Thread: C70 Scooter and CT70 engines the same? Engine woes...

A friend of mine just picked up a 1972 Honda C70 scooter and he's having trouble getting it running.

He said he tested the compression and it only read 50 psi. I advised him to squirt a little oil in the cylinder and check the compression again to see if the compression goes up (indicating bad rings).

He has another friend helping him and this is the email he sent me,

Hey Don – here’s all we’ve done…

Checked spark plug – it looks brand new, dry and for good measure I cleaned it to “as new” condition

• There is strong spark (tested with the plug out and cranking the engine)
• Fuel lines are clear, removed both and cleaned
• Fuel filters are clear, flow tested and seem fine
• Fuel tank was removed, cleaned and filled with fresh gas, as per your suggestion
• Carb removed, disassembled and rebuilt (yes it was pretty gummed up). Float bowl appears to fill nicely, all jets are clean/clear
• Compression tested to about 50psi. (This is my main concern)
• Will do the oil test tonight.

I have a timing gun – but with the engine not running – bit hard to test – any suggestions would be appreciated.

When we bump start it – it “runs”, i.e. it sounds like ignition is occurring, until you slow down/stop pushing. There is NO throttle response and NO power whatsoever."

I have a feeling the problem lies in the fuel delivery or timing. It sounds like it's not firing properly. I'm 2 hours away in a different city so this is the best I could do as far as explaining what's going on.

50 psi of compression is your problem. This is very low! If you have spark and fuel and it does not run. Check your timing it could be way off causing the valves to be open when needing to be closed.
Good Luck

The bad news:
BJs right, . . . 50 psi is not enough compression. If the bike has been sitting for long the rings are probably stuck in the piston, and there are no "magic formulas" for getting them to loosen up.

The good news:
C70s (and their kin, CTs, CLs, SLs, XLs, Z50s, etc.) are easy to fix and parts are plentiful and relatively cheap. And there are bunches of Honda Mini fans here that will be glad to answer all your questions. Tell your friend to go for it.

-kevin