Help! with ignition timing settings, please

A

Alben78

Well-known user
Trevor, it says you can’t use the no spring technique for the hei as it will just totally over advance, the rest is applicable to all distributor types contacts or hei where there are weights springs and vac advance.
 
trevbeadle

trevbeadle

CCCUK Member
I'll try to look at those things......unfortunately busy for the next few days so probably be the weekend before I get back...thanks.
 
trevbeadle

trevbeadle

CCCUK Member
OK so I've reset the initial to 10 BTDC and put lighter springs on the mechanical advance. Now I'm getting 35 all in at 2100 rpm, perhaps a bit low? The Accel can is
 
trevbeadle

trevbeadle

CCCUK Member
1 week stretches to 3! I reset the initial to 10 BTDC and fitted lighter mechanical advance springs. I'm now getting 35 at 2100 rpm which seems a bit low but they look like large weights and maybe I wasn't revving high enough with what was fitted. I do have smaller weights.The can is still over-advancing on manifold vacuum so am awaiting a standard can in the post. Hopefully that will be easy to fit and we'll run things again then.
 
Forrest Gump

Forrest Gump

CCCUK regional rep
As long as you‘re not getting any detonation then all-in at 2100 is okay. You could always change just one of the springs to the next heaviest and push the all-in to 2400/2500 perhaps.
 
johng

johng

CCCUK Member
Trevor, I got that paper and his latest "how to set timing" paper from Lars a few weeks ago. I would post them, but I can't see how to post a document here. If you email him he will just send you a copy
v8fastcars@msn.com
 
trevbeadle

trevbeadle

CCCUK Member
Have fitted a new 'standard' can from Rock Auto, and put 1 light and 1 heavier spring on the advance. The initial is set at 10 and now I'm getting 33 total (10 initial + 23 centrifugal) at 2300-2400 rpm. That all looks fine. Tickover is a little lumpy at idle (set 600). Roadtest shows no 'knocking'.Conecting the new can to manifold vac immediately gives an increase in idle and nice steady rpm when I reduce idle screw back to 600rpm. Sounds much better. I had checked the vacuum from the carb connector at idle and it's 16 in Hg. However, the initial + vac at idle on the rollback light is now 34! The standard can is giving me 24 vacuum advance! If I add the total timing figure, 33,, I'd be up to 57!!?? Should I reset the initial back to 5 so I don't exceed 52 btdc or give up with manifold vac and return to ported vac?
 
johng

johng

CCCUK Member
You can buy a vacuum advance stop plate to limit how much vacuum advance your can provides. I just made one from a small piece of aluminium.
 
Oneball

Oneball

CCCUK Member
From your description it’s doing what it’s supposed to. No idea about theoretical maximums though.

Cruising along at 60 in top you’ll probably find you’ve got over 40 deg advance. It’s supposed to be like that. Plant your foot and the vac will decrease and so will the advance which is why you don’t get any pre-detonation. Some for WOT at the red line.
 
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johng

johng

CCCUK Member
Tim, I'm sure you know a lot more about this than me, but what happens if Trevor is going downhill at 60mph with a closed throttle and hence has 57 degrees of advance? This excerpt from the Lars vacuum advance paper suggests you shouldn't exceed 48.

Most GM V8 engines (not including “fast-burn” style heads) will produce peak torque and power at wide open throttle with a total timing advance of 36 degrees (some will take 38, and some smaller bore engines will take as little as 32, and this number can vary a little with altitude). Also, a GM V8 engine, under light load and steady-state cruise on modern pump gas, will accept a maximum timing advance of about 46-48 degrees (on “vintage” pump gas in the 60’s and up through the 90’s, you could typically run in the 52 – 54 range – this is no longer advised). Some will take up to 50 degrees advance under these conditions. Once you advance the timing beyond this, the engine/car will start to “chug” or “jerk” (aka, “trailer hitching”) at cruise, and may experience occasional misfire, due to the over-advanced timing condition. Anything less than 46 degrees produces less than optimum fuel economy at cruise speed.
 
Roscobbc

Roscobbc

Moderator
johng said:
Also, a GM V8 engine, under light load and steady-state cruise on modern pump gas, will accept a maximum timing advance of about 46-48 degrees (on “vintage” pump gas in the 60’s and up through the 90’s, you could typically run in the 52 – 54 range – this is no longer advised). Some will take up to 50 degrees advance under these conditions. Once you advance the timing beyond this, the engine/car will start to “chug” or “jerk” (aka, “trailer hitching”) at cruise, and may experience occasional misfire, due to the over-advanced timing condition. Anything less than 46 degrees produces less than optimum fuel economy at cruise speed.
Click to expand...
I wonderwhen using a fairly 'stiff' cam if the vacuum advance 'reduction' when going from a cruise condition (high vacuum) to acceleration (low vacuum) is too insensitive. Much the same thing that would prevent quick actuation of headlamps and more important power brake assist? -
 
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Oneball

Oneball

CCCUK Member
johng said:
Tim, I'm sure you know a lot more about this than me, but what happens if Trevor is going downhill at 60mph with a closed throttle and hence has 57 degrees of advance? This excerpt from the Lars vacuum advance paper suggests you shouldn't exceed 48.

Most GM V8 engines (not including “fast-burn” style heads) will produce peak torque and power at wide open throttle with a total timing advance of 36 degrees (some will take 38, and some smaller bore engines will take as little as 32, and this number can vary a little with altitude). Also, a GM V8 engine, under light load and steady-state cruise on modern pump gas, will accept a maximum timing advance of about 46-48 degrees (on “vintage” pump gas in the 60’s and up through the 90’s, you could typically run in the 52 – 54 range – this is no longer advised). Some will take up to 50 degrees advance under these conditions. Once you advance the timing beyond this, the engine/car will start to “chug” or “jerk” (aka, “trailer hitching”) at cruise, and may experience occasional misfire, due to the over-advanced timing condition. Anything less than 46 degrees produces less than optimum fuel economy at cruise speed.
Click to expand...

I don’t know about maximum advisable advance figures etc. You’d need a proper expert for that. But in the scenario you describe the engine is only really experiencing a similar state to a steady cruise (in both cases it’ll be on the max vac advance stop). The reason for what seems a lot of advance is that a weak (ish) mixture burns slowly. When you open the throttle you need a richer mixture which burns quicker so the need to retard the timing from cruise but as engine revs increase you need to advance the timing to “get ahead” of the piston, the mech advance does this.

So in simple terms; vac advance compensates for mixture, mech advance compensates for piston speed.
 
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Oneball

Oneball

CCCUK Member
Forrest Gump said:
This has got to be a much easier way of getting it to run sweet?

View attachment 15741
Click to expand...

On really early carburettors the accelerator was linked to the mixture needle, engine speed was controlled by adjusting the mixture rather than opening a throttle plate. A leather diaphragm would then automatically expand or contract due to engine vacuum and maintain the correct fuel to air ratio.
 
trevbeadle

trevbeadle

CCCUK Member
My initial (base) timing is now set 10° BTDC. The centrifugal advance is, with one light and one medium spring, now giving me an additional 24 degrees at 2500 rpm. (34° BTDC 'all in'.
I have fitted a new Standard VC233 vacuum advance can..This make (Standard) is allegedly made by Dana and has 57 stamped on the flat attachment bar. ( I think this cross-references to VC1859 but can't figure out the '57'). With idle readjusted to tick over, with the vacuum connected to manifold vacuum, this can is giving me an increase of 24° on top of base (i.e. 34°). I've tried most ports on the carb, and it's either 24° (presume manifold) or 0° (presume 'ported). ( setting 34 or 10 on the timing light to align timing reference with TDC).
Do I;
a) reduce the base figure to 4° so IF at any time I'm getting full centrifugal plus full vacuum advance I won't exceed 52°? (4 + 24 + 24).
b) reduce the max advance the can pull to 18° by 'bushing' the pin portion of the vac-can to limit its movement?
c) set the total timing to 36° (12 base + 24 centrifugal) and limit the vacuum to 16 as above?
d) leave things as they are and road test individually with manifold vacuum or ported vacuum connected to the can and see what happens?
e) another course of action. Am I just completely misreading Lars technical papers, measuring things wrongly or completely missing something here?
 
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