1980 CX500custom

Hello all,I just picked up a 1980 honda cx500 custom,one of my
favorites,,had one years back,and decided to get another
one.Unfortunetly it has a small problem.It backfires a bit quietly as
compared to say A BANG,it is more like it is spitting..small pop,would
be a better description....
at exactly 3000rpm,it is a steady popping sound,especially out of the
right side.and after it has warmed up and I start cruising,it goes
well,although I have a small bit of lag at higher speeds 55+,...
the popping goes away when cruising down the highway,and it only
seems to "pop"when am slowing down say around 40 or so...it will pop
while slowing down,and then when I gas it,it really pops,and as I
say,mostly out of the right side for some reason...A friend said it
sounds like it is too lean,and I noticed on the pilot screws the
limiters have been ground off...when I looked at the pilot screws
tonight I noticed that I could see a few threads coming down out of the
carb..
I am thinking of turning them all the way in and bringing them out
the 1-5/8 turn it says to do in my service book..
and just as an aside,,according to what I read in my clymer
manual,they only put the limiters on 81+ models of these,,and well
evidently I have them,,although they are gone...
I know this post is a bit long,but I tried to put in all the info I
could think of..it only has 17,000m miles on it and is really in
pristine condition...I am possibly leaning myself torwards it being
possibly the float on the right side,although I found out recently that
the local honda dealer around here,,does not work on them anymore..too
old..but at 25 years old,and a classic,,I do so enjoy it..
thanks for any help you can give..
Kevin

 

Should of noted that the POP I get after gassing it is only one POP,then
it just speeds right along,..but at 40 or below,,if I let off the gas
hard,and give it gas,,I will get POP everytime..and only after it seems
to be warmed up...
Kevin

 

Lean mixture will make the exhaust sound go "piffle-piffle-SNAP!" as
you roll off the throttle. If the mixture is really lean, it will go
"fart-BANG!" out the tail pipe.

If the mixture is that lean, the engine will use anything it can get
for fuel. It will burn the oil off the cylinder walls and suck it past
the valve guide oil seals. It will burn the carbon off the tops of the
pistons. It will burn up the pistons and valves if you don't feed it
enough gasoline.

Your valves could already be burned...

Usually with diaphragm-type constant vacuum carbs, the pilot screws are
GAS screws, turning them counterclockwise makes the mixture richer. I
can't say for sure whether the mixture screws on your machine are pilot
AIR screws or pilot GAS screws.

But pilot AIR screws have to be turned clockwise to richen the mixture,
that's just opposite of the way you have to turn pilot GAS screws.

Pilot gas screws are normally downstream of the throttle butterflies.
Pilot air screws are normally on the sides of the carbs, upstream of
the butterflies.

Makes sense to me. If some government agency orders anti-tampering
devices installed on carburetors to prevent the owner from tampering
with his own carburetors, he'll tamper with them anyway. I have drilled
out over a dozen anti-tamper plugs myself.

You might also have stuck slides or stuck floats in one cylinder.
Also, check the petcock to be sure it's clean and passing enough fuel
to the float bowls.

Another possibility is that the carburetors are all gummed up. I always
recommend putting 3 or 4 ounces of Berryman's B-12 Chemtool Choke and
Carburetor Cleaner in a full tank of gasoline and riding slowly so the
carburetor has to suck the B-12/gasoline mixture through the idle jets.
Buy the liquid B-12 for mixing with fuel and buy the aerosol B-12 for
spritzing out the carbs if necessary. That will get any varnish out of
the carbs and free up the slides and floats and float valves.

Check www.oldbikebarn.com for carburetor repair parts if you need them.
They manufacture new replacement parts for old carbs.

 

They're pilot gas screws. (downstream side of carb) You need to turn them
out (counterclockwise) to richen the mixture. Pull them out and make sure
you have a spring, washer and o-ring on each of them. Note that the o-ring
MAY be jammed up inside at the top of the orifice.

Your plugs will tell you if you're running reallly lean. They'll be white.

Also, if you're running really lean, check the rubber carb boots. They were
notorious for splitting and admitting air. If you can't find replacements,
you can do a reasonably aesthetic repair by rolling an appropriate diameter
piece of bicycle tire inner tube over the rubber boots and trimming as
necessary.

Brent

 

You're way off on a tangent krusty, as usual.
It's probably just loose or leaking exaust headers

 

Well, it will take a minute or less to check that theory out, won't it?

 

Sounds like your right carb has something wrong with it as you go from
pilot to main jet.


and I noticed on the pilot screws the

Yep, definitely something wrong in the pilot circuit.

I wonder if you have "new" carbs?

Do check the float bowl level (it's really easy to do), but that is very
unlikely. It sounds like a combination of a) the pilot circuit being
lean (due to the 3000rpm deal where you're maxing out the pilot circuit
and moving onto the main jet) and b) possibly the decel valve not
working, if your carbs have that.

Also, the CX'ers have a fan club on the Internet, you might want to try
a Google search...

- Elron