750 big bore carbs

grizld1

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I did indeed mean FCRs, and should have corrected that old post 10 years ago, and thanks for clarifying. But our subject here is no longer carb selection, tasty thouigh that topic can be.There are things that need doing before blaming the carbs.

Rule One: Never, ever, assume. Don't assume that a "professional" shop doesn't make mistakes. Don't assume that good numbers from a simp[e compression gauge mean that your engine can retain and use that pressure. There's a check valve in the line, so that only the highest psi realized will be recorded on a compression gauge. What signifies is how much of that pressure is retained. With a leakdown tester, the operator locks down the crank at TDC, selects the jug that has valves closed, connects compressor to one end of tester to compressor at one end of the tester and cylinder to the other, sets pressure in at 100 psi on one gauge, and reads pressure retained on the other. The minimum standard of health is 90 psi retained for the street, 95 for the track. The test will show where the grief lies. Air out the carb: intake valve defect. Air out the exhaust: exhaust valve flaw. Air out the crank case breather: piston/piston ring defect.

It's too soon to blame the carbies. Just what did those big money professionals have to say about the mess on your hands? Here's a little comparison for you.
D model motor. JE 77.5 mm. pistons for 700 cc displacement. Cylinders sent 800 miles for boring to individual pistons with SiO2 impregnation of cylinders. Shell #1 cam, carefully timed by Yrs. Truly. Crank sent out for new heavy duty rods and bearings, blueprinting, truing, and tack welding to assure against flywheels walking out on the shaft. Assembled by me in an old apple barn. Result? I don't know what the true top speed or max hp is. What I do know is that a well calibrated electronic speedometer will register 118 mph any time the bike is asked for that speed, that it doesn't waste time getting there, that there's around 15% of throttle left at that speed depending on conditions, and that the electronic tach shows 7000rpm. 17/32 sprocket combo. Carbs: 33mm Mikuni flat slide pumpers, bought as salvage, banked and tuned by owner (that would be me).

Sorry that you paid a big tab for shoddy work, and there's no way around the fact that shoddy work is what you got. It's too soon to blame the carbies for your grief. If the shop won't man up and fix their mistakes, you can either take up the work yourself or you can pour more of your money down the "professional" rat hole. I'm very, very sorry, but that's the bottom line for your situation.
 
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Mannyroad

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Nice bike, something is wrong it should rev higher than that with stock gearing. Try a run without those air filters. Some filters that look like the ones in the photo are known to cause problems. The problem is with the mounting flange blocking air circuits. Better air filters are K&N or Uni pods. Unipods are ok but not as free flowing as K & N.
Does your mates dyno have a fuel air meter, you will be able quickly see what needs to be adjusted.
The air filters are K&N's.
Discussing my issues with my mate who has the rolling road, I think.i need first to double check my valve clearances (not been checked since engine was rebuilt), check accuracy of the TDC and timing marks on the Sparx stator, check the the timing thereafter before going further.
My engine was rephased and fitted with the MikesXs 750big bore kit, by Daryl Hutcheon in Oz. He's done many of these conversions to xs650's and according to him he does a "mild grind" of the cam after rephrasing it. My problem is that I do not know what the valve clearances should be for the reworked cam. Has anyone out there got one of his reworked engines and can tell me what clearances I need?
 

grizld1

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Ouch. I run a Probe iggie. Be aware that Mark Whitebook, that good man, discontinued production because folks were choosing poorly designed but cheaper ignitions instead of paying around $50 more for the best.Part of the reason was lack of promotion. Probe Engineering is a big outfit. Mark, a Yamaha 650 owner, designed the original iggie for his own ride, expanded offerings to other machines, and devoted some resources to small-scale production but not many to promotion. A few years back he concluded that he was losing too much money on producing the finest aftermarket ignition package available, and ceased production.

Watch out for that Sparx PMA! Michael Morse sent me one gratis to blueprint the installation and write instructions. I don't like PMAs in general, but I took up the project and even fabricated a template to identify TDC, 15*BTDC, and 40* BTDC.

I knew enough to use high wattage lighting, horns (Fiam duals), and so on to control output at speed. But I should have paid attention to a major flaw in the Sparx design. The OE stator fits snugly on the blades cast into the crank case, providing radial support, while the long, skinny 6mm. screws just hold it against the case. That is all they are meant to do. Period. The Sparx stator is not supported by the blades. The installer must set the gap between rotor an stator and rely on the two 6 mm. (!) screws and a dab of Loctite to hold the stator in place. If the screws, grossly undersized for the job, should fail, the rotor will make contact with the stator and the alternator will be history. It happened to me 250 miles from home en route to a rally. I had a voltmeter and watched it, stopped at a motel in time to preserve just a little bit of function in the terminally damaged alternator, pulled a charger and extension cord out of the bags, pulled the cover, and found the mess. I can't recall just which I cussed louder and more eloquently, myself or the cretin who designed that damn booby trap, but the air was blue all over that parking lot. I set out the next morning for home babying the charging system all the way, mostly taking the risk of running
without lights, and did around 130 miles to a motel for a second night and another battery charge. Home the next morning. That damn free alternator sure got pricy in the end. Reinstalled the OE excited field alternator with rewound rotor and never looked back. You might thicken the mounting blades with JB Weld or tough metal bearing epoxy and give radial support to the stator.
 
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atom4488

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Thanks for all the info guys. Much appreciated. I did a mains change as a quick stab, to see if it made a difference on full throttle. I'm not too clued up with carbs. The motor was professionally rebuilt, big bored and rephased 1500 miles ago by Daryl Hutcheon in Oz, who made a living doing this to XS650s. I acquired the bike with only a handful.of miles on the clock. Now it's pretty much run in its disappointing to find it's so gutless. It has a Probe electronic ignition and my timing light suggests idle and 3500rpm up timing is about right. On my mate's rolling road it pulled through to 7000rpm no problem but yesterday on a ride with a few mates it wouldn't pull more than 4500 in top gear, and it took its time to get there. I so wanted to to get off and kick it's ass. I shall read through the techy info in the links and investigate what other brassware \needle info i can pull from my carbs and report back here for your views. Ive always thought setting up carbs a bit of a dark art tbh and have never been able to understand it like id like to. A pic of the bike is attached. I love it, but not the way it's performing, which is a shame after I've done so much to it.
Beautiful bike, Manny. If I were you, I would forget looking at brassware until I had fully assured myself that everything was right; compression/leakdown (as gggGary and Grizld1 have suggested), throttle opening (full lift of the slides), fuel flow, air flow, etc... Is it possible that the cam timing is off by a tooth? There has to be something really wrong with the basics if it won't pull past 4500 rpm. Good luck!
 

atom4488

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The air filters are K&N's.
Discussing my issues with my mate who has the rolling road, I think.i need first to double check my valve clearances (not been checked since engine was rebuilt), check accuracy of the TDC and timing marks on the Sparx stator, check the the timing thereafter before going further.
My engine was rephased and fitted with the MikesXs 750big bore kit, by Daryl Hutcheon in Oz. He's done many of these conversions to xs650's and according to him he does a "mild grind" of the cam after rephrasing it. My problem is that I do not know what the valve clearances should be for the reworked cam. Has anyone out there got one of his reworked engines and can tell me what clearances I need?
There, I think, lies a major clue: "....according to him he does a "mild grind" of the cam after rephrasing it." You may have a cam that is in some way bad... not correctly timed, not correctly rephased, or correctly ground. In terms of valve clearances, the fact that the cam doesn't have the original profile doesn't dramatically alter valve lash.
 

grizld1

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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....
 

gggGary

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Can't be too far off setting valve lash at .003" and .006"
I'd check vavles and cam chain lash then check compression. You should be over 150PSI

Physician heal thy self? Been wasting way to much time trying to start a "ran when parked" XL500S, pretty sure now the motor's FUBAR, going to go check compression, but a bad valve seems REAL likely. D-OH
 

atom4488

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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....
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
 

Mannyroad

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OK, hear your comments re valve lash and accept points. As for the cam, this is one concern for me in that the guy in Oz had no choice but to close the business due to major health issues. I've tried to make contact but am unsuccessful so far. So I'm not confident I can go to the source anymore for cam timing info. Worries me a tad.
So yes, am going to re-check tdc and 15\40 timing marks, check valve clearances and then more to compression and leakdown tests. The carb slides open fully and K&N's are 1500 miles old. We shall know a little more soon. Keep you all posted and once again, many thanks for all your inputs.
 

Mannyroad

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Also, my rephase is a true 270 degrees, not the usual 277 degrees afforded by the Yam 13 spline cranking. So I'm assuming that a different rephased cam will not be right for my motor in any event. Correct??
 

atom4488

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Can't be too far off setting valve lash at .003" and .006"
I'd check vavles and cam chain lash then check compression. You should be over 150PSI

Physician heal thy self? Been wasting way to much time trying to start a "ran when parked" XL500S, pretty sure now the motor's FUBAR, going to go check compression, but a bad valve seems REAL likely. D-OH
gggGary, recent experience with a "ran when parked" XR250 R that may inspire further checking... engine would not start despite presence of fuel and flame...compression seemed good through the kickstart, but was actually very low. Disassembly of the top end revealed rings firmly stuck in their grooves. Work with PB Blaster and a dental pick freed them up. Engine was reassembled with the now-freed rings and fired on second kick. Runs like a champ to date!
 

jpdevol

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Two cents: Yamaha spec'd .012" as lash when setting cam timing - just to have a std./known value. When finished, reset to 3 & 6.
The lift and duration may be different on the regrind, but the relative timing should follow stock - with one cyl off 90*.

So, degree wheel on the rotor, dial gauge on the valve spring.....

Printable degree wheel with ability to change valve timing shown: https://www.blocklayer.com/degree-wheel
 

Mannyroad

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Nice bike, something is wrong it should rev higher than that with stock gearing. Try a run without those air filters. Some filters that look like the ones in the photo are known to cause problems. The problem is with the mounting flange blocking air circuits. Better air filters are K&N or Uni pods. Unipods are ok but not as free flowing as K & N.
Does your mates dyno have a fuel air meter, you will be able quickly see what needs to be adjusted.
They are k&n filters and not sure what my mates rolling road has. Will ask him.
 

grizld1

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There is absolutely nothing wrong with running K&N filters with your carburetors, and I can tell you that from experience. Problems arise when owners remove the still air box from vacuum diaphragm carbs and install K&N filters that are too small to provide the volume of still/damped air that vacuum lifted slides require for stable operation.
 

jpdevol

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You're welcome.
I don't think that .012" blather is in a Serv. Manual (from memory) - figured I'd better post relevant Bulletin
18891-1405859197-73ab3e5a3c60b2034534371dba1c9d7c.jpg
 

grizld1

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Check your email for the carb manual and let me know if it got to you or got lost in the ozone. Onward thru the fog....
 
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