"87 Honda Elite CH 150 idle problem

[rayt...@hotmail.com]

I recently bought an '87 CH 150. All is well after going through the
body and engine except it idles up when at normal running temp. It
eventually idles so far up that the drive belt begins to engage while
stopped at a light.

I pulled and went through the carb inspecting vacuum diaphragms, jets
and float and saw nothing that would cause a problem. I disconnected
the vacuum line to the petcock valve and the problem went away but of
course the fuel stopped flowing.

This seems to be a carb piston problem and I suspect the vacuum at
idle is causing the piston to open and increase idle speed.

Any ideas about how to get this carb back to normal?

Thanks,
~RayT

[tomcas]

I have to ask the obvious first.... Is the throttle cable slack enough
and did you try adjusting the idle speed screw?
The vacuum at idle is supposed to draw up the piston and this vacuum is
a function of the throttle plate opening.

[paul c]

tomcas wrote:
> I have to ask the obvious first.... Is the throttle cable slack enough
> and did you try adjusting the idle speed screw?
> The vacuum at idle is supposed to draw up the piston and this vacuum is
> a function of the throttle plate opening.

My bet is that the throttle plate is already too far open, ie., the idle
speed adjustment is too far clockwise. I've made this mistake many
times when the real problem cause was elsewhere. And, turning the idle
speed screw the other way may prevent the scoot from starting because
the small jets and transition ports are clogged or because the air or
air-fuel screw has been monkeyed with. Also, could be the automatic
'choke' is bad, but I doubt it from what I've seen, don't know much
about the ones on Honda 150's, but on the 250's the only time I've seen
them cause problems other than increased fuel consumption is in cold
weather, such as 5 C or 40 F. Of course, that's just my two-cent bet!

[paul c]

Perhaps another longshot, but if there is any crack or leak around the
intake manifold (not sure if it's made of rubber on this scoot), the
results of that might have enticed somebody to mal-adjust the carb or
throttle butterfly. Spraying the manifold from a WD40 aerosol can or
even just water in an old kitchen spray bottle when the engine is idling
might temporarily seal such openings and cause the idle speed to rise
noticeably. If so, you know there's some kind of air leak.

[tomcas]

An air leak is possible but not likely. It would produce a unsteady
idle. An easier way to find an intake lead is to fit an length of hose
to a propane torch and probe with the gas coming out while the bike
idles. It will speed up and smooth out when you pass the hose over the leak.

[paul c]

tomcas wrote:
...

> An air leak is possible but not likely. It would produce a unsteady
> idle. An easier way to find an intake lead is to fit an length of hose
> to a propane torch and probe with the gas coming out while the bike
> idles. It will speed up and smooth out when you pass the hose over the
> leak.

Assuming you can point at the leak. Haven't heard of this one before,
but there's no way I'll be fooling around with a propane hose next to a
running engine, or at least no way I'd suggest a stranger doing it, not
knowing how careful or practised in safety procedures he might be.

Still, I agree that the air leak isn't the most likely cause, just one
of several. Didn't think I was suggesting it was likely, just my two
cents, trying to rhyme off the possibilities I could remember from my
own experience. I imagine that scoots are less susceptible to air leaks
than bikes, becasue I think their carbs are removed less often! (body
work serves as a deterrent to backyard mechanics, nothing against them
since I'm one too, but that group has a higher-proportion of people who
are in over their heads than most shops do.)

A subtle air leak on a small engine, doesn't guarantee an unsteady idle.
It may only show up when you twist the throttle and then possibly only
under load.

When it comes to engine physics, there is a larger population of
experienced people on usenet groups like rec.motorcycle.tech as well as
several website such as 'allexperts'. I've found they don't mind
sharing what they know even if the question involves a small engine or a
scoot engine. Some are even willing to share what they don't know, but
it's all free except for one's wasted time trying to understand them
all. My other advice about those groups is to ignore the personal stuff
, because there are some posters who are often ridiculed for spurious
reasons such as their prose or their political attitudes even though
their mechanical knowledge runs very deep.

[RayT]

Thanks for all the input. As far as the propane test I've used and
recommend using ether starting fluid. Although flammable it
evaporates immediately where propane might linger like gasoline fumes
and be a potential hazard. Unfortunately I haven't been able to wrench
on it since my posting but have ridden. The thing runs perfectly, it
just has a very fast idle after 3-4 miles of running down the road. I
feels like a vacuum caused problem in the piston valve area. ~RT

[tomcas]

Ask any experienced auto mechanic and they will tell you propane is the
safest and best way to find vacuum leaks.
Vacuum leaks on bikes do a lot more then then just give you a rough
idle. They will lean out your mixture, cause your engine to run hot, and
in the case of two strokes can lead to severe piston damage.