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Old 07-19-2008, 10:54 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 07-20-2008, 09:35 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, 10:08 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, 10:56 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, 02:43 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, 09:28 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, 10:07 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, 12:50 PM
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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 12:59 PM.
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