Timing Questions

C

corey

New member
Jan 7, 2010
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Happy Sunday. Up to now I've been primarily running the GMC with the timing set by vacuum and/or driveability. I finally put a timing light to it and
it runs best at 25 degrees base timing (1100 RPM)! We have 3.70 gears so I checked total mechanical advance at 2,900 RPM in park with vacuum advance
disconnected Im getting about 36-37 degrees advance. So dist weights are adding 12-13 degrees. I pull full vacuum (hook vac hose to manifold) and get
55 degrees BTDC. So vac can is adding around 20 degrees max. I've verified the timing mark on the balancer matches up w piston #1 at TDC. There
appears to not be any notable slop in the timing chain. I've not been able to detect any pinging under load.
--
Corey P /

Hilliard, OH /
1974 Glacier 26' /
3.70 FD /
ION Wheels
 
If you do a research on total timing you will find that you are in the
correct range.
Total Timing = Initial Timing
https://help.summitracing.com/app/answers/detail/a_id/5157/session/L2F2LzEvdGltZS8xNjA2NjY0NDYwL2dlbi8xNjA2NjY0NDYwL3NpZC9mVVNwb3RVcHVtd3pMUVdtT0RPSlo0ZnJ0cXZOQTQ2RFpJOW1xQmQ4X0d6a2Z0TTdIblhMRG5rTTdzcGczWm5Pekh3dyU3RXI1TXg1MDV0RW5qQWp0SE1fVktMOTYxWXhFOTJ6aUxiZEd2SmQwNkxPTDRKUUpHWW13USUyMSUyMQ%3D%3D
+
mechanical advance
What is timing at 800 rpm with vacuum plugged? That will be your initial
timing.
You may find that at 1100 you are pulling some mechanical weight timing.

From Gene Fishers site

Distributor Specifications, Type: HEI - Modified
Application: GMC Motorhome
Engine: 455 Oldsmobile
Final drive: 3.07
Tow: No

Initial setting: 12°

RPM Advance
Centrifugal
800 2°
1000 5°
1250 10°
1500 12°
1750 13°
2000 14°
2250 16°
2500 18° 30° = Total

Vacuum
6" 0°
8" 2°
10" 6°
12" 10° 40° = Intial + Centrifugal + Vacuum

On Sun, Nov 29, 2020 at 7:35 AM Corey via Gmclist
wrote:

> Happy Sunday. Up to now I've been primarily running the GMC with the
> timing set by vacuum and/or driveability. I finally put a timing light to
> it and
> it runs best at 25 degrees base timing (1100 RPM)! We have 3.70 gears so I
> checked total mechanical advance at 2,900 RPM in park with vacuum advance
> disconnected Im getting about 36-37 degrees advance. So dist weights are
> adding 12-13 degrees. I pull full vacuum (hook vac hose to manifold) and get
> 55 degrees BTDC. So vac can is adding around 20 degrees max. I've verified
> the timing mark on the balancer matches up w piston #1 at TDC. There
> appears to not be any notable slop in the timing chain. I've not been able
> to detect any pinging under load.
> --
> Corey P /
>
> Hilliard, OH /
> 1974 Glacier 26' /
> 3.70 FD /
> ION Wheels
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

--
Bruce Hart
1976 Palm Beach
Milliken, Co
GMC=Got More Class
 
Corey,

As an engine development guy, I have to allow that the Oldsmobile base timing is about as dumb as it can be. The centrifugal advance in my
distributor (a 73) actually starts at about 950. This means that at 1100, you are up on the curve and when you adjust anything, everything changes.
(I bet you knew that.) So, with the manual's set point, you are trying to hit a moving target. I gave up trying very early.

You say that you hit it with the timing light and saw about 25°BTDC? That she ran great and you couldn't hear any spark knock? Well that seems to
be where she wants to be. Leave her alone, but before you quit, do two things:
1 - Get someone with known good hearing (Not ME!!) to ride along and ask if s/he hears knocking at any time in a normal driving cycle. Unfortunately,
with my history and age, this is one thing I can't do these days. Fortunately Mary can still hear.
2 - When you are happy with it, get the idle way down and disconnect the vacuum to the distributor and now look at the timing. WRITE THAT DOWN.
Until your next overhaul, this can be your target.

Wrong timing seems to be a long standing tradition in Detroit. Years ago, I went and did a full tune-up on one of my father's cars. The resulting
performance was terrible. I consulted with a friend at that point and he asked if I had set to the manufacture's specification. To this I replied
that I had and I was very careful to do so. He replied that he guessed that and that was the problem. He told me to bump the base timing up (I have
forgotten how much) which I did and it ran great after that. I was spoiled by the slow turning marine engines that you can time by ear. Many didn't
have external timing marks that could be used.

Maybe this winter I will get to publishing my "gunsight" timing marks for TZE. But, there has been just too much QRM in my life in recent times.

Matt

--
Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit
 
So I must have written it down wrong yesterday. Just double checked and I am getting 25 degrees at SLOW idle (750 rpm) with vacuum advance unhooked.
Mechanical advance does kick in somewhere around 1,000 rpm. I revved up to 3,000 rpm and got the same 35 degrees as I observed yesterday.
--
Corey P /

Hilliard, OH /
1974 Glacier 26' /
3.70 FD /
ION Wheels
 
When the engine has high milage, the timing chain gets sloppy so you can
bump the timing few degrees more as it is compensating for slop.

On Sun, Nov 29, 2020 at 10:34 AM Corey via Gmclist
wrote:

> So I must have written it down wrong yesterday. Just double checked and I
> am getting 25 degrees at SLOW idle (750 rpm) with vacuum advance unhooked.
> Mechanical advance does kick in somewhere around 1,000 rpm. I revved up to
> 3,000 rpm and got the same 35 degrees as I observed yesterday.
> --
> Corey P /
>
> Hilliard, OH /
> 1974 Glacier 26' /
> 3.70 FD /
> ION Wheels
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

--
Jim Kanomata
Applied/GMC, Newark,CA
jimk
http://www.gmcrvparts.com
1-800-752-7502
 
> If you do a research on total timing you will find that you are in the
> correct range.
> Total Timing = Initial Timing
>
> +
> mechanical advance
> What is timing at 800 rpm with vacuum plugged? That will be your initial
> timing.
> You may find that at 1100 you are pulling some mechanical weight timing.
>
> From Gene Fishers site
>
> Distributor Specifications, Type: HEI - Modified
> Application: GMC Motorhome
> Engine: 455 Oldsmobile
> Final drive: 3.07
> Tow: No
>
> Initial setting: 12°
>
> RPM Advance
> Centrifugal
> 800 2°
> 1000 5°
> 1250 10°
> 1500 12°
> 1750 13°
> 2000 14°
> 2250 16°
> 2500 18° 30° = Total
>
> Vacuum
> 6" 0°
> 8" 2°
> 10" 6°
> 12" 10° 40° = Intial + Centrifugal + Vacuum

These numbers are right on the money.


You never want to exceed a total of 40 or 41 degrees total advance.

So if you add more vacuum advance or static advance then you much take some out of one of the other two (Base or mechanical) so as not to exceed 40
degrees total.

So your numbers are way too high.

I am suggesting the the static or base advance number is way off affecting all of the numbers you are seeing. . What you have is probably a loose
outer ring on the vibration dampener / harmonic balancer. I have seen more than one that has slipped making your readings inaccurate.

Here are a few pictures of Roger Black's old and new balancer from about 13 years ago. Once it slips anything you read with the light is useless.

I will let Matt, or someone more versed on doing it to say how to do it, but you need to verify the timing mark as compared to EXACT Top Dead Center
of piston #1.

http://www.gmcmhphotos.com/photos/harmonic-balancer-notes/p5569.html
http://www.gmcmhphotos.com/photos/harmonic-balancer-notes/p5570.html
http://www.gmcmhphotos.com/photos/harmonic-balancer-notes/p5571.html

Ken B.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
Ken - I had the same thought but could have sworn that I was able to verify that the mark on the dampener ring was pointing directly at 0 on the
timing plate when #1 was at TDC but that was months ago and I suppose it very well may slipped since then. Also worth noting, the tool im using for
this is the H.Freight "Cen-Tech" advance timing light. I'm trusting that whoever applied the "Advance Degrees" sticker on the dial lined it up just
right!
--
Corey P /

Hilliard, OH /
1974 Glacier 26' /
3.70 FD /
ION Wheels
 
As long as you are at it, maybe check the accuracy of the TDC mark using a piston stop. Tape a piece of paper several inches long over the timing tag.
Then screw the piston stop into the #1 spark plug hole, and turn the crank CW coming up on TDC compression stroke until it stops. (piston is hitting
that stop) Mark where it stops on the paper timing tag. Turn the crank CCW until it stops. Mark that spot. Half way between the two marks is Actual
TDC. I cut a piece of paper to length between the two marks and fold it in half. JWID
--
Larry
78 Royale w/500 Caddy
Menomonie, WI.
 
Larry,

That actually seems a lot more definite/accurate than what I did last time, trying to eyeball TDC with a borescope camera. I will do that and report
back to the group for possible further advice.
--
Corey P /

Hilliard, OH /
1974 Glacier 26' /
3.70 FD /
ION Wheels
 
> Larry,
>
> That actually seems a lot more definite/accurate than what I did last time, trying to eyeball TDC with a borescope camera. I will do that and
> report back to the group for possible further advice.

Cory,

I can tell you for an absolute fact that the borescope won't get there.
The only good answer is what steam engineers call "Tramel" the engine. A tramel is an uncalibrated measuring reference.
If you can't find out how to do this get back to me and I will walk you through it.

If you have a friend that is an old motorcycle person, he may have a timing gauge that screws into the spark plug hole. That is most of what you
need.

I'll get around to writing up "gun sight timing real soon here.

Matt
--
Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit
 
> Larry,
>
> That actually seems a lot more definite/accurate than what I did last time, trying to eyeball TDC with a borescope camera. I will do that and
> report back to the group for possible further advice.

Let me clarify that...The paper gets taped to the balancer. I then used a piece of coat hanger that I bolted to the block on any handy bolt near the
balancer. Sharpen the end to a point. After you turn CW to stop, Bending the coat hanger wire to point at the CCW portion of the paper, mark that spot
on the damper paper. Turn CCW to stop and mark again. Half way between is TDC. Then compare actual TDC to timing tag and mark on the damper Long
story, short, I got the wrong timing tag from rebuilder on assembly of a new motor. Had lots of trouble with detonation. Finally figured out that the
rebuilder sent me the wrong tag. When I finally did a check for actual TDC, I found it to be 20* off...advanced!! That little SNAFU about drove me
crazy.
--
Larry
78 Royale w/500 Caddy
Menomonie, WI.
 
JWID but I run as much timing as possible without pinging, it may vary based on the fuel. I run the least expensive gas (after changing all the fuel
lines, pump, carb internals...) to prevent problems from the ethanol.
--
Todd Snyder, Buffalo NY
1976 Eleganza II
 
Matt,

I had to Google trammel but after reading a bit, I think i understand and it's similar to what Larry describes - using a pointer affixed to the
engine near the dampener and a piston stop screwed into the spark plug hole. Of course I could start by using the "0" mark on the timing tag as the
"pointer", and verify / check the #1 piston bumps the piston stop on either side of the 0 mark and equidistant from the mark clockwise and
counterclockwise. If not then I need to move to step 2 - finding the true 0 mark and determining whether the dampener has slipped or the timing tag is
incorrect.

How would one determine whether they have the correct timing tag? Mine is white plastic. I have zero maintenence records since the coach was acquired
at auction, but suspect a prior engine rebuild (looks like it's been repainted, and has what appear to be aluminum core plugs). And I have been leery
of the placement of the timing tag as well
--
Corey P /

Hilliard, OH /
1974 Glacier 26' /
3.70 FD /
ION Wheels
 
I am saying that you need to verify that the scribed timing mark in the Harmonic balancer is at 0 when the piston is EXACTLY at TDC. After that you
can worry about a sticker that someone installed and see if it matches at 0. There is just no way that I know of that the engine would run correctly
with the numbers you are seeing.

I have not checked an automotive engine for this in many, many years. So I am hoping that someone else hops in here. I do it all the time on
airplane engines but the procedure is way different. We have this big long lever called a propeller attached to the crank to move the engine by hand
while applying air pressure through the sprark plug hole to find TDC. On an automotive engine they reach through the spark plug hole to figure out
when the piston is at TDC. I have also heard of people using a screwdriver through the hole, but let someone more knowledgable than me here tell you
how to do it.

Ken B.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
You say you cleaned and lubed the weights but that is just the tip of the iceberg. The dist is a shaft within a shaft. If you don’t take apart and
remove the stiction, lube and reassemble the advance won’t be consistent. You can’t set for that moving target.
--
John Lebetski
Woodstock, IL
77 Eleganza II
 
> Matt,
>
> I had to Google trammel but after reading a bit, I think i understand and it's similar to what Larry describes - using a pointer affixed to the
> engine near the dampener and a piston stop screwed into the spark plug hole. Of course I could start by using the "0" mark on the timing tag as the
> "pointer", and verify / check the #1 piston bumps the piston stop on either side of the 0 mark and equidistant from the mark clockwise and
> counterclockwise. If not then I need to move to step 2 - finding the true 0 mark and determining whether the dampener has slipped or the timing tag
> is incorrect.
>
> How would one determine whether they have the correct timing tag? Mine is white plastic. I have zero maintenence records since the coach was
> acquired at auction, but suspect a prior engine rebuild (looks like it's been repainted, and has what appear to be aluminum core plugs). And I have
> been leery of the placement of the timing tag as well

Corey,

You have exactly the right plan. If you do that and zero comes up to the zero on the timing tag (saw) then it is probably correct.

Is it the correct part on there?
Well, the radius of most dampers are pretty much the same, so if it looks close, it is probably the right part. Then again, if you are even a little
paranoid, a tape measure wrapped around the damper will get you the circumference fast enough the calculator in you phone can tell you who long 12°
is when you divide by 30. I say wrap a tape because that is easier than trying to measure the diameter an multiply by 3.141592654.


In the circle I used to travel in, the magnetic timing wheels we had were routinely referred to as "Engine Frisbees" because they were often forgotten
about until the engine was started.....

Matt
--
Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit
 
So today I was able to determine that the harmonic dampener is not the problem. After looking all around the thing for witness marks (to tell me if it
has moved or not) but not finding any, I picked up a piston stop on Amazon for $8. Pretty slick. I was able to turnvthe crank clockwise then CC VERY
slowly and making a mark on the balancers outer ring at both stops, then measuirng a spot halfway between those two marks to locate true mechanical
TDC. Turns out the factory timing mark on the dampener is spot on. It does not appear that the outer ring has slipped at all. So we got out grandpa's
old Matco timing light that hasn't worked in years, and jiggled the wires enough so that we could get it to flash occasionally. It indicated that we
are 12 DBTC. We re-checked the Freight Harbor light (the one with the advance dial) and it was still indicating over 20 DBTC. The HF light is messed
up. You need to turn it to about the 10 or 12 degree mark to get it to read what the Matco reads. This makes sense because the only other vehicle I've
used it on i remember thinking "man this thing really needs a lot of advance to run right". Guess I need to invest in a better timing light!
--
Corey P /

Hilliard, OH /
1974 Glacier 26' /
3.70 FD /
ION Wheels
 
Yep, always a good idea to "zero out" your test equipment, no matter whose
name is on the outside. Meters, timing lights, etc. The whole shebang. Even
my "Fluke 77" multi meter. I always check it out. Those "advance type
timing lights" are notorious for being inaccurate. 2° is a mile off when it
comes to timing. 10°, wow. That's a bunch.
Jim Hupy
Salem, Oregon

On Sun, Dec 13, 2020, 2:20 PM Corey via Gmclist
wrote:

> So today I was able to determine that the harmonic dampener is not the
> problem. After looking all around the thing for witness marks (to tell me
> if it
> has moved or not) but not finding any, I picked up a piston stop on Amazon
> for $8. Pretty slick. I was able to turnvthe crank clockwise then CC VERY
> slowly and making a mark on the balancers outer ring at both stops, then
> measuirng a spot halfway between those two marks to locate true mechanical
> TDC. Turns out the factory timing mark on the dampener is spot on. It
> does not appear that the outer ring has slipped at all. So we got out
> grandpa's
> old Matco timing light that hasn't worked in years, and jiggled the wires
> enough so that we could get it to flash occasionally. It indicated that we
> are 12 DBTC. We re-checked the Freight Harbor light (the one with the
> advance dial) and it was still indicating over 20 DBTC. The HF light is
> messed
> up. You need to turn it to about the 10 or 12 degree mark to get it to
> read what the Matco reads. This makes sense because the only other vehicle
> I've
> used it on i remember thinking "man this thing really needs a lot of
> advance to run right". Guess I need to invest in a better timing light!
> --
> Corey P /
>
> Hilliard, OH /
> 1974 Glacier 26' /
> 3.70 FD /
> ION Wheels
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
 
Well, I am happy that you found it and the numbers make a lot more sense now.

12 degrees static is where mine is set. My vac advance is 10 degrees maximum, and my centrifugal is 17 to 18.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
So the HF tool cost you time, and luckily not your engine. The last thing I bought there was the Jumbo 3/4” drive socket set. Only because real
tools in that size range are way expensive. The ratchet is so bad it skips when least expected. Before that the 134a gauge set was actually ok
except the hose jackets split in like 9 months. No free replacement. My R12 Robinair set is like 30 years no issues I will never buy from them
again.
--
John Lebetski
Woodstock, IL
77 Eleganza II
 
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