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1965 Impala SS 396 Convertible
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Discussion Starter · #1 ·
HI. My '65 Impala SS has a rebuilt 396 in it, all done before I got it, and I am pretty sure it has some type of camshaft upgrade (at least it lopes a lot at idle). Everything works OK but idle is a bit rough. The stock ignition timing calls for 4-6 before. When I check it with a timing light it seems to be set about 20 degrees BTDC (Can't really tell, since it is off the marker). I don't have any info on the cam, but is there any way to figure out where the ignition timing should be? My vacuum gauge needle at @ 800rpm idle reads low at about 17 which the gauge tells me is in the "too far retarded" range, but I don't know if that means anything, since the cam could change manifold vacuum at idle compared to stock. No signs of any vacuum leaks anywhere, everything looks tight, and the vacuum advance seems to working OK, but I don't have a vacuum pump to check it. . Any help here would be appreciated.
 

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Here's a place to start. You need to know where everything is set now to be able to interpret changes that you make and/or return to those settiings, If need be. I'm ignoring cam timing, it's probably correct from when the cam was installed anyway, and not a job for a novice.

There's 3 facets to a distributor. Initial timing, total timing and vacuum timing.

To find out initial timing - disconnect & plug the vacuum port at the distributor & carby, then check timing at idle as usual.
To find out total timing - disconnect & plug the vacuum port at the distributor & carby, then rev the engine until the timing no longer advances also making note of the rpm it ceases at, that's the total timing @ rpm.
To find out vacuum timing - reconnect the distributor vacuum port to full manifold vacuum, not ported vacuum, then check timing at idle as usual but subtract your initial timing value from this reading.

Total timing is 100% the most critical aspect, if it's wrong engine damage can result. It's not simply about having more timing, it's about having the correct timing for your engine. Timing changes will affect carby idle screw & idle mixture screw adjustment as well so timing should be correct before carby tuning. You will be better off attaching a timing tape to the flywheel to be able to read the timing that you can't accurately read now or you could buy or use a dial back timing light.
 

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1965 Impala SS 396 Convertible
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Discussion Starter · #3 ·
Hi Jynx: Thanks for all the good advice; I did some playing around with the timing checks as you suggested and came up with:
Initial timing = 18 BTDC
Total Timing = 38-40 (I only have a tuneup tach that peaks out at 1200 rpm, so I had to guess at ~ 3500 rpm. but this is where the timing stopped advancing
Vacuum timing = 38-18 = 20

I also checked manifold vacuum at idle (800 rpm) = 14-15" and steady Is this high enough to run power brakes? I would think so but wanted your opinion.
Ported vacuum at 800 rpm idle = 7".I was surprised it was this high. It is supposed to be basically zero at idle, isn't it?
 

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For sure the stock timing numbers are out-the-window with a camshaft change. Your total timing sounds acceptable for stock BBC heads, I was thinking BigDave once noted something like this compared to about 36 for stock SBC heads.
I don't see anything wrong with your 20 degrees initial timing for a rumpety cam. I'm biased because I've done it for years, in fact a little bit more, on a big 244/292 cammed 327 on old double-hump heads.
I had power drums and didn't notice them to be bad.
You noted 17" vacuum at the first post and then 14-15 above, I really think the 17" would be absolutely fine. The 15 is probably also fine but I think at 12", that's when it gets to be borderline.
Not to be a broken record but don't forget to adjust carb idle A/F and timing and idle speed for best idle vacuum. Works for me and then only needs a slight tweak or adjustment after a test drive.
 

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1965 Impala SS 396 Convertible
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Discussion Starter · #5 ·
Thanks, B.A. I am pretty sure the heads are stock, but not sure about the two different posts about idle vacuum. I must have gotten the 17" wrong somehow. I checked it several times and steady today at 14-15". I did check the carb with both tach and vacuum gauge (plugged into manifold vac, not the timed port) and adjusted mixture to max vac @ 800 rpm. Seems to run OK. Just worried about the getting the timing right before messing wiht the carb, as you said. I'll double check for any tiny vacuum leaks but is there any way to increase the manifold vac a couple of inches? Is it necessary to try?
 

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Hi Jynx: Thanks for all the good advice; I did some playing around with the timing checks as you suggested and came up with:
Initial timing = 18 BTDC
Total Timing = 38-40 (I only have a tuneup tach that peaks out at 1200 rpm, so I had to guess at ~ 3500 rpm. but this is where the timing stopped advancing
Vacuum timing = 38-18 = 20

I also checked manifold vacuum at idle (800 rpm) = 14-15" and steady Is this high enough to run power brakes? I would think so but wanted your opinion.
Ported vacuum at 800 rpm idle = 7".I was surprised it was this high. It is supposed to be basically zero at idle, isn't it?

Vacuum timing 38-18 = 20; at 3500 rpm? No. Because you opened the throttle plates and ran up the rpm, vacuum advance is basically zero. The 38 is initial plus mechanical. Mechanical is 20. The ONLY variable that impacts mechanical advance is RPM.

Want to measure your vacuum advance; do it at idle. Measure initial first (vacuum advance disconnected and plugged). Reconnect vacuum advance and measure timing; still at idle. Now, that timing reading minus the initial is the vacuum contribution.

A number for initial plus vacuum (at idle) around 22-24 BTDC is a good place to start. Tinker from there until you find your engine's 'sweet spot'.

Here's some reading you may find helpful:

As for the power brakes; there typically is a separate manifold vacuum port for the brakes. Sometimes on the carb, sometimes on the manifold. Sometimes a 'T' off of either.

Just trying to be helpful.

Pete
 

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Magneto - they're noting a difference between the extra timing that comes in due to our distributor<>carb vacuum line versus the inside of the distributor where RPM adds a range of increasing timing due to the rotation of weights and small springs.

So, where you noted 'vacuum timing' - that was really your mechanical advance with those weights and springs. It's still important to total timing so as to not get too much advance in there.
 

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1965 Impala SS 396 Convertible
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Discussion Starter · #10 ·
For sure the stock timing numbers are out-the-window with a camshaft change. Your total timing sounds acceptable for stock BBC heads, I was thinking BigDave once noted something like this compared to about 36 for stock SBC heads.
I don't see anything wrong with your 20 degrees initial timing for a rumpety cam. I'm biased because I've done it for years, in fact a little bit more, on a big 244/292 cammed 327 on old double-hump heads.
I had power drums and didn't notice them to be bad.
You noted 17" vacuum at the first post and then 14-15 above, I really think the 17" would be absolutely fine. The 15 is probably also fine but I think at 12", that's when it gets to be borderline.
Not to be a broken record but don't forget to adjust carb idle A/F and timing and idle speed for best idle vacuum. Works for me and then only needs a slight tweak or adjustment after a test drive.
HI B.A.: I took your advice and have been playing with the timing between 12-20 on my 396. I have it set now @ 20 degrees with my "rumpety cam". I had to cobble up a timing plate extension with stiff paper since the stock plate only goes to 17. (I have an advanced timing light, but I don't trust it). I have the vacuum advance plugged into the timed port on my Holley 750 and confirmed no vacuum advance at idle. Total advance with everything hooked up at 3500 rpm is about 38-40. The car seems to run fine, no throttle lag and no pinging, but a little hard to start and still runs rich (with some carbon fouling on the plugs). The carb seems OK, everything clean and working OK, but could have some problem inside I guess. The previous owner could have put some racing jets in it, but I don't think so since the rest of the car is pretty stock, I set the idle screws to max vacuum at idle, which seems to be about 1/2 way out. With this cam I can only get about 15-16 inches max manifold vacuum, but of course shoots up to abouto 25 when I snap the throttle shut, so the power brakes seem to work ok when I need them. Any suggestions about running rich? Thanks!
 

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1965 Impala SS 396 Convertible
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Discussion Starter · #11 ·
Vacuum timing 38-18 = 20; at 3500 rpm? No. Because you opened the throttle plates and ran up the rpm, vacuum advance is basically zero. The 38 is initial plus mechanical. Mechanical is 20. The ONLY variable that impacts mechanical advance is RPM.

Want to measure your vacuum advance; do it at idle. Measure initial first (vacuum advance disconnected and plugged). Reconnect vacuum advance and measure timing; still at idle. Now, that timing reading minus the initial is the vacuum contribution.

A number for initial plus vacuum (at idle) around 22-24 BTDC is a good place to start. Tinker from there until you find your engine's 'sweet spot'.

Here's some reading you may find helpful:

As for the power brakes; there typically is a separate manifold vacuum port for the brakes. Sometimes on the carb, sometimes on the manifold. Sometimes a 'T' off of either.

Just trying to be helpful.

Pete
HI JAPete. Thanks for the advice. When you say "22-24 initial plus vacc @ idle", I have my vacuum advanced plugged into the timed port on my Holley 750, and I have confirmed zero vacuum and zero advance at @ idle. So for my 396, both inital or initial + vacuum are the same. Would you recommend setting the initial then at 22-24. Seems kinds high and the bugger seems a little hard to start at 20.
 

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Just some thoughts...

With an idle of 800rpm & vacuum of 15-16 Hg the cam is closer to mild than high performance.
40 total is definitely on the high side even if you're always using high octane fuel.
Beware of what you read on the net regarding timing, all the information you will read about timing will relate to high performance engines, which more likely than not were sorted out on a dyno with full knowledge of the component parts used in the engine & their limitations. Also Dyno results don't necessarily convert directly to the real world once the engine is taken off the dyno and fitted to a vehicle. Just remember, there's minimal load on a dyno & water temperature is often controlled to below temps encountered within an engine bay, so take any dyno numbers & dyno timing with a generous dose of salt.
Pinging can be difficult to detect for the untrained ear, but will be most prevalent when the engine is under load, for example when towing or accelerating up a steep incline with a car full of friends.
Damage caused by too much timing is most likely to happen when under load.
20 initial is going to impact starting quality, especially in hot weather conditions, & may require a high torque starter motor if you retain that setting.
You can manipulate the initial timing value by connecting your vacuum timing to full manifold vacuum, by varying the amount of timing added by the vacuum canister.

With those thoughts in mind, personally, I would be more conservative.
I would aim for a total of 36 to be safe. You have 20 in the dizzy so that would mean 16 initial, which should be fine for the starter motor. If you want an initial of 24 as suggested by japete92 then set your initial at 16 still and add another 8 by the vacuum canister on full manifold vacuum, which will not impact your total timing of 36 or make the starter motor struggle. If the starter motor still doesn't like 16 then drop in back to 14 and increase the vacuum canister to 10 assuming 24 initial is your target.

As a side note. Some, not all, dizzy's have replaceable Advance Stop Bushings. These can be used to alter the amount of timing added by the dizzy. So, if your dizzy has this option you could change the bush with another so that the dizzy adds more or less than the 20 yours currently adds, which again allows you to manipulate the initial timing value without affecting your chosen total timing value when set correctly.

Also bear in mind any changes you make to timing settings will require idle screw & air mixture screw re-adjustment, which could be contributing to your current rich mixture if not done and which could also be caused by a dirty or poor flowing air filter.
 

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1965 Impala SS 396 Convertible
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Discussion Starter · #13 ·
HI Jynx. Thanks again. I read some more about timing at the link:
Ignition 101
suggested by JAPete. Good articles but he says you should use ONLY manifold vac on a non-smog engine and plug the timed port. So I did that and came up with the following:
Initial Timing = 12 degrees @ 800 RPM
Initial plus vacc = 32 @ 800 rpm so vacuum advance is adding 20. This sounds like a lot maybe? Fiddling with the timing and the carb has gotten me up to 16" of manifold vacuum at idle. JAPete says shoot for "20-24 combined initial +vacc." If I drop the initial to 4-6 to get to the 24, which is the stock setting, it runs lousy, with backfiring under acceleration.
Timing with vacc plugged = 17 at 3500 rpm, so the distributor is only adding 5 mechanical advance? That seems low? Maybe it's time to pull the dist. and vacuum can and see what's going on inside? My mechanic wants me to install fuel injection and a new distr. for about $5000. That might fix the problem but I was looking for a cheaper fix!
Thanks again for all the help.
Bob
 

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Is this still the original GM dizzy? if so, the only advice i've given and everybody else has given you that would apply is how to determine initial, total & vacuum advance. If it only adds 5 degrees mechanical advance I would go back to what runs best for now & I wouldn't be spending money on fuel injection, but I would be replacing the dizzy so that you can set it up properly for your engine as it is now.
Take a look at Progression Ignition & view some of their videos, you have complete control of every setting from a smart phone while the engine is running, with no need to touch the dizzy at all.

Alternately if you want to stay with a mechanical Dizzy make sure it has the Advance Stop Bushing replacement ability I mentioned above.
 

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HI JAPete. Thanks for the advice. When you say "22-24 initial plus vacc @ idle", I have my vacuum advanced plugged into the timed port on my Holley 750, and I have confirmed zero vacuum and zero advance at @ idle. So for my 396, both inital or initial + vacuum are the same. Would you recommend setting the initial then at 22-24. Seems kinds high and the bugger seems a little hard to start at 20.
I would not recommend the 'timed' port. That 'port' (timed) is above the throttle plates and does not allow vacuum advance at idle.

If vacuum advance is too high, it may (should in my opinion) be 'stopped'/'limited' to approx 10-12 degrees. 20 degrees of vacuum advance is too much.

Suggest doing a little research in Dave Ray's web site. He does a great job of explaining how/why. AND will send specific instructions if one simply asks. Here's the link:


GM 'limited' vacuum advance using a small section of vacuum hose; see the pics in the site. The mechanical 'stop' Dave depicts is more reliable, but both do the same thing.

Mechanical (centrifugal) advance ONLY adds degrees of timing with RPM. The springs and weights determine how much and how 'fast'. It can be changed but not adjusted.

Pete
 

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1965 Impala SS 396 Convertible
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Discussion Starter · #17 ·
Thanks, again, Jynx. I took your advice and purchased a new distributor from Progression Ignition. I installed it over the weekend, and it is exceedingly cool. Install was straightforward and set up is ultra easy with the cell phone app. I have been trying to read all I can about setting timing for a hotter street cam like I have, but still confused about whether to add more vacuum advance or more initial timing. JAPete suggested 20-24 total, a guy on the Chevelle Forum, who seems to know his stuff, suggested up to 30 total. The new dizzy cranks at 15 so starting is not an issue, but I am confused about whether to add more initial timing or more vacc get more total at idle. Does it matter? If I can't get the stinky idle fixed, my wife will make me sell it or move out!
 

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"....the hotter street cam I have...".

Having 20 degrees of vacuum advance at idle (a parameter you stated in a previous post) is inconsistent with having a 'hotter street cam'.

What has you believing you have a 'hot' cam? The lousy idle? That's more likely due to lousy tuning. Actually, the reason the idle is poor with cams designed for high rpm power is because 'idle' tuning/running is not at all the goal. It is sacrificed for high rpm power/tuning. The lousy idle is a fall out of the high rpm tune.

Regardless of how the distributor directs spark to the proper cylinder in the proper time, the basic advantages of timing advance (from top dead center) are present/real.

I am not familiar with your new distributor. Recommend simply follow the directions and tune it for 'normal' and see what you get.

Pete
 

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1965 Impala SS 396 Convertible
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Discussion Starter · #20 ·
Thanks , JAPete, for the advice. I think I have modified cam since it has one of those "rumpety" idles and only gives me about 15" vacc @ 800 rpm. (I think stock should be ~ 18"?) the prev. owner rebuilt it about 10 years ago and also added some Hooker long tube headers, so I think he probably did some work on the inside too. The 20* vacuum advance I mentioned previously was incorrect. Anyway, this new distributor is pretty awesome. It has a vacc port but no can - everything is set electronically from their cell phone app. You can set the vacc any way you want but right now i have it set to start coming on at 900 rpm to a max of 8* and all-in of 36*. The 396 seems pretty happy at 22* initial right now with no vacuum advance at idle, and is running cleaner. I dropped the idle RPM to 700 to try to ensure the throttle plates are 100% closed and have both idle circuits leaned out as far as I can w/o a drop in manifold pressure. (still getting 15-16") I understand your comments about performance cams, but I have got to find a way to lean this thing out or I'll have to drive it with a gas mask! Thanks, Again.
 
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