GS750 rod knock noise (again)?

[OzJavelin]

I've just purchased on EPay (sight unseen) a '79 GS750 "project bike" (unfortunately more project than bike).

It starts runs and rides OK, but when it warms up it seems to have what APPEARS to be a rod knock noise at idle? From what I've read on here this is almost impossible due to the roller-bearing setup in the bottom end? Supposedly the usual culprits are the starter clutch and the clutch itself?

The weird behaviors of this noise are that I can make it almost disappear if I lean the bike over to the right; when it's on the sidestand (leaning left) idling the noise is most apparent. I'm wondering if it's the weird cam-endplay issue? I can't hear it when riding but as you slow up you can hear it when you pull the clutch in and when you got to take off its obvious again (riding without helmet in my field) Any ideas where to start - i.e. don't want to pull down block/bottom-end?

Oh .. other thing was when I FIRST started it, it backfired a few times while running so I suspect the plugs/leads are on their last legs too ..

 

[Guest]

What you just described is a loose clutch basket.

 

[TheCafeKid]

What you just described is a loose clutch basket.

Might be, but that motor had straight cut gears, and since they dont side load, id say it MIGHT be the basket, but I BET its 8v cam walk.. Goes away off idle? It could also be, now that I read the firing issue, one cylinder loading up, and rapping on the crank. But Id bet cam walk. Though, I wouldnt completely rule out Bill's idea either..

 

[OzJavelin]

OK I can probably just try all the obvious candidates first; clutch, starter clutch, test/change leads and plugs, etc.
Two questions:
1. How do I check for cam-walk? Just measure the endplay? What's the max allow limit?
2. I have a workshop manual for a GS550. Will that basically cover me for the 750 for clutch and starter clutch inspection?

 

[waterman]

OK I can probably just try all the obvious candidates first; clutch, starter clutch, test/change leads and plugs, etc.
Two questions:
1. How do I check for cam-walk? Just measure the endplay? What's the max allow limit?
2. I have a workshop manual for a GS550. Will that basically cover me for the 750 for clutch and starter clutch inspection?

Go to BassCliff website and freely download a 750 manual.
http://members.dslextreme.com/users/bikecliff/

 

[srsupertrap]

I have to agree with CTK that the knock you hear at idle is most likely attribtable to cam end play especially if you raise the revs past 1500 rpm or so and the noise goes away. This problems seems to inflict the 8V motors as the miles pile up. Mine started making that awful noise at about 42K.

 

[OzJavelin]

Weird that cam end-play could make so much noise.
Anyone got a "fix", like a thrustplate or something?

 

[srsupertrap]

The answer is on BikeCliff's website, 49er wrote the following article below

http://members.dslextreme.com/users/storagecliff/images/cam_end_float_mod.pdf

 

[OzJavelin]

On old-fashioned V8s there is usually a thrust-plate on the end of the cam (OHV not OHC), but I've also seen some cases where a "bump-stop" has been fitted to the inside of the cam-chain cover to stop the cam/sprocket walking forward - typically the cam is kept in place by the taper on the lobes pushing it back, but overtime the taper deminishes and it can walk forward? I wonder if somthing can be added to the END of the cam to push it back?

Oh well ..

 

[OzJavelin]

Realised today that the backfiring issue was not a once off event; it's doing it when cold all the time. I think I'll be investigating ignition and carb first before looking at this noise

I'm assuming possibly crossfiring or something might be my first problem?