400f Chain tensioner custom fix?

[wilber551]

ok guys, so I am new to the ton. But not to bikes. Been racing mx since I can remember. Decent mechanical skills (fix all my own stuff). I just recently bought a 1977 CB400f and a 1974 CB550 for my dad. so far the 550 is all good, but i had a bit of thinking to do on the 400f. Here goes;

tore the engine down to find loose cam chain so i tried all the tricks that i found on forums about the stripped nut and whatnut, but the cam chain tensioner is toast. I havnt split the cases yet, but i think i have an idea to fix it. since the cam chain tensioner pushes on the back chain slider to tension the chain, why cant i drill and tap a bolt into the top chain slider and put a nut on the bolt and make my own tensioner, give or take some modifications. this is my idea and i dont see why it wouldnt work? what are you guys thoughts??? thanks for reading!

 

[Kanticoy]

The section that you are pointing at is stationary. It is bolted to the inside of the head and doesn't move, nor is there any room for it to move in there. The bottom "horseshoe" tensioner acts by pushing the bottom of the slider up into a bow and arrow shape. It does this by relying on the top being bolted solid and stationary. I don't really see what you're talking about to fix it, honestly, but there aren't really any other ways to do it. What makes you think that it's toast? The main thing that happens is that the bottom horseshoe gets seized and won't allow any adjustment.

 

[wilber551]

its toast because i dont want to split the cases if i can make my own adjuster. i know the top stays stationary, but the bracket im pointing at is what i want to drill through and tap my own bolt thru and use that bolt to push on the slider to bow it out. if the bottom is siezed wouldnt that make it stationary now? one more question, does the motor have to be running to adjust the chain? that might be my problem because i didnt try to adjust it running


thanks!

 

[Swapmeet Louie]

its toast because i dont want to split the cases if i can make my own adjuster. i know the top stays stationary, but the bracket im pointing at is what i want to drill through and tap my own bolt thru and use that bolt to push on the slider to bow it out. if the bottom is siezed wouldnt that make it stationary now? one more question, does the motor have to be running to adjust the chain? that might be my problem because i didnt try to adjust it running


thanks!

I believe it does. I know a cb350-4 does....

 

[Big R]

I believe it does. I know a cb350-4 does....

Yep, has to be running.

 

[Kanticoy]

One thing you can do is take out what is labeled the"blanking bolt"and replace it with a longer one that pushes down on the plunger. My daily rider 400f is done this way since the PO broke off the original bolt on the bottom. You just have to have a nut that tightens down against the case with a crush washer to prevent oil seepage. Works really well.

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[wilber551]

thanks guys all the info helps alot. so just to clarify, the bike does have to be running to make the chain tensioner move??

ps. i will post pictures soon!

 

[Kanticoy]

Re: Re: 400f Chain tensioner custom fix?

thanks guys all the info helps alot. so just to clarify, the bike does have to be running to make the chain tensioner move??

ps. i will post pictures soon!

Correct. It has to be running to get it to tension properly.

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[wilber551]

finally got some pictures, these are about 2 weeks old, the bike is now benched and the motor is all shiny black with polished bolts, brass engine plugs, dyna coils and ignition, loaded gun rearsets, renthal moto 971 bars, custom rear section, regina chain, 2010 Honda CRF250 muffler, the list goes on, but this isnt the right forum for that, thanks for the help guys!

wil

 

[crazypj]

Bike does not have to be running to adjust tensioner, but, you would have to remove rocker cover every 3~4,000 miles to manually adjust it.
It's a half assed shade tree bodge, plus,if you push down on top seat, it could cause the exact same problem that the CB360 had recall for :
You could pull motor, strip, clean fix or replace the lower horse shoe almost as quick as screwing it up (no need to remove cylinder head )
The entire strip and re-build would be easy to do over a weekend, with air tools it can be almost finished in a half day by one person

 

[Kanticoy]

You can let the tension off of the adjuster roof but I always like to let the tension "bed in" while its running that way I can hear the proper tension as the can chain nose quietens. These motors always have some tappet nose, that's why they call them little sewing machines, but cam chain noise is obvious. I agree with you though pj I don't like the homemade tensioner, it is too likely to screw something else up.

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[Kwality]

This is not uncommon on the 400f. A lack of periodic tensioning of the camchain means that the loose chain flaps around and digs into the pivot point of the tensioner arm, causing it to seize. Very loose camchains can also dig/grind into the cases which causes little bits of aluminium to float around your engine in the oil - not ideal by any means.

To check whether your pivot is seized, loosen the locking bolt, and remove the blanking bolt. Then, with the engine running, insert a thin screwdriver down the blanking bolt hole. This brings it into contact with the top of the adjuster. If the pivot is not seized, then you will feel the adjuster rapidly jumping up and down. Applying downward pressure on the top of the adjuster translates the pressure via the pivot arm to the slipper, which bows and tensions the chain. You'll be able to tell if the sytem is working by sound: a loose chain is a noisy high-pitched rattle, so if pressure on the screwdriver lessens the rattle then retighten the locking bolt and you're good to go.

If the pivot is seized then you have decisions to make: you could try, with the engine off, (gently) tapping on the screwdriver through the blanking hole with a hammer to try to break through the burrs on the pivot arm that have caused the seizure, but then you could well have broken steel burrs floating around your engine. You could also live with a loose camchain and its associated problems. Or you could split the motor and refresh the system. David Silver stocks replacement cam chains, slippers and guides.

Good luck!