new owner...need help

[mobilebay750]

Bought a cheap craigslist find 91 CB 750 Nighthawk with the plan of striped down Cafe Racer, however I need to address a laundry list of issues and could use a little "insight".
 Bike has been ABUSED, period.  All cables haven't seen lube in 20 years. Clutch is very tight, throttle cable sticks (fun to ride) head gasket "weeps" oil in front. Fork seals leak horribly rear shocks are totally rock hard and shot. ZERO rebound. Turn signals are held on with electrical tape and string (really not kidding) gauges are shot no functional tach or speedo. Steering head bearings long been shot.
But the most "worry some" issue is a "thump" or bang that comes from the area of the tranny during acceleration. as you move up through the gears it comes through the footpegs (and its audible) thump...thump....thump...thump. doesn't seem to speed up with the speed of the bike and when you pull the clutch in and coast.....it goes away......
Could this be an issue with an incorrectly installed speedo or tach? (since they don't work)
Or a sign of a bad clutch, or bent clutch plate?
Has anyone experienced the horrible lower bearing detonation ?  Could it be a sign of the bearings going south????

Love the bike, it is so light and nimble compared to my 05 Triumph Tiger I feel like I am a kid on a mini bike again   

Any help would be MUCH appreciated.

[geemann]

first off, welcome to the NHF!

secondly, please stop riding until you address some major safety issues like unsticking the throttle cables and greasing up the steering bearings!

I am a huge fan of penny-tech and saving money, but it sounds like it will be a better idea to replace both throttle cables. Also, get a clymers manual, because you are going to need it when you start tearing into the bike for repairs.

for the "thump" sound, is it rythmic or sporatic? does it thump while idleing? when riding, does it thump with the clutch in and the gas rolled on a bit?

i don't think that sound would be caused by an poorly installed speedo/tach.  you can test the tach by unscrewing it from the front wheel and hooking it into a powerdrill. if that doesn't work, it might be time to check out the aftermarket world.

congrats on the steal of a deal!  If you do not mind, what did you pay for it?

also, we need pics!  it helps us with our little NH addiction...

cheers,
-g

[mobilebay750]

Thanks geeman for the info and yes she is parked till repairs are made.  I did manage to ride it home from purchase (as I was told by the p.o. it was a "solid bike", obviously "solid" meant the shocks didn't work and ride was rock hard!)

Will get some pics up today.

The "thump" ONLY occurs when in gear. It is consistent thump thump thump while clutch is engaged if the bike is in neutral (on centerstand) and back wheel is spun ....no thump (I assume this rules out the chain or sprocket originally I thought the chain may have a bad spot where some links where stuck or rusted tight but that would occur while clutch was disengaged or in neutral) going down the road at any legal speed its there thump thump thump --pull the clutch in and coast---its gone, quiet smooth-- engage the clutch its back.
Possibly a bent clutch plate or bad clutch spring????  I guess it could be the drive shaft is bent........I was told (on a different brand bike forum) these engines have a lower bearing that can go out and basically detonate the engine but I (with limited engine mechanic knowledge) don't see how a lower bearing would make this thump thump thump.  It doesn't appear to increase or decrease in cadence with the engine, as I would assume a lower bearing issue would.

All ready on line "sourcing" various things......the first being the Clymer manual---ordered yesterday from Amazon.co.
Paid 942.00.
Here is the current pic as promised..............

[geemann]

looks pretty good!

i think you are right to suspect the clutch since the thump comes and goes with neutral.  however, i am not so sure it is a bent plate. if you have a bent clutch plate then the thump sound would more likely be felt a a vibration that would get increasingly worse as you increased rpm. is this the case? or is there a thump about once per second? or 20 times per second? is the thump the same rate for all speeds, for all gears, and for all rpms?

just out of curiosity, when the bike is on the centerstand, is the thump still present when moving through the gears?

cheers,
-g

[creaky]

Check the chain and sprockets. If they are badly worn, that will cause plenty of vibration and crunching/grinding sounds from the drive sprocket area.

[mobilebay750]

HEY CREAKY    ........thanks for the sprocket chain info!  But I all ready checked that as per ADV forum My "handle" over there is keystiger(used to live in the FL. Keys, and I ride a Triumph Tiger).  lol

Too funny.

geeman thump is about a second apart. doesn't increase or decrease with acceleration.  But goes away entirely when the clutch is pulled in. On road or on center stand.  Result is the same.  Had thought maybe it was a "cushdrive" issue in the rear hub, but that would again, increase and decrease with speed. ANd it really feels to be coming from the area of my left foot, more than my right.

[creaky]

HEY CREAKY     ........thanks for the sprocket chain info!  But I all ready checked that as per ADV forum My "handle" over there is keystiger(used to live in the FL. Keys, and I ride a Triumph Tiger).  lol Too funny.

 .....small world

[geemann]

i feel like we are dealing with a bad riddle... strangely i am excited by this.

so we are looking for something that would cause a rhythmic thump that is independent of engine speed but directly related to the clutch operation and can be felt more through the left footpeg than the right.

i'm not backing down yet, but i need some time to think about this a little more. maybe i'll catch a eureka moment while at work.

until then,
-g

[geemann]

it is not quite an eureka moment, but i think we can eliminate any component from the transmission since both input and output are dependent on engine speed and any bearing in direct contact would have a corresponding change in the rhythm of the thump.

the oil pump cannot be the culprit because it is also dependent upon rpm.

could it possibly be something in the starter (motor, chain, clutch, etc)? perhaps the gearshift drum?

[ariwhiteboy]

Have you pulled the plugs and "read" them yet? I'm grasping at straws but it might give us some insight into what's going on inside the cylinders. 

[mobilebay750]

I have not pulled the plugs.... The head gasket weeps so I ordered a new one.  Figured I would check those with the tank off for the head gasket repair.

It is certainly a riddle.  I have owned over 20 bikes in the past 30 + years of riding and never encountered this. It (the noise) does not seem to affect the power...shifting or handling of the bike that I can tell.  It just makes "noise" .......which is rather a confidence killer as you zip down the road dodging soccer moms in suv"s!!!!!!!!    Is this the thump thats gonna lock the motor ......or this one.....or this one........

I am affraid I am going to have to start removing covers and plunking around inside the engine........when I get the Clymer manual.....I ain't that brave.

I do know (from onine research) this bike has a "sprag" clutch could that be involved?   I wouldn't think so as again that doesn't seem like it would be clutch adjacent in a way that could cause a "thump.........very odd......

[mobilebay750]

O.K. just a guess, however I took her for a little spin today, just in the neighborhood, BUT the thump did increase a bit with acceleration. AND I went sans helmet ( I know I know)  but I wanted to HEAR every sound, and I think it is coming from the cush drive area.  ALSO don't know how it factors in however I have run it on the center stand 6 times now...get this, first time it made the noise, other 5 times no noise when on center stand EVEN shifting up through the 1st. 3 gears.

ANy thoughts gang?   I am leaning towards the "cush"   I had an issue with a cush drive on a single sided swing arm Triumph Speed Triple awhile back, not exactly the same thump just more of an "odd" uneven tug in the torque when I cracked the throttle.  So I was not sure if this was the same thing, I would assume a single sided swing arm would act different than a double sided.

Thoughts????

[geemann]

dude, i am just gonna say this and then let it go:
a friend sans helmet = me

i am glad to hear that the thump changes timing with speed, because i cannot think of any part that would move independently of rpm.

as far as the cush drive goes, and if i am not mistaken* you would be more inclined to check the cush drive if you were experiencing hesitations in the throttle response and then hearing the thump.  since a cush's job is to prevent locking or skipping of the wheel by evening out the torque application.

i would suspect that if the sprag clutch is the problem then you would also experience the thump while you were shifting, and not just when riding.

take some really detailed pics of your drivetrain and post on up, let's see if we can't get a few dozen sets of eyes on whatever the devil this issue is.

by the way, i recommend doing as much diagnosis as possible before/without cracking the case.

goodluck,
-g


*totally and always a possibility

[mobilebay750]

Working 12's this week, sun up to sundown so its going to be the weekend before I can get some pics.  Just to save time, what areas or angles do you feel would be most useful?

I really appreciate all the feedback gang, I am truly baffled by this one.  ANd agreed geemann I would prefer AVOIDING cracking the engine open unless its a LAST resort.  Its one of those thing I "can" do......but HATE doing it.....kinda like repairing plumbing......    

[geemann]

mmm.... +1

12s + plumbing -> crappy!   

for pics, lets start with the chain line, all the teeth on the sprockets (no need to go crazy and get a pic of each individual tooth), the rear brake and foot control, the gear selector foot control.  basically, i am out of ideas for now as to what it could be* and i am thinking that we might be able to catch something silly like a missing bolt. just think of it like a good old fashion fishing expedition!

cheers,
-g


*unless honda secretly installed a metronome on your bike somwhere

[edit]: random idea just popped into my head.  pop your bike up on the center stand and use a stethoscope on the clutch area. i have no idea if this could be helpful, but i am feeling a little slap-happy right now, and it seems like a silly enough idea to actually work.

[mobilebay750]

I will get some shots tomorrow.   The vexing thing to me is the no noise on center stand, noise when on road......

I KNOW the shocks are toasted....like really toasted push down on the passenger seat, the bike stays DOWN ......for like 4 hours   


SO not sure if that could be an issue.   I got a great set of cush drive rubbers, and figure while I have everything apart I will slap em in just for fun.

I mean for less than 10.00 ?????   Why not?

I have seen guys (old school mechanic types) using stethoscopes (those are the well "to do" guys) and even long shafted screw drivers (low dollar shade tree sorts) to "divine" the noise from within an engine....I always likened it to the voodoo of Indians guessing how many cowboys are coming with an era to the ground....but thats just me.

[mobilebay750]

Workin on pics now.  Should have up today.

I have a new front sprocket and also, just to be 100 % certain I just ordered a new chain. So next weekend, new chain, sprocket and cush drive, plus shocks. Only things in the external drive line from the engine "back" that I can think of unless I am missing something................................

[mobilebay750]

o.k. geeman,
Here is what I got.






I notice while the bike is on the center stand, in neutral, and I spin the back wheel (by hand) the front forks "shimmy" left and right.  A steering wobble if you will.  I thought maybe it was "just me" so I put my '05 Tiger on the center stand and did the same thing,  no wobble at all.

I am seriously concerned this may indicate a bent shaft or axle though the rear wheel itself doesn't seem to wobble.......this bike is full of mysteries.

I don't want to run it much till I get this sorted however I did a few laps around the block and I am 100% certain that it has to be chain cush drive sprocket or clutch oriented in 1st. or second "get on" the throttle it (the thump) gets louder or more pronounced/noticeable. And not there at all when on center stand........so it MUST be in the drive line when stress/torque is applied............anxiously awaiting this weeks delivery of the parts and manual so I can get started on some real repairs and not just "tinkering".

[geemann]

"...this bike is full of mysteries."

haha! from this side of the internet, it is more like an inigma.

well, i did not catch anything from a glance over except that your clutch linkage may be in need of some grease.  if you were talking about the front suspension wobbling, then you may want to look into repacking the steering bearings. it will take you about 1/2 a day, but should help a lot!

i am starting to agree with you on the crush drive if the prob goes away when on the center stand.  since you already have a new one ordered, go ahead and replace it and see what happens.  while you are in there, give it a good clean and then re-apply grease where necessary, it looks like your driveline would appreciate it.

cheers,
-g

[mobilebay750]

Geemann
You have no idea how "dry" this bike is.  EVERYTHING that can or needs lube......NEEDS lube.  Never seen such neglect.
Frankly when I got to the bike to view it, I almost walked out on the deal.  But as my 1st. ever bike was a Honda (though I have strayed far away from them over the years) I just couldn't leave the little beastie to the fate that may have befallen it....... so I brought it home for some massive TLC.
I have the chain cush drive new front sprocket and loads of grease awaiting my weekend shade tree mechanic adventures......part one that is.

This will take some time. But I feel it will be well worth it in the end.

Oh I also have a set of rear shocks on the way. THe current ones do NOT work at all.....press down on the seat and in 11 min. (really timed it) they return.  ----amazing---   SO drive line and rear shocks are the 1st. order of business and then I will work forward from there.....hopefully it will clear it up.  I will let you know the results.  on a + note the Clymer Manual came today !!!!!!   

[geemann]

clymers is a good enough reason to do a little dance.   

you could not have choosen a better bike IMHO to be a labor of love.  goodluck with the weekend maintenance and be sure to take lots-o- !

cheers,
-g

[red78ta]

totally a shot in the dark here, but could the noise have something to do with the rear shocks being dead? I mean if the noise happens on the road but not the centerstand, and only when applying power to the rear wheel (in gear, not in neutral), could it be a shock giving what little rebound it has left repeatedly? 

No idea how you would even check that... just a shot in the dark...

[mobilebay750]

This whole bike is a shot in the dark.   

I will see this weekend when I change the shocks-sprocket-cush-and chain  I guess not a very EXACT answer, if I replace all 5 things at once and it goes away I will never know EXACTLY which of the 5 was the problem.........but I will be happy as pie its fixed and won't really care at that point!

If it persists ..................

[red78ta]

Go for it! It would bug me to not know for sure, but a short ride without the noise would help me forget about it and just be 

Good luck, let us know if it clears up!

[mobilebay750]

Well just an update "G" not finished yet.  I thought I had a chain press but in the last move it got lost.  So, new shocks, front sprocket and cush, cleaned (& cleaned & cleaned----oy what a mess) I feel I have traveled back to 1991......some of the bolts on this bike have NOT BEEN TOUCHED since the Manufacturer installed them!!!!!!  Man what a mess.  BUT, I am now awaiting my chain press to complete and hopefully settle the "thump"  mystery......a few more days of waiting won't kill me.....I guess.