Anything to do for low compression?

 

Is there anything simple to do to increase compression.
My old Honda CB125S has low compression.  Hard to start when cold
but runs ok when warm.  Shoot a little oil in the cylinder and
the compression comes right up, so think the valves are ok.

Top end gaskets cost more than the bike is worth.
I'm afraid to ask what a set of rings costs.

Anything simple to help?
I once put STP in a bike.  Helped the compression, but the
clutch never worked again. ;-) Live and learn.
Other options?
Thanks, mike

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mike wrote:

The only way to fix it involves taking off the cylinder and fitting
new rings, also possibly boring the cylinder.  If you think that is
too expensive, the only reasonable thing to do is to break the bike
up and sell it for parts.

I'm surprised that you think $36 for a top end gasket set is too
expensive.  If you're still able to ride it, it's worth at least
five times that much.  A set of rings costs $35.  I'll gladly buy
that bike from you for $70.

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Mark Olson wrote:

Yeah, but we're up to $71...unless I need to have it bored and buy a new
piston.  Then I really should do something about the rust in the gas
tank.  Then it needs a real headlight, and a new battery.  And the oil
leak needs fixed.  Will need a new tire soon...
And a weekend of greasy fingers to do it all.

That's just too much to put into a $20 bike that still gets me from one
garage sale to the next...

I was looking for a quick fix that might help some...maybe...

mike

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mike wrote:

You may want to use some mouse milk like Tecrholine or just run a few
tanks of Mobile gas through it. The rings on the old Hondas had a
tendency to gunk up and stick in the grooves. Don't do anything silly
like trying to rev it to 10k RPM to loosen them, just run it normally.
Rust in the gas tank => water in the top end => rings may be corroded in
place.

If it turns out they are rings are just too worn, maybe switch to
something like a 40wt non detergent oil. It will be thick when you first
start the bike and stay that way until it comes up to operating temp and
parts expand/fit a little better.

If it was me I would consider buying one of the 150cc big bore kits that
are available for that model before I searched out oversized piston and
someone to rebore the cylinder. The kits typically sell for ~$120 and
include piston, sleeve, and gaskets. Some still require you bore out
their sleeve though. There was a lot of work put into those engines. I
think one of the popular hacks at the time was to put in a piston from
something like a Honda 550 four in.

But 'what other people said': I just paid a little over $100 last fall
for a 1975 CB125. It was missing the tank, the seat, most of the
lighting, and wasn't running at the time. That was a pretty typical
price for an old Honda in that condition. I think I've seen clean
runners go for well over a thou$and.

You see a lot of people are coming around to your way of thinking. That
is the old Hondas were good, cheap, reliable, transportation. The CB125
shares many parts with other models like the SL125 so there is a bit of
demand for them. I've spent a lot on mine but it will be worth it to me
if it provides reliable transportation to "garage sales" for the next 20
years. Figure operating cost are less then half that of a car so it'll
pay for itself.

  Things like running, clear title, and registered, mean it would
probably sell for $300+ on eBay. Not all Hondas or bikes from that
period are desirable right now, it's just happens that the one you have is.

 


mike wrote:

March 20, 2005
1975 Honda CB125S2
   1-Cylinder
   4-Stroke
   125cc
Trade-In Value (Good Condition) $300

Trade-in Value is what consumers can expect to receive from a dealer
for a trade-in unit assuming an accurate appraisal of condition. This
value will likely be less than you would receive from a private party
sale because the reselling dealer incurs the cost of safety
inspections, reconditioning and other costs of doing business. Trade-in
values are based on clean units in good condition, with all original
standard equipment. Mileage/condition and additional equipment may have
a substantial impact on the value shown above.

Maybe you might want to clean the bike up, polish it, make it look nice
and sell it for $300 plus whatever the prospective private party would
be willing to pay?


compression, but the clutch never worked again. ;-) Live and learn.

Go down to Walmart and buy a few quarts of Valvoline VR-50 Racing Oil?
It's 50-weight oil. Your clutch won't slip like it did with STP.
It just won't disengage properly until the engine is really warm. Maybe
the heavy 50-weight oil won't circulate when the engine is cold, so you
can keep the engine warm with an electric heater under the bike. Of
course, if your float bowl overflows gasoline and the heater ignites
it, you might burn your garage down...


Want to learn mechanics and fix it yourself? Run down to your local
motorcycle salvage yard and find a Clymer's manual for little single
cylinder Hondas. Motorcycle salvage places don't just buy wrecked
bikes, they buy new old stock from dealers. The first motorcycle I ever
built was from new old stock or NOS, as it's called by the afficionados
of Ye Olde Crocke. Good motorcycle salvage places might even have NOS
for your CB-125, ya never know what you're going to find...

You'd probably pay about $5 to $7 for the manual. Study the manual.
Learn how to find top dead center on the compression stroke so you can
line the piston and valves up properly before taking the cylinder head
off...

Acquire a few good tools.  Spend a few hours on Saturday taking the
cylinder head off and and de-greasing and scraping all the carbon out
of it. Box up the cylinder head and cylinder and piston and take it
down to a local good independant motorcycle machine shop and have the
machinist measure the cylinder and piston to see if the cylinder needs
to be overbored instead of just honed. If you don't need an overbore,
standard rings will fit. If you need an overbore, you need an oversized
piston and oversized rings...

Order the parts and wait...

The next Saturday with all parts ready, reassemble your CB-125 and
enjoy it for years to come...