See if You Can Solve this Puzzler. Rebuild Gone Wrong, But Where?

[Rude Boy]

I know, its a long post. but I'd really appreciate it if you read it. I'm stumped on what to do.I've got a problem that's stumped me and two long time mechanics, Im just about at the point of giving up and parting, but maybe the brainiacs of DTT can save my sanity (and cb350!)


Important Background: Bought a 71 cb350 w/ 6k miles, previous owner bought the bike new and didn't break it in properly. Left piston rings became unseated and scored the cylinder, owner garaged it for 40 years.


When I got it, the bike smoked a lot but ran good w/ one kick start.


I pulled the motor, got bigger displacement pistons from bore-tec and had a shop bore the cylinder according to the piston (this may be the problem)
-Only things I changed inside are the piston-cylinders and cam chain/tensioner.


Put it all back together. Tried starting it, wouldn't (even w/ starter fluid) But I heard the "Ca-phunk" of pistons moving accompanied by a strange honk which was decided to be incorrect timing. (combustion while the exhaust valves were opening, causing vibration)


I've checked several things to get it to run in these past months, but still no avail.
It feels as if the problem has changed over time; For example: the bike no longer honks while kicked, this is because the timing was wrong, not on the points but inside on the camshaft. I really feel that I put it in correctly the first time, but I'll say I didn't because if I did, that means something slipping around in the crank. (could be)


Now, after making sure the timing is correct and the points are correct, the honking is gone. But I have a whole new problem... and this is where I need your help.


Fuel good, air good, points good, spark good. Good recipe for starting, but still wont.
So I decided to tow the bike and try bump starting. This is when it gets weird: While the bike was not moving, it was in 3rd and not able to move. pulled the clutch in, car took me up to 15mph, dropped the clutch... nothing. not even any sound of pistons moving!!! Stopped the car and bike wheels went ridged (it was in 3rd gear)
towed and bumped it again and I could hear a faint "whoring" sound coming from somewhere in the crankcase, but NO piston movement.
It doesn't start w/ kick or starter but the pistons do move.
Doesn't start w/ bump and the pistons don't move...


What is going on!!! am I just fucking losing it?? I figure (but I'm not sure) its something to do with the clutch. but is it? and how? what do?
Thank you for your time

 

[bikeboy]

If it were mine, I'd pull it out and strip it. Sounds like the only way to be certain that everything is where it should be, and doing what it should do is to see it happening. It's not likely to be something simple like a bad plug or loose earth ???


Just the way I'd attack it.


cheers
ian

 

[teazer]

At the risk of stating the obvious, something is wrong.

That "phut" down the pipes implies an ignition event at the wrong time and possibly just a simple 180 degrees out.

BTW never ever tow a bike behind a car. The best way is to run and jump on the seat or roll down a driveway or have a buddy push you. A car has enough power to cause serious damage to a motor and to the rider.

The "Ca=phunk" of pistons sounds more serious and could be pistons and valves hitting each other.

I would remove the cylinder head and see what's going on. What I'd expect to see are bent valves and a camshaft that is incorrectly timed. The cam chain may have skipped or jumped a tooth when it was being assembled but it's more likely that the sprocket is on 180 degrees out. The sprocket can be fitted one of two ways and one way works and the other doesn't. When I say 180 degrees out, I actually mean its back to front and needs to be flipped, not rotated.

 

[crazypj]

Sounds like you have multiple problems.
Possibly bent valves and clutch not adjusted correctly (over center and slipping)
Hopefully you didn't cause too much damage.
You did re-adjust valves when assembling top end?
Pistons are not in backwards?
Looks like your going to have to strip clutch side and top end

 

[Rude Boy]

Thanks guys.
I think what Im gonna do first is open up and inspect the clutch system. hopefully what's going on is the clutch isn't completely engaging and not providing enough force to move the pistons.

I didn't re-adjust the valves, just sprayed wd-40 on the top to see if it'd leak through, seemed fine.

Hopefully this wont turn into a parts bike, but I am already falling in love w/ the sr500 & 400. Plus, Spring is coming quick...

 

[teazer]

And then pop the top off. CB350 is a pretty simple bike to work on but it's also easy to get wrong.

 

[Rude Boy]

No doubt. I was hoping to get riding for spring, it's my first bike. But it's lookin like maybe not...
Is what it is, just gotta be done.

 

[Rude Boy]

is there anyway to check the cam timing without opening the motor? (valve rockers?)

 

[Sonreir]

Pull off the alternator cover and turn the crankshaft counter-clockwise until the LT mark on the end of the crank is lined up with the arrow. The valves on the left cylinder should now both be closed and properly gapped.

Repeat for the right cylinder using the RT mark.

 

[TFWG]

One thing that helped me was to take one of the spark plugs out and shine a light in the hole and see what was going on in the cylinder while I was rotating the crankshaft.

The two problems I had before getting the motor right was I had the timing way wrong and the valve was hitting a piston. Luckily I didn't bend the valve (I did a leak down test with alcohol). You have to be super careful installing the cam shaft because the cam chain slips really easily while you are installing the cam. You have to check it constantly.

The other problem I had was the kick start mechanism was installed upside down and wasn't disengaging. I've totally disassembled the engine 4 times now, but the point is I eventually got it working. It was super frustrating, but that's why they call it a project.

Keep us updated! Good luck!

 

[crazypj]

Pull off the alternator cover and turn the crankshaft counter-clockwise until the LT mark on the end of the crank is lined up with the arrow. The valves on the left cylinder should now both be closed and properly gapped.

Repeat for the right cylinder using the RT mark.

cam runs at half crank speed so that's not entirely correct, you could have it on overlap and valves will be tight.
Better to look at advancer locating pin on cam extension.
Remove points plate
Remove advancer unit (6mm bolt, 10mm head)
At TDC should be inline with cylinder center-line if cam timing is right

 

[Sonreir]

Good to know, thanks pj. My manual doesn't mention anything about checking for the advancer pin.