Modifying Stock Distributor Total Advance

[vwlover77]

Because of the cam I now have in my engine (2 liter FI hydraulic), I'd like to advance the idle timing to approx 12deg BTDC. But that of course raises issues with the total advance being too high.

Is there a way to limit (reduce) the total advance of the distributor so I can set my initial timing to 12deg BTDC and have the total advance be where it should be?

Thanks!

[Amskeptic]

Because of the cam I now have in my engine (2 liter FI hydraulic), I'd like to advance the idle timing to approx 12deg BTDC. But that of course raises issues with the total advance being too high.

Is there a way to limit (reduce) the total advance of the distributor so I can set my initial timing to 12deg BTDC and have the total advance be where it should be?

Thanks!

Why do you want 12º BTDC? This is a figure you normally back into from the real important figure, the dynamic centrifugal timing at 3,000-3,400 rpm. If you have a cam that has introduced serious inefficiency to your idle, try to get the idle airflow and idle mixture close to max before you play with the idle timing.

Set your centrifugal timing maximum advance for 28-30º at 3,200 rpm. What is your current idle timing?

Check to make sure that your engine is thoroughly warmed up and that the auxiliary air regulator is fully closed. Is it? Now see if your idle air bypass screw is at least a full turn from bottomed out. Halfway is better still. What is your idle? If it is close to 1,000 rpm, your current idle timing is fine. If it is above 1,000 rpm, you do not want more idle timing. If it is sluggardly low, like 650 rpm, crank out the air bypass screw some more and see if your idle speed gets up to the 1,000 rpm mark. If it can't reach 1,000, then yes, more idle timing is called for. If your centrifugal advance likes to run the timing mark up the scale real close to idle speed, you might want to pull the spring post back a smidge. It should not start advancing until 1,100 rpm.
Colin

[dingo]

Is there a way to limit (reduce) the total advance of the distributor so I can set my initial timing to 12deg BTDC and have the total advance be where it should be?

How about installing an adjustable stop-screw that halts the movement of the vac-advance mechanism. ? Adjustable, so you can have the max point of advancement to whatever you like.

[vwlover77]

Dingo, remember that the primary advance mechanism is mechanical - the weights flying around inside the dizzy mechanism advancing the rotor shaft as RPM increases. The vacuum can is only good for an additional 10 degrees or so over max mechanical advance in high manifold vacuum conditions like light throttle cruise.

My timing is currently set at the factory spec 7.5deg BTDC. At that setting, my fully warm idle will not exceed 650rpm or so no matter how far I crank open the idle speed screw. I've resorted to adding an additional idle air "booster" circuit by teeing into the hose to the AAR from the S-boot and the big clear hose from the decel valve to the air distributor. The two tees are connected with a hose that has a wooden dowel stuffed in it, with a small hole drilled in the dowel to serve as a metering port for additional idle air.

This setup allows the idle adjustment screw to work again so I can get a 950 rpm idle with a mid-range setting of the screw.

The bad part about this is that the engine falls flat if its not revved a bit when launching the Bus from a stop in first gear. I was thinking some additional idle advance would help that situation, and allow me to get rid of the "booster" circuit too. But, I've got to find a way to limit total mechanical advance.

Thanks!

[dingo]

Dingo, remember that the primary advance mechanism is mechanical - the weights flying around inside the dizzy mechanism advancing the rotor shaft as RPM increases. The vacuum can is only good for an additional 10 degrees or so over max mechanical advance in high manifold vacuum conditions like light throttle cruise.

yeah i was referring to a stop-screw that limits the total travel of the vac-advance mechanism. Someone on ShoptalkForum mentioned doing this

[germansupplyscott]

limiting total mechanical advance is easy. inside the distributor under the points plate are the mechanical advance weights. these weights pivot away from the centre as the shaft spins. the weights have a limiting stop that is bent to a position according to the specs of that distributor. all bosch distributors have these bent stops, except the rare ones with no mechanical advance.

you can access the stops through the oblong metal cover that is pressed into the distributor body. it is a thin sheet metal cover about 15mm long. if you pry it off and rotate the distributor you will see the weights and stops, there are two of each.

bending the stops is easy, what will not be easy is getting them to the exact spot you need. in your case you want them to go inward to lessen the total mechanical advance. you may have to do this step many times to get the exact adjustment you are looking for.

i take no responsibilty for whatever damage you may inflict on your distributor by doing any of this

My timing is currently set at the factory spec 7.5deg BTDC. At that setting, my fully warm idle will not exceed 650rpm or so no matter how far I crank open the idle speed screw.

do you have the original throttle plate / throttle body installed on your engine?

some throttle plates have a hole, some do not. if yours has no hole, you might try one with a hole. i have swapped plates in engines when i build them to non-stock specs and the idle is difficult to set like in your case. some injected engines that came stock with retard on the distributor have the hole in the throttle plate to leak more air into the engine at idle.

[dingo]

Question: at 3200 rpm (and beyond)...how much vacuum is part of the equation...( if any at all.) ?

Ported vacuum only appears as throttle starts to open...but when does it diussappear again ?

[vwlover77]

Dingo, the vacuum advance is designed to allow the timing to be advanced further under light load conditions, at any almost any engine speed. So, cruising down a nice, flat road in 4th gear at 3200 rpm means there is gobs of vacuum to send to the distributor advance to get better fuel economy. Start up a grade and put your foot down and the vacuum signal disappears due to the open throttle so the timing falls back to protect the engine from pre-ignition and overheating. It's the intake stroke of the pistons sucking against the partially open throttle plate that creates the vacuum. Engine at speed, throttle barely open - lotsa vacuum. Engine at speed, throttle wide open, none.

Scott, my throttle plate does not have a hole in it, but I've effectively created one with my "booster" air circuit. But I think the advanced timing at idle will allow me to eliminate that, and also give the engine some more "oomph" coming off idle.

I know it's not the same distributor, but do any of the photos on this site show the parts I need to bend? It looks to me like the total advance on this one is controlled by the slots in the plate under the weights, not the posts that the springs attach to.....

http://www.glenn-ring.com/010/

Thanks!

[germansupplyscott]

Scott, my throttle plate does not have a hole in it, but I've effectively created one with my "booster" air circuit. But I think the advanced timing at idle will allow me to eliminate that, and also give the engine some more "oomph" coming off idle.

I know it's not the same distributor, but do any of the photos on this site show the parts I need to bend? It looks to me like the total advance on this one is controlled by the slots in the plate under the weights, not the posts that the springs attach to.....

http://www.glenn-ring.com/010/

Thanks!

i don't like the sound of your "fix" at all. too messy. the hole in the throttle plate will work much better.

the link is not much help. your distributor comes apart the same as the 010 in the link, but the advance mechanism is quite different.

this photo has a clear view of the oblong cover. ignore the fact that the distributor is for a watercooled vehicle.



get yourself a spare distributor to take apart and play with, that would be my recommendation.

[dingo]

Dingo, the vacuum advance is designed to allow the timing to be advanced further under light load conditions, at any almost any engine speed. So, cruising down a nice, flat road in 4th gear at 3200 rpm means there is gobs of vacuum to send to the distributor advance to get better fuel economy. Start up a grade and put your foot down and the vacuum signal disappears due to the open throttle so the timing falls back to protect the engine from pre-ignition and overheating. It's the intake stroke of the pistons sucking against the partially open throttle plate that creates the vacuum. Engine at speed, throttle barely open - lotsa vacuum. Engine at speed, throttle wide open, none.

Thanks..thats a nice clear explanation.

[vwlover77]

I disassembled my distributor yesterday to see about modifying the settings. It was a lot easier than I thought it would be!

Scott's description of everything was perfect. After looking at how the advance mechanism works, I concluded it would be easier to increase the idle advance by reducing the total advance, retiming the engine at speed for 28-30 deg total advance, and then backing into the idle timing at a higher value, if that makes any sense!

Rather than bending the advance weight stops, I found some plastic air tubing that fit very snugly over the stop posts, which will serve to prevent the weights from fully advancing the timing.

I have not reinstalled it yet to try it out, but if after setting the max advance to 28-30 degrees at 3500rpm (without vacuum line attached) I'll see if I'm in the ballpark on idle advance, which I hope to have at 10-12 degrees.

If nothing else, it was well worth getting in there and cleaning out all of the gunk and goo!