Topic: New to Forum - CB400F resto and front end issue

[rsharp-CB400F]

Hey all,
New to the forum but I have been a lurker for years. Just got done with a year long resto on my 1976 CB400F which I bought in 1982 but had sat since the mid 90s. I have pics attached for before and after, but they don't do justice to how bad it really was. Everything was packed with junk, frozen up or leaking. It did run but only on 2 cylinders and gas was running out the overflows on all 4 carbs. I took everything apart EXCEPT the top end and cases, having done the top end back in the early 90s to fix a head gasket leak and update the whole cam chain issue. The engine did come out and get cleaned and painted, and all removable covers came off and got painted too.

I really tried to keep as much OEM as I could, just cleaning and repairing everything that was worn. New All Balls wheel bearings and tapered neck bearings too. Obviously the tank got repainted along the way, and I actually did two of those so I have a backup. Same with all the covers as I had another set from I scrap bike I salvaged back in the 90s too.

This brings me to where I am today. Bike runs great, pulls well and goes great right up to 30 mph. That's when the front end Pogo starts and it gets really a bit scary at about 50. I have only put 5 miles on it since I rolled it out last week and there is a bunch of stuff that needs touch up. I think the neck bearings are a little loose and I am going to go thru and check all the fork bolts, alignment, etc... I am also taking the front wheel back to the shop that mounted and balanced them to recheck balance because I really feel like that is a lot of weight on there. Also those are Shenko 712 tires and I have seen some reviews that mention this issue.

My problem is that I have made the mistake of literally touching every part of the bike all at once. Forks were rebuilt, but there is not much to that except cleaning, inspecting and reassemble. The shocks are original and therefore by definition shot, so I will likely play with them a little just to see if they are contributing but other than rebalancing and checking the roundness of the tire I don't have much to go on. I know the rims are round, I checked them with a dial indicator and they were good before the rubber went on.

So, what I need to do is rule the front tire out or in. I am desperately  seeking a CB400F in the Raleigh NC area that is running reasonably well already. I will gladly bring my wheel to  you and  you can ride it around the block and see if it starts to hop. I would offer a case of beer or lunch at Redneck BBQ in return for your trouble.

[Ozzybud]

Welcome to the forum

Please define "Front end Pogo"  does it bounce up and down?

I would suspect an out of round or unbalanced tire.
What viscosity fork oil was used?

[BenelliSEI]

Welcome and what a lovely job you’ve done! Your bike looks stunning.

When I first rode my 1976 cb750K6 (after many years off the road and full rebuild like your 400F) I had the same front end symptoms. Here’s what “Hondaman” suggested:

 “the 'pogo' feeling in the front end likely came from the inner dampers not yet being filled with the oil. If you check their level now, you will find them about 3.2 ounces low, which is the oil that stays inside the inner damper when the post-3/1972 forks are drained. That's why they are always so messy when the lowers get removed to replace a seal: they are stingy about giving up that inner oil!

This only happens with the forks after the 3/1972 change. The earlier dampers have external oil valving on the fork tube, which lets air into the lower end of the tube when you drain those - and that's why it takes a long time to drain those, as the later ones drain 'quickly' because they are still harboring about 30% of the oil. ”

I rode it for 10 more kilometres and the bounce completely disappeared!

[BenelliSEI]

Also….. Carefully inspect how the new tire is mounted. It is very easy to get it off centre. Go around both sides of the rim and ensure the bead is evenly seated on both sides. More than one installer has left a portion of the sidewall pinched down, too far in the rim. This usually results in a “wobble” more than a “bounce”…..

[scottly]

With the front wheel off the ground and the brake caliper removed, the wheel should spin freely. Give it a spin, and let it slow to a stop. Mark the bottom of the tire, and spin it again; if it stops at the same place every time, the wheel is out of balance, and that is where the heavy spot is. Also, when the wheel is spinning, look for any out of round or side to side wobbles. A pointer held against a fork leg makes any deviations more apparent.

[newday777]

Welcome aboard the forum rsharp.
Nice job bringing it back to life.
How long has it been since you rebuilt the forks? ( I don't like to assume, not a good practice)
Did you measure the spring lengths, did you make preload spacers? Did you make both total spring and spacers lengths equal?(very critical)

Oil level is very key in forks and as John has said if you didn't prime the forks to push the oil into the inner dampers and air out you won't get a good ride.
I have was taught by a suspension guru when filling with oil, (springs not in), put the fork lowers to stand on the floor, palm of your hand on the top of the fork tube to seal as you push down to prime the forks, when it bottoms release the pressure and pull the tube back up, repeat 3-4 times to get all the air out.
The other critical thing is to ensure oil level is equal to the top of the fork tubes, a fork oil suction tool helps greatly to do this.

[newday777]

With the front wheel off the ground and the brake caliper removed, the wheel should spin freely. Give it a spin, and let it slow to a stop. Mark the bottom of the tire, and spin it again; if it stops at the same place every time, the wheel is out of balance, and that is where the heavy spot is. Also, when the wheel is spinning, look for any out of round or side to side wobbles. A pointer held against a fork leg makes any deviations more apparent.

💯

[rsharp-CB400F]

Thanks for all the responses. I am surprised at the whole filling the damper thing. I did go out yesterday and bounce it up and down a few times, maybe I need to be a bit more persistent. I am going to look at the whole tire roundness thing while I am playing with it and tightening the neck a tiny bit. Fork (and everything else) were just redone, forks being close to last about a month ago. I went with 7 weight fork oil after much back and forth, I just could not bring myself to use ATF as specified and I figured I could drain the 7 and add back 5 if it felt too stiff. Just bouncing it in the garage it did not feel too stiff, but need to ride it aggressively to be sure. Tomorrow is work on bike day, I will keep you advised.

[rsharp-CB400F]

Oh, and fork springs were the same length and within spec to the book, it appears I did not write down the actual number (darn).

[rsharp-CB400F]

Quick update - looks like the tire balance is good but I am measuring .080 inch total out of round (+/- .040). From what I am reading that is really pushing the limit. The rim is less than .020 total on the ID, when I measured the rim on the bead surface before mounting is was better, closer to .010 / .015 total. At the same time I did loosen up all the various fork bolts, tweaked the head bearing a tiny bit tighter and then rattled it around before aligning and retightening. I have also been bouncing it a few times every time I go by to try to make sure the dampers are filling. Will get another longer test ride in later this week before I go give the shop a chance to remount and recheck.

[scottly]

There was a member years ago that had similar symptoms of the front tire bouncing up and down, which got worse the faster he went, also limiting the speed to 50MPH. To make matters worse, he was on a cross-country ride with his new bride while on their honeymoon. While everything pointed to an out of balance/out of round tire, everything looked OK to everyone that looked at it along the way...
The problem was eventually found to be loose spokes: with the front wheel off the ground, the wheel ran true, but with the weight of the bike and rider, the wheel shifted as it rotated from the side with the loose spokes to the side with the tighter spokes, if that makes any sense. A simple test is to get the front tire off the ground, and "ring" the spokes with a small wrench. The tighter the spoke, the higher the pitch, just like tuning a guitar string. If a spoke only makes a thunk, it's too loose.

[M 750K6]

I had a vibration on the front. Finally resolved when I found a very slight bend in left hand stanchion.

[MauiK3]

Use a spoke torque wrench if you get into the spokes, it helps.