Vacuum Advance

[gareth]

I did a title search on the subject of Vacuum Advance and there was a lot of information. A lot of it made my brain freeze and I spent a bunch wincing in stupidity while little pennies of information slooooowly dropped.. buffering....buffering...like downloading a large file on dial-up..

But I still need it spelled out for me..

I have a timing light. I connect it to plug number one. I start the engine and things slow down/ stop at the timing plate. I even have a little degree knob on the timing light that can change the apparent setting at the plate. I have no idea what that is for...

I am using ZRX carbs and will be using a restrictor in the vacuum line to stop the advance from whacking around... How do I determine the ideal restriction needed to allow the advance to operate correctly? No restriction- the mechanism taps away. Too much.. I might as well just cap the line..?

What should I see at assorted no-load RPM's at the timing plate under the light? What should line up with what at 600rpm, 800, 1000, 2500 etc..? Is there a chart somewhere with ideal test values..?

Should I have the colortune on one of the cylinders to check that I am somehow getting a full, ideal burn at some perfect combination of RPM and advance...

I am probably overthinking this... but I am beginning to get a bit bogged down while the circular logic takes over.



Vacuum Sych, Idle, vacuum timing, centrifugal advance/ retard..

If this really means that I need to read up more on the basic physics of engine tuning then reading suggestions/ links are welcome. If there is specific 1979 SF information then that is cool too..

Can anyone advance this retard?

[dbeardslee]

Gareth - When I set mine I don't plug the vacuum advance line. I just hook it to number one and set it to 10 degrees BTDC at 1000 rpms. As you rev it to 5400 rpms it should advance to 36 degrees, if the vacuum advance is connected and if my memory is working right.

My light doesn't have a degree knob, but it sounds like it changes where the light flashes which isn't going to change the way the bike runs. If it has a 'zero' setting, that's where I would put it.

Carbs shouldn't have that much affect on the amount of vacuum drawn, assuming the vacuum port is the same size and in the same location. I took a plastic tubing connector, cut it in two, and stuck it in the vacuum line for dampening but I don't think it was really needed.

[TopCatGr58]

Hey Gareth,

Folks have put a spare old pilot jet down inside the vacuum line going from the intake manifold port to the vacuum advance.
Dbeardslee.....he's using NON-XS carbs, and I believe they do NOT have a vac. adv. port on them, and so that's why he's needing the vacuum info for using his synch port with a restrictor.

There are stats on testing the vac. adv. unit, it should reach full travel range/pull when vacuum applied is 150mm/Hg or 5.9in/Hg.

BUT there's no stats on how much vacuum you should see or measure coming from the carb port at idle, and other rpms, because it varies so much depending on the amount of engine LOAD and throttle positions!

However, you don't want to check the timing at idle with the vac. adv. connected, folks have reported that after a while a vacuum can build up in the adv. unit, and affect the readings by providing some vac. adv. when you don't want to see/measure any when checking and setting the timing indicator for idle!!

You are wanting to measure the STATIC adv. timing which is 10 degrees=F mark on the timing plate vs. the "T" =TDC mark! Then revving to 5200 rpm you should see the timing mark aligned with the pointer at 36 degrees.....26 degrees of max. cent. adv. added to the 10 degrees of static adv.!

The problem with checking the vac. adv. timing is that you can't put it/bike/engine under load when revving in neutral. The vacuum adv. is designed to provide a lot of adv. with minimal throttle input under cruising situations around 4k rpm, it can add an additional 16 degrees for a total of 52 degrees total adv.! When you crack the throttle open, the vac. drops to the adv. unit, retards the timing BACK to the more powerful less advanced position to take advantage of the extra fuel and handle the increased load being put on the engine when trying to accelerate, etc..

Do you have the OEM carbs still?? Sounds like another tech tip idea in the works? Like connecting a vac. gauge to the vac. adv. port of the OEM carbs, and check what it shows at 1k, 2k, 3k, 4k, 5k, rpms. This still might not be as accurate, but actually revving in neutral would be close to the little/no load cruising running mode that the vac. adv. is supposed to generate the most adv. and vacuum levels!!

Then you could compare these OEM vac. port readings to the same rpms using the synch/intake boot ports with the restrictor to see if you can provide enough restriction/flow to simulate the same level of vacuums at those same rpms!?

Feel free to take pictures, and post your results, I'll be sure to give you a tag line/credit for the work!!!
T.C.

[AlanB]

Hi Gareth,

A 0.5mm restrictor hole works fine.

The Vac advance makes NO difference to power at WOT, which is why most bikes don't have it; it should help with economy and smoothness while cruising, which is why almost all cars use it.


My SG-


AlanB

[thewiz]

I use some VM33 smoothe bore ...

carbs and some other mods on my G. The vac advance on this bike has always been annoying ... hammering. Put an old pilot jet in there and it slowed down the hammering some BUT ... either I have vac advance connected to the wrong port on the carbs or something BECAUSE it still pulls the vac advance ON at idle which increases the idle RPM drastically ... ugghhh.. Couple of weeks ago, I just pinched off the vac advance altogether and have been running like that for a while ... the result is: the bike is much more 'street friendly' around town.... tiny bit of hesitation on a 75mph freeway roll on (possibly cuz it's missing that extra 16 degrees of advance that TC was referring to) .... no biggie and the hesitation may not even be related to the vac advance disconnection as I had recently replaced the front crank seal and the timing might still be a bit slow after putting it all back together..

Bottom line: ... I ain't really sure the vac advance is really even necessary on a highly modified bike .... maybe someone else could say more definately ??/

[mro]

I ain't really sure the vac advance is really even necessary on a highly modified bike

My hot rod XS runs just fine without it.

Until you find the right size restrictor
Take a small hose clamp and 1/4 inch nut
Place clamp on vac hose, insert nut between hose and clamp
Tighten clamp until nut won't fall out.
Start your engine.
Slowly tighten clamp while watching advance mechanism.
Tighten until it smooths out.


mro

[gareth]

I have a couple of velocity restrictors from a double-acting pneumatic actuator around here someplace. They have an adjustable restrictor that might do the same thing and would be easy to plumb in. I can understand that some riders, especially those who trust their "seat-of-the-pant" intuition would just RIDE and note and work around the quirky-ness of their carb setup. This is part of the modification process. My bike runs fine at idle with the advance blocked off but I get some spitting back through the carbs. If I let it idle and then apply some vacuum to the mechanism, the idle smoothes out and the RPM climbs. If I remember right, the throttle butterfly on the original carbs also acted as a sort of a shut-off door to the drilled hole for the vacuum connector. Was this done on purpose to reduce the tendency for the idle to climb at low throttle? Although most rider/ wrenchers just get frustrated at the foibles of their bike, especially if it keeps them off the road, I just like figuring this stuff out knowing that I am dealing with a clashing of compromises. If I change THIS, then THAT happens, which might be better or worse than if I had just done the OTHER to begin with. I keep in mind that at some point in the past, a bunch of Japanese engineers sat around a table and decided a lot of this stuff for me. But they didn't have the internet, twenty years of computer-enhanced fuel-air modelling with on the fly computer control etc. or, for that matter, 30 years of owner feedback and mods to refer to. Let alone this forum. These bikes are just so... so....ANALOG. At this stage of my tweaking ability, analog is really cool. I like this throwback, primitive, make the fire-burn-strong, hands-on, approachable LOGIC of it all.

So for the hell of it, considering I just purchased a bunch of cheap vacuum gauges on the Bay, I will reinstall the old carbs and work up something along the lines of what TC suggested...
And then try to use the rudimentary data somehow.

Right now I am wondering if I could install a simple shut-off for the vacuum pipe that would be actuated by one of the throttle screws at idle to stop any possibility of vacuum creep.

See? This is how it starts...

You can make yourself nuts with this stuff...

[dbeardslee]

Gareth - might want to calibrate those gauges. If you put them all on the same tap (one at a time) at the same rpm and check the readings it should give you an idea if you have any variance from gauge to gauge.

[TopCatGr58]

Re: I use some VM33 smoothe bore ...

Originally posted by thewiz
carbs and some other mods on my G. The vac advance on this bike has always been annoying ... hammering. Put an old pilot jet in there and it slowed down the hammering some BUT ... either I have vac advance connected to the wrong port on the carbs or something BECAUSE it still pulls the vac advance ON at idle which increases the idle RPM drastically ... ugghhh.. Couple of weeks ago, I just pinched off the vac advance altogether and have been running like that for a while ... the result is: the bike is much more 'street friendly' around town.... tiny bit of hesitation on a 75mph freeway roll on (possibly cuz it's missing that extra 16 degrees of advance that TC was referring to) .... no biggie and the hesitation may not even be related to the vac advance disconnection as I had recently replaced the front crank seal and the timing might still be a bit slow after putting it all back together..

Bottom line: ... I ain't really sure the vac advance is really even necessary on a highly modified bike .... maybe someone else could say more definately ??/

Hey Wiz,

That extra vac. adv. is active during cruising low throttle input, but when you crack the throttle to do a roll on butterflies open, vac. drops, and the vac. adv. is actually supposed to retard back from the higher adv. to the lesser amount which was determined to be the better amount of adv. to take advantage of the extra fuel/air mix and increased load on the engine. The higher vac. adv. levels are for the very little throttle leaner cruising running air/fuel mix. You said your idle rpm still rises due to the vacuum adv. kicking in at idle, so sounds like you probably still have too much of a vacuum signal going to the adv. unit!

But, with the vac. adv. blocked off, then the hesistation was/is probably due to other carb tuning, but could also depend on what rpm and gear you were trying to do the roll on from?
T.C.

[thewiz]

Hmmmm.

Don't wanna' hijack Gareth's thread here, TC .... but I think I may try more restriction and see if it changes something. I don't know much about tuning these carbs either, frankly .... 'cept for what I've read in the Sudco book and a few other places. There's a guy in a local bikeshop who swears he can sweeten 'em up and I can watch that's good, right? Also there is a second vac port on the carb bank too ... so who knows what it may do.