Topic: Another cam chain tensioner emergency...

[Tacoma CB400f]

Quick back story. The bike is a 1975 CB400F, and I am a newb on most motorcycle maintenance, but mechanically inclined.

Quick backstory. Bike was ride-able but always had mechanical slapping noise at low rpm. Recently I tore down to rebuild carbs, adjusting valves, and other general service, and attempted my first cam chain adjustment. I used the static method and did not get any audible noise when backing bolt out, so I just snugged it and hoped for the best. I got bike back together and running, cam chain noise is worse than before this tear down (maybe carbs out of sync now causing it to be worse?).

Just for kicks I tried dynamic method and I can snug the bolt, or back it out while running, and it makes NO DIFFERENCE in sound. So I removed the adjuster bolt and whatever is directly under the bolt is seized, and i have been riding the bike for a while.

Should I be able to see anything in that bolt hole other than the blunt end of the adjuster mechanism? I assumed when I removed the adjuster bolt I would see a spring. I sprayed some PB blaster in the hole and have given it a few thuds with a drift punch and no movement. I was going to try heat next, although the engine was warm during my other attempts.   

At this point I would love to take it to an experienced Honda mechanic, but I really dont know where to go with one of these old machines. If anyone has a thought on that I am Tacoma, WA, but anywhere in greater Seattle area would be alright.



 

[Bodi]

The adjuster "horseshoe" is probably damaged from the loose chain and seized.
You only see the end of the rod under that upper bolt. The rod and springs go into the hole from below, there's a larger diameter bore where the springs sit. Access to the rod and horseshoe pivot assembly is via opening the main cases, not a small job.
Possibly you can adjust the chain though. Take the top bolt out and get a metal rod that fits in and hits the end of the rod. A large nail with the point filed flat is OK. You also need a hammer, figure out access to the rod/nail so you can beat on it.
Start the engine, get it warmed up to idle smoothly. Loosen the clamp bolt. Tap on the rod, lightly at first... you are trying to force the seized pivot to shift, it may be fairly loose or quite solidly jammed. Tap/bash until hopefully the rod moves.
You will not be able to loosen the chain if you go too far, so do not overdo it!!!
Tap just until the rattling camchain sound gets a bit quieter. You can very gingerly go further until it's not rattling at all but overtight is worse than a bit loose.
Tighten the lock bolt (be gentle, it's actually a weak 6mm bolt although you see a beefy 8mm bolt and nut on the outside). Replace the cap bolt. This should get you on the road for now.

A real fix, dismantling the engine and resolving the problem, is best.
If you can't move the rod at all to tighten the chain... then a proper repair is the best option.

[tuckholladay]

I am trying to adjust the cam chain on my 77 CB400f.  My clymer manual says just loosen the lock nut on the bolt at 1200rpm and the tension will take care of itself. 

Are you saying the part that does the adjustment on yours is seized?

Also the instructions say attach a tachometer to read the rpms.  Is the stock tachometer not good enough?

Sorry if I’m hijacking your thread, But I was literally just about to attempt this. 




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[Bodi]

The auto adjustment method is correct, I think the bike's tach is close enough.
But: after 40 years the tensioner actuator springs have lost quite a bit of tension and the auto method rarely works. Removing the blocking bolt above the lock bolt gives access to the actuator rod: adding a bit of pressure with a screwdriver down onto the tip of the rod makes up for the lost tension. Press down until the chain quiets, and tighten the lock bolt.

There is also a static method where you set the crank and cam angle to guarantee back force on the chain from valve springs against cam lobes, so all slack is at the back of the engine where the adjuster works. I don't recall the exact setting but it's in some manuals and someone here will know.

[awrawr]

If someone has over-tightened the adjuster bolt in the past, the tensioner plunger will have a dent in it. This means that whatever you do, the adjuster will probably always find the same dent when you re-tighten it. If it has been really  abused, the tensioner plunger will have been deformed by the pressure so that it has expanded width-ways in the adjuster tunnel and become jammed. If this is the source of your problem, (very likely) the only solution is to separate the crankcases and replace the tensioner plunger. (And don't ever let anyone else near it with a spanner or over tighten it yourself).

This a very common problem with the 400f

[awrawr]

PS. you might get away temporarily with the method described above inserting something like a nail down the tensioner tunnel from above and tapping it gently but you may find it just seeks the same dent afterwards when you tighten it. I wouldn't use heat or hit it through the adjuster pinch bolt hole. That woulD just flatten the plunger even more, enlarge the dent and make the problem worse.

[cjespersen]

As said above, The tensioner horse shoe could be seized and the pivot function of that part could be damaged. The chain rubs and cuts grooves right at the pivot of that horse shoe eventually locking it up. This guy on youtube has a great rebuild of that 400F motor and also talks about that cam chain issue.

[Tacoma CB400f]

Thanks so much for your input! After looking things over, the auto tensioner has been over tightened in the past, I can see a dent through the hole. I did not try the nail, but I may try that yet. What lead to this was my first major maintenance since owning the bike, and one of the issues I found was carbs needed rebuilt, so I did that previous to chain adjustment, and now I think they need synced. Bottom line is, I dont know if I'm hearing chain, or mechanical garbage from carbs badly out of sync. Tackle carbs first.

Thanks again, Scott in Tacoma!
 

[jakec]

You should better describe when the sound occurs and maybe even post a video. My bike, when its cold (right after startup) it makes a thumping/whacking sound at idle. I think this is just due to the heavy oil (I'm using 20W-50). When I start the bike I hold the RPMs around 2,000 for a while. Once the bike warms up the sound goes away.

I have never heard a noisy cam chain in person. But when you describe a "slapping noise at low RPM, it reminds me of my noise. Which sounds to me like cold jugs slapping cold, thick oil.

[awrawr]

When I balanced my carbs for the first time I was surprised to be able to get rid of a rattling noise which I had assumed was just the clutch rattling around . When I got them in balance the rattling noise completely disappeared, leaving just a beautiful even tickover. I've also had a noisy cam chain. I think the two noises are quite easy to distinguish once you've heard them both. That's not much help to you though, if you've only heard one of them. It may be possible to work out where the noise is coming from if you stick the handle of a large screwdriver on the bone behind your ear and put the tip against different parts of the engine (while it is ticking over on the stand) you should be able to hear the noise getting more distinct in different parts of the engine which could lead you to the source. As suggested above a video with sound may help diagnose it

[minimo]

I’m also dealing with some cam chain tension issues and ended up gutting my whole engine to investigate. I noticed that my cam chain push bar had been jammed down in way that like some of you have noted made sense to try and drift it out with a nail and hammer, which is kinda what I had to do with the good result of finally getting that sucker to pull out.


One of the bike’s previous owners had broken off the Bolt Tensioner screw and as a result of me extracting that screw I had inadvertently put a hole on the flat surface of the Cam Chain Bar...

Notice too that like some others on the forums I fabricated an adjusting bolt (that goes in to replace the upper Oil Check Bolt) to push down/free up tension on the cam chain bar which is supposed to be loaded with a couple of springs. Oh, and that bolt tensioner screw hole then just gets plugged with a screw fitted with a hefty o-ring and washer which I’m told gets very minimal amount of oil to collect in that part of the engine.

I think what was happening to my cam chain system was that the cam chain bar was binding as a result of the marring on the flat; it was getting stuck and not backing off to slacken tension. I believe I had the bar applying too much force on the cam tensioner (horseshoe) that gave me issues. There is now a good amount of wear on my tensioner slipper which I’m going to replace along with some other parts.


DSS has the push bars in stock and I am definitely considering a purchase or maybe make one
https://www.davidsilverspares.com/CB400F-SUPER-SPORT-FOUR/part_312618/


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