Bike Ressurection: Engine Knock

[mack]

easy things

Try to do the easy things first. If it was my bike, I'd do two things right off. The first thing I would do is adjust the cam chain, if it's loose enough to slap then it's loose enough to jump a tooth and start bending valves. The second thing I'd do is get a strobe on that thing and adjust the idle into normal range. The motors have gear driven pumps and they require a certain rpm to push enough oil into the crank towers to properly lube the crank itself and the associated shells. Motoman will chime in with the minimum oil pressure at correct idle. The last thing you might need is a new vacuum pot if the rattle isn't corrected with the first two. Andreas Weiss will help you out there and 3 phase has some great servicing tips for the vacuum advance modules. There is a bearing in there that can get gummy and prevent the required free rotation. But I think it could be the pot itself.

[motoman]

Nothing magic, just that internal combustion engines in general require a min. of 7psi in order to 'hold' and maintain a bearing.

[mack]

Knew

you would be listening in. But you forgot the best part Brant. So at what RPM do the xs11's generate 7 lbs of oil pressure?

[IanDMacDonald]

you would be listening in. But you forgot the best part Brant. So at what RPM do the xs11's generate 7 lbs of oil pressure?

Hopefully never, but uncommon at tick over.

[cmarshall48]

I adjusted the chain, adjusted the idle up, checked my oil, and rode it about a mile, and beginning to become convinced that it's a vacuum advance noise. The bike rode like a dream and pulled hard in every gear, and I'm no expert but it seems to me that if it were something major, performance would be hindered. Also, after getting the bike up to operating temperature, I pulled one plug wire off at a time and noticed no change in the knock. The knock is also much more distinct at the timing cover and disappears anywhere over like 2000 rpms so I'm fairly convinced it is the advance. Or at least I hope it is... Should I just ride it and see what happens? Or is that a big risk to my motor?

[motoman]

you would be listening in. But you forgot the best part Brant. So at what RPM do the xs11's generate 7 lbs of oil pressure?

Actually have not a clue mack, but have to agree with Ian in his follow-up comment...lol.

[motoman]

I adjusted the chain, adjusted the idle up, checked my oil, and rode it about a mile, and beginning to become convinced that it's a vacuum advance noise. The bike rode like a dream and pulled hard in every gear, and I'm no expert but it seems to me that if it were something major, performance would be hindered. Also, after getting the bike up to operating temperature, I pulled one plug wire off at a time and noticed no change in the knock. The knock is also much more distinct at the timing cover and disappears anywhere over like 2000 rpms so I'm fairly convinced it is the advance. Or at least I hope it is... Should I just ride it and see what happens? Or is that a big risk to my motor?

no risk......but I would initially suggest going back through and re-setting those idle mixtures and re-sync for starters. If no change as to how vacuum can reacts, I'd have someone here send you a known good vacuum can for that vacuum advance, and at least be sure that vacuum can hose is hooked to the #2 carb itself vacuum port and not the vacuum port on the carb intake manifold.

[skids]

I adjusted the chain, adjusted the idle up, checked my oil, and rode it about a mile, and beginning to become convinced that it's a vacuum advance noise. The bike rode like a dream and pulled hard in every gear, and I'm no expert but it seems to me that if it were something major, performance would be hindered. Also, after getting the bike up to operating temperature, I pulled one plug wire off at a time and noticed no change in the knock. The knock is also much more distinct at the timing cover and disappears anywhere over like 2000 rpms so I'm fairly convinced it is the advance. Or at least I hope it is... Should I just ride it and see what happens? Or is that a big risk to my motor?

You can run the bike with the timing cover removed to see if the mechanism is bouncing. You could pinch while it is running to see if it makes a difference. If pinching the hose dampens it, I think there have been members here that installed a restriction inline with the hose (from an aquarium manifold?). If you take the ignition plate out, behind it is a weighted centrifugal unit that you might want to clean-up as they get pretty rusty.

[motoman]

BTW, if there's a voltage drop TO the coils, that erratic vacuum drop will also cause that vacuum can to dance and be noisey......BTDT also. Key on only, voltage needs to be the same at coil plug-ins as is at battery. In other words, if (key on) batt. voltage is 12.5v, then also needs to be 12.5v at coil plug-ins. If it is not, I suggest(key on) checking both sides of run/stop switch for starters, as that completes the running circuit. Any voltage drop there is from one side to other, dirty contacts in that switch.

[motoman]

If that voltage is fairly equal(within .2v), then check coil ohms by putting ohm meter across both poles of coil(s). Whether they are 1.5ohm or 3.0ohm coils, more than .3 drop in testing signifies a failing coil(s). A low voltage TO the coils will gradually cause them to reach that failing point(ohm testing), and they don't improve nor come back to their original rating.

[TopCatGr58]

Hey Marshall,

I don't have my service manual handy, ...okay...I lied, just checked it on my computer. If you have a vacuum pump/meter combo, you can attach it to the vac. pot, and apply 150mmHg, or 5.9in/Hg vacuum to it, and it should fully retract the lever, and should HOLD that position as long as you hold the vacuum. IF the pot allows the lever to slowly release and extend from it, then the diaphragm in the Pot is damaged/leaking ie. pinholes!

Also, check to see how much vacuum is being pulled at the carb body nipple at idle, shouldn't be much at all, but if you've set the idle too high, then the butterfly is more open and that allows more vac from the engine to reach the metered carb body port. Some folks have found that their port was DRILLED and enlarged so that it wasn't metered anymore!?

Folks have put a pilot jet down into the vac. hose to provide a metering/dampening affect to the vac....but this was more for when they put on newer carbs that didn't have the metered nipple, and they were attaching the adv. hose to the actual intake boot nipple!?

T.C.