Blue smoke at high revs - are my cylinders / piston shot? | GTAMotorcycle.com

Blue smoke at high revs - are my cylinders / piston shot?

Q

quirco

Member
I ride an 88 honda CB450 and towards the end of last riding season, I started to notice that if I revved my engine to the top end, I would get excessive smoke. I'm almost positive that my cylinder / pistons are worn out, I'm just curious as to what it will take to repair?

I would prefer to fix as much myself as possible. I have the complete manual and have done the odd fix on the bike myself. I'm fairly competent in a shop and can usually figure out what needs to be done.

I am wondering, could I get some pointers or first steps to take to fix this issue? If the cylinder is worn / scratched, I'm assuming I'll have to buy new larger pistons and get the cylinder bored out to fit, correct?

If I brought the bike to a shop, how much would it cost to fix?

Basically, has anyone been through this before and what does it take to fix?
 
honing the cylinders, replace rings. That's just a guess mind, you will have to measure alot of things and check seals once you open it up.
 
Do a compressions check dry.
Then put a few drops of oil down into the cylinders and do another compression check.
If numbers are same it's most likely top end (dried up valve seals or the valves /valve seats).
If compression numbers go up with oil then it's most likely lower end (rings or cylinders walls).
 
quirco said:
boooya, Thanks for the tip! I will do the diagnose as prescribed.
Click to expand...

Won't tell you what's wrong but will point you in the rite direction for tear down and what it will roughly cost.
 
25 yr old engine could have a couple issues, the compression test as booya outlined is first, then valve seals and guides. Once your in there the cam and chains may look a bit tired. If you can do this all yourself that's a major savings, the parts alone aren't "bad" for price. If you have to send it out for a rebuild you'll need to decide how much you love the 25 yr old bike.
 
This is a great "get your hands dirty" project. I did it years ago with a GS750EF that ended up needing new pistons, over-bored cylinders, and new valve seats cut. With a good service manual, the right tools, and a knowledgeable friend to call on if you get stuck, it's not a difficult job.

I decided to take the engine out and put it in my basement where I did the work over the winter. The 2nd time I did it (on the same bike as I had screwed up getting one of the rings properly back into the cylinder) I did it in the bike. Depending on your bike, you may have the option of doing this in or out of the bike.
 
after the compression test, wet & dry, do a leak down test. this will show you all you need.
 
coyo, glad to hear that you've been through this and that it'll be manageable.

Stewart, I'm going to get on a compression test very soon. A leak down test on the other hand is new to me. Looked it up today so thanks for that tip.
 
I'd also recommend a box of freezer bags, a digital camera, and a bunch of tie on tags.

the guys that do this all the time can throw bolts in a pie plate and sort it out after, I put them in freezer bags and put a note in the bag and put a wire tie tag on everything I disconnect.
Everything is easier when a job specific bolt that hasn't been made since '97 is in a bag, not something your looking around in the sawdust for.

Good luck, it's very rewarding work.
 
Just a piece of advice for your rebuild. I would order a new set of case bolts/head bolts/rod bolts ... basically any of the torque to yield bolts. As on a motor that old you do not want to trust reusing those fasteners. Not to mention they should be replaced at every rebuild. If in your shop manual the torque to yield bolts or strech bolts (they are often called) are not used than completely ignore this post.


just my $0.02
 
And don't forget the cornflakes box.
 
BMANN said:
Just a piece of advice for your rebuild. I would order a new set of case bolts/head bolts/rod bolts ... basically any of the torque to yield bolts. As on a motor that old you do not want to trust reusing those fasteners. Not to mention they should be replaced at every rebuild. If in your shop manual the torque to yield bolts or strech bolts (they are often called) are not used than completely ignore this post.


just my $0.02
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Yes excellent advice.
 
BMANN said:
Just a piece of advice for your rebuild. I would order a new set of case bolts/head bolts/rod bolts ... basically any of the torque to yield bolts. As on a motor that old you do not want to trust reusing those fasteners. Not to mention they should be replaced at every rebuild. If in your shop manual the torque to yield bolts or strech bolts (they are often called) are not used than completely ignore this post.


just my $0.02
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I've always heard that.

What is the strength difference between a stretched bolt and a new one?
 
RockerGuy said:
I've always heard that.

What is the strength difference between a stretched bolt and a new one?
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Not a matter of strength, more a
matter of the torque will not be accurate as the bolt has already been stretched to yield.
Allot of head bolts will have a torque spec plus quarter turn or half turn to stretch bolt causing it to have a reverse spring effect so that it will not back off.
 
boooya said:
Not a matter of strength, more a
matter of the torque will not be accurate as the bolt has already been stretched to yield.
Allot of head bolts will have a torque spec plus quarter turn or half turn to stretch bolt causing it to have a reverse spring effect so that it will not back off.
Click to expand...

Now that make more sense
 
boooya said:
Not a matter of strength, more a
matter of the torque will not be accurate as the bolt has already been stretched to yield.
Allot of head bolts will have a torque spec plus quarter turn or half turn to stretch bolt causing it to have a reverse spring effect so that it will not back off.
Click to expand...


Yep ... what he said. I've seen guys re-use them in the past... not a good idea for the miniscual cost. You'll be chasing vibrations forever ...
 
Very few fasteners on older bikes like that are stretch bolts with the likely exception of the con-rod cap bolts, and if you are only doing the top end of the engine, you won't be touching those anyway. It is not necessary to touch the con-rod bolts to do a piston ring job on that engine.

Any fastener that calls for a specification such as "X N.m plus Y degrees" is likely to be a stretch bolt.

The factory service manual will specify which bolts are to be replaced every time.
 
I definatley haven't rebuilt an old honda engine, but how do you reinstall the pistons without taking them off the crank?
 
BMANN said:
I definatley haven't rebuilt an old honda engine, but how do you reinstall the pistons without taking them off the crank?
Click to expand...

I'm not familiar with that engine but sounds like you just pull the air cooled cylinder rite off the piston.
Like so
ae71163d-dc22-7d63.jpg
 
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