CT70 Carb issues

TheGarBrah

Member
So I've built my K0 CT70 and had it running well for a bit and then I went to take it out for its first off road excursion and ... it didn't want to stay running. Ran fine before, but now it won't.

Current issues: Takes forever to get it kicked over and running. The gears feel very soft , but thats another matter. Once the motor gets under way the carb will start to pop when the throttle is turned , and there is no increase in rpm of the engine (or if it does it barely does). The bike will idle seemingly fine but thats about it.

Remedy: I've replaced the oem carb with a chinese carb to single out any carb issues. The carb works fine on another identical CT70 so ive ruled out carb issues for the time being. Compression is around 60 psi which seems low, but it should still probably run. I cleaned out the gas tank, put in fresh gas, new fuel line filters, and new spark plug. I did notice that the plug shows the bike running a bit rich but not sure that is what my problem is.

Any thoughts here on what I might be missing or where I should start trouble shooting this ?
 

TheGarBrah

Member
60 psi compression is why it's running like you describe, I can't believe it idles/starts at all.
So I thought that seemed waay too low but I have an xr75 that fires right up and runs no problem and is at about 70-75 psi. Both of these seem well below what they should but the difference in how they run is kind of surprising.
 

hrc200x

Active Member
Does the compression gauge ever register up around 150 psi when checking other small honda engines? If you rebuilt the top end what parts did you use?
 

TheGarBrah

Member
I have not rebuilt the head on this particular motorcycle yet. My gauge is a Harbor Freight and I hardly ever have it hit 150 psi on other small engines. I figured to rebuild the engine over time but I didn't know if the popping of the carb was caused by some sort of vacuum issue or an air leak somewhere. Everything else such as the points, condenser, electrical is all brand new and tuned so I don't think thats the issue.
 

hrc200x

Active Member
Did you rebuild or swap out the piston/cylinder? Air leaks can be tested when bike is idleing and spray starting fluid around the intake pipe and carb at the joints and listen for RPM to change. Just make sure to keep it away from the spark plug wire incase there is a break in the plug wire and it sparks. If the points cam on the flywheel is rusty it can wear out the rub block on the points and throw the adjustment off quickly. Did you double check the points gap?
 
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