Topic: 350 twin engine issue....

[mikedialect]

1972 cb350 twin

So, I've had this engine apart more than once. Valves were bad on the last go 'round. This time I replaced pretty much the lower case up. Used some of the same old parts, but checked all the bad ones. I finally have this thing back in the frame with carbs on it and everything ready to go and I go through the first set up tune. I get the tensioner and move on to do the valves...... none of the valve adjusters are moving. Not at all. This is a problem. A big one.

Any ideas? What the hell could have gone wrong here? Anyone have any ideas short of me taking this engine out of the frame?

[bryanj]

Cam chain broke? Does cam turn?

[mikedialect]

Cam chain broke? Does cam turn?

cam chain? Yeah, it turns. Engine kicks over. Starter cranks it. Has compression. It's the roller rods, or 'cam bearings' that adjust the valves that don't move. The engine was freshly put back together. Hasn't ever been run.

[754]

You can only turn them when the valve is closed..

try that..

 been a long time since I set a 350 valve...maybe around 76 ish..

[Raul CB750K1]

You can only turn them when the valve is closed..

try that..

 been a long time since I set a 350 valve...maybe around 76 ish..

+1. Furthermore, the rocker shaft is oblong, it has some kind of "cam" shape so when you rotate it the distance between the rocker and the valve stem changes. Obviously the locknut must be loosened. I don't remember but I recall, even with the valve closed, that you couldn't fully turn the rocker shaft, it had only a given amount of free play until the rocker touched the stem and then you couldn't go past it.


God, I should have today the DMV card for my CB350. The day of having it registered and road worthy is closer now...... 

[lordmoonpie]

Aren't the rocker shafts marked with a tiny dot? You can't turn them 360 degrees when installed - only about 180 degrees ad the position of the dot indicates what is a tighter or looser clearance? It's also harder to turn them when the cam is not in the right position - if you try it on full cam it'll be as tight as a gnat's chuff!

[mikedialect]

When i say I cant adjust them I mean that I've turned the alternator around several times checking positions and they don't move a millimeter. At this point I'm thinking that the cam is installed wrong. I honestly can't think of what the hell else could be going one here.

The rockers all move up and down except the left cylinder intake. That rocker never gets loose from the valve. And besides that, at no point when the rockers are loose or tight can I adjust the valve clearance.

[lordmoonpie]

Hmmm it doesn't sound good - and the 350 twin is a relatively simple beast to work on top end. Might be worth starting by taking it back to camshaft out and go from there. Has it got a slotted cam sprocket or plain holes? Sometimes a slotted sprocket can slip and cause issues but you'd hear a click if that happens.

[mikedialect]

So, I rebuilt another 350 twin that I had in my living room and I pulled it apart to see if I had the cam in right on that one - never ran it, just polished it and rebuilt it. I had the cam in WRONG on that one too! I bet the one in the garage has the same problem! Doh!

I'll update as I'll be pulling it tomorrow night and checking it out.

[lordmoonpie]

....ah but look on the bright side - you can now fix two motors at the same time - result! 

[mystic_1]

Would I get tarred and feathered if I admitted that, years ago, I  adjusted these on a CB350 by ear while the motor is running?  I found that if I held the shaft with a screwdriver, I could crack loose the locknut, adjust until the tapping noise was just gone, and then tighten.  It was fairly easy to tell when the clearance was correct, from both tactile and audible feedback.  Checked the clearances afterwards and they were right on.  Was I just being young and stupid?

mystic_1

[lordmoonpie]

Or young and gifted?!

I know a guy who adjusts his camchain the same way - run it up tight so it's whining, slacken it off until it just starts knocking and then tighten half way between the two! Not something for the faint hearted I think!!

I know a guy over here Clive Padgett who can do the same with 2-stroke race motors. He was an ex British champion contender years back and one of the gifted ones tipped for world stage glory until a crash shattered his arm and ended all that. One of his business lines now is renting out 2-stroke GP 250s for Isle of Man racers.

We replaced a carnk in an RS250 Honda and took it over to him to check, running and blipping the throttle. He was talking to someone as we approached. Before we got to him and said a word he turned road and shouted - bring it back when you've synched the left carb properly. We were completely benused so went back to the paddock to take alook. Sure enough, the left carb slide was rising 1mm ahead of the right - 1mm!!

He could tell that just from sound alone - amazing...

[tbpmusic]

Would I get tarred and feathered if I admitted that, years ago, I  adjusted these on a CB350 by ear while the motor is running?  I found that if I held the shaft with a screwdriver, I could crack loose the locknut, adjust until the tapping noise was just gone, and then tighten.  It was fairly easy to tell when the clearance was correct, from both tactile and audible feedback.  Checked the clearances afterwards and they were right on.  Was I just being young and stupid?

mystic_1

We did that for years, back in The Day.

Don't ever try it on a 450, but it does work on 350's.

bill