rbond Posted August 31, 2018 Share Posted August 31, 2018 In reading several well written posts on this topic... I understand how the spark advance system works... or is supposed to work... but in my case, I'm scratching my head... F350 7.5L with CA emissions shenanigans making life more complicated... I've read that after setting timing/idle (8˚@ 800 for me) you should be able to re-attach the vac hose to the advance diaphragm and the idle shouldn't change... but mine does... increases to about 1100 from 800 rpm and I'm suddenly at almost 30˚ of advance I have the usual 15" of vac (a bit higher actually) at idle at the vac advance diaphragm... which, according to my calibration decal, is plumbed as straight manifold vac once the engine has reached OT. Before OT, the vac advance is getting vac source through a restrictor. So, I popped the dist cap off and put a manual vac pump directly on the advance diaphragm just to see how much advance is applied at 15" and the plate basically rotates to full advance. When I apply a timing light at idle with the vac hose attached I'm at 29˚ advance and my idle jumps up by 300rpm. I would think that this much advance at lowish rpm would cause the motor to come to a complaining halt. This can't be right... right? Shouldn't I only be getting about 5" of vac at the advance diaphragm at idle? At this extreme amount of advance, the system has nowhere to go at higher rpm. The Vac Advance unit is new... and the spring tension was set to mimic the replaced unit The Spark Coil is new... and rotates freely ... so what am I missing? Link to comment Share on other sites More sharing options...
Rembrant Posted August 31, 2018 Share Posted August 31, 2018 This can't be right... right? Shouldn't I only be getting about 5" of vac at the advance diaphragm at idle? At this extreme amount of advance, the system has nowhere to go at higher rpm. The Vac Advance unit is new... and the spring tension was set to mimic the replaced unit The Spark Coil is new... and rotates freely ... so what am I missing? Firs things first, the vacuum advance and centrifugal (RPM) advance are activated separately, so even if the vacuum advance is maxed out, the centrifugal advance will still work. I don't know how this is supposed to work on a CA emissions vehicle, but I know that with normal ported vacuum, you won't have any vacuum at idle. You can adjust the vacuum advance spring until it is not activated at idle. If you tighten up the spring, it will still advance the full 20 deg, it just won't begin until later. I can't say much more than that...maybe one of the other guys on here is familiar with the CA calibration... Link to comment Share on other sites More sharing options...
PetesPonies Posted August 31, 2018 Share Posted August 31, 2018 I would bag the stock mess and run it the way it should be done. Who is going to know the difference? The hourly people at inspections? yeah right. Link to comment Share on other sites More sharing options...
rbond Posted August 31, 2018 Author Share Posted August 31, 2018 I would bag the stock mess and run it the way it should be done. Who is going to know the difference? The hourly people at inspections? yeah right. wish I could... but I have to pass smog first So... what would be the "way it should be done"? Link to comment Share on other sites More sharing options...
rbond Posted August 31, 2018 Author Share Posted August 31, 2018 Firs things first, the vacuum advance and centrifugal (RPM) advance are activated separately, so even if the vacuum advance is maxed out, the centrifugal advance will still work. ... right... so if the vac adv is maxed out all the time... why even have it? You could simply adjust for it in timing setting I don't know how this is supposed to work on a CA emissions vehicle, but I know that with normal ported vacuum, you won't have any vacuum at idle. that was my understanding as well... hence my confusion... You can adjust the vacuum advance spring until it is not activated at idle. If you tighten up the spring, it will still advance the full 20 deg, it just won't begin until later. True... and I may try that. But still baffled by the full vac draw at idle... I can't say much more than that...maybe one of the other guys on here is familiar with the CA calibration... Link to comment Share on other sites More sharing options...
Rembrant Posted September 1, 2018 Share Posted September 1, 2018 ... right... so if the vac adv is maxed out all the time... why even have it? You could simply adjust for it in timing setting Well, you could adjust it to account for the maxed out vacuum advance, but then it would sort of work in a backwards fashion. Remember, the reason that vacuum is there is because the throttle plates are closed at idle. Once you step on the gas and open the throttle plates, vacuum decreases. That would cause your timing to retard terribly IF you had it set to account for max vac advance. So...don't do that. The vacuum advance is beneficial, you want it to work correctly. I think what Pete was suggesting was to run the vacuum advance as it was normally run (outside of CA, I guess) with a ported supply off the carb. This would ensure zero vacuum at idle. Or, in my case, using the ported vacuum on my Holley carb, I was still seeing about 10" vacuum at idle, so I simply adjusted the advance so that it doesn't activate until it pulls 10" vacuum. The vacuum advance adjustment doesn't change the amount of vacuum, it just changes when the vacuum is applied. There's about 20 degrees available there, and it takes about 10" vacuum to go from 0 to 20. This is why at 15", you're already pegged at full advance. Adjusting the spring tension simply moves the 10" of travel. Mine is activating now (approximately) between 10" and 20" of vacuum, instead of zero to 10". Mine may not be correct where it is, but the truck is working really well, so I left it alone as it is. I'm going to do more ignition curve tuning later on, so I consider my current set-up a temporary one. Or...last option, is to leave it all as it is, and try to find out what the California calibration is exactly. I assume it is in the service manual...what year is this thing again? Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted September 1, 2018 Share Posted September 1, 2018 In reading several well written posts on this topic... I understand how the spark advance system works... or is supposed to work... but in my case, I'm scratching my head... F350 7.5L with CA emissions shenanigans making life more complicated... I've read that after setting timing/idle (8˚@ 800 for me) you should be able to re-attach the vac hose to the advance diaphragm and the idle shouldn't change... but mine does... increases to about 1100 from 800 rpm and I'm suddenly at almost 30˚ of advance I have the usual 15" of vac (a bit higher actually) at idle at the vac advance diaphragm... which, according to my calibration decal, is plumbed as straight manifold vac once the engine has reached OT. Before OT, the vac advance is getting vac source through a restrictor. When I apply a timing light at idle with the vac hose attached I'm at 29˚ advance and my idle jumps up by 300rpm. I would think that this much advance at lowish rpm would cause the motor to come to a complaining halt. This can't be right... right? Shouldn't I only be getting about 5" of vac at the advance diaphragm at idle? At this extreme amount of advance, the system has nowhere to go at higher rpm. ... so what am I missing? You always need to reset the throttle stop after reattaching the vacuum line. With this system you will see quite a bit of advance at idle. This is an emissions engine and with more advance there is more time for fuel to burn before the exhaust valve opens. The VCV should apply restricted vacuum to the advance can unless the truck starts overheating. At that point full manifold vacuum is applied, which increases idle speed -making the water pump and fan turn faster- to help cool the engine. Link to comment Share on other sites More sharing options...
baddog8it Posted September 1, 2018 Share Posted September 1, 2018 I would bag the stock mess and run it the way it should be done. Who is going to know the difference? The hourly people at inspections? yeah right. wish I could... but I have to pass smog first So... what would be the "way it should be done"? Wow! A smog test on an '86! Are you in California? Can you clue us in to what that entails? Here in Missouri, years ago, we actually had to put our vehicles on a dyno with a sniffer up the tailpipe. What a pain. Now it's just a matter of checking the OBDII for codes. If there are no codes, it passes. Of course, only vehicles with OBDII are required to do this. This is part of the reason that I insisted on my (then) next vehicle (The Beast) be pre-1995. The emissions test is only required in some counties in/around the St. Louis area, and some counties in the Kansas City area. There are also some exemptions for SOME diesels and vehicles over a certain GVWR. Link to comment Share on other sites More sharing options...
Steve83 Posted September 1, 2018 Share Posted September 1, 2018 ...the plate basically rotates to full advance. ... ... so what am I missing?The centrifugal advance (as Rembrant noted), and the fact that the restriction SLOWS the vacuum advance's action after the engine is started, and the detailed instructions printed on the REST of the VECI label. Link to comment Share on other sites More sharing options...
rbond Posted September 1, 2018 Author Share Posted September 1, 2018 ...the plate basically rotates to full advance. ... ... so what am I missing?The centrifugal advance (as Rembrant noted), and the fact that the restriction SLOWS the vacuum advance's action after the engine is started, and the detailed instructions printed on the REST of the VECI label. ok... thx for all the feedback. I always think of vacuum advance as working "in conjunction" with centrifugal... that it would "add to" the effect of the centrifugal... but in this case, it's working to compensate for "load"... bringing the point of ignition closer to TDC as the throttle opens more. Rembrant... your reply was very helpful to clarify my question... thx. Your solution makes sense. As it is right now, my advance unit is set to start pulling a 5"... seems a bit low. I'll tighten up the spring on the advance unit to 10" and see how it responds... adjust for the faster idle. Thx, Gents. Link to comment Share on other sites More sharing options...
Recommended Posts