timing and full or ported vac
Thread Starter
3rd Gear Member
Joined: Oct 2007
Posts: 671
From: Florida
timing and full or ported vac
well, initially i'm redoing my timing after i replace the plugs and wires (since i burned the #6) and i was wondering, the book calls for 6 deg BTDC. so i'm adjusting that with the vac off and plugged up. what should i shoot for with vac on and should i switch from ported vacuum or full vacuum? i've heard varying sources but i'm curious. also i can't find the information on my 4160c on to which is the ported and full vac, right now i have the advance plugged into the small port on the front metering block which i'm guessing is the ported. any help with this is greatly appreciated. 
6th Gear Member
Joined: Oct 2006
Posts: 6,417
From: Cork, Ireland
RE: timing and full or ported vac
to adjust initial timing do _not_ connect any vacuum from carb to distributor.
on carb side try to block them for the time you adjust as otherwise the carb will pull air there and ebngine will run superlean on ignition timing. you can block them with a small rubber hose that fits over the hose and carb and plug the other end with a screw.
When you have initial timing set and you have a strobe where you can adjust the advance angle, you can always rev up the engine slowly and see where you reach total advance. this should not be more than 38 degrees.
At the same time the 289s tend to run better with 10 degree iniktial advance instead of 6 or 8.
You can change springs and put in plates in the mach advance of distributor to adjust the curve there, but forget about that for the moment. we just need to know what your max advance (without vacuum).
When that's all done, hook up the vacuum from carb to distributor.
ported vac is on top of throttle plate, manifold below so in most cases the manifold vac hose on carb will be below ported.
If unsure, hook up one to the distributor and check for timing.
if ported vacuum is connected, then the initial advance shouldn't change much or not at all.
when hitting the accesserator hard (quickly) the advance should increase (far more than mech alone) and go back again
if manifold vacuum is connected, then the initial advance should change drastically as soon as it's connected
hook up to ported vacuum.
Make sure the diaphragm in distributor is working.
on carb side try to block them for the time you adjust as otherwise the carb will pull air there and ebngine will run superlean on ignition timing. you can block them with a small rubber hose that fits over the hose and carb and plug the other end with a screw.
When you have initial timing set and you have a strobe where you can adjust the advance angle, you can always rev up the engine slowly and see where you reach total advance. this should not be more than 38 degrees.
At the same time the 289s tend to run better with 10 degree iniktial advance instead of 6 or 8.
You can change springs and put in plates in the mach advance of distributor to adjust the curve there, but forget about that for the moment. we just need to know what your max advance (without vacuum).
When that's all done, hook up the vacuum from carb to distributor.
ported vac is on top of throttle plate, manifold below so in most cases the manifold vac hose on carb will be below ported.
If unsure, hook up one to the distributor and check for timing.
if ported vacuum is connected, then the initial advance shouldn't change much or not at all.
when hitting the accesserator hard (quickly) the advance should increase (far more than mech alone) and go back again
if manifold vacuum is connected, then the initial advance should change drastically as soon as it's connected
hook up to ported vacuum.
Make sure the diaphragm in distributor is working.
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