4000 rpm knock

[Guest]

Hello all, happy spring.

I've been trying to figure out an issue I've been having and would appreciate some help.

As my handle says I own a 1982 GS550LZ(all stock). Over the winter I split the motor and freshened up the trany and top end. It's been running well except for a very annoying knock which rears its head around 4000 rpm. This issue was there before the rebuild and I thought it was gone but no it's still there.

I can't hear anything and all I get is a knock I feel on the handle bars and foot pegs. From 1100 - 3800+rpm nothing, 3800+ - 4200rpm knock, knock, 4200+rpm nothing.

Exhaust, engine mounts, gas tank, foot pegs, steering head, rear swing arm, bearings, forks, frame spot welds, all torqued to spec and or checked ok. Carbs have been synced three times, carbs adjusted to highest rpm, iddle screws approx 2.5 turns out.

I am aware of the cam-end float issue but I believe it's supposed to be heard, felt only at iddle?

I can't determine the cause/source of this knock and would appreciate your help in figuring this out, thank you in advance.

[tom203]

Does this happen with no throttle- as in coasting down hill holding engine at 4000 rpm? any gear? You're sure it's not tires, wheel bearings?

[BillGS550LZ]

Hello tom203,

It happens during accel, descel and while coasting, in all gears. Tires are new Avons RoadRider, the knocking happen with the old tires too. Front bearings were inspected and greased, swing arm bearings and bushings replaced, rear wheel rubber shocks and hub bearing replaced, other wheel bearings inspected and greased.

Thanks for the input. At this point I'm reaching randomly, I wonder if a miss fire or flat spot in the carbs is causing a cylinder to stop working intermittently and slap around? Idk

[BillGS550LZ]

Hello all, happy spring.

I've been trying to figure out an issue I've been having and would appreciate some help.

As my handle says I own a 1982 GS550LZ(all stock). Over the winter I split the motor and freshened up the trany and top end. It's been running well except for a very annoying knock which rears its head around 4000 rpm. This issue was there before the rebuild and I thought it was gone but no it's still there.

I can't hear anything and all I get is a knock I feel on the handle bars and foot pegs. From 1100 - 3800+rpm nothing, 3800+ - 4200rpm knock, knock, 4200+rpm nothing.

Exhaust, engine mounts, gas tank, foot pegs, steering head, rear swing arm, bearings, forks, frame spot welds, all torqued to spec and or checked ok. Carbs have been synced three times, carbs adjusted to highest rpm, iddle screws approx 2.5 turns out.

I am aware of the cam-end float issue but I believe it's supposed to be heard, felt only at iddle?

I can't determine the cause/source of this knock and would appreciate your help in figuring this out, thank you in advance.

[tom203]

".... wonder if a miss fire or flat spot in the carbs is causing a cylinder to stop working intermittently and slap around? Idk "


Have you tried hitting the kill switch off when it's in this 4000 rpm range?

[BillGS550LZ]

No I've not tried that and it would be easy enough but isn't that the same as pulling the clutch? Btw/ the knock stops when I pull the clutch. What should I be looking for when I switch it off?

Thanks for the help.

[mrbill5491]

No I've not tried that and it would be easy enough but isn't that the same as pulling the clutch? Btw/ the knock stops when I pull the clutch. What should I be looking for when I switch it off?

Thanks for the help.

The knock stops when you pull in the clutch you say? I would start looking in the clutch area then.

[BillGS550LZ]

Thanks for the input mrbill5491. I'll add it to things to look at. During last work I did I replaced the clutch (bucket, plates, bearings and countershaft) with one from a donor motor. It shifts through the gears like butter. Since this is my first time working on one of these motors I could have screwed something up so I will take a listen see if I hear anything.

Any thoughts on a possible intermitten miss fire, cab flat spot that could cause a knock?

Also, you are the first person I heard of being the original owner of one of these beauties. I don't think I really know what they are supposed to sound like when properly tuned or what the cam-end issue is supposed to feel/sound like. Is it possible the knock I feel is the camshaft sliding back and forth in the bearings?

Thanks again.

[mrbill5491]

Thanks for the input mrbill5491. I'll add it to things to look at. During last work I did I replaced the clutch (bucket, plates, bearings and countershaft) with one from a donor motor. It shifts through the gears like butter. Since this is my first time working on one of these motors I could have screwed something up so I will take a listen see if I hear anything.

Any thoughts on a possible intermitten miss fire, cab flat spot that could cause a knock?

Also, you are the first person I heard of being the original owner of one of these beauties. I don't think I really know what they are supposed to sound like when properly tuned or what the cam-end issue is supposed to feel/sound like. Is it possible the knock I feel is the camshaft sliding back and forth in the bearings?

Thanks again.

I don't know, never had that problem.

[BillGS550LZ]

Thanks for your time and input.

I went for a ride earlier and wounded out to 7500 rpms in 2, 3, 4th gear. Just as the tach approaches 4k I feel the vibration/knock starting, by the time it hits 5.5k rpm she vibrates and maybe knocks but at a much higher frequency and begins to scream as if the motor is going jump out of the frame.

Perhaps this is normal but I'm worried the motor is going to blow if I keep running it like this. I'd appreciate input from any and all fellow gsers.

Thanks a bunch.

[tom203]

"....Perhaps this is normal but I'm worried the motor is going to blow "

If this was normal, most of us would own hondas! Time to examine clutch area, since you said pulling in clutch stops knocking, right??

[BillGS550LZ]

I plan on taking a closer look at the clutch area as soon as i have time to open the side cover, which i'm dreading.

Also I was researching clutch issues and came across a few threads about twisted cranks. I'm going to look into this as well even though the current crankshaft is from a different motor and appeared to be in good shape. Murphy could be at it again I guess.

I'll let you know what I find out. Mean time your input/suggestions are very welcome so please keep'm coming. Thanks

[mrbill5491]

I plan on taking a closer look at the clutch area as soon as i have time to open the side cover, which i'm dreading.

Also I was researching clutch issues and came across a few threads about twisted cranks. I'm going to look into this as well even though the current crankshaft is from a different motor and appeared to be in good shape. Murphy could be at it again I guess.

I'll let you know what I find out. Mean time your input/suggestions are very welcome so please keep'm coming. Thanks

Kind of wondering now if you might have a bad 5th gear in the tranny? Once you pull in the clutch and unload the clutch and tranny it stops. Gotta be one of those or maybe even a combination? Don't know about a twisted crank, seems like it would knock all that time? I'm sure someone here has had a similar problem and can give you some suggestions.

[BillGS550LZ]

5th gear is is also from donor motor. One of the dogs had a slight chip on the leading edge not on the top but the lateral outside edge which I filed smooth. Aside from this the splines were solid on both gear and shaft, and gear teeth were smooth and clean. Definitely an area that could have worsen since except the trany seems reasonably quiet.

It is a shame after all the work i've put into this bike but feeling ready to throw the towel after reading the posts about twisted cranks. Anyways, I was just tinkering a bit. Pulled 1 and 4 plugs, rotated crank 90* from TDC and measured distance from top edge of spark plug hole to top of piston and I get a 1.5 mm difference between the two. I know these machines are not known for tight tolerances but I wonder how much imbalance/vibration/knocking, forgive my idiotic babbleling, this could cause?

How soon before she gives up her ghost (and possibly mine) if I continue to run it this way?

[zuzu]

Bill he states that it happens in all gears.

It happens during accel, decel and while coasting, in all gears.