Vacuum advance Question

[John]

Has anybody ever thought how to remedy the problem we have with the wires to the ignition pickup coils breaking? I had that problem also with my 80SG. a simple splice worked, but I was thinking about the cause. The pulsations caused by the advance getting it's vacuum from just one cylinder. It would seem that a more constant vacuum supply would be a good fix. How about a vacuum check valve like that used on some automotive applications, or would that not allow the advance to retard when needed? Or how about the resrictors used in a carb balancing vacuum gauge? They seem to allow vacuum to the gauge, but without the wild fluctuations.

John
80SG Twilite Special

[skids]

Re: Vacuum advance Question

John, someone once recommended using a small oriface to help dampen the bouncing effect. An inline fuel filter might provide a bit of air to act as an air shock absorber of sorts. You are connecting the vacuum hose to the No. 2 car nipple rather than the intake manifold boot, right? A simple compression check might help do determine if the valves are working. I am not going to talk about the ramifications of a worn-out baffle, but it is a possibility...

[skids]

Re: Re: Vacuum advance Question

That should read, "carb nipple." =:-/

Originally posted by skids
vacuum hose to the No. 2 car nipple

[John]

Vacuum advance

Skids, I'm using the correct vacuum port for the advance. I was just wondering if anybody has come up with a fix for the advance to keep it from jumping around so, and breaking the wires to the Hall effect switches. Mine had broken INSIDE the insulation. Had a hell of a time troubleshooting that. A simple crimp splice and some heat shrink took care of it. I swapped out the engine a few years ago, and used one from an XJ11 but I couldn't find a wiring diagram to wire in the XJ's ignition system, so I retained the ignition from my XS, and also kept the alternator from the XS. I just bought a 78F model, as a parts bike, but discovered that not too many pieces from a standard will fit an a special...especially the exhaust pipes, but I have a plan...
He he he...

John
80SG Twilite Special

[MartyA]

Over on the Venture web site, there is a vacuum modification they refer to the Jason mod, after the guy who came up with it. It involves tying #1 and #2 vacuum nipples together with a "T" and then routing the third end of the "T" to the vacuum advance. Then just connect #3 and #4 together with one piece of hose. They claim more low end pull, better mileage, smoother acceleration, higher RPMs at lower throttle openings, etc.

Now, the Venture is a V-4, but it has 4 carbs, as does the XS, and I am not sure if this applies. But I wonder what effect doing that mod to an XS would have? Would it pulse twice as much? Or would it smooth out? Would it pulse further, causing more stress on the coil wires?

[TopCatGr58]

Okay Marty, I'll bite!

I just came up with this little diagram to map out the cycles for each cylinder:Power, Exhaust, Compression, Intake.
Now I can't find the firing order in my Clymers or Owners manual, so I'm assuming it's 1-3-4-2, but it could be 1-2-4-3? Talking about the Xs, not the V-4 Venture!

1 2 3 4

Pwr Ex Cm In

Ex In Pwr Cm

In Cm Ex Pwr

Cm Pwr In Ex

But, with the Venture, what nipples are used for the vacuum advance? If one of the main synching nipples is used, then linking 2 together might work for it, but not necessarily for the XS.
On the XS,the carb nipple has a smaller orafice and is further away from the direct vacuum pulse of the head, which provides a smoother more evenly controlled amount of vacuum to the advance unit!

Linking the synch ports together would provide a longer vacuum pulse, but perhaps smoother since it would also be pulling on the other carb reducing the amount of vacuum sent to the advance unit. Also, it would allow more air into the mix both for the "T'd" carbs, as well as the other pair that is just linked, and would probably lean out the mixture, which would provide the better mileage, but not so sure about the other claimed performance enhancements!? I would think you would have less effective vacuum pulling on each carb since air would be leaking into the now open linked synch ports. I'm not sure if a leaner mix would help the low end, or cause a stumble!? Okay, I've rambled enough, now we'll let the real GURU's have at it!!
T.C.

[MartyA]

Hi TC.

I think the firing is 1-2-4-3 on the XS.
The carbs on the Venture, as far as I know, are basically the same. Synch is done the same way with the same nipples. The set-up on the Venture for the advance is to take the vacuum from #2 cylinder, which is the left side rear cylinder. The other 3 are capped, just like the XS. The left front cylinder is #1 on the Venture.

Philosophically, the intent is the same as the 2-1 or 4-2 exhaust, except you are trying to balance the vacuum. I don't THINK this does anything to the mixture. I wonder if it just adds a little more advance a little earlier?

I would like to hear from the carbie gurus on this also.

The short explanation is here:
http://www.venturers.org/NextGenTech/V2tech.html

on the left side top of the page, click under '1st Generation | Engine | Jason Mod.

[John]

Ported vacuum

I'm sure that the vacuum to the advance is ported, not applied unti the throttle is opened, just like a car. If it were not, you'd have full advance at idle, and across the entire throttle range, and I think that's why using the ports on the intake boots would not be a good idea.

John
80SG Twilite Special

[fusionking]

John you are soooooo right!

[wildbill]

Let me think,talking about linking vacum lines from 1,2,3,4 to a junction and then run one line to the advance.Might work but where are you going to run the hoses?If I have this right.

[MartyA]

Forgive my technical ineptness, John, what is 'ported' and how does that relate - or not relate - to the XS?

[CMA1]

You'd have a better result just joining the 4 intakes together into one line and leaving the Advance line on the ported nipple of carb#2. Joining the 4 intakes will act as a "cheater carb balance" and you 'may' feel a seat of the pants difference (?) but bike would run smoother (if the carbs are un-balanced to begin with).
Don't forget ... actual manifold vacuum drops when you twist the throttle! (throttle vaves open, slides go up... easing the air restriction that induces the vacuum in the first place) so the signal to the vacuum advance if using the intake nipples would DROP, retarding the advance slightly and maybe give a little higher top end response. But you'd have full advance at idle, not really noticeable but hard on the piston tops and valves.!
The old Corvettes wth dual-point distributors had a similar system... when you'd hit Hi-Revs, one set of points would cut-out and the vacuum sensor would cut the vacuum to the advance unit (retarding the timing) at those Hi rpms and the driver would experience higher top end and Hi rev response.
I know I rambled a bit, hope it made sense!!
Leave the advance line on the ported source!! (Carb#2)
<edit> ported vacuum is the source, because of channeling in the carb, that vacuum INCREASES as throttle is opened. Manifold vacuum is created by the piston going down on intake stroke , combined with the restriction of the throttle plate. (without getting into Venturi principle and all that!!!!) I edited this when I saw your question MartyA.
Ride safe.....

[skids]

Isn't this basically how the YICS system works on the XJ?

Originally posted by CMA1
You'd have a better result just joining the 4 intakes together into one line and leaving the Advance line on the ported nipple of carb#2. Joining the 4 intakes will act as a "cheater carb balance" and you 'may' feel a seat of the pants difference (?)

[pgg]

The 'twitching' below 3000 revs is caused by the overlap from the cams causing a small 'blowback' into the carbs. Overlap equals inlet and outlet valves momentarily open together. The larger the cam duration , the larger the overlap. Once the revs are up and the cams 'come on,' everythings sweet and smooth. The earlier 78e's have a higher duration and I found my 78e's were twitchy below 3000, no matter how much carb tweaking I tried. Anyone out there had a 78e with grunt below 3000?

[sixtysix]

Originally posted by skids
Isn't this basically how the YICS system works on the XJ?

The Venture is a YICS bike (At least the early ones). There is an expasion chamber arrangement bolted right on top of the front bank. I had one, I know.