baja 90 loss of power compression
HarryHydro
I was looking at someone's Baja 90 which was not starting. After
adjusting the intake valve clearance (tight), it started, but not very
easily, and lacks low-rpm torque. It looks like the cam-timing is
right. Compression feels weak with a finger over the spark plug. It
barely pushes my finger off. It's not smoking or noisey when running,
but difficult to idle. Could the rings be wiped out? We pulled the
head off. The valves look OK except for carbon buildup. I think they
were running with choke on, as it will almost only run with choke on.
Carburator was taken off and cleaned out, not very dirty. Changed
gaskets from carb to head.
The primary rider is a 6YO who beats it well!
Thanks for any leads.
Harry
GTD
IMO, you should get a real compression tester, the finger over the plug
hole isn't very accurate. The idea that running an engine with the choke
on will make it so it only runs with the choke on is a myth: Engines
don't get 'addicted' to having the choke on or such. If it only runs
with the choke on, check for leaks between the carb and the motor, as
that's a a likely reason for needing the choke, as is a mis-jetted carb.
A richer mixture will conceivably help if the engine is losing lots of
compression, in theory anyways.
A real compression tester will tell you for sure if the compression is
suspect. Any way to check ignition timing and such? what do you mean
that the cam timing "looks right"? When a timing chain stretches, thus
retarding the cam, it shifts the power curve toward the higher RPMs,
which could explain the loss of low end torque.
What did you see when you adjusted the valves? Do you mean it WAS tight,
or you adjusted it TIGHT in comparison to the recommended specs?
HarryHydro
> HarryHydrowrote:
>
> - Show quoted text -
Hi:
I never heard of an engine getting addicted to having the choke on..
There was a home-made spacer (broken) between the carb and intake
manifold, but there is an o-ring on the carb-face so we just took that
spacer off. There was another heat-spacer/gasket between the head and
intake manifold. It's a cool day, the carb wasn't hot.
I don't have a compression tester with that small thred. I'd think a
9:1 should at least be 130psi. I think I'd have to fight to hold
100psi even on this small plug-hole. There was nothing to it.
We pulled the cam-cover off the side and seen the marks were aligned
with the flywheel. Our first suspicion was the timing chain slipped
and the piston impacted a valve.
One thing we didn't do was pull the cylinder, which shouldn't be very
difficult. The head gasket looked good, but we didn't check the
cylinder-face of that gasket, either. It was late and dark.
If he still has it the next time I go down there, I'll delve deeper.
Harry
HarryHydro
>
> - Show quoted text -
Opps. The valve.. The intake rocker was tight. I adjusted it to
about .002". We didn't pull the valves. The seats might be scored,
too.
Harry
GTD
> It's a cool day, the carb wasn't hot.
> I don't have a compression tester with that small thred. I'd think a
> 9:1 should at least be 130psi. I think I'd have to fight to hold
> 100psi even on this small plug-hole. There was nothing to it.
Not really, The smaller plugs ATVs use means that 100psi cylinder
pressure only means about 8 lbs of linear push on the area of that plug
hole.
Ed Medlin
about .002". We didn't pull the valves. The seats might be scored,
too.
Harry
Did you check the cam lobes for scoring or wear? If everything else seems
ok, that is where I would look.
Ed