wierd issue with a buddys monkey bike

[two-stroke-brit]

A good friend asked me to look at his 1970 honda CT70 trail .
he has had since he was 6 but let his kids/grandkids loose on it now it wont start.
he said the kickstart is acting strange too.
so i rebuilt the carb (blocked jets).
and checked comp .low only 60 psi (poss bent valve).
so i pull the clucth cover and everything looks good nothing broken or suspect.
i rechecked the comp and its reading 125 now.
i kicked it over and it starts.(shut off quick as no oil with cover off).
put the cover back on .
wont start and low comp again 60.
is there a de-comp mechanism i am not aware of .
any help is greatly appreciated.
cheers mark

[gpaddict]

Check the valve clearances.

[evan_calgary]

Valve clearances should always be checked in any old 4 stroke but could also be sticking rings on an old bike. Do a wet compression test. If you've got changing compression valves something odd is going on. This is not something that should happen. It is more like a change in how the compression test was done. Did you have the throttle wide open both times when doing the compression test?

Decomp on a tiny bike like that would be unlikely.

[two-stroke-brit]

thank for the help ,i did the comp test in the same manner each time.
i will check the valve clearances.
its a strange one for sure.
thanks again mark

[85RZwade]

No compression release on those engines

[evan_calgary]

Tight valves are unlikely to result in the issue mentioned regarding the compression changes unless you were doing the test hot once and cold other time. It simply has to be something to do with the piston side seal if you are getting varied readings or a wobbly cam.

[two-stroke-brit]

Thanks to all, i will dig into it this weekend.
i was really hoping not open the motor up .
side covers/carb rebuild is one thing but full disassembly on a motor i am not familiar with is getting deep into the weeds .
hoped it would be an easy fix ,not looking that way .
cheers mark

[silverstrom]

The fact that the compression improved to 125 and then dropped to 60 again sounds like stuck or sticking rings. I would expect a valve clearance issue to give constant low compression. If the rings are sticky the problem may come and go.

[RuZty]

I have never tried this so take it as free advice worth nothing, but I have heard this from people who have bought old bikes with engines with stuck rings that wouldn't turn. Put a SMALL amount (say an ounce) of ATF (detergent) or even diesel in the spark plug hole and let sit for a day or more, would probably help if the cylinder was vertical to get even ring coverage. I wouldn't think you could bend a valve on those without breaking the timing chain.

[evan_calgary]

That could help RuZty. followed by some oil in the cylinder. Crank it for a while with spark plug out to clear.

It should also be quite simply to remove the cylinder on a monkey bike and replace the rings. If they are sticking it hsould likely be done.

[two-stroke-brit]

thanks to everyone for the advice.
i noticed while i was getting my years worth of cardio exercise kicking it over ,that sometimes it kicked through but the magneto cover didn't spin .
turns out if the clutch is worn the readings can slip and not spin the crank.
hence the strange comp readings and failure to start.
new clutch plates ordered
and a special castellated tool too.
hopefully this will sort it.
cheers sweaty mark

[Darrell]

Wow, that's interesting! I wouldn't have thought rings would stick, unstick, and then stick again, but I didn't have a clue what the problem could be. Shows to go ya that Murphy really does exist! Good job figuring THAT one out.

[two-stroke-brit]

Well new frictions and springs sorted the non starting issue.
it starts easy and idles great now .
but will not rev under load feels/sounds like it is drowning.
i thought it was fat on the mail jet (#72),put in the #60 and it will not run without choke.
so back to the 72.
will look at the needle position but i think its fixed.
could it be the points as that's something i haven't touched (don't like points ).
any hint/pointers would be great.
mark

[MK]

As the engine-knockoff is used in any of the available pitbikes, you may convert to a CDI system to get rid of the points?

For my IMR 155 I even mounted an Ignitech DC CDI P2 and modified the ignition curve & rev limits.
Looks like this when in action :
https://youtu.be/RBDDeDlVgCU

But for me it sounds like the usual carb issues. Almost all of them have that.
Mine hardly started and ran rich (attached a Lambda probe to confirm).
Instead of re-jetting I resolved it with getting a dedicated Mikuni VM26 kit pre-jetted for this engine (around 130 Eur incl rubber manifold).
The stock Mikuni had the idle fuel adjustment screw pointing down, so you'd have to remove the carb for every iteration of jetting. It was easier to get a new one that had an idle air screw sideways.

Another frequent and annoying issue is bogging in upper revs due to ground contact issues. Check all electrical connections, clean and ensure they have good contact.

[two-stroke-brit]

thanks Martin great advise as always.
will look at it again this weekend .
cheers mark