Torque peak

yamahammer

yamahammer

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I got the bike out with the newer cvk's re-jetted with the 40/140/JJJ synced and running really well, did some calculations with the help of a roadside radar detector (my tach and speedo skip like crazy) and found my torque peak to be right at 4,500 rpms. That's where the engine runs out of steam, stops making power, and starts to vibrate more than is tolerable. It's a rephased 447 with an 80mm Mikes kit, 447 cam, stock exhaust, and cvks. My timing is slightly retarded to match the 18/30 final drive ratio, but both cylinders are spot on with a newtronics system.

My question for you guys is, does that sound about right for a rev ceiling? It was a few hundred rpms lower with the bs38's but the curve felt the same, the cvks are just snappier and rev more quickly. When it was a 360 and second oversize it seemed to like to rev higher than that 5,500-6,000 rpms and developed a "balanced" feel under high load where the primary and secondary forces seemed to compliment each other, the big bore rephase seems to like to be lugged more and is happier at low to moderate load.

Is this everyone else's experience as well?

BTW, I use this calculator http://www.nightrider.com/biketech/calc_speed_rpm.htm it is very easy/helpful.
 
Experiment a little more with the slide needles. When I found the sweet spot on mine the motor ran silky smooth on the highway and revs clean and quick. I think I'm using the HKJ needles now on clip 5 with a shim. Adding the shim was a night and day difference in how the motor feels and responds.
 
Richen it up then, my pipes are looking a little yellow at the ports so it may indeed be running lean, i forgot to mention that. My jjj's are in the middle right now with shims underneath that came in my carb set .006 I think they are. I'll try raising the needle first.

Thanks for the input!
 
With a 750 kit you should be able to rev way higher than that, perhaps the gearing is a bit much, I used to run 18/33 with my 750 with standard cam, never really felt it was it's limit until it was going too quick to worry about. used 32mm and 34mm carbs not much difference between them
 
HJK's are leaner than JJJ, but going from clip position 3 to 5 is richer.

So the difference is a little of both.

Last report I had seen from Mr Riggs was;
"38 pilots, 138 mains, GJH needles, clip #5, plastic donut spacers"

And this means the jetting is changed for both pilot and main.

As you can see there is still some trial and error to tweak these carbs for best results for each application.

And some more notes from Mr Riggs testing, he wrote somewhere:

"Set the main jet for best WOT performance, set the pilot for the best idle, then ALL the other tuning can be done with the needles.

Off idle and low speed is tuned by changing needle diameters.
Half throttle is tuned with the length (clip position).
Three-quarter throttle is tuned with the taper.

Float height - 17 mm or with fuel level gauge .5mm above gasket surface"

Merry Christmas.
.
 

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OP, a few points:
1. You have a mistaken idea of what "torque peak" means; it isn't the point at which the motor falls on its face. Google for explanations.
2. Single wall pipes are going to color, and if you see only a little yellow at the ports, you're running too rich, not too lean, somewhere in the cruising range.
3. Do you mean that peak rpm is 4500 in all gears, or just in 5th, where you're ridiculously overgeared? If in all gears, you may have more problems than carb tuning and ignition timing to deal with.
 
'Scuse me if I'm being a bit thick here but the link seems to calculate speed for a particular gear ratio at certain rpm's. I don't see how you have used it to predict the point of maximum torque.

Also not sure if it is just the way you have phrased your comment '...and found my torque peak to be right at 4,500 rpms. That's where the engine runs out of steam, stops making power' but:

Power and toque are not the same animals - power = torque x rpm. Peak torque is usually much less than when peak power occurs. If you have a play with the calculator in 'How to compare engines' here you can see how power increases as revs rise.

So to your issue. You are saying that the motor won't rev much beyond 4500? That suggests that there is something very wrong here as in standard form peak torque occurs around 6K and maximum power at over 7K. With a re-phased crank I would expect it to be able to re quite happily beyond the normal rev limit of a 360 degree crank.
I suspect there isn't an easy answer, it will be a case of working through all of the variables, e.g. Cam and ignition timing, valve clearances, fuelling exhaust restrictions etc.
 
Correct on all points but one, Max; rephasing won't enable the engine to achieve higher rpm. The rev limit is going to be imposed by the strength of the crank assembly and cases. 360* motors in full race tune (with competition rods and big end bearings, welded cranks, etc.) will wind up to 10K, but by that point very bad things start to happen. Read posts by Craig Weeks (650performance). Craig's engine modification manual is something that anyone intending to build one of these motors for more power should read carefully; it's the best $40 you'll ever spend.
 
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perhaps you have an ignition issue. The stock bikes at 4500 rpms is where the power really starter to happen. The vibes also increase at this point.
The ignition timing at this point should be at max advance. I might suggest you check the timing at idle with a timing light. then as you slowly rev the engine watch for the timing marks to advance, move left, they should reach to the max advance at 3000-3200 rpm.
If you have no advance then that just might be the issue.
Leo
 
I'm thinking along the lines of Leo's suggestion, mostly because I have no idea what this means: " My timing is slightly retarded to match the 18/30 final drive ratio, but both cylinders are spot on with a newtronics system."

Why would you retard timing to accommodate a sprocket ratio change? What is a newtronics system?

I can tell you that 4000-4500 is where it stops revving if your ATU is jacked up.
 
XSJohn (RIP) recommended retarding the timing a few degrees, to reduce the "ratcheting" as he called it. Especially with higher geared sprockets.
The Newtronics is a fancy ignition system. It probably has electronic advance. Similar to the Boyer. If set up wrong it may not be advancing as it should.
Leo
 
grizld1 said:
Correct on all points but one, Max; rephasing won't enable the engine to achieve higher rpm. The rev limit is going to be imposed by the strength of the crank assembly and cases.
Click to expand...

:agree:

Agree with you Mr Grizld1, the maximum revs will be limited by the components. I didn't make my point very well.

To try and clarify, my understanding is that a re-phased engine, due to the reduced inertial loads brought about by both pistons not being stationary at the same time, will tend to pick up revs faster. It is easy to go beyond the original rev limit, which is what I would expect Mr Yamahammer's motor to be capable of.

You are absolutely correct to point out that unless associated components have received appropriate strengthening this is not a good idea to do on a regular basis! :eek::eek:
 
Leo, the Newtronics ignition does not offer electronic advance, nor is it particularly "fancy;" if anything it's the crudest aftermarket ignition available for the XS650. Re. the late John Underwood, his work on the BS34 carburetor was brilliant, and he offered many interesting suggestions. Most of them were useful and valid, some of them were questionable but gave food for thought, and some of them--let's just say they weren't what I'd do if I wanted to keep my motor in one piece. John phoned to chew the fat from time to time, and one point we used to go around on was my contention that the XS650 motor was not designed to lug, and John's contention that it could be made to do so. Caveat lector.
 
Well my "fancy" was a bit of sarcasm. I'm not that familiar with the Newtronics ignition, heard it referenced on here a few times. I didn't know if it used an electric advance or used the stock mechanical unit.
As far as XSJohn, it was sometimes hard to agree with all he said. I didn't particularly agree with the oil cooler in the oil line to the head.
He did come up with some good ideas.
Checking the timing and advance function would be my next step.
Leo
 
So my perceived torque peak is 1000 rpms lower than stock it seems, according to that helpful old pamphlet, thanks Twomanyxs1bs. I dial indicated my cam very carefully on install the new endless chain has most likely stretched, retarding the cam. I should redegree it.

It's not so much an issue, it's a wonderful ride under 81mph, and who sustains speed higher than that for too long?
I probably misinterpreted the torque peak. That's just where my senses told me, "stop revving higher, pumping losses/friction/volumetric efficiency/scavenging/whatever are starting to win ." But you've all confirmed what I suspected, that it should rev higher, more freely. You guys are probably right about my gearing being too high, it doesn't climb as fast in 5th.

Thanks for all the input from everyone and merry xmas!

Edit, I like the newtronic system, optical trigger, led static timing aid, no e advance sadly.
 
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My bike but not my work, with VM38 carbs, cam and a 750 kit the hardest thing to do was keeping it from busting redline in the first 3 gears. It loved to rev and would overpower the clutch at about 5500 RPM. So my take is the motor should LOVE to rev....
Note past tense AFAICT it now needs a welded crank. :shrug:

 
Right you are, gggGary; with a few mods these motors will pass stock redline like it wasn't there. Yep, Leo, you've named one of the things I wouldn't do--that and increasing the size of the oil pipe orifice. As Craig points out in his engine modification guide, long experience has shown that these motors don't require modification of the lubrication system for full race duty, apart from installing an oil cooler plumbed the right way, in the output side of the pump.

OP, I don't know how you're achieving your perception of the torque peak in the absence of a dynamometer. A bone stock XS650 motor that won't move the bike into triple digits needs tuning and/or repair. BTW, retarding cam timing on these motors, as on others, will move peak torque and peak power higher in the rpm range, not lower. The first XS650 motor I modified (many years ago) was a stock 447, except for VM34 carbs, 1-1/2" single wall pipes, and head dropped .030" to boost compression. The cam sprocket wasn't moved from the stock position, so that valve timing was significantly retarded. The result was a flat spot from around 3200 to 3400 rpm and strong pull to about 6K, after which point a crazy burst of power kicked in. I have no idea where power would have dropped off; the crank hadn't been welded, so I made a point of shifting at 7500 and never wound it up past 8K, at which point the motor was still making power like depravity unchained. Anyway, retarded valve timing is emphatically not the problem with your motor.
 
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Figures, bass ackwards lol. You guys don't think a pilot can sense a bikes torque peak? I have no evidence, just my butt dyno. Some people can feel the holy spirit, I get stuck with esp for a torque curve. :shrug: ... but I think torque is the holy spirit lol.

No more blasphemy, I used to work at a Yamaha dealership with a dyno, maybe I'll buy an hour and shake it down all scientific like.

And I'm Kyle, BTW. Forgot to say, but OP works for me, I'm also a /b/ro
 
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