Valve Timing Check - Engine in bike

RobDat

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Hi guys, my first post.

I have a CB750K '76 US model. Lovely bike with just 10k miles (think it's genuine). Recently had it in part ex for a Datsun Z. I've spent a few weeks checking over the bike and sorting a few things before I tried to run it. I've had the carbs apart and checked/cleaned everything, ignition timing is fine (electronic upgrade), new plugs etc etc. Compression is 170,170,168,168. Valve clearances 3 and 4 thou (1 thou large after reading the internet)

The bike runs fine (static on stand) and seems ok however when I checked carb sync with a new set of vacuum gauges the reading is 10 (across all cylinders) instead of the advised 20 Hg expected. According to my research that indicates a valve timing or air-leak problem. I don't think I have air leaks and if I did would all cylinders be the same?

The other issue is that the slow running airscrews make no difference, I can even screw them all the way it. However the bike will tickover ok at 1000rpm.

So guys how can I check the valve timing without removing the engine. I have access to a dial gauge but I'd rather try to see the cam timing marks. What have you guys done and can I lift the cam cover enough to see the marks?

Do you think my problem is cam timing anyway? Taking the engine out is a huge issue just to check the marks.

Anybody used an endoscope (just ordered one) i.e. lift the cam-cover and insert a scope?
 
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dirtdigger

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The only marks for cam timing are on the point side end of the camshaft(cylinder 4 side). Crankshaft on T mark and the 2 index lines on the end of the cam should be parallel to the mating surface with the notch pointing up. Its in the shop manual. You may be able to pick the cover up enough to see them but you cant remove the valve cover in frame. You will probably ruin the cover gasket then have no way to replace because the cover wont come off in frame. I would doubt that it is off though if it runs good. The reading your expecting is for an engine in perfect condition at a certain elevation, temperature etc.
 

RobDat

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Thank you for the reply.

The engine appears to be in great condition.

What a shame there isn't an inspection aperture in the end of the cam-cover. I'm not going to do anything hasty but I'm seriously considering drilling the end of the cam-cover (but obviously being careful not to get swarf into the void - grease on the drill?). After checking the timing I could use a grommet?
 
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RobDat

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Update, problem over. Well it appears I never had a problem. I was reading the wrong scale on the gauges. I saw the green area and because that related to a figure of about 20 I assumed that was the correct scale. Wrong: cmHg is the outer scale so I'm fine. These gauges are not for motorbikes?
 

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dirtdigger

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To do it correct you need 4 individual gauges so you can see what all 4 are doing in real time. An adjustment on one cylinder will affect the others even if it is a small amount. All four being as perfectly even is more important than the overall vacuum number.
 

RobDat

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Yes I used 4 gauges. I'd got the balance very good by eye so they needed very little further adjustment. I balanced the chokes by eye too - they were out.
 

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