I apologize that I don't have all of the information available tonight. I
don't know an awful lot about the ignition system but think I know enough to
get the engine to run right, and think that is what its all about. I don't
have the new advance specs for the distributer but know someone in our club
who has them. I will describe the general method of setting the curve and
get more info later. I am doing this from memory (don't have any technical
manuals here tonight and have not done this in a along time) so maybe
someone can correct me if they see a mistake.
Here is what I know. You need a tach and a timing light, and mark the timing
marks on the harmonic balancer with white out( same kind secretarys use to
correct mistakes) so you can see the marks clearly with a timing light. you
are going to be checking the advance at idle with the vacuum to the
distributer disconnected and plugged at the manifold first,, and than at a
higher RPM.
1. You have to recurve the distributer to take into account the
reformulated gas of today since it burns at a different rate than it did in
1976 or so.
2. the parts that you need to do it correctly are, all ordered from summit
at 1-800-230-3030 (fits both 403 and 455):
a. adjustable vacuum advance, part # CRN-99600-1 @ $22.95
b. HEI advance curve kit part # SUM-G5212 @ $4.50. this kit consists of
two weights, center plate,three sets of color coded springs, nylon
bushings,set up manual. I believe the manual describes the process in enough
detail
3. With the engine warmed up and the vacuum disconnected from the
distributer and the vacuum line to the manifold plugged you need to set the
idle speed to 850RPM (this might or might not be correct, I told you I
didn't have the specs) and the timing light should show the advance to be 8
dgrees BTDC (don't know if this is correct either). You vary the distibuter
to get the correct timing at idle.
4. run the engine speed up to 2000 RPM (can't remember this RPM either) and
note the timing mark on the harmonic balancer now (10 degrees BTDC), and
than 3000 RPM (19 degrees BTDC) and note the readings each time. If your
distributer curve does not agree with these readings, we can change the
curve either up or down by selecting either a weaker or stronger spring set
depending on whether the weights are advancing to fast or to slow, and
varying the adjustment on the vacuum can. Keep playing with it until you get
the curve right at idle, 2000RPM and 3000RPM correct(or what ever the specs
call for).
Thats the general idea.
What you are really trying to do is stop the engine from pinging when you
really get on it, but still have lots of power.
>>
>> I am trying to figure out what to do with my distributor. Thanks to the
>> GMCnet I now know that I should not be using a Toronado distributor on my
>> 455. I really don't want to spend the $$ for a new/recurved one. It has not
>> been that long since I upgraded from the points ignition to the HEI. My new
>> distributor has very few miles on it.
>>
>> Can anyone on the list tell me what parts I have to change to get the
>> curves closer to what they should be for the GMC? I know that the vacuum
>> advance is different. Does someone have the part number or an application
>> for the correct vacuum advance? I assume that the springs and/or weights
>> are different too. Any part numbers or suggestions for the correct
>> springs/weights?
>
>Dave,
>
>>From Duane Simmons presentation at Marion, NC:
>
> Vac. Advance Servo No. 1973577
> Cent. Advance 0 deg. @ 900rpm
> 10 deg. @ 2000rpm
> 19 deg. @ 3100rpm
> Initial timing 8 deg. @ 850rpm
>
>The trick is finding someone in your neighborhood that still has a
>distributor test machine. Try calling the local speed shops and race
>engine shops. After chasing all this stuff down, I ended up sending my
>distributor to Bob Lamey to rebuild. Bob is very knowledgable
>concerning GMC's and I didn't feel like educating the local speed shop
>distributor spinner. AAMOF, why don't you call Bob first at
>909-982-7747. Might save you some leg work.
>
>Patrick
>
>
Tom & Marg Warner
Vernon Center NY
1976 palmbeach
"The beautiful Mohawk Vally"
don't know an awful lot about the ignition system but think I know enough to
get the engine to run right, and think that is what its all about. I don't
have the new advance specs for the distributer but know someone in our club
who has them. I will describe the general method of setting the curve and
get more info later. I am doing this from memory (don't have any technical
manuals here tonight and have not done this in a along time) so maybe
someone can correct me if they see a mistake.
Here is what I know. You need a tach and a timing light, and mark the timing
marks on the harmonic balancer with white out( same kind secretarys use to
correct mistakes) so you can see the marks clearly with a timing light. you
are going to be checking the advance at idle with the vacuum to the
distributer disconnected and plugged at the manifold first,, and than at a
higher RPM.
1. You have to recurve the distributer to take into account the
reformulated gas of today since it burns at a different rate than it did in
1976 or so.
2. the parts that you need to do it correctly are, all ordered from summit
at 1-800-230-3030 (fits both 403 and 455):
a. adjustable vacuum advance, part # CRN-99600-1 @ $22.95
b. HEI advance curve kit part # SUM-G5212 @ $4.50. this kit consists of
two weights, center plate,three sets of color coded springs, nylon
bushings,set up manual. I believe the manual describes the process in enough
detail
3. With the engine warmed up and the vacuum disconnected from the
distributer and the vacuum line to the manifold plugged you need to set the
idle speed to 850RPM (this might or might not be correct, I told you I
didn't have the specs) and the timing light should show the advance to be 8
dgrees BTDC (don't know if this is correct either). You vary the distibuter
to get the correct timing at idle.
4. run the engine speed up to 2000 RPM (can't remember this RPM either) and
note the timing mark on the harmonic balancer now (10 degrees BTDC), and
than 3000 RPM (19 degrees BTDC) and note the readings each time. If your
distributer curve does not agree with these readings, we can change the
curve either up or down by selecting either a weaker or stronger spring set
depending on whether the weights are advancing to fast or to slow, and
varying the adjustment on the vacuum can. Keep playing with it until you get
the curve right at idle, 2000RPM and 3000RPM correct(or what ever the specs
call for).
Thats the general idea.
What you are really trying to do is stop the engine from pinging when you
really get on it, but still have lots of power.
>>
>> I am trying to figure out what to do with my distributor. Thanks to the
>> GMCnet I now know that I should not be using a Toronado distributor on my
>> 455. I really don't want to spend the $$ for a new/recurved one. It has not
>> been that long since I upgraded from the points ignition to the HEI. My new
>> distributor has very few miles on it.
>>
>> Can anyone on the list tell me what parts I have to change to get the
>> curves closer to what they should be for the GMC? I know that the vacuum
>> advance is different. Does someone have the part number or an application
>> for the correct vacuum advance? I assume that the springs and/or weights
>> are different too. Any part numbers or suggestions for the correct
>> springs/weights?
>
>Dave,
>
>>From Duane Simmons presentation at Marion, NC:
>
> Vac. Advance Servo No. 1973577
> Cent. Advance 0 deg. @ 900rpm
> 10 deg. @ 2000rpm
> 19 deg. @ 3100rpm
> Initial timing 8 deg. @ 850rpm
>
>The trick is finding someone in your neighborhood that still has a
>distributor test machine. Try calling the local speed shops and race
>engine shops. After chasing all this stuff down, I ended up sending my
>distributor to Bob Lamey to rebuild. Bob is very knowledgable
>concerning GMC's and I didn't feel like educating the local speed shop
>distributor spinner. AAMOF, why don't you call Bob first at
>909-982-7747. Might save you some leg work.
>
>Patrick
>
>
Tom & Marg Warner
Vernon Center NY
1976 palmbeach
"The beautiful Mohawk Vally"
