Major camshaft help needed!!! - CB250 k4

[Green199]

I know it could put it constantly a few degrees out, but could it make it vary? As in when I rotate it once its 5 degrees off, and then rotate again and its 10 degrees? So it isnt keeping constant?

I applied the tensioner at top dead centre, but a few people on honda twins mentioned about the 10 ATDC method. I didnt do this, which is why I thought that could be the problem.

Havent counted the crank teeth yet, been busy packing for Christmas, but will be checking it later today. So stock is 17 teeth?

 

[xb33bsa]

I know it could put it constantly a few degrees out, but could it make it vary? As in when I rotate it once its 5 degrees off, and then rotate again and its 10 degrees? So it isnt keeping constant?

I applied the tensioner at top dead centre, but a few people on honda twins mentioned about the 10 ATDC method. I didnt do this, which is why I thought that could be the problem.

Havent counted the crank teeth yet, been busy packing for Christmas, but will be checking it later today. So stock is 17 teeth?

i don't think this has been stated yet but it is an absolute
CAMSHAFT TO CRANK RATIO MUST BE 2 TO 1 that is what matters
in that relationship
whatever your crank has the cam needs double, 17 to 34 is your drive ratio with a 34 up top

 

[Green199]

Yep, understand that it should be 2-1.

Crank has a 17t sprocket, making my 34t cam sprocket correct.

My Christmas present to myself is the opportunity to try and rebuild this from scratch and see if I get the same problem again.

 

[Green199]

Merry christmas everyone! Hope you all enjoyed your holidays.

So, I finished assembling the motor this morning...and the problem seems to have disappeared . The timing is bang on, so Im guessing it much just have been slipping off the crank sprocket...with the tensioner done and adjusted correctly, it seems to be on there solid and wont jump!

However, there is no compression on the left cylinder...not sure why!

[EDIT}

Poured a spoon of oil down the bore, and checked to see the compression. Was the same, but the I noticed some bubbling from the intake side valve. So its valve seats which are the problem. I also then noticed worryingly a crack running down the intake port...

.. so the head is in need of repair or replacement now. Shit.

 

[crazypj]

You probably bent a valve, the heads can 'move' pretty easy
Never seen a crack as you describe, your sure it's not just a casting mark?

 

[Green199]

Interesting. Could be a bent valve. Im assuming its just a bad seat to be honest as I was rotating it over very slowly when it was skipping teeth, and I cant imagine they would be that easy to bend.

It sure looks like a pretty nasty crack to me, Ill try and get a picture to show you PJ when I can find my camera.

 

[crazypj]

You would be surprised how easy the head bends on stem when it touches piston

 

[Green199]

Ahh ok, thanks for the info PJ


Think I'll try and get the valves out, and then replace them with some over ones I have cleaned up and put in a spare head. I would just swap that one on as the valves and seats are all pretty much spot on, but one of the carb mount manifolds has the edge cracked off. May try and find someone to build it back up with weld and then re-tap the hole....