Rookie Ignition Timing Question - Micrometer Adj?

[Andrew1]

Hi Team:

I have been mostly a structural and finishing handyman on my BJ8 and found the forum amazing at sharing ideas and tips over a 2-year spruce-up of the car.

I took my first steps at playing with an otherwise fine overhauled engine completed 15-years ago (only 5,000miles since) this year be tweaking the HD8 carbs. This with encouraging, "go ahead do it yourself", phone advice from a enthusiast.

I lingering issue with the engine has been light-moderate pinging under acceleration in 3 - 4th gears under engine load. To date this seems to only be reduced if I find 94 octane gas +++++ tons of octane booster additive.

After hearing my experience, someone suggested that my ignition timing might be a bit off and have to be adjusted.

If this is the case, with the car otherwise running mostly well, should I be tempted to play with the micrometer adjusting mechinism on the distributor? Is that the purpose of the device? /ubbthreads/images/graemlins/canpatriot.GIF

Which way should I turn it first and how much to start? I have access to a timing light and a dwell meter....

I am an ambitious learner.... how should I get started out, or should Andrew look for an experienced tuner?

- Andrew 1 -

 

[Dave Russell]

Re: Rookie Ignition Timing Question - Micrometer A

Hi Andrew,
Yes, you can probably use the micrometer adjuster. Marked on the distributor, near the vacuum advance, there will be two arrows With A & R marks. Turn the adjuster knob in the direction of the arrows to advance or retard timing. This really should be done in conjunction with a timing light.

With the vacuum advance line disconnected, run the engine up to about 4,000 rpm & check advance with the timing light. Maximum advance should be somewhere around 36 degrees.

This method will be more accurate than trying to set the timing at idle speed.

If you don't have an adjustable advance timing light, 36 degrees would be about 1.9 inches from the TDC mark on the front pulley.

If there is not enough adjustment range on the micrometer adjustment, loosen the distributor clamp & rotate the distributor body a bit. Clockwise to advance, CCW to retard.
D

 

[Andrew1]

Re: Rookie Ignition Timing Question - Micrometer A

Wow, thanks Dave, that's great! Very clear for me to understand.

Now a follow-up question. 36 degrees is that ahead of the TDC arrow in the direction of engine rotation or behind?

Is 36 degrees or 1.9 inches a subjective measure & guideline or is there a more accurate method using protractor measuring tool of sorts?

I would alo guess that 36 degrees is also the "maximum" to adjust at 4000 rpm with the vaccuum hose removed, because under normal circumstances the vaccuum advance mechinism will move the advance to other figures in between depending on engine conditions?

- Andrew -

 

[Dave Russell]

Re: Rookie Ignition Timing Question - Micrometer A

Hi Andrew,
Looking at the front of the engine, it would be 36 degrees clockwise from the TDC mark. The engine rotates clockwise viewed from the front so the 36 degree mark would be before the engine reaches the TDC mark.

1.9 inches is based on dividing the circumference of the pulley by 360 degrees to get the inches per degree. It's pretty accurate if you measure carefully. It would actually be 1 & 29/32 inches, based on a 6 inch pulley diameter. You "might" get closer with a protractor, but it's pretty hard to get full access to the pulley. The formula would be (pulley diameter x 3.14)/360 degrees x 36 degrees = 1.9 inches for 36 degrees.

You are correct on the vacuum. The 36 degrees will include the static advance plus the centrifugal advance. The vacuum advance varies according to throttle position & engine load.
Good luck,
D
PS, If you have to move the distributor body, it would be good to first return the micrometer to the center of it's adjustment range.

 

[Ed_K]

Re: Rookie Ignition Timing Question - Micrometer A

Andrew,
My BJ8's front pulley had 6 scribed timing marks cut into the pulley with no number markings. After reading several sources, I was able to determine that the marks were at: 15a 10a 0 10b 15b 35b .
They were not very easy to see, if you have them, clean the pulley surface with a solvent of your choice, then get an old white crayon and fill in the marks with it. Wipe off any excess crayon on the pulley outside of the
marks leaving nice white lines. Now you can use an old fashioned timing light to check your timing. I thought the last mark was at 35 degrees perhaps it is 36 as Dave said to check for. I didn't have any problem adjusting the tiiming at a slow idle with the vac advance line disconnect to 15 degrees before TDC @ 600 RPMs but that is how I learned to do it. Dave,s way is probably more accurate.

 

[Keoke]

Re: Rookie Ignition Timing Question - Micrometer A

HI ED, No that is correct the Max advance Specification is 35 DEG, Dave was close enough.However, If you reverse the color scheme you will be able to see the timing marks easier. First paint the marks black, then paint the unmarked spaces white and a bit on each end.The dirtier the marks get the better they show up. Somebody did you a favor by adding the additional marks on the damper. If the Dizzy is in good condition which most are not or if you have an electronic ignition adjusting the timing as specified in the shop manual will work out ok. The old fashion timing light will only work on your car.Most others have only the 0 DEG mark which will require that they calculate what distance on the damper corrosponds to 15 DEG-Fwiw--Keoke

 

[Dave Russell]

Re: Rookie Ignition Timing Question - Micrometer A

Hi Ed,
I don't know that setting advance at idle is more or less accurate, but possibly ok.

Depending on the particular distributor & it's condition, 600 rpm is possibly in the region of the centrifugal advance starting point. If so, even minor errors of engine speed can make several degrees difference in the timing.

I believe that the maximum timing advance is the most critical part of the equation, as it determines whether the engine is going to get into detonation, either heard or unheard, & damage itself. 36 degrees at maximum advance is usually a safe setting. Let the idle timing fall where it may. Unless of course, you want to recalibrate the centrifugal advance to get the best of both worlds. Better to be slightly retarded than advanced too much. At higher rpm, it's possible to have detonation that is not heard but it will still do damage. Just trying to give "safe" advice.
D

 

[Ed_K]

Re: Rookie Ignition Timing Question - Micrometer A

Yeah Keoke, That would show up better under a timing light. I'll try that next time I need to adjust the timing on an old something... I always learn some good stuff from you and Dave and the other members of this forum. ...
I finally got my windshield back on and one of the front fenders . One more fender to go, then the doors, then the top.... Then .... I get to actually DRIVE and ENJOY this subject of all my free time and $$. I will NOT know how to act !!

 

[Keoke]

Re: Rookie Ignition Timing Question - Micrometer A

"I will not know how to act"---Just- /ubbthreads/images/graemlins/grin.gif---Keoke- /ubbthreads/images/graemlins/cheers.gif