So you want an adjustable vacuum advance?

[Crazcnuk]

Mine is loud enough other bikes move away.

I actually finally eliminated almost all of my vibration noise from the Windjammer, this spring.

personally, if you want to know if it's the vacuum advance, just unplug it and go for a ride. it isn't goig to urt anything for a short hop to check for the noise.

"and these things do rather badly enough as it is..."

I think that 45mpg is actually pretty good....

[Ivan]

Well, the advance kits I have installed were mostly different springs and an adjustment to the timing. Of course they differed from one model to the other. On the 4R0 TCI, I don't have the luxury of adjusting spring weights, since RPM based advance is controlled in the TCI. I have never seen an adjustable rate dash pot. I am sure they exist, but my experiance has been that they are open or closed. Even at that, the XS has a different setup for the vacuum advance. Most autos used a check valve and unported vacuum and it was for the sole purpose of reterded timing for starting. Most of my tuning was on domestics, so imports could differ. When I first got the XS, it seemed odd to me that it did NOT have a check and was hooked up to ported vacuum. Its not hard to see that the idle quality improves with the vacuum advance actuated, so I have no idea why Yamaha decided to leave the advance closed at idle. From what I can tell, the 3500 rpm surge is mostly caused by the retarded ignition getting advanced when the vacuum kicks in. On the old engine, plugging the advance made the throttle much more linear, but killed the mileage.

I am always ready to learn the whys of how something is done, so if you know why the strange advance strategy, please educate me. It just seems counterintuitave to normal procedure, but seems to work ok.

So, the search continues.

[3Phase]

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

Accel and others make adjustable vacuum advance pots and the rough advance adjustment settings are on the web. They come with a hex key that fits a screw inside the vacuum nipple.

Most of the vehicles I've worked on use ported vacuum from somewhere ahead of the throttle plate for the vacuum advance signal so the advance port receives no signal until the throttle is opened. The Ford Load-O-Matic distributor uses a mixed, proportional, valve-controlled, vacuum signal from the carburetor main venturi cluster and manifold vacuum from below the throttle plate.

[crazy steve]

...I am always ready to learn the whys of how something is done, so if you know why the strange advance strategy, please educate me. It just seems counterintuitave to normal procedure, but seems to work ok.

So, the search continues.

The setup on the XS is actually an 'old school' type, as in if it were a car, pre-emission. Now, I don't know if the later carbs are different, but my '78 carbs have the advance vacuum connection between the throttle plate and the valve on just one carb, i.e. 'manifold' vacuum. This gives you high vacuum on lower, lighter load throttle openings, and the vacuum goes down as load/throttle opening goes up; if everything is working as it should this will be proportional. Under WOT, you should have little or no vacuum advance. The one anomoly with the XS is it's low idle vacuum, but I'll guess that's due to the fact you have no common plenum like an automotive application and just the one cylinder doesn't develop much vacuum at idle. It may also be due to the fact that they fitted a dampening restrictor at the carb. The big advantage to this type is you apply advance according to load; too much advance under load, and you get pinging. So this is why you noticed no big difference in the power curve when you disconnected the vacuum advance, but disconnecting 'manifold' vacuum advance will kill your mileage, as you noticed. The better fuel economy is due to the longer burn time from the advanced timing, promoting efficiency; ease of starting is just a side-effect. This type will usually give you the best combo of power/mileage without knocking. But it had one failing; emissions-wise, it was 'dirty' when the vacuum advance was low. Too much unburned fuel went out the tailpipe when applying power.

So enter the 'ported' vacuum advance. This works exactly opposite; the bigger the throttle opening, the more advance. Great for emissions, but pinging could be an issue and it introduced more heat into the motor. That's why you'll see very few vacuum advances connected only to ported vacuum; the motor will overheat under high load conditions. Nearly all 'ported' vacuum either is switched on/off or switched between manifold/ported via a temp controlled valve. This type of vacuum advance became pretty much standard on nearly everything with four wheels starting in the late 60s.

As to your problem Ivan, the FSM merely states that you should have 'full travel at 5.91 inHg' but says nothing about how much advance it adds, or it's rate. I suspect that your pinging problem is you're getting a bit too much, probably at a lower number than recommended. I'd try turning that screw in a bit and see if the problem goes away. Put a bit of tape over the screw while you're testing, then re-seal it with some RTV if you get it 'right'.

'78E original owner

[randy]

I had an '81SH (4RO) around 1991 and it pinged too. I thought it was a lean condition under load. My solution then was to add washers under the slide needles. The ping disappeared. JAT.

[3Phase]

Steve, the vacuum and mechanical advance curves are in the Yamaha '78E PDF.

Appendices [7] / Maintenence Specifications / Section 4. Electrical / Page 7-7
There is a similar page in the fine manual for each of the later models.

The vacuum advance port is not uncovered at idle unless the carburetors are out of adjustment. It is a ported vacuum source that, along with the transfer ports, is located slightly ahead of the throttle plate and it is not normally exposed to manifold vacuum at idle. The throttle plate does have a notch machined into it to give better response with the sharper, shaped edge.

Click the pictures for larger images:-

Now you see it; now you don't!

The vacuum advance port and three transfer ports are clearly visible with the throttle slightly open.
The idle and enrichener ports are visible in the foreground.


The vacuum advance port and the first transfer port are barely visible underneath edge of the closed throttle plate.
The idle and enrichener ports are visible in the foreground.

[Ivan]

I had an '81SH (4RO) around 1991 and it pinged too. I thought it was a lean condition under load. My solution then was to add washers under the slide needles. The ping disappeared. JAT.

I did this last night, as I have a dynojet kit. Even in an overly rich condition, it still pings. Plugging the vacuum pod also resukts in the same issue.I also switched over to the 2H7 reluctor and advance weights along with the 2H7 TCI. This made the noise lessen, b ut not go away. FWIW, the 2H7 and 4R0 TCIs fire the coils opposite each other. I had to swap the pick up coils when I first tried to start it with the 4R0 and then cack when I put the 2H7 TCI on. How weird is that?

At this point, I have eliminated every common source of detonation, so I pulled a plug and the pistons are black, but the plugs look fine. Maybe time to run it on some seafoam. If that doesn't work, I will have to say its from a piston slapping a bit. Which is confusing, because it does it at specific times. Sounds just like the old engine and my 79, but louder. May be the fairing causing it to be overly loud. The thing runs like a raped ape and even with the gargantuam fairing and the ginormous rider will get to 100 in just over 10 seconds. I haven't let it wind up all the way, cause the popos scare me. around here 30 over is automatic suspension and a week in the slammer. While the free rent is appealing, I hear the food isn't all that good and the beer sucks.

Also, Steve, the stuff they put on top of the screw is almost like body putty, but a tad harder. I think resealing it would be better done with a bit of epoxy, or even bondo. I don't think the RTV would hold the screw in place as well, but I could be wrong. I was once before, but that involved a cat, lots of whiskey and a Piper J-3.

[crazy steve]

Also, Steve, the stuff they put on top of the screw is almost like body putty, but a tad harder. I think resealing it would be better done with a bit of epoxy, or even bondo. I don't think the RTV would hold the screw in place as well, but I could be wrong. I was once before, but that involved a cat, lots of whiskey and a Piper J-3.

I ran into a very similar problem on one of my Sportsters; it would ping, but just in a certain RPM range and at certain throttle openings, mostly during the transition between steady-state cruising and whacking the gas (this is after I raised the compression ratio and changed cams). They use what they call a 'VOES' (vacuum operated electric switch), but it works pretty much the same as the later Yammy box; at a particular vacuum signal, it would change state and tell the ignition box to swap curves. These were sealed with RTV, so I dug it out, made my adjustment, then resealed it. I suspect that Yammy used what they did more to keep you out of there, rather than a real need to 'lock' the screw (or when did silicon come into common use? Maybe it was too expensive/unavailable at the time...). I'd at least try some RTV...

3Phase; yup, I missed the curve charts in the back of the FSM. But I'd still hesitate to call this 'ported' vacuum, as the port isn't really either above or below the throttle plate. That location does explain no vacuum at idle though. And I'd almost be willing to bet money (I'm not a gambler, as my luck is uniformly bad) that the screw Ivan uncovered is for calibration, but you'd have to get in there with an accurate vacuum gauge and a timing light to nail that down.

Of course, all this may be nothing; Ivan apparently hasn't run a windshield before, and they can magnify/focus sound a lot. So this may be a 'normal' noise that he just never noticed before... I know when I put the first one on my Sportster, it freaked me out as all sorts of strange noises appeared (those make lots of mechanical racket), but eight years later it still sounds the same.

'78E original owner

[Ivan]

My original though upon discovering this was "This is probably what Yamaha did for the updated ignition assembly." My thought was that it would have been the cheapest route to solve the ping, and that is usually what companies do when they mess up. I thought it may help someone down the road. From my tinkering with it, id does definitely change things, but just didn't work on my scoot. I really think it would help on others.

Being these are air cooled engines, they are probably built "loose" because of the extra thermal expansion, so its a wonder why they don't have more uncommon noises. Some of the aircraft engines I have helped my dad with would sound like they were going to explode until they got warmed up, most notably the radials.Of course, the continentals sounded fine, but would crack at the prop flange on the crankshaft and cause a sudden loss of thrust and oil pressure

This has been an enlightening discussion though.

[3Phase]

But I'd still hesitate to call this 'ported' vacuum, as the port isn't really either above or below the throttle plate. That location does explain no vacuum at idle though.

Steve, it's hard to see in the 2D picture but the advance port is slightly ahead of/above the throttle plate. You can only see the leading edge of the port because of the cameraphone's flash so it's genuine ported vacuum. I'd guess it's that way for instant off-idle response on a, relatively speaking, lightweight, high performance motorcycle. A stock big block Mopar with its 2,000 lb suburban grocery getter trim and a carburetor big enough to stick your head in and have nice a look around wouldn't even notice it.

My original though upon discovering this was "This is probably what Yamaha did for the updated ignition assembly."

Mine was, "Right on! Tuning!" It'd be cool to find a dead vacuum advance and cut in half to see how it works like the guy does on that new History Channel show, "Sliced."

Thanks for finding this, Ivan!

[Crazcnuk]

The noise I have is definately not because of my fairing. I've had my fairing the entire time.

The vacuum port IS ported. To test it slowly turn up the idle on carb #2. Raise the idle too much and it incovers that port and your idle will rocket to the 3000+ rpm range. (because you allow vacuum to the port and it pulls the advance on.)

It just occured to me that maybe my top end rebuild just finally 'settled' in and now my compression is higher than it was before?

That could certainly cause a ping when coupled with timing etc?

[petejw]

without throwing a spanner in the works,
my bikes doing the same thing at 4-4500
only under load, but mine sounds more like the
valves knocking.
i know its hard to describe a noise but
how best would you guys describe the noise
thats coming from your engine?
pinging i would describe as more of a rattle.

ive also messed about with fuel mixtures and that
hasnt made any difference.

ive got a fixed timing plate that ive elongated the holes
so i can adj the timing, but havent had a chance to fit it yet.

[petejw]

iridium spark plugs.

well ive been fiddling with the bike today,
i took out the iridium spark plugs and had to lean my
mixture screws in a little (quarter turn) and ive got rid of my knock
the bike is running better and smoother under accleleration so im wrapped atm.

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.

i hope this helps some of you.

[crazy steve]

...The vacuum port IS ported. To test it slowly turn up the idle on carb #2. Raise the idle too much and it incovers that port and your idle will rocket to the 3000+ rpm range. (because you allow vacuum to the port and it pulls the advance on.)...

While this port is located at or slightly above the throttle blade when the throttle is closed, as soon as you open the throttle the opening is now between the blade and the intake valve, which would make it manifold vacuum. If you moved the opening 180 degrees in the carb bore, then you would have 'ported' vacuum as the throttle blade is now between the opening and the valve.

Maybe I'm not explaining this right, or maybe not using current terms. I was taught (and it's always worked in practice) that ported vacuum (AKA spark ported) is low under no-or-light load, small throttle conditions, and manifold vacuum would be exactly opposite. The above statement by cnuk is a textbook example of 'manifold' vacuum; no load, small throttle opening = high vacuum. Ported vacuum under these conditions should be minimal. It would be easy enough to check; connect a vacuum gauge to this nipple and take the bike for a short ride. Cruising at about 30-35 mph in high, watch the gauge. Without shifting down, open the throttle. If the gauge drops, it's manifold vacuum. If it goes up, it's ported vacuum.

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

'78E original owner

[Crazcnuk]

Heh, no problem. The operation of these carbs is sort of backwards to what we've come to expect from cars (it isn't, actually, but we don't see those rubber things as an intake manifold, which they are).

"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 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?