Backfiring

The drill for compression testing is to do the test with the engine
warmed up and both spark plugs out. Open the throttles wide open and
maybe even block the vacuum slides up so the engine gets all the air it
can to compress. Since a rubber hose connecting the gauge to a fitting
in the spark plug hole has a lot of extra volume, one turn of the
engine won't raise the compression pressure to what the engine does
with the spark plug in the hole.

Like, if you have 10:1 compression and you're at sea level, the engine
would normally compress the air to 10 X 14.7 = 147 pounds, assuming
that the valves and pistons are making a good seal.

But, as I said above, the extra volume of the compression tester hose
and the bourdon tube inside the tester make the pressure lower than
what I calculated, so it takes several turns of the engine to build up
the pressure inside the tester to what the engine normally does in one
stroke.

If the pressure doesn't come up to the manual's specifications in about
five turns, the next thing to do is squirt a few drops of oil down the
spark plug hole of the cylinder that tests low and turn the engine over
a few turns to distribute the oil. If you put too much oil into the
cylinder and don't distribute the oil, you can blow up your tester, the
compression will be sky high.

Then repeat the test. If the compression pressure increases, the rings
are worn out or broken.

Another way to check for whether it's burned valves or worn out rings
is to turn the engine to top dead center on the cylinder you want to
test and pressurize the cylinders with a source of compressed air and
listen for leaks at the exhaust pipe, the carburetor inlets and the oil
breather hose. If it's the gasket, you'll hear the hissing where the
head and cylinder mate.

You could use a small electric compressor like the emergency tire
inflation compressors they sell at K-Mart or Wal*Mart. It would
probably cost about $30.
You would need to hook up your compression tester's hose to the
compressor outlet hose.

(I tried to do a leak down test on my pickup truck's cylinder head
about a year ago, using a tire pump. I figured I could pump one
cylinder full of air a lot faster than I could hand pump a tire. But I
heard the air hissing out through the open spark plug hole on the
adjacent cylinder and that's how I knew I had a blown head gasket. The
tire pump trick would have worked for valve
leaks or worn out piston rings, but the head gasket leak was just too
much for my hand pump.)

The piston is going to try to move when you pressurize the cylinder
through the spark plug hole, it can throw the wrench you use to turn
the crank across the room, so put the transmission in gear and block
the rear wheel from turning before you pressurize the cylinder.

If you're hearing air leaking out the exhaust pipe, the exhaust valves
are leaking. Air out the carbs means the intake valves are leaking. Air
out the breather means the rings are worn out.

I'm glad to hear you are progressing little by little. I never even
thought of ignition points because most Japanese engines didn't have
ignition points after about 1980. If you have any spare change, you
might even consider installing an aftermarket electronic ignition and
get rid of those maintenance-intensive points.

One thing about getting that deep into an engine that I should mention
is to follow the repair manual's instructions for setting the pistons
to top dead center before removing the camshafts. And, when you go to
reinstall the camshaft bearing caps, for gawd's sake, clean all the oil
out of the holes the cam bearing cap bolts go into and clean all the
oil off the bolts themselves before you try to torque the bolts to the
manual's specifications. You can RUIN the cylinder head by trying to
torque OILY BOLTS!!!!

 

Thanks for the tips, krusty.

One more thing on the carbs - if the pilot screws are missing the
o-rings or washers and air leaks past them, would that make the mixture
too rich or too lean?

I'm still concerned that the bike is running too rich, even on the right
(strong) cylinder, because of the sooty plug and the occasional popping
from the exhaust.

 

The pilot screws are actually adjusting a mixture of fuel and air, but
when the pilot screws are located downstream of the throttle
butterflies like they are on your CV carbs, we think of them as
adjusting the flow of fuel. So if the o-rings are missing, the mixture
will be too rich according to the thinking of professional tuners.

The only way you'll really know whether the mixture is too rich is by
the
way the engine runs, whether the idle goes too high when the choke is
on, or hangs up when the throttle is closed suddenly.

 

While cranking, either keep the ignition off (assumes you have a kick
starter) or ground the spark plugs wires to the engine. That is to
avoid damage to the ignition system.

 

First thing is to resolve the compression problem.

 

Yeah, maybe it'll work things out. Valve cover and head gaskets on the way.

I tried dripping some oil into the sick cylinder, bumped up compression
5 psi to 95 psi...still way off from the other cylinder's 150.

 

That sounds like good news.

Hopefully you ordered a complete gasket set. It would be good to
replace the valve-stem seals if you are getting any oil fouling.

 

Question about the CV piston - if I plug the air intake to the piston
with my finger and try to move the piston, should it still move or kind
of "get stuck"?

One of the pistons on the carb "gets stuck" in position, while the other
still moves pretty freely.

 

.... when you plug the "air intakes", presumably.

Each of my pistons (slides) has a couple of large (~1mm) holes in the
bottom. There are also air jets in the carb body below the diaphragm.
If your carbs are similar, which "air intakes" do you mean?

If one carb behaves differently from the other, it should be pretty
clear that something is wrong with at least one of them.

If movement of one piston is impeded when you plug its bottom hole(s),
and that doesn't happen for the other carb, maybe air jet(s) in one are
plugged? Under different assumptions, maybe it means you have a hole in
one diaphragm.

You should have cleaned the sliding surfaces of the pistons and the carb
bodies thoroughly with carb cleaner so that they feel smooth to your
finger. Careful not to get carb cleaner on the rubber diaphragms unless
you want to spend $130 and send them off to England for a rebuild.

 

There's an intake above the main carb barrel, an oval shaped one. This
is the one I was referring to.

Does this mean that the carb cleaner also wrecks havoc on rubber orings
inside the carb?

 

The oval hole is the source of atmospheric air pressure that actually
lifts the piston when the engine is running and engine vacuum pumps the
air away from on top of the diaphragm.

But blocking that oval hole while the engine isn't running should not
make the slide stick. The air on top of the diapragm should be able to
escape through holes in the bottom of the vacuum slide. If you have the
slide out, you'll probably see two holes about 1/8th of an inch in
diameter. They are the "vacuum" holes that allow air to be pumped from
on top of the slides while the engine is running.

It's hard to imagine those hole getting crudded up so bad they get
plugged, but you might check it out. Spray some aerosol carb cleaner
through the holes. Maybe there's a dead bug in one of the holes?

It might just be gum and varnish on the slide. You can clean it off
with aerosol carb cleaner while the carbs are installed or just wash it
off with clean gasoline if you have the slide out of the carburetor.

Those are pilot air jets in some CV carbs. Other types of CV carbs have
the pilot air jets in the intake bell mouth.

It's actually a kind of bent oval...

I've been cleaning my carbs with aerosol carb cleaner about once a year
for 30 years and have never hurt a diaphragm with the stuff yet.
Constant exposure to gasoline will eventually degrade rubber parts,
especially when they are weeping gasoline just enough that they are
always wet with it but a drop never forms. The little o-ring on float
bowl drain plug will deteriorate for that reason, and will eventually
come apart when the float bowl drain screw is opened. But right there,
the softened rubber gets ripped and torn by the screwdriver turning the
drain plug.

If you are always tampering with your idle mixture, turning the screws
back and forth, you could conceivably wear the tiny o-rings out. But,
what the heck. Most shade tree mechanics tweak the idle mixture screws
a few times, then they get tired of messing around under the shade tree
and go for a ride and they quit messing with the idle mixture screws
and the tiny o-rings can go back to sleep ;-)

Some needle jets have an o-ring on them and the o-ring seals fuel air
mixture for years and years and never deteriorates. But very few people
even know they can push a needle jet out of their carb, so they never
see that the o-ring looks just like brand new.

 

No matter what the design of the carb, obviously they should both do the
same thing.

Is it resisting going up or failing to come back down? If you've sealed
all of the vacuum intakes to the top of the piston, it should resist
going up.

As Matt wrote, there's often a small hole in the bottom of the piston
too, which presumably would keep you from sealing the area at the top of
the piston completely.

Can you pull out the pistons without removing the carbs from the bike?
If so, I'd do that and check for crud, tears in the diaphragms, etc. You
might also try swapping pistons to see if the trouble follows the piston
or the carb body.

 

Maybe the OP's carb has some path other than a hole in the bottom of the
slide for sucking air from above the diaphragm, and maybe that path is
blocked.

Anyway he should be able to use a wire to see that any bottom holes are
clear.

 

Sorry for not specifying this - the carbs I was referring to in the last
reply are another set of carbs that the seller threw in with the bike.
They aren't in the best shape, all of the parts were tossed in a
cardboard box, but I wanted to give it a try nonetheless.

I figured that the slides would feel the same as the other. I'll have to
make sure the passages are REALLY clean (although I did blast the whole
thing down with carb cleaner).

BTW, update - filled the tank up today, 3.7 gallons, 66 miles on the
odometer. thats like...17 miles per gallon? I have the smallest SUV in
the world!

The strange thing is that the exhaust doesn't smell like raw fuel, and
it sputters almost like its running lean, although its still sooting the
plugs. Weird. Complete gasket set on the way, and i'll try the second
set of carbs at the same time.