Valve timing issues 750 GT

R

riccardo chiorboli

New member
Location
south africa
Advice please. I am unable to set the valve timing spot on 750 GT. It appears to be a 1 tooth or half a tooth out either way. Cannot get it to line up on the gear/head mark. The motor has been assembled as a stripped motor. Work done new chain (tensioner removed) head torqued down using spacers new original gaskets cylinders resleeved cyl head rebuilt TDC using dial gauge this is not my 1st 750 engine I have rebuilt. As far as I can see it is all std and no mods done. Thanks in advance. UPDATE>> piston at tdc mark T at crank ,cam gear is 3mm off either side.

Admin note 8/1/26
Riccardo originally emailed with his query and provided a bit more background info as follows:
Hi, all new to the group. 750GT. Timing valve marks are not lining up. Engine stripped and assembled. Piston is at TDC using dial gauge the cam gear line does not line up, tensioner removed brand new original gaskets fitted and head is torqued down using spaces. At TDC the cam timing is half a tooth out it is either retarded or advanced cannot get it spot on. No mods have been made to the engine. Note I have rebuilt another same motor some time ago and have not experienced this problem. What's up? thanks in advance.
 

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riccardo chiorboli said:
Advice please. I am unable to set the valve timing spot on 750 GT. It appears to be a 1 tooth or half a tooth out either way. Cannot get it to line up on the gear/head mark. The motor has been assembled as a stripped motor. Work done new chain (tensioner removed) head torqued down using spacers new original gaskets cylinders resleeved cyl head rebuilt TDC using dial gauge this is not my 1st 750 engine I have rebuilt. As far as I can see it is all std and no mods done. Thanks in advance.

Admin note 8/1/26
Riccardo originally emailed with his query and provided a bit more background info as follows:
Hi, all new to the group. 750GT. Timing valve marks are not lining up. Engine stripped and assembled. Piston is at TDC using dial gauge the cam gear line does not line up, tensioner removed brand new original gaskets fitted and head is torqued down using spaces. At TDC the cam timing is half a tooth out it is either retarded or advanced cannot get it spot on. No mods have been made to the engine. Note I have rebuilt another same motor some time ago and have not experienced this problem. What's up? thanks in advance.View attachment 101740
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There are competent people on this forum, which is not my case.
Are you sure that the key between cam halves is in place and not broken?
Do you know what the camshaft is? From there, you can find the diagram and check it. The timing can be out from one cam half to the other.
Very precise measuring and work to get it right.
If it turns out that it's just the mark on the cam gear that isn't correct, you can slot the gear to adjust.
Paul
 
Paul Marx said:
There are competent people on this forum, which is not my case.
Are you sure that the key between cam halves is in place and not broken?
Do you know what the camshaft is? From there, you can find the diagram and check it. The timing can be out from one cam half to the other.
Very precise measuring and work to get it right.
If it turns out that it's just the mark on the cam gear that isn't correct, you can slot the gear to adjust.
Paul
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Thanks Paul that square key is in good order. When I assembled the head I placed it in the oven to warm it up at the given temp and the bearings/cam in the freezer, upon assembly there was no hassle all lined up as it should and evenly bolted it up. Thanks for your help much appreciate it I will just wait and see what others have to say.
 
A whole tooth seems more logical than half a tooth. Did you check if the PM mark on the starter clutch is in the right position?

Marnix
 
I’ve been reading this and the only thing I can think of is either the starter clutch is slightly lose/ worn key way or the cam is suffering in a similar way. Have you tried rocking/ rotating the motor back & forth slightly - maybe this might show up the problem.
 
Exmoor beast said:
I’ve been reading this and the only thing I can think of is either the starter clutch is slightly lose/ worn key way or the cam is suffering in a similar way. Have you tried rocking/ rotating the motor back & forth slightly - maybe this might show up the problem.
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Hi chain has no free play it is tight hence the new chain. All good.. I have done several 750 motors up this is the only one i have a prob with.
 
Exmoor beast said:
Have you had the head or barrel skimmed? If the head or barrel were shortened it would effectively make the chain longer, moving your timing mark.
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I don't think it would make that much diff - I ran my SF 1.85mm shorter (1mm off the head, 0.15mm base gasket) to up buretted CR from 8.5:1 to 10,25:1 and never had a prob lining up the cam sprocket mark.

Given the 'do I run it advanced or run it retarded' situtation you are in I think your best bet would be to actually go the extra mile (km :cool:) and check the actual cam timing at max lift with a degree wheel.

You'll need to know exactly what cam is in it and find the figures for that cam (I think the numbers in the green book are accurate?). Best to determine TDC using the piston stop method (google it) - can easily be 5 degrees off trying to ascertain perfect TDC with a dial gauge on The psiton.

Apart from the need to get it 'close' you could be pleasantly surprised at how well it will go when it's dialled in. You'll need to slightly slot the holes in the cam flanges - it runs at half crank speed, so a few degrees at the cam is double at the crank. Check bnoth sides to make sure they are 360 apart (180 at the cam). If there's a discrepancy worth worrying about you might want to go the whole hog and cut the square key in the middle so the L and R sides can be set independently. You'd be amazed just hiow inaccurate the setup can be - not that important on a Laverda harvester but definitely improtant on a moto!
 
I always use a wire pointer to the T mark at confirmed TDC instead of using the mark on the crankcase several cm away.
It is possible that when the crank was rebuilt that the centrepiece was put back in slightly wrong, but as mentioned that can be worked around.
Was the bike working ok before this rebuild now, or is this the first time it is put together with the rebuilt crank from 25 years ago?
 
riccardo chiorboli said:
Hi Piet yes it was about 25 yrs ago been shelved ever since not sure to what extent,it appears all the main bearings have been replaced.
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Could be that that's your problem. The centre journal may not be in the correct position, off-setting the timing at the crank.

The splines of the journal are cut randomly, the journal is then pressed into each of the inner webs, cutting those splines in the process. This makes each and every pair of inner webs unique, the reason inner webs and centre journal were never supplied seperately as spares, only as an assembly. The journal has a 15T sprocket, whereas there are 45 splines engaging with the webs. There are only 3 positions in total where a tooth will point directly straight to the crank pin, ie TDC, as it should. Any other position of the journal will cause incorrect cam timing.

piet
 
Tippie said:
I always use a wire pointer to the T mark at confirmed TDC instead of using the mark on the crankcase several cm away.
It is possible that when the crank was rebuilt that the centrepiece was put back in slightly wrong, but as mentioned that can be worked around.
Was the bike working ok before this rebuild now, or is this the first time it is put together with the rebuilt crank from 25 years ago?
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Hi Tippie the engine was a basket case, 25 yrs ago I had it sorted regarding all the sed engineering work from a reputable engine builder back then and shelved it still in pecises . Whilst the head was off i checked TDC and the pulley T lined up perfectly. As to answer your question it is the first time putting it together. As mentioned when the T mark is corresponding with the case (which it is TDC ) the cam gear now sits half tooth in advance..it measures 3mm from the 2 lines the head and gear. Please note the pic shows 3mm in retard I have now set it 3mm (half tooth) in advance.
 

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sfcpiet said:
Could be that that's your problem. The centre journal may not be in the correct position, off-setting the timing at the crank.

The splines of the journal are cut randomly, the journal is then pressed into each of the inner webs, cutting those splines in the process. This makes each and every pair of inner webs unique, the reason inner webs and centre journal were never supplied seperately as spares, only as an assembly. The journal has a 15T sprocket, whereas there are 45 splines engaging with the webs. There are only 3 positions in total where a tooth will point directly straight to the crank pin, ie TDC, as it should. Any other position of the journal will cause incorrect cam timing.

piet
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Wow thanks for that,is there a way around it ??
 
riccardo chiorboli said:
Wow thanks for that,is there a way around it ??
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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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