Distributor tuning

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Distributor tuning

Postby Asolepius » Mon Jun 16, 2008 9:44 am

I have determined that I need more centrifugal advance. I get best power on the dyno when the static timing is 8 deg BTDC, but then starting is slightly tricky (coughs and kicks back when cold). Starting is OK when I go for 5 deg, but then top end power is a bit down. Not getting any detonation. So I think I need a few more degrees advance at about 5,600 rpm. Can anyone point me to some easy instructions for modifying the advance curve? I am using a 45D distributor with Pertronix electronic pack.
Les Rose, Salisbury, UK
1959 Sprite, owned since 1978
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Re: Distributor tuning

Postby Charlie Tolman » Tue Jun 17, 2008 12:56 am

The usual method to increase the centrifugal spark advance vs. rpm, is to install weak (light) springs in the advance mechanism under the contact-breaker base plate.

However, at 5600 rpm, the centrifugal advance mechanism in your distributor might have reached its mechanical stop. This mechanism, which also has the distributor cam, is available in a wide range of maximum centrifugal advance values. There usually is a number stamped on the plate that indicates the maximum number of degrees of advance. I believe the numbers are distributor degrees, which are one-half of crankshaft degrees.

Observe/measure the total spark advance vs. engine rpm to determine when the advance has reached its maximum.

If the advance mechanism has reached its mechanical stop, before your desired total spark advance at a particular high rpm value, and you do not have a replacement with a larger value, then a small amount of material can be removed from the end of the arm on the mechanism, to increase the maximum advance.

Charlie Tolman
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Re: Distributor tuning

Postby Charlie Tolman » Wed Jun 18, 2008 12:47 am

The spacing of the posts on the mechanical advance / cam plate, the posts that hold the outside ends of the two advance springs, is slightly less than 2.0 inches, being close to 1.94". The arm on the advance mechanism rotates until it hits one of these posts, and this regulates the maximum centrifugal spark advance.

To increase the maximum centrifugal advance by 2 degrees at the crankshaft, as an example, one needs to increase the advance by 1 degree in the distributor. With a spacing of 1.94", or a rotation radius of 0.97", then use the formula,

X = A x R / 57.3 = (1.0) x (0.97) / 57.3 = 0.0169" or 0.43 mm,

where A is the angle of advance change in the distributor, R is the rotation radius of the spring post with respect to the center of rotation of the distributor, 57.3 converts the angle from degrees into radians, and X is the amount of material to be removed from the distributor mechanism arm. For an additional 2 degrees of maximum spark advance, therefore, remove about 0.017" from the mechanism arm.

For a 4 degree increase in the maximum spark timing at the crankshaft, remove about 0.034", or 0.86 mm, or a little more than 1/32".

Remember to use distributor degrees in the formula (one-half of crankshaft degrees).

Charlie Tolman
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Re: Distributor tuning

Postby Asolepius » Wed Jun 18, 2008 2:58 am

Many thanks Charlie - what would we do without you on this board? I'll try that and let you know what happens - but it may take a little while as there are other jobs in the queue.
Les Rose, Salisbury, UK
1959 Sprite, owned since 1978
Asolepius
1098cc
 
Posts: 489
Joined: Tue Jan 25, 2005 6:50 am
Location: Salisbury, UK

Re: Distributor tuning

Postby Charlie Tolman » Wed Jun 18, 2008 11:04 am

I had the reverse situation with my 1963 Sprite race car, where there was an excess of centrifugal spark advance.

To reduce the advance, I removed the outside end of one of the springs from its post, and installed short pieces of small diameter (telescoping) brass tubing over the post, and re-installed the spring. The advance mechanism arm would now contact the tubing, which resulted in a reduced centrifugal advance.

If one is concerned about the tubing weight adding a small unbalance, add the same tubes to the other spring post.

With a 1/64 inch tubing wall thickness, each added tube decreases the centrifugal spark advance by nearly 2.0 crankshaft degrees.

Charlie T.
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Re: Distributor tuning

Postby Charlie Tolman » Tue Jul 22, 2008 1:13 am

Mr. A.:

It has been over a month since the last posting on this topic.
Any news to report ?

Charlie T.
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Re: Distributor tuning

Postby Rot » Tue Jul 22, 2008 1:54 pm

Hi Charlie:
He has been on holiday for a few weeks but I'm sure we will see him soon! I only know because I sent a 1275 harmonic balancer in care of his neighbor.
Roy
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Re: Distributor tuning

Postby Asolepius » Tue Jul 22, 2008 2:25 pm

Yes, I am back from hols now and tried to post here this morning but the site would not accept my post. Trying again! No, I haven't pulled out the dizzy yet as I have been away, but I'm anxious to try your ideas Charlie. Will report back soon.
Les Rose, Salisbury, UK
1959 Sprite, owned since 1978
Asolepius
1098cc
 
Posts: 489
Joined: Tue Jan 25, 2005 6:50 am
Location: Salisbury, UK


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