Honda Twins banner
1 - 17 of 17 Posts

· Registered
Joined
·
8 Posts
Discussion Starter · #1 ·
I got a free 1973 cb350 that I've been working on as a senior project I've been slowly tearing down the problems and I'm fairly certain that I have stuck valves since I have spark and fuel but not fire and it blows back through the intake on compression stroke if anyone has some tips on freeing up that valves that would be nice I plan on removing the head and cleaning the valves and valve guides when I have the time
 

· Registered
Joined
·
1,711 Posts
The valves on most if not all ohc honda engines, and other manufacturers, are inclined, not parallel with the cylinder, meaning if a valve is stuck, open usually, as the piston rises, it will bend the valve and possibly break a valve guide.

This is why I harp on about this very real possibility particularly with an engine that has been in hibernation for some time.

A stuck valve can turn a viable engine into scrap in one stroke.

Check for very large valve working clearances, a sure sign of a stuck/bent valve.

Remove the cylinder head, check for piston crown damage, bent valves and broken valve guides.

Pictures pictures pictures.

You may be lucky in some much that crud/bad valve seat and/or valve faces.

A stuck open valve and an opening valve can tangle/interfere with each other again causing damage.

By what you say you have been cranking the engine for some time.

Pictures pictures pictures...
 

· Registered
Joined
·
254 Posts
One way would be to remove the valve caps, watch the carb side or intake side valve go down, then begin to move back up or closes the valve.

Meanwhile, you look down at the stator marks for which side you started with, then lineup the T make. Chopstick down the spark plug hole to make sure you are at TDC-Compression stroke, then lift a rocker arm to see of it moves a lot.

 

· Registered
Joined
·
42 Posts
On my bike (CB125K3 parallel twin) you can actually see the valve opening from the plug's hole with the plug removed. Shine a bright light in there while turning the flywheel - which should be easier with both plugs removed.

Ben
 

· Registered
Joined
·
1,711 Posts
Good little vidio pio, any pictures of the mechanical carnage ?

Good point Ben...

Wade, did you look thru the plug holes ? Do any of the valves look odd and any damage seen seen on the piston crowns ?

As you know the engine has to come out to remove the cylinder head, however, from memory, the cylinder head "cover" is removable with the engine in situ giving a good view of the camshaft, rockers and valves, but if you do that, place washers over the studs to make up the lost "thickness". Remember that the "rocker arms" alloy carrier casting held on by peripheral 6mm bolts ( 10mm hex heads ) is being "forced up" by the valve springs when valves are open....

There may be some benefit in removing the lid, but I don't have your engine in front of me.....

Pictures pictures pictures with the engine "as is"....

Do your checking with the engine "as is' for now.

With plugs out and turning the engine over by hand, or gently with a ring spanner, any tight spots felt ?

I presume you have cleaned the cb points only and have not introduced an incorrect ign timing ?

As said, it could be crud stopping that inlet valve from closing completely.....how are the valve working clearances ???

A bright torch shone thru that inlet port ( carbs off ) when that valve should be closed, any light seen thru plug hole ? Valve working clearance large at that point.

Feedback as it helps others.....

Hope no carnage.....

Dd23🤓
 

· Registered
Joined
·
1,711 Posts
Good little vidio pio, any pictures of the mechanical carnage ?

Good point Ben...

Wade, did you look thru the plug holes ? Do any of the valves look odd and any damage seen seen on the piston crowns ?

As you know the engine has to come out to remove the cylinder head, however the cylinder head "cover" is removable with the engine in situ giving a good view of the camshaft, rockers and valves, but if you do that, place washers over the studs to make up the lost "thickness". Remember that the "rocker arms" alloy carrier casting held on by peripheral 6mm bolts ( 10mm hex heads ) is being "forced up" by the valve springs when valves are open....

There may be some benefit in removing the lid, but I don't have your engine in front of me.....

Pictures pictures pictures with the engine "as is"....

Do your checking with the engine "as is' for now.

With plugs out and turning the engine over by hand, or gently with a ring spanner, any tight spots felt ?

I presume you have cleaned the cb points only and have not introduced an incorrect ign timing ?

As said, it could be crud stopping that inlet valve from closing completely.....how are the valve working clearances ???

A bright torch shone thru that inlet port ( carbs off ) when that valve should be closed, any light seen thru plug hole ? Valve working clearance large at that point.

Feedback as it helps others.....

Hope no carnage.....

Dd23🤓
 

· Registered
Joined
·
8 Posts
Discussion Starter · #8 ·
I recently had my wisdom teeth removed so I haven't been able to really look at it at all in the past few days I'll try and take some pictures when I get home I've already removed the carbs so I'll do the flashlight thing too I still plan on pulling the motor tho I've turned it over like u said previously and It does feel like it has some compression but I don't have a tester and I didn't notice any tight spots other than a little resistance on compression stroke which I'd assume is normal thanks for the helpful information
 

· Registered
Joined
·
8 Posts
Discussion Starter · #10 ·
I took of the valves covers, pulled the plugs and spun it over by hand I couldn't see the exhaust valves themselves but they looked to be moving nicely looking through the valve covers and the intake valves are definitely moving I have videos I just need to figure out how to post them
 

· Registered
Joined
·
35 Posts
Engines that sit for years and years usually have pitted, corroded valves and seats. Usually needs a top overhaul. I did that to a 73CB450 spare engine I had that sat for 20 years, replaced the timing chain and tensioner, carb rebuild, points, filters etc. Still runs great after 10,000 more miles. It might work for you before tearing everything apart. The job was a senior project with my son.
 

· Registered
Joined
·
8 Posts
Discussion Starter · #13 ·
Yea I plan pulling the motor and rebuilding the top end the valves need a good cleaning and new gaskets never hurt I don't think I'll replace the timing chain though that seems like more work than I'm really comfortable with
 

· Registered
Joined
·
1,711 Posts
Before you take the head off, shine bright torch into inlet track when that valve fully closed, look thru plug hole at all angles, any light seen ?

Perhaps a liquid on piston crown to aide reflection....syringe to extract liquid after....

Any marks on piston crown to suggest a valve strike ?

In no, do a dry and wet compression check.
 
1 - 17 of 17 Posts
Top