Change stroke on XJ700 X?

Thinking and typing here Simmy, so bear with me.
The pistons are lifted by suction induced by the airflow underneath. From Mr Bernoulli, increase velocity, maintain mass airflow, get lower pressure - or increased depression.
Now, feed this depression up through the piston into the area above the diaphragm - hence lifting the piston. More flow equals more lift, but there is a problem - the lift goes up with a square of the flow. More about this later. Also, more about this hole later.
What about the pressure below the piston? Ignoring the bottom of the piston, because the pressure there is balanced by that from above, we look at the pressure under the diaphragm. Where does this originate from? Sure enough, the airbox.
So, the piston will lift by an amount equal to the differential between the inlet and the depression above the diaphragm, times the area of the diaphragm. So why doesn't it just hit the top stop? The spring of course.
Now, to go back to the inlet pressure. if, at a speed and load somewhere above pootling along, theinlet pressure is reduced, (ie by removing filter or adding a free flow one), what happens?
The inlet pressure increases, the suction induced lift stays largely the same, so the piston lifts higher.
So in simple terms, to prevent the piston topping out, you need stiffer springs. Otherwise the cv, or cd as the original designers labelled them, is lost.
To add complications, the spring pressure adds the more it compresses, which offsets the square law issues from Mr Bernoulli. The size of the orifice is critical to the speed of response of the piston. On SU carbs, there is a hydraulic damper, or dashpot. If you leave this empty the piston moves too quickly, reducing the pressure above the jet, which reduces fuel flow when you need it most. Add some oil, the piston slows its rise enough to actually restrict the airflow, which sucks more fuel out of the main jet - result is smooth acceleration.
So opening up the hole in the bottom of the piston is likely to be bad news.
There we are then, my take on it - less filter depression needs stiffer spring..

 

Damn! now you've gone all sciencey on me.
I had one semester of thermodynamics/fluid mechanics in college 35 years ago and I barely soaked up 50% of it then.
I'm just repeating what my dynotuner told me and the instructions from Dynojet and Sigma.
I know when to bow out

 

Must not have hit the "Post Reply" button last night? Hopefully not too confusing with the delay.....

I believe you are correct on the piston height, but no need for a spacer. That's the reason for using the FZR1000 rods because they are shorter to give it the proper deck height. Sorry I don't have all the measurements available.

Tony

 

Wow Minimutly, that's some serious pressure theory being applied!

Have to say that all I know is the reaction of the carb slides on the CV's is very dependent on smooth airflow into the carbs.
And that too much of a good thing (airflow) can be bad, if other components aren't changed or tweaked to compensate.

Tony

 

Minimulty is corrrect.
Cutting coils from a spring increases the spring's stiffness (more actually goes on when coils are cut out, but for the purposes of this discussion I think my explanation is sufficient).

 

Actually this site explains things better than I ever could.

https://aminoapps.com/c/motorcycle-...on-cv-carbs/PQ6Q_8Bhmu0NlQ3nWM1dvoeMJ3Vj1Zn2d

 

If you cut a chunk off the spring wouldn’t the hat be easier to assemble? Wouldn’t there be less preload acting against the slide?

 

Something to do with spring rate? Lower when less is coiled against the hat? I dunno... me brain is hurting now!

 

Mine to. I can understand 2 springs of equal length, one with more tightly wound coils will be softer but I don’t think that’s the same thing here.

 

Just did a little reading, and It sounds like it depends on how many inactive coils were before and after the spring is cut. Inactive, meaning they were pressed up against the hat & not part of the pressure, because the first coil that contacts the surface is taking the pressure.

... that’s as far as I got before I decided it’s time for me to tap out!

 

This thread has bounced all over the place, that is my fault.
Back to the original topic of X stroke, that would not surprise me if all the parts for an 840-X just fit together, Yamaha stuff is like that. Since the XJ cases already accept the stroke from the 900 the 56mm stroke from the FZR1000 will probably go, maybe a little help from a Dremmel, aluminum is soft. That machining required on the crank is nothing an average machine shop can’t handle.

 

I feel that minor mods would be needed once a test assembly dictates anything I haven't thought of.
And yes, the machining so the timing plate will fit shouldn't be a problem for a qualified machinist.

Tony

 

I will join in the appologies for taking this discussion away from expanding the xj engine.

What I will comment on as a non proffessional machinist is that it's not about quallifications, more to do with having the kit to set the job up for machining - modifying the end of a crankshaft isn't an everyday job. If I were doing it I would be looking at setting it up on the lathe, with a drill in a collet. Very much a one-off.

 

One-off yes (so setup costs will be a bit higher), but not at all an unusual operation in terms of what needs done, just in terms of what it's being done to (though reworking crankshaft noses is not that uncommon in the U.S., but usually it just involves recutting woodruff keyways).

If it were mine I'd either get a scrap crank to work things out on and practice (and to see if the wife would notice that I "had" to buy a bigger lathe), or a bottle of decent scotch and make friends with someone at a small machine shop.

 

Just put the new lathe further away... so it appears smaller!

 

If you do it yourself you can be sure it's right.
I built a v8 a couple of years ago - bored, honed, surfaced, gas flowed big valve heads, cam etc. The car builder was fitting a different manual 'box, so I surfaced the front of the flywheel, lightened the back. The car builder was having a "professional" fit the clutch. I asked for it all so I could get everything balanced, the machine shop with the ballancer called me and said the thing wasn't half up to speed and it wanted to walk out the door!
I checked it in my lathe, the thing was 1/8 inch out - a full 1/4inch throw! Had to move it around and redrill the wheel.
So much for professionals...

 

There are all sorts of professionals, and all sorts of amateurs. Finding a good one can be tricky in either case.

 

I do have a second FZR1000 crank, but it won't be me practicing on it. Hopefully I can get "the trusted machinist" to do both, however it may require two bottles of scotch for the best results.

Tony

 

Hopefully applied after the job is done!