Help! with ignition timing settings, please

[trevbeadle]

Hi,

I need help in explaining what to me seem ‘strange’ figures I’m getting on checking the ignition timing on my 1980 C3. The car was a Californian ’80 so originally had a 305 cu in (5.0 L) V8 engine rated at 180 hp (134 kW),. The carburettor and ignition timing were controlled by Chevrolet's new Computer Command Control system.

The engine was replaced with a 350 Goodwrench crate motor (“Hecho en Mexico”), engine number 1M0615 9VP, block casting 10066036. My research says this was a new Goodrich 350 engine 1971-1985 Car and light Truck Replacement Engine, 350 cu in, 8.5:1 comp ratio, 4.00” bore 3.4.7” stroke, 250 HP @4300 rpm. i.e the standard Goodwrench 350. With GM HEI ignition, Base ignition timing for this engine should be 10 before TDC (vacuum advance disconnected) and total ignition timing around 36 @3000 rpm. The car stayed in California (with all emission control items fitted) and presumably met California tight emission limits. It was imported to Britain in 2019 when I got it.

I have replaced the ‘electronic’ quadrajet with a ‘new’ manual only quadrajet part number 17085226 1566 HLY. The GM HEI distributor has also been fitted with an Accel (brand) adjustable vacuum ‘can’. Still has EFE, A.I.R and EGR fitted although I don’t have a clue if they’re now still working as the CCC engine loom has been removed from the old carb and A.I.R. pump. However EFE and EGR are vacuum operated anyway and still connected.

I have a roll-back ignition timing light and a vacuum gauge.

Now, checking Initial (base) timing, engine warm, at 600 rpm I have 12 BTDC with vacuum advance disconnected and plugged (12 on the light dial, timing marks aligned on 0). OK, so 2 more than my quoted figure of 10, but this is just a check. Now having read articles, I’m expecting as engine RPM increases I will get an increase caused by the centrifugal advance weights of around another 24 degrees., so I’m expecting around 36 BTDC all in or so (initial plus centrifugal/mechanical). Setting my strobe lamp at 36, sure enough the advance increases but then stops, and I have to wind the setting down 7 to get the timing marks aligned on 0, the timing light is therefore showing 29 all in, so it appears that the weights are only giving me a maximum of 17 degrees of advance rather than the 24 expected. Curious! I do have a kit with various springs, but if it’s all in, its all in? If I were to turn the distributor to give me 36, I would have an initial timing of 19 BTDC!!!!!(I haven’t done that!)

Next query! The car, being Californian born and bred, it has the vacuum advance pipe connected to ported vacuum. Testing the suck with a vacuum gauge I’m getting no vacuum at idle (expected) but vacuum increases as I open the throttle (again expected). Now I’m advised that I should be using Manifold vacuum for vacuum advance so I seek out an alternate port on the quadrajet. Connecting to a spare one low down on the carb below the throttle blades I now pull 18 inches hg at idle on the gauge. Now I’m expecting around another 15 degrees of advance at idle according to articles if I connect my ignition can to this Manifold vacuum source, so giving an idle total advance of Initial (12 in this case) plus 15 from the vacuum equalling 27. However, when I do this timing goes to 36 BTDC, 24 from the can! If I left it there, wouldn’t I then get another 17 degrees from my centrifugal advance making 53 total advance? I read that initial + centrifugal + vacuum should not exceed 52, so if I drop my Initial from where it is now (12) to what my spec says (10) is this going to be right with 51??? I know individual figures look wrong but totals look rightish!!!

Opinions invited as to if I’m talking bollocks or my car is all wrong!,…Thanks

 

[johng]

Trev, mine is set to 15 degrees at tickover (without the vacuum advance) and it is all in at 36 degrees at 3200rpm, so 21 degrees of mechanical advance. I'm pretty sure I have read that you can limit the total mechanical advance, so it could be that has been done on yours. Alternatively 17 degrees might just be what your distributor has as standard, I'm not expert enough to know that. My vacuum can pulled 18 degrees as standard, but I've limited it's travel to 11 degrees. You say yours is an adjustable can, so presumably you can adjust it to suit.

 

[Roscobbc]

Wonder if someone previously had limited the total mechanical advance by doctoring the distributor plate? TBH I'd go along with John's figure as a maximum.
Mine a far larger engine with a few performance modifications so comfortably runs with a static timing of 22 degree - and a total of 38 degrees. The vacuum can on mine 'pulls' another 15 degrees so I have a theoretical total of 53 degrees. This total will only ever be at idle or on a trailing throttle - as soon as the throttle is opened, vacuum decreases and the 15 degrees will recede. The idle vacuum on mine (if I recall) is either 12 or 14 hg. I sometimes wonder if the the vacuum can (or perhaps the distributor linkage connected) is a little tight or binding as occasionally if flooring the gas pedal it pinks........indicative to me that the distributor plate isn't retarding as quick as it should. I'd be interested in the details of your adjustable vacuum can.
Back in the 'old days' the first thing to buy when looking to 'perk-up' the engine was a Moroso spring and bob-weight kit. Usually putting-in the weakest springs and heaviest bobweights to get the quickest advance rate possible (but only on manual cars). Did that when I first got my '69 427. Max advance came-in at 2000 rpm. Really sharpened things up!

 

[Forrest Gump]

It is a bit intriguing. Your current setting of 12 degrees initial plus the 17 mech is only giving you 29 all-in, so you are leaving a lot on the table in terms of optimum performance (aiming for 36) Someone may have set the initial timing and not checked the all-in, or possibly something has gone amiss with the mechanical advance - like it is sticking. If it is modified you might be able to see a spot of weld on one of the slots on the distributor plate. There’s no harm increasing initial to 19 as long as you are sure the mechanical advance is reliably giving you 17. Then as John said, limit the vacuum can to 14 or 15.

 

[Oneball]

Both mech and vac advance are limited in dissy. Take a photo of the dissy with the rotor arm off and put it up, see if anyone can spot anything amiss. It could be that what you’ve got is the setup of your dissy.

 

[Roscobbc]

Is the dizzy the oem unit or aftermarket?

 

[teamzr1]

Is it possible that the CAM was replaced in the past of a custom CAM was installed
and they did not align the CAM straight up or degree'd the CAM when installed as that would affect the timing ?

Just an example of timing, this is a GM calibration for a 1985 Corvette with 350 CI engine that has total advance timing
higher than your C3

 

[trevbeadle]

Very much the OEM GM HEI distributor I think Rosco. It is an aftermarket can. The Accel vacuum can is set to 4 1/2 turns from fully counter-clockwise so I was expecting, from the chart, 15 degrees. Tomorrow I'll wind that in and out and see if that has any effect on the 24 I seem to be getting now. I'll also pop the cap off the dizzy and take some photos. Cheers.

 

[Forrest Gump]

According to that chart the maximum advance you can get from the vac can looks to be just over 20 certainly not 24!
I wonder if you are actually getting some mechanical advance coming in at idle rpm. That could account for why it appears to only gain 17 mechanical. The springs could be too light for the Bob weights or one spring missing. A good test would be to fit stronger springs and see what happens to the timing curves then.

 

[Alben78]

When sorting the hid on my 78 I found the attached most interesting.

http://www.corvettefaq.com/c3/HowToSetTiming2.doc

This is part of a larger file some of which may make interesting reading if anybody has a specific problem.

Index of /c3

Hope this helps someone!

 

[johng]

That's sort of the same paper I used, but if you email Lars ( v8fastcars@msn.com ) he will send you the latest version which is quite a bit longer than the one Alben has pointed to

 

[Chuffer]

Hi,

I need help in explaining what to me seem ‘strange’ figures I’m getting on checking the ignition timing on my 1980 C3. The car was a Californian ’80 so originally had a 305 cu in (5.0 L) V8 engine rated at 180 hp (134 kW),. The carburettor and ignition timing were controlled by Chevrolet's new Computer Command Control system.

The engine was replaced with a 350 Goodwrench crate motor (“Hecho en Mexico”), engine number 1M0615 9VP, block casting 10066036. My research says this was a new Goodrich 350 engine 1971-1985 Car and light Truck Replacement Engine, 350 cu in, 8.5:1 comp ratio, 4.00” bore 3.4.7” stroke, 250 HP @4300 rpm. i.e the standard Goodwrench 350. With GM HEI ignition, Base ignition timing for this engine should be 10 before TDC (vacuum advance disconnected) and total ignition timing around 36 @3000 rpm. The car stayed in California (with all emission control items fitted) and presumably met California tight emission limits. It was imported to Britain in 2019 when I got it.

I have replaced the ‘electronic’ quadrajet with a ‘new’ manual only quadrajet part number 17085226 1566 HLY. The GM HEI distributor has also been fitted with an Accel (brand) adjustable vacuum ‘can’. Still has EFE, A.I.R and EGR fitted although I don’t have a clue if they’re now still working as the CCC engine loom has been removed from the old carb and A.I.R. pump. However EFE and EGR are vacuum operated anyway and still connected.

I have a roll-back ignition timing light and a vacuum gauge.

Now, checking Initial (base) timing, engine warm, at 600 rpm I have 12 BTDC with vacuum advance disconnected and plugged (12 on the light dial, timing marks aligned on 0). OK, so 2 more than my quoted figure of 10, but this is just a check. Now having read articles, I’m expecting as engine RPM increases I will get an increase caused by the centrifugal advance weights of around another 24 degrees., so I’m expecting around 36 BTDC all in or so (initial plus centrifugal/mechanical). Setting my strobe lamp at 36, sure enough the advance increases but then stops, and I have to wind the setting down 7 to get the timing marks aligned on 0, the timing light is therefore showing 29 all in, so it appears that the weights are only giving me a maximum of 17 degrees of advance rather than the 24 expected. Curious! I do have a kit with various springs, but if it’s all in, its all in? If I were to turn the distributor to give me 36, I would have an initial timing of 19 BTDC!!!!!(I haven’t done that!)

Next query! The car, being Californian born and bred, it has the vacuum advance pipe connected to ported vacuum. Testing the suck with a vacuum gauge I’m getting no vacuum at idle (expected) but vacuum increases as I open the throttle (again expected). Now I’m advised that I should be using Manifold vacuum for vacuum advance so I seek out an alternate port on the quadrajet. Connecting to a spare one low down on the carb below the throttle blades I now pull 18 inches hg at idle on the gauge. Now I’m expecting around another 15 degrees of advance at idle according to articles if I connect my ignition can to this Manifold vacuum source, so giving an idle total advance of Initial (12 in this case) plus 15 from the vacuum equalling 27. However, when I do this timing goes to 36 BTDC, 24 from the can! If I left it there, wouldn’t I then get another 17 degrees from my centrifugal advance making 53 total advance? I read that initial + centrifugal + vacuum should not exceed 52, so if I drop my Initial from where it is now (12) to what my spec says (10) is this going to be right with 51??? I know individual figures look wrong but totals look rightish!!!

Opinions invited as to if I’m talking bollocks or my car is all wrong!,…Thanks

If it helps , I have a 1980 C3 with engine swap to a 1985 Camaro IROC -Z 305 cubic inch 9.5 : 1 compression ratio. The original TPI was replaced with an Edelbrock Torker 2 manifold and an Edelbrock 650 cfm carb and it is set up 34 degrees `all in`.

 

[trevbeadle]

Alben, thank you, I think that is for non-HEI dizzy?
Chuffer, yeah I'm expecting 34-36 all in.
Forrest, as the can is set 4 and half turns I'm only expecting 15 not 20+.
Teamzr, so that grid shows a maximum of 48.2 all in? My engine would be circa '86 or a bit later.
Oneball, here are some photos..........

I think you can secure the weights with an elastic band to stop them moving, so I will 'tie' them and check it's not putting in mechanical advance at idle.
I have an HEI curve kit with weights and springs (photo 5) so I have 3 different colour and tension springs, the the black ones look like the ones already on, so I could try a lighter/heavier pair, however, the weights look a lot smaller than the ones that are on there already. They are all free to move and as far as I can see, functioning correctly (I used my fingers!)
If anyone can see anything 'funny' with the centrifugal advance or the vacuum can, please comment.
Thanks

 

[Oneball]

I don’t think your weights are returning completely to rest. Can you close them up by hand and the springs go loose?

 

[teamzr1]

Teamzr, so that grid shows a maximum of 48.2 all in? My engine would be circa '86 or a bit later.
Oneball, here are some photos..........

Thanks

Yes that timing table is inside the calibration as to what GM set timing to which is common values for an example of 1985 C4
This is depending on RPM and engine load

Is the RPMs steady at cold or hot idle, and is running smooth or lumpy at idle ?

 

[Chuffer]

Alben, thank you, I think that is for non-HEI dizzy?
Chuffer, yeah I'm expecting 34-36 all in.
Forrest, as the can is set 4 and half turns I'm only expecting 15 not 20+.
Teamzr, so that grid shows a maximum of 48.2 all in? My engine would be circa '86 or a bit later.
Oneball, here are some photos..........

I think you can secure the weights with an elastic band to stop them moving, so I will 'tie' them and check it's not putting in mechanical advance at idle.
I have an HEI curve kit with weights and springs (photo 5) so I have 3 different colour and tension springs, the the black ones look like the ones already on, so I could try a lighter/heavier pair, however, the weights look a lot smaller than the ones that are on there already. They are all free to move and as far as I can see, functioning correctly (I used my fingers!)
If anyone can see anything 'funny' with the centrifugal advance or the vacuum can, please comment.
Thanks

I forgot to say that mine has a HyperTech Street / Strip dizzie .

 

[johng]

Out of interest Trev, at what rpm do you get the 17 degrees of mechanical advance?

 

[Roscobbc]

Trevor - when I first used the advance kit (on my stock dizzy) - as noted previously I used the lightest springs and the heaviest weights to get the advance asap.
I plotted the advance/revs with a 'dial back' timing light every few hundred revs from idle right up to the rpm where maximum advance was occuring (1900 or 2000 rpm I think)
Sometimes the 'curve' doesn't quite work as you would expect and 50% of the mechanical advance was all-in by 950 rpm (from an idle of 550 rpm)
Would it help you to 'plot' both 'ends' of the advance curve perhaps tie the weights in both fully closed and full opened positions and (using the dial back timing light) measure the respective readings? - it will at least confirm the full mechanical advance range.

 

[Forrest Gump]

Forrest, as the can is set 4 and half turns I'm only expecting 15 not 20+.

…Yes but in your first post you said you were getting 24 from vacuum? I was meaning something is amiss there too because the vacuum can isn’t capable of giving that much according to the Acell chart, plus you have it adjusted for less.

 

[Forrest Gump]

perhaps tie the weights in both fully closed and full opened positions and (using the dial back timing light) measure the respective readings? - it will at least confirm the full mechanical advance range.

Would the car actually start with the weights tied to give full mech advance? Be careful of kick-back and chipping flywheel teeth! You sometimes see on racing cars where the timing is locked they spin the engine over then hit the ignition.