Vacuum advance Question

[skids]

Yes I do. 78E with Jardine 4-to-1 pipes. Standard airbox with washable K&N filter. NICE low end torque.

Originally posted by 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?

[ratbyk]

Pulling below 3000

My Ratbyk also pulls very well from as low as 2200. Stock air box and exhaust. I've played with the jets, (1 step richer mains, 2 step richer on the pilots with a slightly lower float level)
It will run smoothly and with good power from as low as 2200. I don't crack the throttle wide open down there but just start twisting it on and opening the throttle more as the revs build.
The carbs have been color tuned and synch with a 4 vacuum guage set up. It really runs smoothly and I don't feel the need to drop a gear or two while at lower RPM. Simply start feeding it in and let the torque pull it out. Easier to ride and saves the transmission.
Ken/Sooke
78E Ratbyk
82 FT500 "lilRat"

[wildbill]

The 78E I have,last time I rode it pulled like a tractor at low rpm.Going back to the vac. advance,mine would really do a snake dance at low rpm. and then would smooth out as rpm. climbed until it was steady.Running those lines,would you join all 4 or could you say join 1-2 or 3-4.The 2 outer cylinders rise and fall at the same time and the 2 inner cylinders rise and fall together.Wouldn`t that smooth the advance some since it would be coming from 2 instead of 1. Wouldn`t that work?

[Lawrence]

Vacuum Advance VS Inadequate Baffle…

Hi All

My 83S is wearing a 4-1 with no silencer and an old baffle ‘shoved’ down the pipes throat. It runs real nice, is ‘smooth N sluggish’ under 3K then the power comes in all the way to the top.

Unfortunately, it will only pull to 6.5K in Top gear.

The Vacuum Advance pipe was disconnected from Carb No2 – reconnection didn’t improve top revs?

So… What should the XS run like if the Vacuum Advance is/isn't working correctly??? – I don’t have any misfiring/hesitation etc.

Also, Skids mentioned the ‘worn-out baffle’ ???

I wanted to appreciate these dynamics before looking at the jets.

Cheers and E/:?)njoy to all

[pggg]

Dunno how 78E owners get useable grunt below 3000. I had 2 78E's up and running, (aussie spec models) but with 78' cams neither would pull strongly until 3000. Slammed in some 80' cams and the twitchy fluffy tendency between idle and 3000 disappeared. My current XS pulls strong and smooth from 2200 in 5th. Open the gas at 2000 in 5th and I get a drivetrain snatch. With lighter throttle use it'll chug along effortless from 1500 in 5th. My standard 78E's were basically 'dead' at those low revs.

[MartyA]

"Dunno how 78E owners get useable grunt below 3000."

Don't.

[ratbyk]

To get "grunt" on 78 below 3000 you drop 2 gears then go for it.
Ken/Sooke

[MartyA]

Shoulda said
Don't go there.

[81xsproject]

I have been pondering this from a physics stand point and not really getting anywhere. Insted I thought of it this way. The negative pressure is created as the number two cylinder drops with the intake valve open, then it closes and pressure begins to equalize back to atmospheric pressure (this is what causes the twitch) until the intake valve opens again on the next drop of the #2 cylinder.
But, and I could be wrong here, at the same time the #2 intake closes and the equalization to atmospheric begins, the #1&4 cylinders are begining thier downward intake stroke.
So, if you T'd the #2 with either the #1 or #4 it should just make for a much more continuous negative pressure.

As far as connecting ALL of the cylinders as previously mention in the thread, that sounds off for some reason to me. I think if you had two cylinders both pulling the advance at the same time you would have 2x the (-) pressure, but I am not sure about this either. I think I will ask my physics insructor as I recently caught him at a gas station on HIS motorcycle.
JC

[ratbyk]

I think that you are wasting a lot of Grey matter on something the engineers and the guys in white shop coats at Yamaha have already worked out. They have made several hindered thousand (at least) XS1100's. Long before they started going down the assembly line all the factors relating to vacuum from one carburetor, diaphragm size, spring tension and any other factors were worked out. They ran fine when they were released. I don't think any shade tree mechanics are going to improve on the original design. Just make all of the factors as they were in the beginning and the vacuum advance will work just fine. If the linkage is worn out or the diaphragm has a hole in it then replace them.
Ken/Sooke

[skids]

Well said, Ken.

Originally posted by ratbyk
Just make all of the factors as they were in the beginning and the vacuum advance will work just fine. If the linkage is worn out or the diaphragm has a hole in it then replace them.
Ken/Sooke

[TopCatGr58]

Hey Lawrence and JC, and other's interested!

Here's a quote from the 1/78 Cycle Road Test magazine article explaining the ignition's vacuum timing characteristics, and I think it will help you understand why you can't get over 6.5K in top gear WITHOUT the vacuum unit connected....you'll note I didn't call it an "Advance" unit!! Read on!!

The Eleven's Ignition system's second important feature, and one that's unique to motorcycles, is the vacuum-actuated advance/"RETARD" mechanism. Actually there hasn't been an automobile produced in at least 30 years without some form of inter-connection between manifold vacuum and ignition timing, simply because spark advance otherwise has to be compromised between the conflicting demands of power and economy. Engines running under slight throttle openings have a thin, low-pressure mixture in the their cylinders, and they don't run efficiently unless the fire is started long before the piston reaches top dead center. Much less ignition advance gives optimum results at large throttle openings. Yet, all motorcycles but the Eleven effectively have a fixed spark advance, with a centrifugal device to retard the timing for easy starting. And their off-idle timing has to be compromised, with enough advance to give reasonably good efficiency under cruising conditions but not so much as to cause detonation when the rider uses full throttle.

The Eleven we tested had 10 degrees of static ignition advance, and another 26 degrees from the centrifugal mechanism, which adds up to the 36 degrees other engines of similar configuration have needed for maximum power. But then there's also 16 degrees built into the vacuum-advance device, and that pulls the timing around to 52 degrees BTC when the bike is cruising along at freeway speeds. Wind the throttles open, and it drops back to the power-setting; roll off throttle, and it auto-advances for economy; use some in-between throttle position, and the ignition adjusts itself to suit conditions. It's a big setp out of the technological Dark Ages, and one that's long, long overdue.