Topic: CB750F handling issue?

[rockandride6]

Hi all!

So, I have had two 750Fs for a while but embarrassingly, this is the first time I have actually gotten any sizable ride now that I have finished one of them.  While I am utterly in love, I did have a few scares due to handling.  The suspension is completely stock and the only handling related modification I can think of is that I have swapped out the bars for clubman bars.

Now, I understand I will have a lot less torque on the bars than the stockers which could potentially impact this, but the front end dances around like a twerking teenager over any bumps.  Is this just a result of not having as much control over the front end from the clubmans?  Is it common for the front end to dance around over bumps?

The only thing I noticed (while riding on the tank to look down at the forks) was that the forks didn't seem to cycle more than about 2.5 inches of travel, even over some good-sized bumps.

Any thoughts?

[PeWe]

Too much oil in forks? Out with old maybe dirty oil and refill with new fork oil

[flybox1]

old fork springs
old fork oil sludge
rim out of round/balance
bad tire/bearings
loose spokes
bent forks
....it could be many things.

[70CB750]

How tight are swingarm bushings?  And the steering head bearings?  Those two objects are critical for decent handling.

[seanbarney41]

which f?...f2/3 frontend is very different from f0/f1

[rockandride6]

which f?...f2/3 frontend is very different from f0/f1

Sorry, it is a F0 (1975).

The front wheel is true and the spokes are all tightened.  The tire is old and has a slow leak, but saving up to buy new ones shortly.

The forks on this could be all of those things, except perhaps the bent part (I checked them the best I could when swapping them out). I had been waiting on the oil and springs issue until I could save up a bit more money to buy new springs.

The bushings front and rear seem to be in good working order.  They are all in good shape and I notice no slop front/rear, left/right, or up/down.  The front end has remarkably little resistance when the front wheel is up and I turn the bars, but I just took that to be a characteristic of the bike.

[Jerry Rxman Griffin aka MuthaF'er]

How tight are swingarm bushings?  And the steering head bearings?  Those two objects are critical for decent handling.

Whether or not these may contribute to your problem an upgrade will NOT hurt and could just be your answer. All Balls steering stem tapered bearings and new bronze swing arm bushings. Mine has both new and it's rock steady. Of course there are other possibilities like tire and wheel bearings too. You have apparently eliminated the wheel and spokes already. At the least try tightening the front steering stem ball bearings until you get a little resistance.   

[johnny_dallas]

... The front end has remarkably little resistance when the front wheel is up and I turn the bars, but I just took that to be a characteristic of the bike.

Did you actually take the front-end off and check the headset bearings? I have a '78 F and had a slightly different handling issue (bouncing/pulsing). When I had the front wheel off the ground it moved smoothly, did not have any notches, and also sounded good with a stethoscope. When I visually inspected the bearing it was clearly no good; not much grease, and any that was left was old and hard. Replaced it with All-Balls tapered bearings and the bike handles way better. No more weird bouncing or pulsing and handles great.

If you still have the stock bearings in there they're probably not good and you might as well swap them out for new ones. Especially since new bearings are only like $20 bucks on Amazon.

[rockandride6]

Well....okay....

First off, I am assuming that when you unplug the drain hole on the fork, oil should not shoot out 3 feet and spray everywhere? Secondly, I assume that oil should actually come out after the initial spray attack?

Last night I checked my forks further and sure enough, I could barely get them to move when grabbing the front brake and pushing forward/down.  I don't know how I didn't check that before....ideas on why my forks could be so locked up?  I removed the spring on one of them and it *SEEMS* fine.  The oil was also the viscosity I would have expected for fork oil...not old nasty sludge.

Any ideas what could cause the forks to be totally locked up?

[Davidov]

Well....okay....

First off, I am assuming that when you unplug the drain hole on the fork, oil should not shoot out 3 feet and spray everywhere? Secondly, I assume that oil should actually come out after the initial spray attack?

No, but it will do that if the front end is weighted with the bike on the side stand.
You're suppoed to service forks with the bike on the center stand, or on a service stand.

Open the drain bolt, and pump the forks down a few times.

[rockandride6]

I know 

I had it up on the center stand with my ever-so-patient girlfriend sitting on the back of the seat to keep the front end off the ground.

When I then put it back on the ground and pumped, I could neither get the forks to cycle at all nor could I get more than a weak pump of oil out from the drain hole.  In case anyone is wondering, I am a 6'2" 200 lb. rugby player, so I was getting a lot of force on those forks. Haha.

[rockandride6]

By the way, most rugby players are sissies. Better have your GF pump the forks for you 

Haha   Definitely debatable, though the possibility of being injured in some way, and thus unable to push on the forks, is quite likely!

There was still oil in there I believe, as I was able to get more out once I uncapped the top.

[rockandride6]

Everything, to my knowledge, is stock. These were off my other 750F (since I am replacing those with modern forks) and were apparently gone through (and definitely polished) by the previous owner.

I ran out of time last night, so couldn't perform all of the tests I wanted.  While I never could confirm that I did get all of the oil out, after draining what oil I could from both forks and with both forks drain-plus open, I still could not compress the forks.

I know on a dirtbike I bought a few years back the PO had forgotten a spacer on the wheel/axle, thus pinching the forks and preventing them from really cycling smoothly.  I am wondering if that could be a culprit (though I double and triple checked while putting the front end back together last month).

[flybox1]

you can easily 'wash' out your old fork oil with mineral spirits....to make sure nothing is stuck in there.
drain bolt out and fork top nut off.  pour it in and watch the discolored stuff come out.  repeat until clean.
its not as good as a full rebuild, but it works well.

[rockandride6]

Thanks again everyone.

Any idea why I simply get no suspension travel when pushing on the forks?  I would think that I could get SOME movement after opening up the drain plugs.

I already told the lady not to expect me in the house tonight

[rockandride6]

Okay, update.

I got the forks drained, removed the spring, and took them off. The left fork cycles no problem and all seems good. The right fork won't move at ALL. When I flip it upside down, there is something loose in there (not sure if that's normal or not). Ideas?

[rockandride6]

I haven't gotten beyond removing the forks, spring, and oil. Candidly...not sure how to get in there.

[scottly]

The forks on this could be all of those things, except perhaps the bent part (I checked them the best I could when swapping them out).

How did you check the straightness of the fork tubes? Is the right side leg stuck fully extended, or halfway through it's travel?

[rockandride6]

You'd have to drop the wheel, loosen the triple clamps, slip the fork out and down, then invert it to what comes out.

Would "nothing comes out" be a bad sign?

And the forks are fully extended. My dad layed them out on a jig and checked them for straightness.

[rockandride6]

As a temporary solution, I am going to swap back to the known-good forks that were on there originally (now on my other 750).  Before I do so, I would like to set the Preload.  Suggestions on setting this up?

Thanks!

[Jerry Rxman Griffin aka MuthaF'er]

Typically done with installation of a PVC tube cut as a spacer, placed on top of the springs. Makes installing the top nut a challenge, but it's the best method. Now, to figure out how long to make it...?

And that's how my Progressive spring kit came.

[Oldtech]

I have a CB450 vintage racer with a late CB550 front end on it. When I first installed the front end it would chatter and bounce on rough corners and nothing I did seemed to fix it. Then I took off the front wheel, took out the springs and pushed the lowers up as far as they would go to check that they moved smoothly. The right side would only go up half way and then bind. The tubes checked out as straight. It was the fork lower that was distorted and binding on the tube. At some point the donor bike must have been in an accident although there was no indication on the outside. If your tubes are straight then I'd suspect one of the lowers.