issue. Stock or race what ever.

No more hot start, no more burned out starters or broken starter noses.
No more hot running loaded up motor. No more goofy flairing idle.
No more crappy throttle response. All for a $15 switch and or a $85 starter crank retard box. No more I hate this car!

Try it.

The lil switch also is a great anti theft device.

Last edited by F-BIRD'88; 07-21-2008 at 09:56 AM.
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Old 07-21-2008, 11:50 AM
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Thanks!

Yesterday I installed an HEI cut-off for starting .
I have to put the timing light on it, and am hoping that I was able to limit the centrifugal advance to 10-12 degrees with my modification.

If the timing tests prove this out, then I can dial in my initial timing at 24 degrees, and that the total works out to 34-36 degrees.

Roger on the ported vacuum for the vacuum advance, thanks!

I have not started her up today because my morning has been filled up with some DR. procedure stuff that makes me have to stay close to the john

I appreciate the in depth explanation you have taken the time to illustrate. This is theory I can go with! I am going to go with it... it has already been proven to me that what you offer is the stuff that I have been looking for.

Thanks.

Last edited by DENCOUCH; 07-21-2008 at 11:59 AM.
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