Timing Question

[Dan Hodges]

Ignition timing

Originally posted by pggg
Dan, running a 2H7 centrifugal unit(131-01) advanced 5 deg in a 3H5 motor gives a high rev boost but not by much. Running a 3H5 centrifugal unit(131-02) at 5 deg advanced in a 2H7 motor doesn't seem to change anything. A 5 deg advanced 2H7 unit in a 2H7 motor also gives a small high rev kick. 3H5 cams in a 3H5 motor with 2H7 carbs running a 2H7 centrifugal unit advanced 5 deg gives similar performance to 2H7 cams in a 2H7 motor running a 3H5 unit at normal advance. Running 3H5 cams in a 2H7 motor with a 3H5 unit advanced 5 deg with 3H5 carbs...

Ah F*#* it! I'm going to bed.

The 81 had a non adjustable pick up base and the timing as such was not adjustable like the previous models.You checked the timing by letting it idle and by use of a timing light you check to see if the pointer falls between the designated marks on the timing plate.Actually,the 81 will have 5 degrees at 1200 rpm and 38-40 at 3900-4000rpm with out the vacume advance hooked up.I modified my pick up base on a bridge port where I can advance or retard it and installed a 79 timing plate which is degreed in five degree increments up to 60 degrees.I can now set my ignition timing at whatever I want.I have run it at 10 degrees initial and 45 advanced but it will ping/rattle on a hot day.I have also ran it without the vacume advance hooked up,but that is not kosher on a street bike.It has more snap at 45 in the midrange,but the 5&40 works best all around which just so happens to be stock.I have run both the 78-79 cams and the 80-81 cams plus an after market grind with the intake advanced 5 degrees.All other factors being equal,more ignition timing will help the low and midrange but hurt the top end.How much timing a motor want's,depends on the combustion chamber design,the shape of the piston dome and the pumping compression which will also determine the octane requirements of the motor.My initial question was,what is the timing on somebody's stock 79 special.The shop manual in one place say's 31 degrees at 5200rpm and in the spec section says it has 5 degrees initial and 31 centrifugal and 36 total.I have never checked a 79 and I am curious to find someone who has actually checked a stock one.If any body has, I would like to know what it is.2H7+3H5=I'm totally confused!!

[Dan Hodges]

Re: Ignition timing

Originally posted by Dan Hodges
The 81 had a non adjustable pick up base and the timing as such was not adjustable like the previous models.You checked the timing by letting it idle and by use of a timing light you check to see if the pointer falls between the designated marks on the timing plate.Actually,the 81 will have 5 degrees at 1200 rpm and 38-40 at 3900-4000rpm with out the vacume advance hooked up.I modified my pick up base on a bridge port where I can advance or retard it and installed a 79 timing plate which is degreed in five degree increments up to 60 degrees.I can now set my ignition timing at whatever I want.I have run it at 10 degrees initial and 45 advanced but it will ping/rattle on a hot day.I have also ran it without the vacume advance hooked up,but that is not kosher on a street bike.It has more snap at 45 in the midrange,but the 5&40 works best all around which just so happens to be stock.I have run both the 78-79 cams and the 80-81 cams plus an after market grind with the intake advanced 5 degrees.All other factors being equal,more ignition timing will help the low and midrange but hurt the top end.How much timing a motor want's,depends on the combustion chamber design,the shape of the piston dome and the pumping compression which will also determine the octane requirements of the motor.My initial question was,what is the timing on somebody's stock 79 special.The shop manual in one place say's 31 degrees at 5200rpm and in the spec section says it has 5 degrees initial and 31 centrifugal and 36 total.I have never checked a 79 and I am curious to find someone who has actually checked a stock one.If any body has, I would like to know what it is.2H7+3H5=I'm totally confused!!

Well, I got my answere right here on channel Eleven.Blaine Hoops has a post where he checked his 79 and he got 36 degrees at full advance,end of story.Thank you Blaine.