I have a K0 ct70. It ran fine last summer and suddenly died on a ride. I assumed it was electrical. Replaced points and condenser and questionable ignition winding. Spark was fine. Cleaned carb. Didn’t start even with a shot of starter fluid. Bought a harbor freight compression tester and it showed no - zero compression. Pulled head and cylinder. Intake valve wasn’t seated, piston, rings, and cylinder looked fine. Intake valve pulled and there was rust in the valve seat and valve stem is slightly bent. Can I have a new valve seat cut? Do I need to have the valve guide replaced?
Have spark, have gas, have zero compression!
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it looks like that intake valve has hit something from the photo, are you sure the piston didn't get tapped? how tight is/was the valve lash in the rockers?
valve guides usually hold up, might as well do both exhaust and intake seats, both valves, along with new rockers and springs.
valve guides usually hold up, might as well do both exhaust and intake seats, both valves, along with new rockers and springs.
I agree there is a small nib on the top of the valve. First thing I noticed. Not sure what caused that. Then I noticed it didn’t look like the valve was seated and I could see light looking through the valve adjustment hole. The valve came out fine. When I roll the stem on a granite countertop it is bent.
In trying to get the bike running again I didn’t have spark. All of the electrical parts replacement solved that. I adjusted the valve clearance and they were both very tight and did not have the correct clearance.
thoughts on what bent the valve stem? On what took the nib out of the top edge of the valve? You have me concerned about the piston now. How can I tell if it took a hit?
In trying to get the bike running again I didn’t have spark. All of the electrical parts replacement solved that. I adjusted the valve clearance and they were both very tight and did not have the correct clearance.
thoughts on what bent the valve stem? On what took the nib out of the top edge of the valve? You have me concerned about the piston now. How can I tell if it took a hit?
Robert thran
Well-Known Member
You should see a spot on the piston dome ( clean piston good and look for a witness mark) if it hit the piston..make sure your cam chain isn’t slap wore out..if it is it could jump time and possibly bend another valve.? Or weak valve springs and over reving? Both are long shots but I would still check.....
Yes I looked this morning and there are scratches on the dome like you said. I should have noticed. All of my rebuilds in the past have been frozen top ends that were encouraged to release and not a diagnosis of what happened. I will take the cylinder to a machine shop along with new valves to get that resolved. I will check cam and rockers for integrity and tolerances. Can I use this piston and jug? Replace cam chain? What else to get this back together?
Here are better pics. Looks to me like piston was printed correctly with arrow pointing down and dimple on the plug side. I noticed the scratches on skirt of both top and bottom. I do see very light scoring in the cylinder when looking at it in sunlight. I cannot feel it. what are thoughts - honing and a new piston and rings?
OLD CT
Well-Known Member
That is the right piston, The cheapest way out, I would transfer the dowels into a new standard bore cylinder from ebay and throw out the piston from the kit. You could harvest the gaskets and buy a ''new correct piston'' kit ''standard bore'', separate. Nothing wrong with the cylinders though. $29 plus shipping.. Or upgrade a little and buy a basic K0-81 88cc piston/jug. Wouldn't be a good idea if you are rebuilding the original head and cutting corners with a hopeful hone job.
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JHminitrails
Well-Known Member
One other item you should check is your connecting big end bearing. You want zero radial play, meaning up and down. Side play is normal, but if there's any up and down play, the crank will need to be rebuilt. This play coupled with the connecting rod expanding at operating temperature and a high RPM run, could spell disaster for piston to valve interference.
Rod doesn’t seem to have any radial play. I found a machinist to cut new valve seats and place new guides on the cylinder head and bought an aftermarket jug and piston and rings. The cam lobes and rockers are within tolerances. Hopefully have it back together and running in a week or two. Thanks for all of the help!
Thanks for all of the previous help. Need some help with current issue. I bought a standard cylinder jug, piston, and rings from Dr. ATV. I also replaced the timing chain. I took cylinder head into machine shop had new guides installed and valve seats cut and put in new valves. I checked camshaft and was within tolerances. All went together well and it started up no problem. It ran and idled then died. Couldn’t get it to start and had low compression again. Took off cylinder head and intake valve is not seated, looks like valve stem bent again. What do you think is causing this? Crankshaft?
Do I need to replace crankshaft? Can I replace valve without recutting seat?
Do I need to replace crankshaft? Can I replace valve without recutting seat?
OLD CT
Well-Known Member
You probably had timing marks incorrect. No way of telling, now that you took it apart again. Do you know how to do this? You can try to replace the valve and lap in the new valve. I don't think it's the crankshaft at all. Do you know how to check the piston height in the bore or check this type of stuff as you are building this engine? Ask this type of stuff as you are building, not after it's too late.
Robert thran
Well-Known Member
Did you see if your cam gear marks were still lined up before you tore it all back down.?…it sounds like it jumped time… The only way the valve can get bent is if it hits the piston. I dought the valve stuck itself in the down position so it had to of been out of time or jumped time..if your guides or rubber bumper are worn out it can make the chain loose …if you installed the wrong chain it would have been sloppy lose when you put it together..a bad crank would be a long shot to cause that problem in my opinion..you should be able to just buy a new valve and lap it in with some valve seating compound. I would personally buy a new Honda valve not a cheap Chinese replacement.
Unfortunately I did not check the timing marks before taking the head off. I quadruple checked that the 0 was aligned with the notch at TDC. The cam chain was really tight and hard to get on the cam shaft.
old CT- how do I check the piston height in the bore? What else do I need to check for with reassembly?
Must say I was really surprised that this happened again.
old CT- how do I check the piston height in the bore? What else do I need to check for with reassembly?
Must say I was really surprised that this happened again.
Robert thran
Well-Known Member
Did they install a new valve guide on the intake side of the head? If so maybe when it got hot it tightened up and the valve didn’t shut fast enough or stuck itself open? If your chain was tight and your marks were lined up I can’t really think of anything else that would bend a valve that fast..…but like old ct said check piston height.???
Thank you for all the help. So rookie mistake. I did not bend valve stem. I overtightened valve tapper screw pushing the valve out of the seat - explains the gap I saw. I have put the cylinder head back on and am taking pictures of timing alignment. While putting the stator plate back on I picked up this small round very strong magnet on my screwdriver. I inspected the stator plate and flywheel
and can’t tell where it came from. Is this just something I picked up off my garage floor or does it go somewhere?