Topic: CB175 twin. Horrible engine noise. *head gasket :(*

[fmctm1sw]

I had a lot of hope for this bike.  Got my head back from a guy after I had him put a helicoil in it.  Turns out, I should have had the other plug done too but oh well, the top end has to come back off anyway.

I put it all back together and attempted to start it.  I got a loud clacking sound from seemingly the top end.  It ran on one cylinder for a moment before I shut it down.  I noticed a few things when I was putting it back together.

First, I had a hard time getting to the "T" mark on the rotor.  It would move freely right up to it and I would feel a little resistence.  It then slipped past the "T."  Also, my pistons seemed to have a lot of slop in them at the pin.  The service manual says the pin "is offset 1mm" so I don't know if the play is design or not.  I know for a fact that the valve timing is right on:  "T" mark on rotor and small "O" on the cam sprocket.  I get a lot of air out of the breather too.  Almost like it's the exhaust  

My kickstarter is also acting up.  Kind of like it's stripped or something.  Moves right down at times.

My game plan is to take the motor back out and look at the timing again.  I want to find out if that resistance around the "T" mark is a fault of the bottom end or top end.  What I plan on doing is disconnecting the cam chain and turning the rotor.  Resistence = bottom end (piston pin play maybe  ).  Smooth movement = valve timing or something with the camshaft perhaps.  Actually, I could take it down all the way to the pistons and jug and turn the rotor and watch the pistions too...

[kslrr]

I had a 175 twin many years ago and don't remember any strange resistance while setting the timing.  I never had the engine apart so there's not much more I can tell you, except "a lot of slop" does not sound right.

[fishhead]

First, I had a hard time getting to the "T" mark on the rotor.  It would move freely right up to it and I would feel a little resistence.  It then slipped past the "T." 

 I had the same problem with a CB 350 twin. Spin the crank towards the T mark and right before it wants to "slip" past the T mark, adjust the cam chain.
 This took all the nasty noise out of the 350 that sounded like yours seams to.
 I have a 1970 CB 175 bottom end if you need any parts, PM me.

 

[tbpmusic]

I had a lot of hope for this bike.  Got my head back from a guy after I had him put a helicoil in it.  Turns out, I should have had the other plug done too but oh well, the top end has to come back off anyway.

I put it all back together and attempted to start it.  I got a loud clacking sound from seemingly the top end.  It ran on one cylinder for a moment before I shut it down.  I noticed a few things when I was putting it back together.

First, I had a hard time getting to the "T" mark on the rotor.  It would move freely right up to it and I would feel a little resistence.  It then slipped past the "T."  Also, my pistons seemed to have a lot of slop in them at the pin.  The service manual says the pin "is offset 1mm" so I don't know if the play is design or not.  I know for a fact that the valve timing is right on:  "T" mark on rotor and small "O" on the cam sprocket.  I get a lot of air out of the breather too.  Almost like it's the exhaust 

My kickstarter is also acting up.  Kind of like it's stripped or something.  Moves right down at times.

My game plan is to take the motor back out and look at the timing again.  I want to find out if that resistance around the "T" mark is a fault of the bottom end or top end.  What I plan on doing is disconnecting the cam chain and turning the rotor.  Resistence = bottom end (piston pin play maybe  ).  Smooth movement = valve timing or something with the camshaft perhaps.  Actually, I could take it down all the way to the pistons and jug and turn the rotor and watch the pistions too...

Sounds like a bent valve is causing the breather tube blow-by and noise - maybe happened when you first turned the alternator to line up the cam mark. I'll bet if you check the compression, it's zero on one side now. It might also be way loose tappets, but probably not.

The pistons may have a bit of side play (no cylinders in place), but should have no up and down play. The 1mm offset has nothing to do with this.

The "run by" when approaching the "T" mark is typical of the Twins, and is related to the 1mm offset you mentioned. It's much more pronounced on the 180-degree twins (350/450) than on your 360-degree 175.

[fmctm1sw]

I checked compression and I had 150 on one side and around 125 on the other.  Still an awful lot of air coming out of the breather.  It's going to be the weekend until I can get a good chance to look at it.  The kickstarter seems to work for a few kicks, then just pushes down with no resistence.  I guess I should try to notice exactly what happens and when with the breather and kickstarter...

[Hasenkopf]

blow by from the rings maybe?

[fmctm1sw]

Well, though I had a breakthrough but I guess not.  I took the engine back out of the frame and tore the top end down again.  The pistons seemed good and tight up and down wise and twist-wise.  There is no sound turning the motor over with just the pistons and jug on it.  Matter of fact, you can put the top end all back together and turn it over quietly too.  But the second you put any torque on the bolts that hold the top end together, the clack comes right back along with the weird "T" mark thing I mentioned before.  I moved the valve timing one link to both the left and right without any change.  I cleaned and colored the ridge of the piston where the valve comes close to with a black marker and put it all back together.  No ink transferred so I have no idea what the hell is making that noise.  


*edit*  I was convinced a valve(s) had to be striking my piston(s).  I went back out and smeared bathroom caulk all over my valves waiting to see white on my piston crown.  Put it all back together:  nothing...    At least I know my valve timing is good and my valves aren't crashing into the pistons.  As noted before compression was decent too so I really shouldn't have been surprised by that.  Tomorrow I think I'm going to pull the cam out of it and rockers and look at them.  I noticed a small shard of metal (looked like threads from something) in it.  Maybe a rocker or messed up rocker shaft...  I'm just grasping straws now...

[Staggerlee]

I would double check to make sure you have the correct head gasket in there; I had the exact same problem on my CL175 when I rebuilt the engine.  I think I somehow ended up with a HG from an earlier 160 motor; when I compared the bore diameter of the offending gasket with the old one, it was obviously narrower.

[fmctm1sw]

I would double check to make sure you have the correct head gasket in there; I had the exact same problem on my CL175 when I rebuilt the engine.  I think I somehow ended up with a HG from an earlier 160 motor; when I compared the bore diameter of the offending gasket with the old one, it was obviously narrower.

I have two head gaskets and I actually thought about trying them both at the same time.  I'll compare the two for size.  I think I'm going to try to take the rockers out for one side at a time to see if I can localize the noise to one cylinder..

[fmctm1sw]

Did a whole tear down of the cam and rocker arms and shafts.  I didn't see anything that would lead me to believe something was drastically wrong with anything I saw.  My next step is to torque it down and then loosen it up.  Then I'll roll it past the "T" mark again and see if the head pushes up in hopes that I can find out what could be bumping together...

[fmctm1sw]

Problem discovered.  I tightened the bolts on the head in different patterns to localize exactly where the collision was taking place.  All indications pointed to my intake valve (#1, or left side of the twin engine) so I took the head back off.  Again, I saw nothing to indicate the valve was bent or anything.  Looking at the pistons though, I noticed something.  One is sitting higher than the other...  noticeably.  Closer inspection showed the front area of the piston, supported by the location of the head bolts that worsened the condition, was rubbed silver.  That piston is slamming into the bottom of the head.  I strongly doubt one connecting rod is longer than the other but I don't know how I arrived at this condition.  So there's the answer but I don't really know what to do about it.  

[fmctm1sw]

Well, found it.  Somebody put the wrong piston in it...  Unbelieveable.  I started measuring distances (with my rudimentary metal rulers) between different parts of the piston and other things.  If you look at the photos, you will notice the area above the top compression ring to the bottom of the crown on one piston is larger than the other.  Approximately 1/32".  The actual area from the top of that ring to the very top of the piston crown, however is the same.  So the valves were spared but the piston whacks the bottom of the head.  I also noticed one piston has 2 oil holes on each side and the other has 4.  The markings on the pistons follow:

"313 3" and "313 7"

I'm betting that piston that's too big ("313 3") is from a different year or maybe a CL bike.  Off to eBay I guess....

*edit*  Maybe that 7 is a 1....   

[tbpmusic]

Pistons are the same for CL/CB/SL 175

313 is actually an SL175 number, but interchangeable with CB/CL, they all used the same parts.

Not sure what the 3 or 7 might indicate - you need to measure the bores, just to make sure it's not even crazier than you think........

[750goes]

Well you found the problem - and glad you found it now instead of when you would have very shortly after trying to ride it a few miles..
interesting that it appears very similar -

if you have to would it be possible to remove a small amount of material from the top edge of the piston?? - without weakening it of course..

[fmctm1sw]

Well you found the problem - and glad you found it now instead of when you would have very shortly after trying to ride it a few miles..
interesting that it appears very similar -

if you have to would it be possible to remove a small amount of material from the top edge of the piston?? - without weakening it of course..

That thought crossed my mind.  I have a brand new dremel tool that I probably could use to shave some off.  Wouldn't hurt anything I guess, I'm just going to trash the piston anyway...

[bryanj]

There were CB and CD models which had different compressions and the cd only had 1 carb so could it be a mix-n-match job?

[fmctm1sw]

If you look at this auction for CB 175 pistons on eBay, they look different too.  WTF?

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?VISuperSize&item=380178388010

*edit*  and those oil holes in that auction.  Different... Just like mine.  Maybe they need to be swapped?  I may try that before doing anything drastic...  Same part #s in the fische though..

[fmctm1sw]

Ah man...  My head gasket...  Smaller than it needed to be, I'll post pictures later...