Change stroke on XJ700 X?

Just a curiosity question. I don't have an X... yet. But I'm sorta tempted to hunt one down, once I finish the current project. Something to up my game on, as far as working on the engine.

Is it feasible, and what would be needed to change the stroke on an XJ700 X, to get it up to 750cc? Short of just finding one up in Canada? Is it doable, feasible? What's needed? pistons, rods, ...cams? Blood? Tears?

If I understand correctly, all they are is a 750 with the bore sleeved?

And would the result be? Worth it?

 

Lots of money and no

You might find a place that will weld up the journals and regrind then but it will be expensive, most likely it would be cheaper to get custom pistons made if the liners can be bored to suit

 

yeah... and the more I'm reading up on this bike, I get a sense I'd just be messing with a good thing.

 

You would need the 750 pistons, rods, crank and likely some crank bearings.
The bore is the same for both motors btw

 

Simmy is correct that you'll need the parts he listed. That's because the difference is the 700 has a 48mm stroke and the 750 has a 51.6mm stroke. Both have bore sizes at 68mm so not sure why you thought they were sleeved?

And if you do want to upgrade, you'll have to use a Maxim X crank, not one from an FZ750 because those are not machined to accept the timing plate that mounts on the left side.

Tony

 

Is the 48cc THAT much slower..my 650 Seca, when revved, seemed at most 3-5 MPH slower than my 750's.......and that 100cc's!!

Less torque..yes, thats almost a given, but still....

 

The 750 redline is 11000 and the 700 only 10,000, probably because the 700 has taller and heavier pistons. I suspect the TCI’s are different also. This is likely where any serious hp gain will be had. Considering a nice clean 750X can be had for 2 grand Canadian any consideration of these mods doesn’t make sense.

 

Hey Tony, as per your sig I see you have a custom 911 FZX.
85 750X - "RestoMod"
85 700X - Building w/ spare parts
86 FZX - 911cc Custom Naked Build
can you tell us about this one? How far along is it?

 

Well I don't want to hijack the thread but have to say it's really just in the planning stages. Have 2 700cc engines and an FZR1000 piston/cylinder/head/cam set for the build. Plan is to set the bike set up initially with one of the 700 engines while the 911cc one is built.

Lots of other projects (house related stuff) has slowed the pace of my motorcycle building, but have been picking away at it whenever I can.

Have plans for an 840cc X engine as well (69mm bore with a custom machined 56mm FZR crank / using FZR1000 rods). However, that's way down on the list of things I want to get done in the next couple of years.

Tony

 

I'm sure turpentyne won't mind the hijack as it is similar theme

I'm sure you know this, you'll need a 750 crank for 911.

Exact same issue here.

I'm curious to learn what modifications you've seen are necessary to adapt an FZR1000 crank into the X?
Where would you get 69mm pistons? Yamaha several sizes up? Why not just use 75mm pistons from the FZR1000 for a full 1000-X?

 

Hijack away!

 

OK then, let me take the poduim......

The Fazer has the identical pickup for the timing, and as you said, a good FZ750 crank is needed. Have to be careful though as the US had an FZ700 in 1987 and could be mistaken as a 51.6 mm stroke crank cause some people are clueless and they're actually 48mm.

69mm pistons are pricey, but these are top notch:
http://dynoman.net/engine/pistons/custom.html#FZ750

Regarding the full 1K cc build, the 75mm pistons / 1000cc head combo has a different valve angle and port sizing. Using the X head, because of the unique engine angle and port sizing, would prevent from getting the proper carb set up (33mm Mukunis) for the extra fueling to support 1000cc's. Fuel injection is NOT something planned for either engine.

The 911 build is getting the 75mm pistons and cylinders, along with the larger ported 1000cc head/cams with another custom x-pipe exhaust that I'll be building. It will include 34mm Mikuni downdrafts (that came stock) that'll be jetted the best I can according to the engines requirements. The stock airbox will include a K&N filter, with the short intake tubes (sourced from an FZX750 in Greece).

The pick up plate will mount on the end of the 56mm (1000cc) crank, after it is machined to the correct depth, and a dowel hole is drilled to accept the X plate.

One of these days I'll set up pictures in my photobucket to show the differences and mods needed.

Tony

 

I agree with you, a 1000-X breathing through the 33 mikunis would not be ideal. I have a 911-X and considered XJ900 Seca carbs, going as far as buying a bank and jetting them for my bike.
Just prior to mounting them I noticed that the X had special Mikunis with a shorter body to clear the water pump on carb #4. The 33mm carbs are not ideal but the big bore surpassed all my expectations. It sure made lugging a sidecar much easier.

Not that I'm considering an FZR1000 crankshaft but I appreciate your comments. I need to look at the fiche to understand what you're saying needs machining.
A stroker X sounds interesting.

 

Well the FZ and the Maxim X have different ignition triggers and pickup assemblies. I'll have to take a picture one of these days to show what I'm talking about, and how to modify it to work. Definitely a job for a professional machinist though.

Yes, the BS33 carbs are very unique and the other roadblock is the intake/carb holder configuration, and trying to connect up to the stock air box (pods are a no-go with CV carbs).

Being that the X was the only Genesis 5valve with shaft drive on the XJ, the cylinder inclination angle is specific too. Swapping over any other models carb/intake parts is basically out and custom fabrication would be a serious undertaking.

Sticking with the plan I've come up with after a ton of research.

Tony

 

It can be done. The trick is to install softer throttle slide springs.
I think since my podded carbs are now too small for my big-bore motor they respond much better.
Instead of the restriction at the airbox it's now the mouth of the carb. Before the big-bore it stumbled pretty bad at 5-5500 rpm.
Now I can power right through the mid range, sidecar loaded up in 5th gear.

 

Ooo, stiffer springs surely - less depression on the inlet side of carb lifts the piston higher for the same suction at the jet?

 

The extra suction created by the air box helps lift the slides. When you eliminate the air box the slides no longer cooperate with your wrist, they don’t open so the rider gives it more throttle then the slides slam wide open, terrible thing to ride. Sometimes the stock springs will not even allow the slides to fully open.
Good stage-3 jet kits should come with softer springs, the cheap kits instruct you to clip half a coil off the oem spring until you get acceptable results.

 

Another thing the tuners do is drill the hole into the vacuum chamber bigger. I think this just quickens the response of the throttle slides. Without the air box the delta between the pressure at the carb inlet and the pressure inside the vacuum chamber is smaller and probably slows down the reaction of the slides. It might show some benefit to a stock bike but I wouldn’t want to disturb a perfectly stock bike for this. You can’t drill it smaller.

 

OK, so here's a view of a Maxim X crankshaft:


And this is one of an FZR1000:


Both have a bolt hole drilled in the center which is good. Not sure if the depth is compatible to accept the pick-up plate from the X.
Both have a dowel pin hole, but there is some machining required to get the correct distance from the #1 journal for proper case alignment. Like I mentioned, it's a job for a professional.

BTW, the crankcase may have to get clearanced for the rods, but the 56mm stroke (with the FZR1000 rods) along with the height of the X cylinders (which will be bored to 69mm) gives it the correct deck clearance. That will yield 840cc's. I think the compression should be around 11.8/1, but not certain until I cc the X head after machining the gasket surface.

See you got on the subject of the carbs, and one huge challenge is to get correct throttle response with either the stock airbox or the dreaded pods. You mentioned springs and vacuum orifice size and how important that is. Going up a size on the pilot jet helps but one thing that is blatantly missing on the CV's is an accelerator pump and squirter jets.

Here's a good example a set of Mikuni Flat Slides on an XS1100 so you can see what I mean:


Tony

 

As I recall the 750-X piston was taller than the FZR1000 piston.
I used a 1000 block and machined it down. By my calcs I strived for a 10.5:1 CR. This was all 20 years ago.

I would have thought an 840 stroker would require a spacer under the block.

 

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!