i am trying to check total timing as per the articles written by John and Lars which indicate 36 degrees. Mine exceeds that but the initial timing is 10 deg on my 365 per the book. Does this mean 40 deg would be the number for a 365?


from what I've read in searches it seems that would not be the case. therefore i would need to change out springs, heavier? i believe. i had this set up by a guy with a sun machine so i wonder what went wrong?

 

If you had the distributor set up on a machine, the first thing I would do is make sure the timing light is reading right. I've have taken 3 different timing light and 2 read the same at idle, 1 was different. All 3 where different at 3000 rpm.

 

i may do that since I dont have an advance light and used the tape method.

i checked everything again dwell, static and reved engine. timing mark will go beyond 36 deg. assuming light is good, springs next?

also, vac advance does not seem to be working at idle. there is no change in timing with it attached. new unit provided by tuner of distributor. i suppose i need to find out what it is and get a lower vac unit?

 

If I'm not mistaken, you should have 24* (or 26* w/ TI) of centrifugal advance. The figure of 36* total does not include any vacuum advance.

The springs on the distributor only regulate how quickly the centrifugal advance comes on.

 

ok, to clarify- with the vac disconnected I read in excess of 36 deg, when reved as indicated by the 0 tab vs. the tape mark.

i found some info on the web about limiting the advance?

 

In order to recurve the distributor to conform to the correct map you might need to install a combination of weights, springs or autocam. Also a B28 VAC is the appropriate canister for a 327 365 .

 

Adjust the timing until you see only 36 degrees when revved. This will alter your base timing at idle, but will put the timing where it needs to be under power.

 

That's correct. To expand on this:

Your advance curve is too long. Make sure your advance stop bushing is not missing - most are missing, making the curve too long.

Once you get the bushing installed, set the total timing to 36 and take whatever initial timing you get. If it's below 10 or 12 degrees, you should consider shortening the advance limit slot so you can run at least 12 degrees initial with 36 total.

Lars

 

Check the little rubber bushing under the cent. mechanism that limits the advance. The get old, hard and fall off.

Lars was one minute ahead of me. LOL

 

...at least we're consistent in our recommendations...
Lars

 

lars and others provided the probable answer, the bushing, but I wrote this and it may help understand the tuning goal (so take what makes sense).

These old iron head engines usually produce the best power at 36-38 degrees total advance at wide open throttle (WOT). There is little vacuum (and no vacuum advance) at WOT, so total advance testing is done with the vacuum advanced hose removed and the port on the manifold plugged.

If your engine idles great with 10 degrees static at idle, and then you run the engine up to 3500 rpm and you watch it run past 40 degrees total, the distributor allows too much centrifugal advance for your engine combination. The advance springs have no effect on the total degrees of advance allowed by the distributor (the springs only define the speed that they allow the degrees of advance to rise at a given RPM).

The centrifugal advance is controlled by a pin that rides a slot in the advance plate where the rotor is attached, inside the distributor. You cannot easily see the pin because it is under the rotor & advance spring plate. The pin is smaller than the slot, and GM (Delco) designed it this way to allow the use of a soft bushing that is pressed on the pin (to provide a soft bearing surface between the steel pin and the slot in the steel plate). A common problem is the bushing wears and falls out (they are soft brass, or hard rubber). With no bushing there is too much pin travel in the advance plate slot, due to the skinny pin, sometimes resulting in way more than the originally intended 22 to 28 degrees.

The easy solution to resolve too much advance is to buy and install a new bushing. Remove the distributor cap and pull the rotor & advance weights off. Reach under the advance plate with a finger tip to feel for the pin. Rotate the advance plate and feel the pin move along the slot. You may be able to feel pieces of an old worn bushing, but more often than not there is no bushing. The new bushing can be pushed on by hand, and if you can, lightly squeezed with a pair of needle nose pliers (deformed to help hold it on the pin). You should be able to rotate the advance plate and feel the more limited travel in the advance slot, with the bushing bottoming or limiting travel at one or both ends of the slot. Put the weights and springs back on with the rotor and cap.

Since the new bushing now limits the advance at a different place in the advance slot the distributor needs to be timed from scratch. Reset the 10-degrees of advance at idle and see what you end up with (hopefully less than before).

In the old hot rod days you could buy different size & shape bushings to further tune the pin's travel in the slot (less centrifugal advance for high performance) when you needed more advance at idle and still maintain 34-38 degrees total advance. Racers also used to file the bushings to fine tune the advance travel.

Some racers would run no bushings to allow more centrifugal advance, good for power when using alcohol as a fuel where you can run 45 or more degrees of total advance. Since a lot of centrifugal advance was cool for "racers," many people had their distributors jacked up to duplicate the "racer tune" by people who do not understand what they were doing (a practice that continues today).

There are also people who tune distributors who do not trust using the GM advance bushing (including me). It's not that the bushing is a bad design, it's just that the stock bushings do not always limit the pin's travel on the advance slot exactly where the tuner wants the total advance to be. The engine may have a wild cam and need 18-degrees of advance at idle, that becomes a problem when combined with the factory bushing's centrifugal advance, because it ends up with too much total advance (more than the desired 32-38 degrees the engine might prefer with aluminum or iron heads). If the distributor needs fine tuning, it was common to disassemble the distributor and braze one end of the advance slot to fill it with brass. The shorter slot provided less centrifugal advance. The soft brass brazing material also allowed the tuner to file the slot to the exact advance travel they wanted. Not enough advance, pull the distributor apart and run a jewelers file across the brass to open up the slot and reassemble to test. With a Sun Machine you can properly tune a distributor's centrifugal advance this way in a very short time, to provide the correct amount of static advance (at idle) and total advance (at the rpm when you want best WOT power advance).

This process is what we used to consider getting a distributor properly "tuned" in a Sun machine. You tested the engine for the best advance for a clean idle, and either guessed or had a dyno run to learn the best total advance for power at WOT (or made lots of timed back to back tests on a deserted road). Once you determined the idle and WOT advance limits you would head for the Sun Machine and take any plain-jane GM-Delco distributor and tune it for whatever engine combination you had.

Once the advance slot was tuned, you fitted the lightest advance springs that did not allow pinging when lugging the engine (a few more WOT runs down that deserted road).

Remember to put the vacuum advance back on when you are done tuning the centrifugal advance, because the engine will actually run better at part throttle with more than 38 degrees advance. This is the advantage of the additional vacuum advance, it adds more advance when the engine can benefit from the added combustion pressure without the potential for detonation that occurs with the heat of WOT power.

 

Good write up there with some good tips.

 

Ditto. It would be a good post to be added to the FAQ up top.

larry

 

ok thanks for the response:

The complete unit was rebuilt on a SUN and he is supposed to know what he is doing. I will need to call him on this to see what he says.

I'm not sure how it will react at idle should I reduce the timing, how I take it, is you suggest this only temporarily until I shorten the advance? ...but I"ll give it a try and look into the bushing issue as stated.

 

I would set the timing for the best idle (and document the degrees of static advance when it idles best), and keep my foot out of the throttle. There is little to fear if you just keep your foot out of full throttle, until you get the distributor properly matched to your engine. It's also better in the short term to tune for a quality idle, than tune for the best WOT performance, because you don't need to drive at WOT but you do need the car to idle clean.

Your distributor tuner can then take the idle timing information you give them and tune the centrifugal advance in the distributor to achieve a total advance of 36 crankshaft degrees.

If the engine idles best at 12-degrees of advance, then have the distributor tuner set the centrifugal for an additional 24-crankshaft degrees of advance (12 static + 24 centrifugal = 36 total degrees). If you need more static advance for a clean idle then tune the centrifugal advance for a corresponding shorter amount to result in the same 36 degrees of total advance (for example with a long duration race cam you might need 18 static + 18 centrifugal = 36 total).

When the tuner is done you should be able to set the idle timing where it needs to be for a quality idle, and then flog the throttle with no fear of detonation and total timing advance over your target of 36-38 degrees.

 

Ok, here comes the stupid question time.

Initial timing is set at the timing tab be it 6*, 10* advance or what ever is needed, correct?? My initial time is unknown cause its way off the timing tab, but let me make a guess first. If I set my initial time at 10* adv., that will get me my needed 36*-38* total time?

But, how do you change or set total time?

I have a dial back light, and with the vac. adv. plugged, RPM's up so timing does not change anymore, I have 54* of total time.

 

Have you read the instructions that came with the dial-back timing light? It's not possible to have 54* total timing - that would give you around 30* of initial advance, and the engine wouldn't even start.

 

Since the vac advance does not engage at idle does it make sense for me to plug it to limit any additional advance?

 

???? Sure it engages at idle, or it should. When you connect the vac. advance does your timing and RPM increase at idle? If not have you confirmed the vac. advance is working at all by testing it for holding a vacuum?

 

there is no change to idle or timing with it attached or plugged. I assume the engine vac is too low to engage it.

I,m all ears if you would advise how to test it but I assume it is using a mighty vac tester