750 big bore carbs

grizld1

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That's all well and fine, JP, but stock is stock and regrind is otherwise. Shell Thuet never altered his recommendation of I .006" and E.012" for the very streetable short track-intended torque oriented #1 grind, and if memory serves, he knew a thing or two . The old maxim applies: Better to hear those valves than to smell 'em. Sure, you can play with those settings a bit. I set the lash on my Shell #1 at .004 I and .011 E, but I wouldn't feel comfortable inside those numbers.
 

Jim

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I just kinda buzzed through this, so apologies if it's already been mentioned...
There's some PMA's out there that don't use a woodruff key to lock the rotor to the correct crank position.
I'd add to the list verifying the TDC mark is actually TDC. If there's no key.... well, no tellin' where ignition's at.
 

jpdevol

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That's all well and fine, JP, but stock is stock and regrind is otherwise. Shell Thuet never altered his recommendation of I .006" and E.012" for the very streetable short track-intended torque oriented #1 grind, and if memory serves, he knew a thing or two . The old maxim applies: Better to hear those valves than to smell 'em. Sure, you can play with those settings a bit. I set the lash on my Shell #1 at .004 I and .011 E, but I wouldn't feel comfortable inside those numbers.
Yes sir, I was mainly throwing in the concept of using .012" to set the timing as that's what the factory used to get opening degrees etc. I don't pretend to know the proper final adjustment for that valve-train (just that it should be readjusted). Me counter you, Mr. Thuet or others of similar regard - no way!
 

grizld1

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Uh, valve timing and ignition timing are both set with reference to degrees of rotation of the crankshaft, but they're independent of each other. The service bulletin you shared (very useful, by the way) refers to running valve lash, not to valve lash set for inspection of valve timing, which specification remained at .012" for both valves to the end of production.
 

jpdevol

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Sorry - should've been more specific - I was referring to cam timing and the "Note" at the bottom of the Bulletin - which BTW, also contained the factory's misadventure in "new" clearances to .002 & .004":)
 

Mannyroad

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I just kinda buzzed through this, so apologies if it's already been mentioned...
There's some PMA's out there that don't use a woodruff key to lock the rotor to the correct crank position.
I'd add to the list verifying the TDC mark is actually TDC. If there's no key.... well, no tellin' where ignition's at.
Verifying this is one of my next tasks. Cheers.
 

atom4488

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Agreed grizld1, but as a start, you could check valve timing one cylinder relative to the other, to see if they match. Secondly, you could check the rephased cam's valve timing against stock valve timing and see how that compares. I would expect that the "mild regrind" has a little more lift, and a little more duration, with earlier opening and later closing of the valves. If the cam timings were plotted (resembling bell curves) and overlaid, the mild regrind should basically be simply a higher and broader bell. During the regrind process, an error could also have been introduced in the lobe centerline angle, dramatically altering the behavior of the cam
An anecdote from a fellow local gearhead who used to race a Formula Ford; FF rules require a stock Ford 1600 cam, but allow the cam timing to be "tuned" for optimum performance. Cam timing tuning is accomplished with offset dowel pins, limiting any adjustment to what can be achieved with an offset dowel which is only 0.250" in diameter or so. After such a cam timing adjustment and the subsequent test run, it was found that the engine accelerated hard up to 4500 rpm and then fell totally and completely on its face, to the extent that my racer friend was miming heavy braking effect. The cam timing was readjusted and the car behaved normally, pulling its normal redline.
The moral of the story, if not obvious, is that relatively small discrepancies in cam timing can have a dramatic effect on engine performance. If you have exhausted the checks of induction, ignition timing and compression, I would focus on the cam.
 

grizld1

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Amen to that, Atom! Installed a Shell #1 cam with a few degrees of error to the retard side on the intake opening. I tried to split the difference with the exhaust opening. Didn't work so well: the setup didn't make the motor outright fall on its face, but it was weaker than it should be when wound up and liked a lot of short shifting. Went back in, set up to Shell's spec of 45* EO, and let the rest of the valve events fall where they may. With rev limiter set for 8K rpm, the motor still has giddyup at 8K. The crank and rods I used will take it, but there's no point to stressing a street bike that way. Anyway, very little movement of the cam sprocket resulted in a big change in engine behavior.
 
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Mannyroad

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Does anyone know if the Sparx PMA on my bike is likely to be keyed to the crank? The Sparx fitting instructions appears to suggest it is keyed but it's not clear.
 

grizld1

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It's been a while since Michael Morse gave me a Sparx alternator in exchange for blueprinting the installation, but I think the rotor was keyed.

In the OE system the stator is firmly supported radially by the vanes cast into the case. The two 6 mm. screws provide only unstressed axial support. In the Sparx system those two 6 mm. screws are all the support the stator gets. If they loosen or otherwise fail, the air gap between rotor and stator closes and the alternator is destroyed as a result. Happened to me 200 miles from home on the way to a rally (and yes, I installed those screws with a hand impact driver and blue Loctite). First sign of trouble showed on the onboard volt meter. Pulled in at the first available motel and checked in. Pulled the cover and saw the obvious. Turned the air blue 50 feet up. Pulled charger and extension cord out of saddle bag and hooked up. There was a bit more than nothing coming from the alternator, so by turning off lights where I figured law enforcement would be thin on the ground, keeping an eye on the meter, and being allowed to use a socket by the obliging convenience store owner where I stopped for gas, I got100 miles down. Stopped at a second motel for a full charge and full rest. Home the next day.

The experience cost me a nonrefundable deposit at the rally venue and two motel charges. gggGary's observation on free motorcycles applied: that free alternator turned out to be damn expensive!
 

gggGary

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The vape rotors aren't keyed and turning the rotor on the crank is the timing mechanism. It's a pain to get real accurate timing. Break the taper loose, try to move it "just a little", retighten rotor check timing, repeat. I marked a degree scale on the rotor to ease the process, still not "easy".
 

gggGary

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Crank has a keyway "matching" is an interesting question.
push rod.jpg
 

jpdevol

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I'm not experienced with the Sparx, but appears to have a keyway. One would assume in the correct location if the unit was sourced for an XS650 https://www.650central.com/Sparx install.htm

I have some experience with rephased motors, but not the set-up you have. The timing marks on the Sparx rotor will need to be verified with a dial gauge and a degree wheel to see where they correspond to your 270* rephase. Was it timed (crank and cam) off right or left and was it left leading or right leading? Does the Probe ignition support your particular rephase?

Was cam timing ever verified? And was leakdown test accomplished?
 
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Mannyroad

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I'm not experienced with the Sparx, but appears to have a keyway. One would assume in the correct location if the unit was sourced for an XS650 https://www.650central.com/Sparx install.htm

I have some experience with rephased motors, but not the set-up you have. The timing marks on the Sparx rotor will need to be verified with a dial gauge and a degree wheel to see where they correspond to your 270* rephase. Was it timed (crank and cam) off right or left and was it left leading or right leading? Does the Probe ignition support your particular rephase?

Was cam timing ever verified? And was leakdown test accomplished?
Good questions.
I've checked timing marks (left cyl) against true TDC and they are in the right ballpark but not bang on. Marks are approx. 5 deg retarded.
Haven't checked cam timing yet, or measured the cam as I'm still awaiting assistance from a friend on this.
Not carried out leakdown test yet, for same reason (friend's a very busy lad). The Probe ignition I have is indeed for a 270 degree rephased motor, not a 277 rephase.
Will post more info when I'm a bit further down the line.
I did swap out the #32.5PJ's for #25s but had to turn the aircrew in to 1\8 - 1\4 turns from closed, and it still runs and sounds shite, so I wl probably put the #32.5s back in, and reset aircrew back to 1 1\4 turns out as they were. Idled MUCH better with that set up.

Thanks for your interest and comments guys. All appreciated.
 

rockhead

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Ah, the mystery cam. Can't check valve timing if you don't have the specs. Be a shame to replace it with an expensive but known rephased cam and have the same grief afterward. I know you're in Britain and the wizard is in Oz, but unless the business is defunct it shouldn't be hard to get the specs from them. Gotta be an email addie somewhere. Or have you tried and gotten crickets? Feeling your pain, as Slick Willie used to say....
Darryl got all of his cams done by " a local cam guy". Otherwise known as Clive at https://www.clivecams.com.au/. Has 50 years experience. All Clive has ever done is cam building/ regrinds. Bought out the company he did his apprenticeship with when they closed. Stared as an apprentice aged 13. Does quality work. Designed, made & ground cams for a V12 made from two LS1 Chev V8s. I'm sure he'd be happy to chat by phone or email.
 
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