Ignition timing - Team Camaro Tech
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post #1 of 13 (permalink) Old Oct 12th, 14, 04:13 PM Thread Starter
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Ignition timing

Quick question. Im getting a little dieseling when I shut down.
496, HR- 296- 2S- 12 IG cam, 10:1 or so compression, Holley 830 annular carb.

Starts and runs fine, idles great. Im 12 degrees initial timing, and about 34 degrees total at 3500 with the vacuum disconnected. I never ran it with the vacuum connected yet, but I plan on hooking it up to manifold.

68 RS/SS 496 T56 6 Speed Tuxedo black w/ white bumblebee. Strange Dana 60, Spicer 3.73 Helical Gears
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post #2 of 13 (permalink) Old Oct 12th, 14, 04:52 PM
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Re: Ignition timing

The throttle blades are probably open a bit too far. What is your idle rpm? If it will idle lower that should solve the run-on. If it idles as low as it can now then you need more initial timing which will raise the idle and then you can close down the throttle blades and get back to the current idle. Careful increasing initial timing increases total timing...

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post #3 of 13 (permalink) Old Oct 12th, 14, 04:58 PM Thread Starter
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Re: Ignition timing

Pretty sure im in the 850-900 rpm range for idle but can look to be sure. I think I can go lower for sure. Whats a safe max to put initial timing? I can start with a degree or two to see if it helps.

68 RS/SS 496 T56 6 Speed Tuxedo black w/ white bumblebee. Strange Dana 60, Spicer 3.73 Helical Gears
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post #4 of 13 (permalink) Old Oct 12th, 14, 05:20 PM
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Re: Ignition timing

Also run a bottle of gas treatment with a full tank of gas to get rid of any carbon buildup.

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post #5 of 13 (permalink) Old Oct 12th, 14, 11:50 PM
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Re: Ignition timing

Quote:
Originally Posted by mxquattro View Post
Pretty sure im in the 850-900 rpm range for idle but can look to be sure. I think I can go lower for sure. Whats a safe max to put initial timing? I can start with a degree or two to see if it helps.
Every engine is different for example I have 34 locked out timing see what it likes

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post #6 of 13 (permalink) Old Oct 13th, 14, 10:22 AM
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Re: Ignition timing

At 850-900 rpm idle I would try lowering it to about 700rpm if it will idle there. If you add the vacuum advance back in on a manifold source it will raise the idle more and allow you to close the throttle blade some more.

Just keep in mind any time you change the timing either by vacuum advance or mechanical adjustment it will have some effect on the engine at other rpm ranges. Always do one change at a time and test drive thoroughly before moving on...

...Dennis

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post #7 of 13 (permalink) Old Oct 13th, 14, 01:36 PM
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Re: Ignition timing

If u cant get the idle tpo drop on the idle speed screw with a stable idle, then that indicates the secondary butterfly are a little to far open (or have a vaccuum leak, or butterflys are not aligned in the bores correctly.
To drop the secondary while engine running, a rivet with the end hammered into a small blade screwdriver and a near 90 deg bend in the shaft... this allows access to the little screw going up under the edge on the opposite side to the throttle....

12 degs initial and 34 degs total thats 22 in the centrifigal... good start
10:1 compression u may need a little less total... more around the 32, and maybe slightly faster all in around 3200 rpms

How many degs in the VA? I will make a ball park assumption of 7 to 10 degs for now.

So u have set the carb idle advance with around 16/18 degs (lets say 18)
u want around 32 total and 22 in the cent.
So if u set the initial to 10 (easy fire up, low starter loads, amp draw, armature loading)
plus 22 cent thats your 32 total
The VA needs to have the idle vaccuum pull it fully all in at idle...so say 10 degs in the VA initial+VA gives idle advance of the 18 degs.

It is important not to have all up... initial+cent+VA above 40/ 42 degs unless dialing in the engine with knock sensors...
intial 10+ cent 22 +VA 10 = 42

To establish your ideal total....lock the counterweights with a good rubber band then set the timing... engine running to around 36 degs....
do 3 running times over a measured distance... preferbly up a steep hill....
then drop the timing back 2 degs... repeat till u suddenly see an increase in times... u should feel it to.
Your total should be 2 maybe 3 degs above this.

Remove the rubber bands, and that will be your initial....and with your current setup my assumption of 7 to 10 deg in the VA u will still be under the 40/42 all up....and a idle (intial+VA ) 16 to 18 degs ball park.

IF u do have a heap of degs .. too many, in the VA, make a small tag that mounts under the top mounting screw of the VA... long enough to stop the full movement of the VA lever.

My Spelling is not incorrect...it is creative

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post #8 of 13 (permalink) Old Oct 17th, 14, 09:44 AM
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Re: Ignition timing

Quote:
Originally Posted by mxquattro View Post
Quick question. Im getting a little dieseling when I shut down.
496, HR- 296- 2S- 12 IG cam, 10:1 or so compression, Holley 830 annular carb.

Starts and runs fine, idles great. Im 12 degrees initial timing, and about 34 degrees total at 3500 with the vacuum disconnected. I never ran it with the vacuum connected yet, but I plan on hooking it up to manifold.
When you connect the vacuum advance to full manifold vacuum, the VAC will add 15*-16* of timing at idle, for total idle timing of 28* (12 + 16); that will increase idle rpm, and allow you to back off on the throttle opening, which will stop the dieseling.


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post #9 of 13 (permalink) Old Oct 21st, 14, 09:10 PM
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Re: Ignition timing

Quote:
Originally Posted by JohnZ View Post
When you connect the vacuum advance to full manifold vacuum, the VAC will add 15*-16* of timing at idle, for total idle timing of 28* (12 + 16); that will increase idle rpm, and allow you to back off on the throttle opening, which will stop the dieseling.

What he said.
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post #10 of 13 (permalink) Old Oct 22nd, 14, 03:31 PM
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Re: Ignition timing

Quote:
Originally Posted by JohnZ View Post
When you connect the vacuum advance to full manifold vacuum, the VAC will add 15*-16* of timing at idle, for total idle timing of 28* (12 + 16); that will increase idle rpm, and allow you to back off on the throttle opening, which will stop the dieseling.

Quote:
Originally Posted by Straight-line-69 View Post
What he said.
What they said.

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post #11 of 13 (permalink) Old Oct 23rd, 14, 03:33 PM
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Re: Ignition timing

Which is the same as what is above.. except above the timing is dialed in...and the then the VA advance added.

My Spelling is not incorrect...it is creative

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post #12 of 13 (permalink) Old Oct 26th, 14, 09:50 AM
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Re: Ignition timing

34* on a BBC? BB like about 40* total timing.
Go to the strip and keep increasing the timing until the MPH drops, or it rattles. then back up to the best MPH. You may need to run 91 or 93 octane.

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post #13 of 13 (permalink) Old Oct 26th, 14, 04:01 PM
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Re: Ignition timing

That is obsolete old school stuff... one starts high then decrease the advance till there is a sudden drop off within a couple degrees....
The reasons why , how etc are discussed many times in old threads.
Start working with 21 century data loggers )(2 , TPS IMAP knock sensors u will see why.
Also under load one doesn't have any VA actuated....NEVER configure/run a a engine over 40/ 42 max all up (initial+cent+VA) without dialling in with a knock sensor and data logger.
If the engine is on the street and doesnt have EGR, one runs VA on manifold and is also part of the idle advance

My Spelling is not incorrect...it is creative

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