750SF cam timing is off

TeddyG

TeddyG

Hero member
Location
Portland Oregon
Well, after chasing down several possibilities for the poor engine performance and excessive vibration of my 750SF, I finally took heed of advice given by Forum members, and I removed the cam cover (after tilting the motor). With the TDC marks aligned on the engine case, there is no mark visible on either side of the cam gear. But further down a few teeth, the master link is visible with no doubt the timing mark. What could've caused this- a loose timing chain or poor work when the engine was supposedly rebuilt previously?
So my question is how to advance the timing just on the cam gear, while keeping the timing aligned on the engine case. With 15K miles on the odometer, I'm assuming a new timing chain is in order.
Next will be removing the cylinder after seeing excessive wear using a boroscope.
 

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It's not enough to simply align the TDC mark in the crank. You also need to be on the correct stroke (remember that a 4-stroke has a 720° cycle). If no mark is visible on the cam sprocket, you may be at the end of the exhaust stroke. Turn the crank another 360° and hopefully the timing mark on the sprocket will appear. It should point (roughly) vertically upwards and align with that mark on the rocker cover mating surface.

Don't worry about the location of the master link. It has nothing to do with cam timing. It'll be in a different spot each time you turn the engine over. The only time you need to be concerned about the location of the master link is when you go to split the chain. You need it at the top where you can get at it. You'll likely have to crank the engine over several times to arrive at a condition with timing marks aligned and the master link at the top.
 
What Cam said. I'm also looking at that split link and think I'm seeing the spring clip is on the wrong way around.

Anyways, rotate the crank 360 and see where the mark is. if it does need to be retimed, as Cam said, just keep rotating until the split link is pretty much at the top and the mark is visible. Remove the split link (rags down the cam chain tunnel). Once undone, secure the ends with wire and move the cam wheel until all marks align - refit the chain, with the split link open end TRAILING (ie open end at the back ).
 
The mark on the sprocket will be facing either straight up or straight down so do as said rotate the crank another turn. With the tensioner removed, to open the chain you sometimes need to compress the head down a bit to get some slack in the chain by using spacers, washers or whatever. If you can rock the crankshaft fwd/back a bit without moving the cam you have enough slack. As said, with rags in place draw out the split link enough to lift one side of the duplex and get a length of wire in each side for safety.
 
Once again, you all are spot on- after rotating the motor another 360 degrees clockwise, the cam sprocket timing mark is at 12 o'clock.
But after all the work I've done to rebuilding the carbs, etc, etc, the motor is still vibrating excessively and struggles past 60. I'll try to post pics of the cylinder wear marks I'm seeing from using a boroscope. 2 previous compression tests w hot engine showed 130 lbs each cylinder.
 
Check the tappet clearances. These engines can suffer valve recession and loss of clearance run on lead free fuel. When cold you'll get good compression that then goes away when things get hot.
Very worn cams can give the same lack of power symptom, use a dial gauge to see that all valves open as expected.
As for "vibrating excessively", compared with what - they do vibrate!!!!
 
Arancia- Thanks for that info. I've checked the valve clearances twice before and they're in spec. Not sure how to use a dial gauge though. The motor revs freely at start up/idle w no stumbles or flat spots, but struggles past 50-60. I'm not accustomed to an old parallel twin, so the vibration I'm feeling must be normal.
 
The twin specialists will be able to help out here, but I recall that the engine could have been ‘tuned’ with, for example, SFC Electronica pistons which would drop the compression ratio heaps?
 
Have you looked in the carburettors to see that the slides are actually opening completely? This is an odd one. Max I have seen SF with wrongly fitted electronica pistons, still went like a normal SF, as do bikes with very worn motors. Something specific is impeding this bike.
I had an SF2 in once that struggled with speed, the fuel tank had been falling back onto the seat and the fuel hoses were kinked almost shut as a result, as well as it's front brakes were binding. The owner had it in for a rebuild it didn't need.
 
I had the fuel tank breather hose pinch off under a tank bag, it stopped the bike 4 times before I found it, 3 minutes later it would let the air pressure back and run till it happened again. It's hopefully a stupid simple cheap fix if you're lucky. BTW Tippie is right, Laverdas typically have huge movements required, more than one wrist twist to get full throttle to happen, check the slide completely clears the carby on full wack.
 
So if I understand correctly, once you got the cam wheel mark visible, it aligned with the marl on the cylinder head when the crank was at top dead centre? So you've eliminated cam timing as the culprit.

If the bike struggles to get of 50-60mph I'm not surprised it's vibrating badly. Something is impeding it, something isn't right, and that will cause vibration in itself - until it's rectified there's no point worrying about vibration. I reckon once you sort it you'll be astonished at how rapidly it goes from 60 to 115mph and how smooth it is compared to what you're experiencing now.

So it doesn't misfire, just won't go - correct? I'm stumped for ideas as to where to look next. I would have been a lot happier if you'd taken the motor out, removed the head and barrels, and had a really good look at the innards ... you could have checked valve and valve seat condition, bores for wear and parallel, rings, piston ring lands etc etc, broken piston rings ... you got half way there and now you're back to square one - that must be frustrating! You will just never know until you look further.

If I'd bought a used bike and didn't have an absolutely clear record of its condition from a trusted previous owner I would probably pull it down for peace of mind. Don't give up - it'll be worth the wait when you experience a good SF!
 
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Timing correct on BOTH cylinders? Full advance/TDC marks at sprag clutch coincide with BOTH pistons, ie, crank phasing? Is the woodruff key positioning the sprag clutch intact? I've found these sheared and the timing set way out from where it should be. Cam phasing? Check cam timing on both sides, as suggested. Check valve LIFT at both sides for worn cam lobes.

Differences will all cause undue vibration with a more or less loss of power.

VHB slides can be fitted wrong way 'round.

If all this checks out OK, tear it apart. Probably shagged beyond belief.

piet
 
Thanks again everyone,

The bike is still apart w the cam cover removed, and I plan on inspecting the cylinders, etc, next. I'll use a friend's boroscope to look at the cylinders again today and post pics if possible- last time I saw white wear streaks and scratches. I will also try the other suggestions- so far the cam lobes appear to have no wear marks. The carbs have been completely rebuilt w new gaskets/O rings/slides/float level checked/synched/new fuel lines/spigots. The bike instantly starts up w/ even idle, but has poor acceleration past 50 mph- motor doesn't feel like it has any more power and is straining.
 

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In your profile pic the bike has what look like original Conti mufflers. They should be straight through, i.e. you can poke a broom handle all the way up into the header pipe with no restriction.
 
Apologies - I got the impression you found the cam marks all aligned and put it back together and (obviously) still had the same problem. All Piet's suggestions are valid.
 
Thanks guys- Tippie- it's got a Malcolm Cox exhaust. Here are some of the boroscope images. I see the normal honing/cross hatching pattern, but the "striped" wear mark lines, pitting, and scratching look bad to me. The piston crowns appear clean w/o carbon deposits. My plan is removing the cylinders for further inspection/evaluation. When I received the bike w/ 15K miles, I immediately drained the dirty oil that contained some meat swirl, but no copper flakes. The cam lobes appear undamaged w no wear marks or scratches. The cam chain tensioner was left loose w no lock nut engaged, but I see no wear marks on the cam chain.
 

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Odd looking borescope images. There's some cross-hatching still evident so the bores don't look too badly worn. Those vertical streaks are a mystery. The cross-hatching is visible within the streaks so they appear to be some kind of staining rather than damage.

The last time I rode a 750 twin that wouldn't go past 60mph and vibrated a lot, it was only running on one cylinder. Start and run the engine for a maybe 20 seconds then check the exhaust header temperatures. If one is significantly hotter than the other, there's your problem. Note that even a dead cylinder will heat the header up a little because of the compressed gas venting into the pipe. It's basically an air pump.
 
Main jet size? (Mine cleaned up at the upper end, significantly, after leaning-out the main a bit)
 
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