Real techy question

[5bikes]

Looking at the ignition advance curves for the 1st Gen's in the service manual show high timing advance (15- 48.5 degrees) at low vacuum (1.57 in. Hg). And low advance (5-30 degrees) at high vacuum (9.84 in. Hg). This is opposite of cars.

NORMALLY:

High vacuum is low load on the engine therefore allowing more ignition advance. ie. cruising along

Low vacuum is higher engine load therefore less timing advance is needed. ie. hard acceleration

Can anyone explain this? Are the 2 curves mislabeled?

[pegscraper]

Are those units of pressure rather than of vacuum? Y calls the thing a boost sensor too, when I think it should be called a vacuum sensor because it senses manifold vacuum. Maybe they're also referring to spark advance in terms of pressure. There's no way it operates backwards to what you're familiar with. There's just no way it could work like that. That would be all wrong for the engine's requirements.

[5bikes]

Relooking at the curve on the timing diagram says vacuum. The so called "Boost Sensor" is hooked up to the intake manifold. I put a vacuum gauge on the same fitting under the carb and get 3 to 10 inches of vacuum. The boost sensor can read "positive pressure" too but that would retard the timing according to the diagram in the boost sensor section.

So why is the timing diagram labeled backwards?

[BuddyRich]

Language barrier ?

[Dano]

Another thing the factory manual is wrong on again...........

[93 venture]

Looking at the ignition advance curves for the 1st Gen's in the service manual show high timing advance (15- 48.5 degrees) at low vacuum (1.57 in. Hg). And low advance (5-30 degrees) at high vacuum (9.84 in. Hg). This is opposite of cars.

NORMALLY:

High vacuum is low load on the engine therefore allowing more ignition advance. ie. cruising along

Low vacuum is higher engine load therefore less timing advance is needed. ie. hard acceleration

Can anyone explain this? Are the 2 curves mislabeled?

 

Ok the way i look at this it is right,a car uses venturi vacuum, it gets that vacuum from a port located above the throttle plates.when the plates are shut little to no vacuum,and when the plates are open full vacuum, on our bikes it is port or manifold vacuum,while the throttle plates are shut we have full vacuum and with open throttle plates we have low to no vacuum.

I think im right on this but im just a diesel mechanic, i dont have to worry about vacuum advance.

[pegscraper]

All car engines used manifold vacuum up until the 70s. Ported vacuum above the throttle plates was an emissions controls stunt. The idea was to remove all the spark advance at idle, making the engine run hotter and supposedly burn cleaner. Really all it did was make the engine use more gas. I always switched my cars back to manifold vacuum for spark advance. They idle and run better.

[5bikes]

1. Don't really know

2. Thinks it is ported vacuum, wrong, it's below the carb, has to be manifold vacuum.

3. Curves labeled wrong, as I suspected.

 

At cruise (moderate load) we are getting about 4" vacuum; at idle, very low load, we get about 8"; and on deceleration, no load, we get about 10+" 4"=35 degrees, 10"=45 degrees Reference p. 8-14 of Service manual (pdf)

 

More vacuum = more advance, without going too far (dentonation) we get more free power, mpg and acceleration. I vacuum T'ed off the left 2 cylinders to the "Boost Sensor" but no increase in inches of vacuum occured. Even tried coming off the YICS 1/2" vacuum line to the manifolds did not work.

 

Ok, now how can be get more vacuum (OR more advance)?

[Gearhead]

5bikes,

 

Well, it could be a simple typo. While on that subject, I question the advance curve anyway. It shows the "vacuum advance" as effective right off idle. IIRC, in practice it doesn't kick in until almost 2k rpm. Not because there's no vacuum below that, but because that's how the TCI is programmed.

 

Second thought is to echo what pegscraper said at first. Maybe Yammy is quoting absolute pressure rather than vacuum. High vacuum = low absolute pressure. They do call the thing a "boost sensor" after all. What kind of a name is that for a vacuum advance, anyway?

 

Third idea: The manual listed for 83-85 shows the same timing diagram as the manual for 86+. I know from this site that the 83 was connected differently than all the others. Vacuum comes off the carb somewhere. It could simply be ported vacuum which is about the same once you're off idle, but I think I've read that it operates backward, as in, you get more vacuum at the connection point as the throttle opens. You know, as I type that it really doesn't make any sense. But if true, maybe the manual was written for that and never modified.

 

I think to modify the ignition curve you'd have to either:

 

1) Rotate the pickup coil plate in the engine case cover, which I've read here that there's really not room to do.

 

2) Be really clever with electronics and alter either the boost sensor's output or the TCI's ignition curve.

 

Jeremy

[pegscraper]

I thought the lost in translation idea was pretty good. I have some old hifi magazines with some ads for Japanese equipment in them. It was obvious that whoever put those ads together didn't know a lick of English, and translated them directly and literally from a Japanese to English dictionary. They were just hilarious to read. Now that I'm reminded of those, I do see a little bit of that kind of thing in these shop manuals for our bikes. Suddenly I understand why they call the thing a boost sensor and refer to manifold vacuum as pressure, even though it still doesn't make any sense.

 

How to play with the spark advance? In the 2nd gens, it's pretty easy to modify the mounting bracket to advance the pickup coil relative to the crank. I advanced mine 4*. I don't know what the 1st gens look like here. I've also been thinking about making an offset keyway to adjust the position of the flywheel on the crankshaft. This might be another option for a 1st gen.

 

Playing with the vacuum advance would be trickier. Electrically, the vacuum sensor is a potentiometer with the wiper controlled by manifold vacuum. The only way I can think of to play with this would be to add some resistance to one side or the other of the potentiometer. That would make the wiper seem to be farther to the opposite end of the scale than it really is, adjusting the load based spark advance either sooner or later, depending on which side of the thing that resistance was added to. You would just have to trial and error this and see what happens.

 

Does Dyna make an ignition module for the 1st gen? I also can't help but wonder if the ignition module for the '85 - '89 V Max is the exact same thing. I don't know that for sure though. But I know that Dyna makes an ignition module for the '85 - '89 V Max. If it wasn't an exact fit, I would think it wouldn't take too much to make it fit.

[BuddyRich]

Dyna 3000 ? for the V-max ?

 

http://www.dyna3000.com/

[5bikes]

I like the idea of a Dyna 3000 ignition advance if they are not expensive, over $100?

Another VR member and I are trying to add the potentiometer to the output of the boost sensor. Nice if it would mount on the handlebars and be adjustable to load. I've have not been succeeded in this yet. If I can make it work I will post here. I would hate to mess up the boost sensor or TCI and have to replace them.

Keep thinking we will find a way.

[David Strnage]

ignition timing in a car is affected by 2 things on an older car, first there are a set of weights in the distributer that cause the timing to advance, the vacuum advance mechanism on the side of the carb actually retards the timing with low vacuum when accellerating, when ingine speed gets high enough the weights will not allow this retarding to have an affect. So to simplify this as engine speed increases the timing should advance.

 

On the bikes vacuum would increase as engine speed increases if venturi vacuum is used on the carbs not the manifold so as rpm's increase thenso does timing advance.

 

 

David

[5bikes]

All of the older vacuum advance cars I worked on were like this: Centrifical advance was engine speed dependent, and independent of vacuum advance. ( My Father was an engineer for GM, Ford and Packard) Faster RPM more centrifical advance. Lower load, high vacuum more advance, opposite true also. Some post 70's did have vacuum retarding and advancing spark but that's not the way the graphs in the VR service manuals suggest.

 

Again any suggestions on how we can get more advance, by getting higher vacuum or lower voltage to the TCI?

[Neil86]

First of all....Yamaha does not refer to the sensor as a Boost Sensor....its a Pressure Sensor...as in SENSOR, PRESSURE 22N-82380-10-00 (from parts list).

So if its measuring the "vacuum" as absolute pressure, the curve in the elec specs is correct....low abs pressure, high vacuum, light load, more advance.....high absolute pressure, low vacuum, high load, less advance.

Want more advance?...synch your carbs so one runs slightly higher on vacuum and use it as the sensor input.

[pegscraper]

(Bozo alert.)

 

They most certainly do refer to the sensor as a boost sensor. It says "boost sensor" right on the daggone thing. When is the last time you looked at one?

 

Unsynching the carbs only "slightly" is not going to change the vacuum enough to make any difference in how the vacuum advance operates. Unsynching the carbs to the point where it would make a difference in now the vacuum advance operates is only going to make the engine run like crap.

[Neil86]

Yup I have a spare sensor and coil rack too...did you notice that Hitachi built it....Boost sensor is Hitachi's name..(its probably used on several applications, not just Yamaha). Look in the service manuals and parts diagrams, Yamaha always calls it the pressure sensor.

I'm not suggesting massive synch offset....any increased vacuum will help...since you are advancing timing on all 4 cylinders. This was just a suggestion to 5 bikes without getting into something hard to undo. At the lower speeds (3000 rpm)1" vacuum changes timing 2 degrees.

Thats part of the reason synching makes so much difference...if your reference intake (usually #2) is running low compared to the others, you have the double hit of carb unbalance plus delayed ignition advance.

Edited October 10, 2008 by Neil86

[mbrood]

If I'm reading this right, the static pressure (engine off) produces about a 2v output from the pressure sensor and as vacuum is applied the voltage drops... if you wanted to "cheat" and have it "shifted" to a lower voltage, wouldn't it be as simple as putting in a 10 or 20k pot with the wiper feeding the TCI?

 

http://www.bergall.org/temp/venture/boost-help.jpg

[Gearhead]

You can probably get the vac advance to kick in sooner, but not more of it. The total amount is determined by the programming of the TCI, so I'd think the max is the max. All you can theoretically do is convince the TCI that the boost / pressure sensor is showing a higher vacuum than you really have.

 

Jeremy

[Neil86]

Jeremy...

according to the curves...48 deg BTDC is the maximum advance.....I would question how much vacuum you pull at speed, on the road. Yes the vacuum rises when you increase rpm at rest...but you are at very low load....so if you can bring the TCI input down at road load it should increase advance. It would be interesting to be able to monitor the sensor voltage output under riding conditions.