1975 CT 70 compression issue

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KROY33

Member
Hi...Bought a bike that was not assembled was told the engine was free. It sat for a few months and when I went to turn it over it was stuck..Soaked in mystery oil for a few days it it freed up...Now I have almost no compression on my gauge. The carb was not on when I did this but from the spot where the carb bolts on I have what apperas to be alot of suction....Are the rings bad?? Am I missing something..Did the oil get where it shouldnt go ?

Thanks
 
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b52bombardier1

Well-Known Member
Check to see that the valves are set to .002 inches cold. This will most likely eliminate valves not fully closing as an immediate concern. What are you getting for compression numbers? A hundred PSI would be a bare minimum with 170 PSI as the spec for a brand new engine although I'm happy at 150 PSI after a rebuild.

Try squirting some engine oil into the cylinder to see if your compression goes up.

Rick
 
K

KROY33

Member
Yes good idea..I am thinking valves may be open.....I will check them next. Basiclly no compression right now....see you have a few st 90's....me to ...mine are green. Are those rare?
 
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b52bombardier1

Well-Known Member
Rarer than CT70 and CT90 bikes but still relatively easy to find and get parts. However, they only made the ST90 in 73, 74 & 75. Dr ATV also keeps a good supply of parts. An automated EBay search usually turns up everything else I need.

My Mighty Green ST90 is the favorite bike of my niece and grandson.

Rick
 
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6

69ST

Well-Known Member
Sounds like rusted cylinder walls, rings and at least one valve & seat. I wouldn't expect anything but a top end rebuild to get this motor running properly but, you've got nothing to lose by trying the Mystery Oil, a little gasoline down the plug hole and seeing if it'll fire. I've successfully rebuilt many of these engines with cylinders that looked like they were used for saltwater storage. As long as the lower end isn't rusted, they rarely are, there's not all that much in terms of parts or work, needed to rebuild the upper end back to tip-top specs, even if it looks horrible.
 
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KROY33

Member
Put oil in spark plug hole. Still no compresion.....is there away to check valve seats? Thinking that's the problem. Can dirt mr causing this. Should I try and soak valves again?
 
6

69ST

Well-Known Member
Most commonly, the valve problem will a "tuliped" (worn-out) intake valve; that happens over time, allows progressively more reversion into the port and manifests as a blackened intake port. In extreme cases, the intake and even the carb throat will turn black. With this, specific motor, however, I suspect that one of the valve faces & the seat are rusty. It only takes one relatively tiny flake of exfoliated rust to keep the vale from fully seating and,voila!, massive compression bleed-off.

You could try rotating the crank to the compression stroke, which leaves both valves closed, then introducing solvent into one port at a time. Brake cleaning solvent will find the smallest gaps. That said, going by your description, I'd pull the head. It's a quick job and will answer your questions in a fraction of the time we can spend trading guesses in print. You're going to end up having to do this anyway, may as well get to it ASAP...you'll be glad you did.
 
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