So you want an adjustable vacuum advance?

[3Phase]

I'm really not trying to start a pi**ing match, I swear.

Heh heh heh! Techie fight!

The ported vacuum advance signal in the XS100 is a simple analog throttle position sensor that is most useful at small, part-throttle positions. It reacts faster and more precisely to small changes in part-throttle position than manifold vacuum.

As the throttle is opened the timing is advanced as the edge of the throttle plate moves across and uncovers the vacuum advance port to help burn the lean part-throttle mixture provided by the idle and transfer ports and increase power.

When the throttle is closed there is no vacuum advance because no one in their right mind cares about making good power at zero throttle except Toyota.

At higher speeds and larger throttle openings the fuel/air mixture is no longer a part-throttle lean mixture and further advancing the ignition timing will create lots of heat and high cylinder pressures. The engine will work against itself so the total amount of vacuum advance allowed has to be limited.


Okay, 35 MPH in 5th gear is only about 2,000 RPM. Let's see what would happen!

As the throttle opens past ~1/4 travel, the area of intake manifold pressure starts to extend up the carburetor throat ahead of the throttle plate to the carburetor venturi slides. The velocity of the air flowing through carburetor starts to increase and the idle and transfer ports begin to get some help in the form of more fuel from the main jet and more air from the carburetor venturi as the carburetor slides and the jet needle begin to rise.

If manifold vacuum stays above 6 inHG with the throttle wide open the vacuum advance is essentially 'all in' and will add to the centrifugal advance as the engine RPMs increase.

'78E Vacuum Advance
0* @ 50 mmHg [1.97 inHg]
16* @ 150mmhg [5.91 inHg]



As the speed of the engine continues to increase above 2,000 RPM and the mechanical advance starts to work, manifold vacuum will continue to rise and the vacuum advance will stay fully advanced unless the throttle is closed.

'78E Mechanical Advance
1* @ 2150
26* @ 4750




At 35 MPH in 5th gear the engine is only turning about 2,000 RPM. The mechanical advance has not yet begun to operate and there is only static ignition timing plus vacuum advance. If the throttle is cracked wide open, vacuum advance might drop, briefly, but at 2150 RPM the mechanical advance will begin to go to work.

The engine will accelerate, slowly, and as the main jets begin to provide additional fuel and the carburetor slides open to provide more air it will produce more power and accelerate faster until the blue light begins to flash and the real fun begins.

[3Phase]

... my bike wasnt pinging it had a definate knock to it at 4-4500rpm under acceleration, putting in a set of stock plugs and leaning
the mixure screws in solved my problem.

so if your running iridium spark plugs it wouldnt hurt to try swapping them
out for a stock set of ngks,

i also tried running a hotter plug bp5es (its coming on winter here) and my bike burns oil but that also caused the engine to knock.

Pete, that's some good information about the spark plugs, thank you!

"You know, a Sun distributor tester would be really handy"

I have access to one. We used it to recurve the distributor on the Monte. I'm not sure how it could be used on our bikes, I will have to ask.

I was mostly kidding because to use the Sun tester you'd have to find a way to put the XS ignition parts in something cobbled together to look enough like a regular automotive distributor to fit and run in the tester. Truthfully, I think it would be a lot of work.

I have a 79SF parts bike. I could take the entire timing assy from it, put it on the 80 and see if it makes a difference? As I understand it the 79SF has 5deg less timeing?

It might help but I don't remember the two advance curves off the top of my head and it's almost bar-O'clock. I need to get some sleep but I'll check tomorrow if you haven't found it already, sorry.

[crazy steve]

... If the throttle is cracked wide open, vacuum advance might drop, briefly, but at 2150 RPM the mechanical advance will begin to go to work...

Ok, let me take another shot at this. You're saying what I'm trying to explain in the added 'bold' above. When you're running at a light load/small throttle opening, the motor wants to draw in more air, but is prevented from doing so by the throttle blade restriction. So you have a high vacuum on the engine side of the blade (manifold vacuum), and a low vacuum on the inlet side (ported vacuum). Crack the throttle open, and now conditions start to reverse; the high vacuum below the blade (engine side) goes down, and the low vacuum above the blade goes up as they try to equalize, and more air/fuel flows into the motor. So if your vacuum advance is connected to manifold vacuum, this will retard the timing, as you stated above. Ported vacuum would advance the timing under these conditions. So in spite of the fact that the location of the port appears to be 'ported' (and is, under closed-throttle conditions), as soon as you open the throttle it 'switches' to being manifold vacuum.

'78E original owner

[Crazcnuk]

Just to clarify it a bit more... (numbers for the 80G)
Neither the vacuum or mechaincal advance can retard the timing. Both advance the timing as needed.

The mechanical advance advances the timing at a linear rate from 0 to Max (the maximum it has been setup to do)

In our case the initial timing is 5deg BTDC (except maybe 79SF?).

The mechanical advance adds in another 30deg at 3900rpm +/- 300 rpm for a total of 36deg at 5200rpm (the test engine speed).

The mechanical advance will add timing in a linear fashion from off idle to ~4000rpm. Throttle position makes no difference here, and the mehcanical advance timing is always the same at any given engine speed. This means that the system is adding 30deg over a 3000rom range (1000 idle to 4000max advance) or about 10deg per 1000rpm. So technically timing should be approx 5deg at idle, 15Deg @2000rpm, 25deg @ 3000rpm, 35deg@4000rpm) roughly.

This is to give the fuel/air mixture a bit more headstart, as engine speed increases, to insure that the 'explosion' is occuring at the optimal time for maximum power output.

Vacuum advance does nothing at wide open throttle ( I suppose until your at max speed and the engine can't push anymore?)

It operates at any partial throttle opening. It is designed to add more timing (up to 17deg?) at various part throttle settings.

This means that at 4000rpm if you let the throttle off completely your max timing would show 53deg advance...

In practical terms your timing will be a combination of X mechanical adv @ rpm + Vac Adv/throttle opening.

The sole function of vacuum advance is fuel economy. Any time your trying to make power (ie opening the throttle a lot) vacuum adv will be negligible.

If you want to test this hook up your timing light, and rev the engine while watching the timing plate.

This is why I wonder when people say their bike runs better at idle, or has more power with the vacuum adv hooked up.

In drag racing, we unhook the vaccum advance becasue the last thing we want is over 50deg timing at the end of the track. Very hard on the engine. It's the leading cause of engines exploding at the end of the track.

I would assume in normal practice however, that this extra advance is responsible for much of the 'brake effect' of your engine when you let the throttle off, as the engine will now be firing an idle mixture while the piston is still coming up the bore on compression (ie: too early)

there is a very good explanation of advance systems here:

http://en.wikipedia.org/wiki/Ignition_timing

[3Phase]

Ok, let me take another shot at this.

<ducking> Swing and a miss!

Seriously, Steve, if manifold vacuum was all that was required then Yamaha would have saved 0.00001 yen per bike and omitted the drilled vacuum port, the machined notch in the throttle plate and the pressed-in orifice restrictor and brass nipple on Carb#2. They would have just hooked the vacuum advance line with a restrictor in it to the manifold nipple on one (or all) of the manifold boots and been done with it; called it a day; gone home and drank a bottle of Saki or whatever Yamaha engineers did for entertainment back in the late 70s/early 80s.

Manifold vacuum is all manifold, all the time full stop. That is why it's called manifold vacuum and it's what the synch nipples on all four rubber manifold boots provide: manifold vacuum.

Vacuum to the distributor advance is ahead of the manifold and is regulated by the throttle plate. Even though that port is at times at the same pressure as the manifold it is not at manifold pressure 100% the time. It is a ported vacuum source. I believe the throttle plate also amplifies the low vacuum signal (Bernoulli) when the air increases speed as it passes through the restriction between the throttle plate and the carburetor throat but I could be trying to make bricks without straw.


I really am genuinely curious to see how it works under load for tuning purposes and not just sitting in the garage so I'm going to go for a ride (probably many rides, actually) and try to get a video. I'll hook up a couple of tees and vacuum gauges: one gauge to the #2 manifold nipple, the other to the vacuum advance port. I'll try to use the readings as a baseline before removing the epoxy on the advance pot so that, hopefully, if it all goes pear-shaped I can put it back the way it was before I tried to 'fix' it.

If nothing else it should be technically challenging and because I don't have a mount for my phone camera, just lots of duct tape, it should be interesting to see if I can pull it off without breaking anything, including me!

Now where did I put those stupid tees? ... I'll try all of this when I get back from the beach, I'm going for another ride!

[3Phase]

Just to clarify it a bit more... (numbers for the 80G)

Thank you very much for the '80G timing numbers, Craz! I haven't done any racing myself but I do know that too much advance will kill engines; seen it happen and helped clean up and rebuild the aftermath.

I was using the '78E curves because they're easier to read and Steve has a '78E. The vacuum advance on my '80G works at WOT because I've watched it with the timing cover removed and managed to not hit anything while I was looking at the advancer instead of where I was going. I need to look around and find a suitable location to use as a test track because sooner or later I'm going to get into trouble.

The Book of Yamaha says the '78E has 10 degrees static timing. The '79F/SF, '80G/SG have 5 degrees static timing. From what I've been able to find the '79SF may have 31 degrees of mechanical advance, not 36 degrees like the '78E/'79F? Take it for what it's worth and measure it with a timing light before accepting the numbers. Even if they're correct, someone may have dropped in the ignition assembly from a different year/model.

The book also shows that the '80G/SG vacuum and mechanical advance kick in earlier than the '78/'79 and the vacuum advance seems to max out earlier with less advance than the early models.

I haven't messed with the dynamic timing yet on my '80G. It looks like there are two curves per diagram in the manual for the mechanical and vacuum advance -- one for the 80G and one for the SG or it could be a 'range' of advance? I don't know what they're trying to illustrate because the reason for using two curves per diagram is not explained and I haven't been interested enough to dig any deeper with a bike that is working just fine the way it is now. I think no one was really supposed to mess with it because of the new emissions rules; replace any broken parts and make sure they do something to the timing when the engine is running and trust Yamaha that the new parts work to Yamaha spec.

[crazy steve]

...if manifold vacuum was all that was required then Yamaha would have saved 0.00001 yen per bike and omitted the drilled vacuum port, the machined notch in the throttle plate and the pressed-in orifice restrictor and brass nipple on Carb#2. They would have just hooked the vacuum advance line with a restrictor in it to the manifold nipple on one (or all) of the manifold boots and been done with it....

...It is a ported vacuum source. I believe the throttle plate also amplifies the low vacuum signal (Bernoulli) when the air increases speed as it passes through the restriction between the throttle plate and the carburetor throat but I could be trying to make bricks without straw.


I really am genuinely curious to see how it works under load for tuning purposes and not just sitting in the garage so I'm going to go for a ride (probably many rides, actually) and try to get a video. I'll hook up a couple of tees and vacuum gauges: one gauge to the #2 manifold nipple, the other to the vacuum advance port...

Well, I'll agree that this isn't a 'pure' manifold vacuum design; Yamaha managed to come up with a 'hybrid' design. From Cnuks' note about high idle speed if the port is uncovered, I see that as the explanation for the design. Actually a good trick; normal advance while riding the bike, none when idling, and done with no extra moving parts.

But what we really seem to beating on is a definition; to me, 'manifold' vacuum decreases when you open the throttle, and 'ported' vacuum increases when you open the throttle, irregardless of where the port is located. You can affect the amount of change compared to a 'pure' design by where you locate the port, size of the orifice, etc, but it shouldn't alter the basic definition.

Your test will tell the tale, so I await the results; I'd do this, but mine is still in parts all over the shop...

'78E original owner

[Crazcnuk]

Np, I just happen to have my 80 supplemental manual handy.

The only reason I am worrying about this is because of the 'knock' that my engine picked up recently, and I am just trying to figure out how I am going to test it.

Right now, I have to check total mechanical advance, then see what my vac adv is doing, change back to regular plugs from my iridiums, and take her for a test drive.

I will have to watch it to see if I am going lean, for some reason, as well.

[3Phase]

Well, I'll agree that this isn't a 'pure' manifold vacuum design [...] Actually a good trick; normal advance while riding the bike, none when idling, and done with no extra moving parts.

Yes, it's brilliant!

Your test will tell the tale, so I await the results; I'd do this, but mine is still in parts all over the shop...

Aww, come on, that's no excuse! Hook up a couple of gauges to the carb' and boot nipples, then go outside and swing them around over your head really, really, fast and see what the gauges (and possibly the neighbors) say!

I'll get started tomorrow and see what I can find.

The only reason I am worrying about this is because of the 'knock' that my engine picked up recently, and I am just trying to figure out how I am going to test it.

You and Ivan and a couple of others are all having a similar problem at the same time. Yours is the worst if people riding next to you are putting in earplugs when it happens. Maybe it's the new spring weather you all need a new spring!

[petejw]

hey scott,
a little while back i did exactly that and hooked
up the vac guages to the no2 carb and intake nipple and
took the bike for a good ride
from memory other than at cruise the needles acted differently,
it was a good way to see how it all works.

[petejw]

i think each of us that have/had that knock,
have had iridium plugs fitted,
i was amazed at the difference the stock plug had
over the iridiums.

ivan - are you running iridiums atm?

[crazy steve]

....Hook up a couple of gauges to the carb' and boot nipples, then go outside and swing them around over your head really, really, fast and see what the gauges (and possibly the neighbors) say!

What?! And give the neighbors yet another reason to wonder about me??

'78E original owner

[3Phase]

hey scott,
a little while back i did exactly that ....

Thanks, Pete! I'm going to remove the timing cover and try to mount the camera and gauges so the video will show both vacuum readings and the advancer movement with one camera. I don't know if I can keep them all in focus and still have enough detail to be useful. I'll try to get some video with a timing light on all of it, too.

What could possibly go wrong?

What?! And give the neighbors yet another reason to wonder about me??

Mine gave up long ago and moved or found similar odd hobbies and personal foibles. The folks in the two houses across the street do circle track racing and sand rails, respectively. It's a lot of fun when I'm working on my XS, one's tuning a new engine before a race (he gets 'em pretty cheap for only $20K a pop) and the other's getting ready to head out to go dune hopping on the same weekend.

[petejw]

scott,
when i did it, i couldnt watch what the vac adv
was doing, but when you watch the guages moving
itll give you a good idea of what would be going on.


i took the bike out again and gave it a good wrap,
the difference was huge, the knock is definately gone,
just waiting on 1 of you guys to swap out ur iridium plugs to see if its
the same cause.

[Ivan]

I am running the BP6ES plugs. Although I might try a step cooler, just for giggles.