Valve timing issues 750 GT

piranha-bro2 said:
Didn't know the crank had been apart - good pick up Tippie and Piet. I'd be slotting the cam flanges and setting L and R cam timing correctly.
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Hi, slotting the 2 cam flanges but leave the gear alone as to not slot it... ?? Key as to cut it flush as to not interfere with the cam gear.. Will do as piet says pull the barrels off and look at teeth orientation on crank.
 
sfcpiet said:
If it is indeed the issue, not really. The crank needs to come apart again, which can open another can of worms. Depending on how the centre journal was prepared prior to re-assembly, the splines in the webs may suffer, causing a weakened connection. In my experience, the press fit of the crank pins also often leaves a lot to be desired on cranks that have been apart. Taking it apart once more will only make things worse... but there's no turning back actually, you currently have no options other than altering the cam sprocket and flanges to suit.

Pull the barrel and remove the timing chain damper wheel. Rotate the crank so the rods/pistons are at TDC and verify there is a tooth of the crank sprocket pointing straight up towards TDC. The half-tooth difference at the cam sprocket should be apparent at the crank. The slightest off-set will cause incorrect cam timing.

Providing the centre journal has been replaced in the former L/R orientation, there will not be a phasing difference between left and right cylinders. The scalloped flange of the centre journal was always fitted to the primary drive side of the crank. You may be able to verify this when viewing from the crankcase mouth.

piet
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Thanks that is sound advice... just asking!! what will happen if I run it as is.?
 
Generally it is better to advance cam timing than retard it. If you slowly pass TDC rotating and check that all valves have at least 1mm clearance to the pistons I would do as Piet says, suck it and see. I have slotted my cam flanges for the studs and the duplex sprocket for the keyway and run more cam advance, my preference. Mine was a file job, any good machinist could do it with machinery. Re the cam flanges removing/refitting them weakens the fit on the cams, more risk of them loosening.
 
Tippie said:
Generally it is better to advance cam timing than retard it. If you slowly pass TDC rotating and check that all valves have at least 1mm clearance to the pistons I would do as Piet says, suck it and see. I have slotted my cam flanges for the studs and the duplex sprocket for the keyway and run more cam advance, my preference. Mine was a file job, any good machinist could do it with machinery. Re the cam flanges removing/refitting them weakens the fit on the cams, more risk of them loosening.
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Hi, how far advance did you run yours ?
 
As Piet and others have pointed out, just asking randoms on the internet isn't going to tell you much since nobody has the slightest idea what you've really got there.

The only way when dealing with a build like that which is coming together from a pile of bits is to go back to first principles and trust nothing. Use a piston stop and degree wheel and firstly check the phasing of the crank, note any variance from left to right.
Now measure the valve timing using degrees from 1mm of valve lift, these should vaguely compare to something from the cam timings which are available from the green book or probably on the interwebs here somewhere.
https://uk.laverdamania.net/OLD/cams.htm
Since the numbers are reliant on many variables and can get very confusing, I take the quoted figures and work out a lobe centre from that, then take my measured figures and work out a lobe centre of those. Then compare theoretical to actual to see how far out you are. Do not try to actually measure the point of max lift, it is an unreliable method.
Since it is very easy to get messed up with all the measurements and numbers BBDC, ATDC and so on, I either draw it out on a circle or these days cheat and use this great online calculator here:
http://www.wallaceracing.com/camcalc.php
So for example the SF 5/1 cam has quoted inlet opening 42 (BTDC), closing 74 (ABDC), and exhaust 64 (BBDC) and 52 (ATDC). That gives you lobe centres of Inlet 106 ATDC and Exhaust 96 BTDC. Put a dial indicator on your valve and note the point it has lifted 1mm from closed, and again 1mm before closing. By comparing the lobe centre not the actual open/close numbers, you take out all the various inconsistencies that may exist between how the factory measured those figures and how you are. They are all cancelled out as long as you take the degrees at the exact same amount of lift (around 1mm, but exactly repeatable). Only then will you actually know what your real timing is and how much of a problem it is. Just for fun, do it on both sides, allowing for any deviation from 360deg of the crank.
On a 750 of course inlet to exhaust is not variable and generally I figure it's more important to make the inlet right, but worth checking the exhaust as well just to sanity check and also possibly make the best compromise to be closest to factory figures as possible.

On my race bike I made a sprocket arrangement which allowed me to dial in both sides independently. One side was a vernier drilled arrangement to the sprocket and the other side was slotted and bolted to the first. It's fine to do away with the keyway, that is only for assembly convenience when not actually trying to set it up properly. The original three very slender (from memory M5) studs and nuts you can't properly tighten up aren't the best design. Again on my racebike I converted to 1/4UNF 12-point head ARP bolts each side which can be properly tightened with a ring spanner, but that's not something easily done with the original arrangement.
 
Thanks to SteveB you have a useful esson in checking/setting cam timing. It's a good skill to have. The piston stop method is dead simple for establishing actual TDC and the same principle is applied to valve timing (using before and after degrees to determine 'middle'). Nearly all cams will have max lift exactly halfway between the opening and closing figures. I haven't used Steve's 1mm from open method but I like it. I've done it using 1 or 2mm either side of max lift (dial gauge on the tappet adjuster or rocker). Both methods rely on the curve of the cam being symmetrical around either max lift or about-to-open/close.
 
Or you could do what I did, barter some valve timing for the construction of a clean room to build race engines in. Best deal I ever did.
 
riccardo chiorboli said:
Hi, how far advance did you run yours ?
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I just dug out my notes from the freezing garage. I ran 7 deg advanced from spec. This was on my race bike with a 6C cam I did for more mid-range. Check what your valve timing is as described by Steve (Breganzane). It is very important to check piston to valve clearance through overlap. I seem to remember having other SF motors that the cam-wheel mark was not in line with the mark on the head.
 
Tippie said:
I just dug out my notes from the freezing garage. I ran 7 deg advanced from spec. This was on my race bike with a 6C cam I did for more mid-range. Check what your valve timing is as described by Steve (Breganzane). It is very important to check piston to valve clearance through overlap. I seem to remember having other SF motors that the cam-wheel mark was not in line with the mark on the head.
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Lee,

The cam timing mark of the 750s is rarely more than 1 mm off, which comes down to about 2-3° in total. This will not make much difference in a road motor but may be of concern if searching for the last 0.1hp.

As the cam wheel has 30 teeth, 1 tooth equates to 12°, ie 24° at the crank. The OPs cam is a half-tooth out, so around 12° at the crank, more than enough to normally throw basic timing out the window. But, as this is all about a GT with "8" cams struggling to muster 45 ponies once the planets align, it may well be a pretty moot point. With the low and sedate lift of the cam there's not much danger of piston contact either.

Might even idle OK if the compression is low enough... :rolleyes:

@Steve, the studs joining the cams are M6 with 9mm AF nuts. They're a pita, true, but since ensuring the mating faces of the flanges and cam wheel are dead flat, as well as using Schnoor-type washers under new(!) nuts, I haven't had one come apart, not even under racing conditions. The press fit of the flange to cam has proven to be a major problem though, this must be carefully checked prior to assembly.

piet
 
Agree totally Piet. I (and probably you) have had SF come in for service with cam timing a full tooth out, running and ridden bikes with owners assuming that was just how they go.
My 270 motor has the flanges bolted to the chainwheel with allen screws and schnoor, no keyway through, just the flanges keyed.
 
When I was doing a machinist course for hobbyists at Bocky Tech (Box Hill TAFE by then) a guy racing Honda twins in classic told me I could adjust my cam timing and just bolt up the slotted flanges and it would hold absolutely no problem. Dead right. I would never have believed it. Amazing how strong clamped pressure over flat flanged surfaces can be.
 
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