Plugging Timed vacuum advance hose
I should really start taking notes when I'm making adjustments, im pretty good when tuning my carbs but never wrote down timing numbers. I'll double check hopefully this week some time. I'm pretty sure I adjusted the can to open at 9" vacuum, because I was using manifold vacuum and wanted it full open with the low idle vacuum. I will raise it up some with the switch to ported vacuum but I Need to drive with a gauge to see what my ported vacuum readings are under different throttle conditions. I guess my question would be after figuring all that out, where to set the can to start advancing the timing ?
Notes are good!
When I get in, I usually rewrite the mechanical timing & rpm numbers into a graph.
I would not touch the vac advance with a change to ported source of vacuum.
* One change at a time, unless there is some particular relationship that demands otherwise.
In this case, the only change will (should) be the loss of advance at idle. As soon as the throttle is slightly off idle, it will bring the vacuum advance the same as before.
* If anything, you may decide to change the initial timing.
Example:
Lets say with your 340 has the same generation Chyrsler built MP distributor that I used in the test linked above. If it was set to begin adding vacuum advance around 9", then by 11" Hg it will be adding 4 to 5 degrees. At 15" and higher it will be adding around 16 degrees advance.
Lets say it currently idles at 800 rpm with 12 degrees initial and that distributor's mechanical curve begins advancing at 850 rpm and adds 20 degrees before 3000 rpm.
Therefore,
It generally idles with 12 initial + 5 degrees from vacuum = 17 deg BTDC. That's ballpark for tuning in that size engine/cam/compression.
Full mechanical advance would be 12 initial + 20 advance = 32 degrees by 3000 rpm. That's decent, safe, although a little more advance might result in quicker WOT acceleration at the strip. It also might result in quicker response off the line - but that also depends on the slopes and shape.
Interstate cruising will probably be around 2600 - 3000 rpm and 16 to 20" Hg vacuum.
So timing will be 12+20 degrees BTDC mechanical, plus 16 degrees BTDC from that vacuum advance. Cruising 48 degrees BTDC. Again not bad, engine will nice, although maybe not quite as sharp as it possibly could with a little more.
Now if all that is done is change the vacuum source to ported. The only change will be in the idle timing. It will drop to 12 degrees. Nothing you do with the vacuum advance adjustment will offset that.
The engine may run 50 to 150 rpm slower at idle, and it may like a slightly richer mix. If it runs too slow and wants to die, you will be tempted to increase the throttle opening with the idle speed screw. However this will change the transfer slot opening. If its opened too much, it messes up the entire off -idle and carburation (AJ has a whole write up on that).
So, IF you find the engine is running to slow, and your measurements show it likes more timing, give it the timing back by adjusting the initial.
IF, as in the example I gave above, the total timing and cruise timing and rate of advance may allow for additional advance, go ahead and test everything.
However, IF the total timing is already 35 degrees, adding 5 more degrees into the initial will put the total at 40. DO NOT run it at WOT like that.
Even changing from 32 to 35, you want to be pretty careful about your next outing at the drag strip. If the mph go up and there's no pinging, great! But do check the spark plugs for any visual signs of detonation.
If there is any audible pinging, get out of the gas. Finish the run at less than full throttle and reduce the timing. In this scenario, the mechanical advance will need to be shortened or at minimum slowed down.
