Backfiring

I am so glad I have a vacuum petcock on my bike. If I ddn't have one, I am
sure my varnish problems would have been much worse. They were bad enough as
it was.

pierce

 

Hmm, bad news!

I went out to my bike earlier, confident after being bestowed with new
knowledge that might finally fix the stupid thing...

....and come to realize that, after some fiddling and lots of stalling,
that the jets were all the way screwed in, and unscrewing them at all
slowed the idle and made the bike run even rougher.

This made me think that bike was always running rich. I removed the
spark plugs to find them completely covered in soot! It wasn't oily,
just powder. But it was soot and a lot of it, too. I cleaned them off
and it seemed to run slightly better but certainly not perfect.

Does this mean my carbs have jets that are too large? I can't lean them
any more then they are now. The air filter is new, too. Would oil
leaking past the head gasket cause soot on the plugs?

 

The choke on the bike is working properly too, I think - it's not stuck
closed or anything - because it definitely makes the bike run
differently when cold.

 

As long as the pilot mixture screws are screwed all the way in, you
might as well unscrew them and make sure each one has a spring, small
washer, and little rubber o-ring on it. If the o-ring isn't on the
screw when it comes out, look up into the hole and see if it's stuck up
in there.

You have to have that little o-ring to seal the fuel/air mixture around
the tapered end of the pilot mixture screw. The washer is #81 on the
diagram, the o-ring is #82.

Check to be sure your spark plugs are the correct heat range, or no
more than one number colder or one number hotter. I had a hell of a
time getting a Jaguar engine to run right. Even a professional mechanic
was scratching his head at his inability to synchronize the carbs and
couldn't understand why the plugs kept sooting up. It seemed that I had
followed the
aftermarket camshaft manufacturer's recommendations and had installed
spark plugs that were the correct range for Daytona, but I was driving
on the street. So, maybe you have a set of "cold" spark plugs in there.

It's possible that the pilot jets are the wrong size. The parts diagram
at www.buykawasaki.com shows that item #41 is the pilot jet, with two
sizes available, a #40 and a #45. I suppose the #40 was the original
equipment pilot jet, and #45 pilot jet was optional. You can probably
see the number stamped on the pilot jet. If you try to remove the pilot
jet to see what you have, be sure that your small slot screwdriver fits
the pilot jet perfectly and you may need to push down hard on the jet
while unscrewing it. If you bugger up the slot on the pilot jet, you
may have a hell of a time getting it out. They are put in tight at the
factory and they usually come out with a snapping sound as I turn them
with a lot of force.

Item #45 on the parts diagram is the main jet. The diagram shows that
#115, #120, #125, #130, and #135 jets were available. I suppose that
you'd be just OK on the street with the #115 main jet, it would be a
*round* mainjet with a screw slot, NOT a hex-shaped jet, those were for
dirtbike carburetors. You would be able to see the jet number stamped
right on the jet without taking it out.

Other possibilities are that the floats are set too high, or the float
valves don't seat properly. Do the carbs overflow a lot of gas? Check a
repair manual to see what the float level setting is. It's probably
somewhere between 14 and 17 millimeters, and a millimeter difference is
critical to get the fuel level in the float bowls correct.

If there is a tiny bit of crud on the tapered tip on the float valve,
it can't seal and the carb will flood.

The float level setting operation is done on the bench with the
carburetors upside down, and you need a caliper to measure the from the
top of the float (the carb is upside down so the bottom of the float is
on top) to the gasket surface on the carburetor body. Not the gasket
itself, but the surface it goes against.

So far as the needle goes, the e-clip should probably go in the second
slot from the top. Check to see if somebody has stacked a bunch of
small washers underneath the little round doodad the needle goes
through. If the needle is raised too high, the carburetors will be
running off of the tapered part of the needles too soon and will be too
rich...

If you had oil leaking past the head gasket (or the valve guide oil
seals) you'd see wet black carbon, at least around the end of the
threaded part of the sparkplugs where it sticks into the combustion
chamber. More oil would cover the porcelin insulator, even clear down
to the center electrode. If you were burning oil, you'd be seeing blue
white smoke.

 

A choke is a flat piece of metal that closes off the carburetor air
intake and makes the carburetor have to suck harder to get air through
it, so gasoline gets sucked out of the float bowls into the stronger
vacuum with the choke plate closed.

Your carburetors do not have a choke. Each carb has a starting
enrichener system.

If you go to www.buykawasaki.com and look at the carburetor diagram for
the 1980 750 LTD-2, you'll see that #25 "plunger, starter" does the job
of a normal choke.

When you work the "choke" lever to the position that would ordinarily
CLOSE a plate type choke, you're actually pulling that starter plunger
up, and you are opening an air passage in a bypass around the throttle
butterfly.

The starting enrichener system has a starter jet down in the float bowl
and when you pull the "choke" lever up, the starter plunger is pulled
up and air is sucked around the closed butterfly and gasoline is sucked
right out of the float bowl. It's a very rich mixture, it's about 1:1
fuel to air ratio.

When you open the throttle butterflies with the starting enrichener
plunger
open, vaccum drops off and the enrichener sucks a lot less gasoline.
You can actually ride the motorcycle for miles and miles and never
notice that the "choke" is still on until you stop. Then you notice
that the engine idles a little rough.

It is possible that the starter plunger would leak on *one* carburetor,
but if it leaked on *both* carburetors, and that was causing your spark
plugs to soot up, I would recommend checking the mechanical linkage
that pulls those starter plunger up.

If they were actually worn out and leaking for that reason, you could
remove the screws holding the #28 "body, starter" from the side of each
carb and try to blow air past the starter plunger to prove that the
plungers leaked. But, if you try that test, be careful you don't ruin
that #29 gasket, it might be expen$ive to replace.

One other possibility that might cause the spark plugs to carbon up
with fluufy soot is holes in the rubber diaphragms on top of the
slides. If engine vacuum cannot lift the slides because the diaphragms
are torn, the vacuum will suck extra gasoline out of the needle valve
that the tapered needle goes up and down in.

 

I checked the plugs - they're NGK "regular" heat range plugs and were
the correct model and gap.

Will have to look at those.

Probably have to take the carbs out to get to them? I'd be willing to,
but if there's any way to leave 'em on...

Gas used to pour out of the carb before I got them "rebuilt" by the
mechanic. Now, they are dry even when I leave the petcock "on".

Why would someone do this? Is there any benefit at all to making it run
so rich?

Hopefully it'll be fairly obvious when I tear the thing down, and
hopefully my fuel economy will improve over the 40 mpg I am getting now.
Thanks for the help, krusty.

 

You might be able to remove the float bowl on one carb if you have a
short phillips screwdriver or a phillips tip that fits in a 1/4 inch
socket. Then you might be able to read the number stamped on the main
jet with a mirror and try to figure out what the numbers mean while
viewed backwards. You won't be able to read the numbers on the pilot
jets that way, though

Enthusiastic shade tree tuners read the side panels in a motorcycle
magazine and they see the part about how they can improve the
mid-throttle operation by stacking washers under the needles, but they
really don't understand that a CV carb's slide isn't directly connected
to the twist grip, it has a mind of its own. That fact, combined with
the strange interactions between exhaust systems, air boxes and
camshaft timing often results in an engine that's puffing black smoke
all through the throttle range and the shade tree mechanic never knows
exactly what caused his problems, whether it was the huge main jets he
put in, the stack of washers under the needle, or the five turns out on
the pilot mixture screws

You could probably get 45 to 50 mpg, if everything was working just
right. 40 mpg is not all that bad. Big inline 4's get from 35 to 42
mpg. When you start getting 25 mpg, you know the bike is running way
too rich.

 

Sometimes it stalls, but it had poor ridability either way, especially
when cold. Power delivery is not smooth and the bike kind of "bucks".
The plugs were covered in soot after only 500 or so miles since I
changed them last.

 

Here;s the update...

Yesterday I filled up my tank and averages 23 mpg. Wow.

Today I pulled the carbs off and throughly cleaned them with either B-12
Chemtool or Gumout. The stuff came out brown after it washed around the
jets and stuff. One of the vacuum pistons was closing pretty slowly too,
now its better since I stiffened the spring.

Now the bike can run at around 800-1000 RPM but still stalls pretty
repeatedly from a misfire or something. What would be the next logical
step to take? What would cuase misfires or poor idling? It seemed to run
better before it fully warmed up.

 

That reminds me of when one motorbike started getting 25 mpg. The
ignition points were burned and dirty and out of adjustment. I usually
don't think of 1980's motorbikes as having ignition points, but the
ignition parts fiche on www.partsfish.com shows the 1980 KZ-750G1 as
having points!

Have you cleaned and regapped the points and checked the timing? You
can just hold a transistor radio next to the points as you turn the
engine over with a wrench. You'll hear a pop of static when the points
open. This should occur as timing marks line up. The ignition key has
to be on.

And there's an ignition advancer on the points cam. Have you checked to
make sure it's working right? A shot of WD40 or penetrating oil will
free it up if it's sticking in the full retarded position.

After you get the ignition points cleaned and set, you can fiddle with
the carbs. Motorbike ignition systems are weak enough, without having
dirty points. They need the idle mixture to be set fairly rich in order
to have enough mixture in the spark plug gap for the spark to find any
burnable mixture.

I suppose the idle mixture screws should be around 1/4 to 1/2 a turn
open.

Did you check the float level to see if it was right? If the fuel level
in the float bowls is too high, the engine finds it too easy to suck
gas out of the float bowls and the engine will run too rich.

 

Shit. Yeah, I did a little - the piston was closing so slow and wasn't
even closing all the way...

 

I couldn't check it, I didn't have a fuel level gauge with me...one of
the floats was higher than the other though, so now they are pretty even.

I checked the points briefly a few weeks ago, wasn't able to change them
because my wrench wasnt small enough, but they seemed at least
servicable. I didn't check the ignition timing or the advancer, though.

Could misadjusted valves cause any of this? I was planning to do that soon.

 

So, instead of finding the actual problem, you just bent the spring?

Have fun synchronizing those carbs.

 

How would you suggest "finding the actual problem"?

The diaphragm looked fine, sealed fine, all passages were just cleaned
and had air blown through them. My Kawasaki manual doesn't mention this
issue at all. I thought the spring could have gotten weak with age.

 

Sometimes it's necessary to improvise a tool, if you don't happen to
have a
vernier caliper with a depth gauge attached. Like an ordinary six-inch
steel scale will work for measuring float height. It doesn't even have
to be a metric scale, if you know that a millimeter is 0.0394 inches.
If the repair manual calls for a float height of, say, 15 millimeters,
15 X 0.0394 = 0.591 inches; 0.591 divided by 0.03125 = 19/32nds of an
inch...

Well, if you have ignition points, sooner or later you're going to be
changing them, so you might as well figure on buying a wrench that fits
the bolt that holds the terminals on the points. I used to have to
clean and regap the points every 3000 to 4000 miles, and I would use a
set of points until the springs gave up and wouldn't close the points
anymore.

If you can see the points sparking, they are burning, and the contacts
will look rough and grey. They need to be smooth and shiny, so I used
to buy a thin points stone to clean them.

If the intake valves are too tight, the engine will spit back through
the carburetors. If the exhaust valves are too tight, it's possible for
the exhaust to backfire.

 

stretching ths spring is a big no-no. Something is causing the slide to
move slow. Find out what it is. Usually the slide is gummed up when it
moves slow.

 

Cleaner came out brown? Some rebuild. Whatever that mechanic did, it sure
as hell wasn't a rebuild.

pierce

 

When some of my idle jets were clogged and I tried to rev it up, it would
backfire. Seems that there wasn't enough gas to properly fire, but it
would ignite in the exhaust pipe.

pierce

 

Review your assumptions.

Assumptions:

1 - The diaphragm sealed fine. Unless you've got see-through carbs, you
can't really confirm that a diaphragm is sealed properly except through
performance.

2 - All the passages were cleaned. How do you know you got every
passage? If you missed one, you wouldn't know it, would you?

3 - The manual is accurate and complete. The latter is a bigger problem,
IME, than the former. Trust your own eyes and brain more than the
manual.

4 - The spring had weakened with age. I've never heard of this happening
to a diaphragm spring. Also, why would it only have happened to *one* of
them? They're all the same and have had the same life. And it's not
like they're under a lot of stress.

5 - The CV piston and its cylinder are clean. There's not much clearance
in there, and a small amount of crud can interfere with the piston moving
correctly.

 

Today's update...

I should've learned after I checked the carbs NOT to trust my mechanic.
"Head gasket is leaking, but its not bad, compression is OK" he said.
150 psi on the right cylinder...90 psi on the left cylinder. 90 PSI! And
that took a good 5 seconds of cranking, it wanted to stay down at 60 psi.

Looks like that'll be the next step. Is there any way to be sure that
the problem really is the leaky gasket, or maybe piston rings or
something more severe? Or do I just need to go in there and find out?

I replaced the points and condenser. The points were, in fact, charred
over, and there was no lubricant on the point cam or the felt. The
timing was also retarded as far as it would go.

Now it idles closer to 1000 rpm, not smoothly, but it doesn't seem to
want to quit. The carbs are out of sync again after my episode trying to
equalize the pressure between them...

Maybe my fuel economy will improve as well. Valves and gaskets are next!
There is light at the end of the tunnel.

Thanks to everyone who's contributed to this thread -- its helped a ton.
I'm definitely returning to the carbs after I have adequate compression.