Running Great...then Died

1967 Guy

New Member
Throwing this out there before I do a bunch of unnecessary repairs and replace things prematurely. Looking for some (more) advice from y'all.
I recently got my 72' CT70 running beautifully (started 1st/2nd kick, had good throttle, etc.), BUT after riding it for about 3-4 miles, it sputtered slightly, died, and now it won't start. Just prior to dying it sounded like the valves were tapping extra loud and the engine was smoking a bit too.
Background info.....the timing was near perfect, new points/condenser, plenty of oil, rebuilt carb, new rings. I'm getting good spark and when I pull the plug, there is fuel on it. Every 5-10 kicks it pops pretty loud, but won't turn over.
Bent Valves? Obviously things got hot (too hot) or it wouldn't have been smoking, but what could have been the cause and what damage? Thank you in advance for any knowledge you're willing to share.
 

Robert thran

Well-Known Member
i would check the compression 1st..if it’s low or none at all I would check the valve adjustment..a bent valve would cause the adjustment to seem real loose and a tight valve would cause low or no compression… just my thoughts on what I would do 1st.
 

1967 Guy

New Member
Thank you both. I got ambitious last night and pulled the motor. When I initially had the problems I pulled the plug and used the kickstart to check compression, which it seemed to have. Last night, after pulling the plug and hand-turning the flywheel, and couldn't feel any real compression (it was slight/way less than "normal"). When I took off the tappet cover, I noticed the valve seems a little extra loose at one point when I rotated the flywheel. Not sure if that fits the "a bent valve would cause the adjustment to seem real loose" and the low/no compression. I will dig into the valves today/tonight and update the post. Thank you again
 

Robert thran

Well-Known Member
Before you pull the head check the cam chain adjustment and timing..if the chain is wore out and loose it could have jumped cam timing or so loose it retarted or advanced the cam to the point where it made it easy for the valve to make contact with the piston..if everything is looks good there’s a chance someone installed a bigger cam and didn’t install stronger valve springs??? I’ve personally over reved the hell out of these motors and have never bent a valve but I have pulled apart used motors that did and they all had loose cam chains…
 

OLD CT

Well-Known Member
A symptom of a bent valve would be you can turn the engine over only a half a turn and the flywheel will not rotate anymore, lock up.
If you can spin the engine over with no interference, you do not have a bent valve. Doesn't sound to me like you have that problem.
You may have a worn tuliped valve that is not fully seating....
 

Old Guy Too Many Bikes

Well-Known Member
"Real loose" indicates a valve is not opening properly. You didn't indicate in any of your posts that you adjusted the valves. Do that and see if you get your compression back. I wouldn't have pulled the motor yet. Also you could check the timing and timing chain and tensioner. Rotate the engine to the T mark and put a phillips screwdriver carefully into the spark plug hole and verify the piston is all the way up as the flywheel crosses the t mark. You can also take off the cam cover and see if there's excess play in the chain. These motors are pretty stout and it takes a lot of abuse or neglect to break one.
 

1967 Guy

New Member
Thank you, Gents! I honestly saw your responses after I worked on the bike today.
I tend to get frustrated when I know things are correct, but still second guess myself, so I over-react and overdo things. As I mentioned, I pulled the motor. Today I pulled the head. Didn't notice any "tagging" on the valve @lukelaw1. I'm assuming this means contact between the valve and the piston? Cam chain seemed perfectly tensioned/no slack. At TDC the stamped circle on the cam timing chain gear was at the correct mark on the head. Did not do timing with static timing light prior to disassemble today/yesterday. I believe it is the stock cam @Robert thran. Flywheel never got stuck or locked up and as of now I don't have a bent valve (only had time today to check 1 of them) @OLD CT. I did lap the valve with compound to make sure it seats properly while I had it out. Gonna do the other one tomorrow when I check it. Did not seem to be tuliped and appeared to seat well.
I did adjust the valves prior to all of my other work and because the bike was running so well prior to this mishap, I assumed they were still set properly @Old Guy Too Many Bikes. I will remember the "screwdriver in the the spark plug hole" trick next time (if there is one).
My gut tells me the valves "went out of adjustment"? I will update the post again tomorrow. Sorry for my long response.
 

1967 Guy

New Member
Well.... I had a brief chance to dig into the other valve (intake) last night and sure enough... it is bent (not badly....but bent slightly). I will order and replace it.
While in there, I also replaced the valve guide seal (exhaust only) and found that my throttle needle wasn't being pushed down with the spring. The spring wasn't "seating" against the throttle slide and was allowing the needle to move up and down slightly (now it's fixed as well).
Before I put everything back together, I have a few questions for y'all.
1) what are the most common reasons for bending a valve? Heat? If so, why so hot?
2) should the valve guide seal be "somewhat" snug around the valve? It slides nicely, but snugly.
3) I can't say that I remember readjusting the valves after I replaced the rings, points, condenser, and did the timing. I think I did the valves prior to... months ago, when I thought that was originally the problem. Would that affect the valves adjustment in being correct or not? (probably an obvious answer/dumb question)
4) how do I know I have the correct throttle slide in my carb? I found another carb I had (from the same year bike) and the slide is a different length.
I wish I could buy y'all a cold one for all of the help and advice you provided. Thank you in advance.
 

Robert thran

Well-Known Member
Any chance you have the piston in upside down? Look closely for a witness mark on the piston.. If not check for a badly worn valve guide..I don’t think heat could bent the intake valve being the exhaust valve gets way hotter and didn’t bend…but let others pipe in for there opinions..
 

Old Guy Too Many Bikes

Well-Known Member
Usually a bent valve happens when something like a small bolt, some piece of metal or piece of a carburetor falls into the intake manifold and is stuck between the valve and the piston. Did you see any evidence on the top of the piston? It may be possible that a valve was adjusted so tight that the valve itself hit the piston on TDC of exhaust stroke.
 

1967 Guy

New Member
Might be over-doing the thank yous.... but thank you again for your responses.
@Robert thran- Piston is in correctly (large indentation on piston up, smaller one down to correspond to each of the valves) AND the spark plug "dent" is on the correct side. Valve guides upon inspection look to be in good shape. I tested both with a straight valve.
@Old Guy Too Many Bikes- nothing was in the combustion chamber at all. No new marks, no other new damage, scratches, dents, or dings. I suppose it could have been too tight (re: valve), but there are no marks on the piston. Valve may have been too tight, but no evidence on the top of the piston.
Here are a couple of pics. Both valve pics are of the intake valve. Tried to capture the bend.
Please keep the responses coming if you know something or see something I'm missing.
 

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lukelaw1

Active Member
I honestly think that the exhaust valve area of the piston next to the cylinder wall looks rough/dinged up. It appears to be marks in the piston that are not there when piston is new.
 

1967 Guy

New Member
Thank you lukelaw1. I see/saw that too. The problem I am facing is that those marks are on the exhaust portion of the piston, caused by the exhaust valve. The exhaust valve and guide are in near perfect condition (no bend, no dings, no irregular marks). The bent valve is on the intake valve, which appears to be undamaged on the piston side. Am I missing something?
Here is a picture from a few months back, before I did the previous work and buttoned everything up. It actually looks more damaged now... i agree. Just weird that the opposite valve is the one damaged.
Thoughts?
Easy fix.....replace the valve/s. Just wanting to learn "why" this happened and obviously prevent it from happening again.
 

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1967 Guy

New Member
Wanted to update this post....it's been awhile.
Replaced both valves (intake was bent), then reassembled. Rechecked valve lash and timing. Bike started right away and ran great. Rode it for 10-15 min in the neighborhood, then the same/similar problem occurred (see above/smoking and died). Didn't take off the top end and check for bend valves b/c it still started, but repeatedly died and wasn't running good. After digging in a bit more I found that the cam chain was SUPER tight. Possibly the cause of the excessive heat, etc.? I'm hoping Robert Thran and Old Guy Too Many Bikes were right and THIS fixes the ongoing problem. I'll give another update after a test ride.... should be tomorrow.
 

1967 Guy

New Member
Update...
Loosened the cam chain tensioner/cam chain, rechecked timing and valves. Started right up. Ran great. 15 minutes into the ride, it died again and was smoking from the head (again). Luckily had a friend with me and he suggested we check the temp and see if it really was that hot and compared it to his (which hadn't been running quite as long as mine). Mine was around 310 degrees F, his 275-280 (is this excessively hot?). Went home and pulled it apart (again). Checked the oil screen, which I had not done yet. It was filthy….I mean loaded with guck. Hoping that was the cause of it “overheating” due to restricted oil flow. Thoughts anyone? Anything else I'm missing?
 
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