750/650 hybrid

Hmm so I begun noticing that my engine has two different part numbers, the block, head and upper crankcase are 5G2 which is correct for my bike, but the lower crankcase is a 4H7 I noticed today. I also noticed when I had the selector shaft cover off that the selector shaft/drum was marked 4H7. I have no doubts the crankcase was dissassembled at some point, there's some very old orange FIPG showing slightly around the seam in the crancase which I'm certain the assembly plant didnt use sloppy orange RTV.

I also noticed that the valve cover was painted black(there's some overspray underneath on the inside as well as some runs in the paint), and I also noticed there are reminants of black paint in the cooling fins on the cylinder-jug block.

I wonder if these could be the reason why I sometimes hear the cam chain hit the valve cover(confirmed by the marks inside the valve cover) and why I have what feels like excessive play in my drivetrain. I can rotate the rear wheel perhaps an inch or two by hand with the motor in gear. When I had the selector shaft cover off I noticed that the slack wasnt present in the rear end, driveshaft or final gear(I dont know what to call it, where two gears form a 90degree transfer into the driveshaft under the gear selector cover). So I know the slack occurs either within the gearbox, clutch or engine.

Anybody more knowledgable have any thoughts on this? I dont worry about the rear end slop so much as the annoying sound of the cam chain hitting the valve cover. I've replaced the head(I had it decked which Im sure doesnt help) so I know the tensioner works but I'm not keen on dissassembly of the crankcase just to replace the timing chain.

 

You don't disassemble the crankcases to replace the timing chain.

You "break" the chain; and re-rivet the new one.

Don't freak out about the part numbers. A LOT of internal parts common to the whole series will have part number prefixes different from your specific model.

For instance, I have 550 Secas. They're a "4U8." A LOT of the parts on my bikes have cast in numbers beginning in "4G0" which is the 400 Seca from which the 550s sprang. There are also some "4H7" part numbers on my bikes as well.

 

Does the chain scrapping the valve cover necessitate replacement, or is there a chance that the vlave cover may be shallower since it appear to have come off another XJ(it was silver and painted black, OEM for mine is black).

Thanks Fitz, youre the best as always

 

Yamaha painted the black covers black only on the outside and they had black overspray inside from the factory. That's no indication somebody did it.

I don't think there's any difference between the YICS 650/750 valve covers and a 550 cover wouldn't fit.

How many miles are on the motor?

Are you sure the automatic tensioner is functioning correctly?

What shape are the cam chain guides in?

You can do the "worn out chain test" the same way you do for a drive chain; see below. Grab a link pin at about the halfway point of the chain's engagement with the sprocket. Pull the chain away from the sprocket and if you expose 1/2 tooth on each side (or more) the chain is worn out.

 

I also have a patch where the camchain has been hitting the valve cover, though since the chain seems to have been replaced perhaps it was the old chain. I might well dress it back so that it is smooth, though.

My XJ750's exhaust cam has a worm on it for driving the tacho cable, yet there is nothing for it to drive. There are no signs that the cam is not the original. Maybe they had old cams left over, maybe the decision to run an electronic tacho was a late one.

 

There was no reason to manufacture a different cam just for the switch to an electronic tach (the 650 and 750 have the same cam profiles). You'll find that worm gear on all the XJ750 exhaust cams.

Production costs didcate that a manufacturer get as much use out of a part as possible. If you take a close look at any vehicle, you'll see parts that are used on other models from the same manufacturer. If each model required all the parts to be unique to that model, vehicles would still only be the playthings of the rich.

 

"There was no reason to manufacture a different cam just for the switch to an electronic tach (the 650 and 750 have the same cam profiles). You'll find that worm gear on all the XJ750 exhaust cams."

Okay, thanks. I had wondered, because there's no telling what you might find with a 33 year old bike!

 

I thought the 650 MAXIM had a milder cam profile than the 750's and the 650 SECA.

How much would shaving the head advance the cam timing??
Would "chain stretch" also advance it??

 

+1 I had my head decked too, I'll have to upload some pics but they shaved it scary close to getting into the valves.

I never realized the worm gear was for a tach cable, I thought perhaps it would help push the pooled oil into the return.

 

The cam profiles are different (I goofed on that), but the cam bodies are the same.

 

The cam profiles are different (I goofed on that), but the cam bodies are the same. In any case all of the 750 exhaust cams have the worm drive on them.