Backfiring

My KZ750 backfires (or that's what i'm guessing...a pop-pop-pop when I
coast at higher speeds). Every once in a while there'll be a loud one.
Is this likely to be caused by a carb that needs adjusting? Maybe too
rich or lean?

The carbs were just rebuilt and adjusted but I don't completely trust
the mech who did it.

The bike also has trouble idling at anything below ~1300 RPM - it'll
just stall.

Any ideas? Thanks in advance.

 

You need to clean out your idle jets and idle passages and adjust the
pilot mixture screws, which are probably concealed under EPA
anti-tamper
plugs that need to be drilled out.

As I have covered this subject a bazillion times in this NG, I won't
write all that up again, you can google for it.

Alternatively, you might try running 3 ounces of Berryman's B-12
Chemtool Choke and Carburetor Cleaner through a full tank of gas.
repeat until you've used up the whole 15 ounce can. You can get the
liquid B-12 at Wal*Mart or any good auto parts store for about $3.00.
You'll know the stuff is working when you have to keep turning the idle
speed down...

 

Maso,

How long had the bike been sitting?

BTW krusty is spot on. Although I have not tried Berryman's liquid, I have
tried the Yamaha liquid carb cleaner. It did a great job and it seems
fairly gentle on plastic parts. it just took a long time to dissolve that
ancient varnish.

Krusty,

Have you tried the Yamaha liquid carb cleaner? It smells unlike anything I
have encountered in the automotive field. I think it smells something like
SpotShot(tm) carpet cleaner which is very mild. BTW SpotShot is a great
product. I consider it the best carpet cleaner available.

pierce

 

Mark Olson recently said that the Yamaha cleaner contains xylene.
Berryman's
B-12 also contains xylene and acetone.

There's no Yamaha $tealer$hip here in Cowpoop...

If I went to Yamaha to buy their product, I'd have to drive 60 miles
round trip and probably spend five times as much money to get what I
can buy locally from Wal*Mart for $3.00...

 

The bike hasn't sat long since I had the carbs rebuilt.

Do the idle mixtures usually need to be adjusted after the carbs are
rebuilt?

 

I think i am confusing two products. MarineClean and Yamaha carb cleaner.
Yamaha may indeed contain Xylene and Acetone.

The Berrymans B-12 Chemtool I picked up contains Methanol, Tolulene,
Acetone, mixed Xylenes (I didn't know there was more than one kind), MEK,
and Butoxyethanol, and Isopropanol.

If that doesn't clean it, nothing will.

 

How long did it sit before the carbs were rebuilt?

It depends on who did the rebuild and if they noted where the mixture
settings were and if they know where a good starting point is. Generally a
one turn or a fracton thereof is all it should need to make the idle miture
optimum.

Krusty is right. The idle jets are clogged completely. The cheapest way
to fix this is to follow Krusty's instructions to the letter.

Once the bike idles good, then generally the bike will run pretty good
under load and at speed.


pierce

 

Be sure the headers and exaust are not leaking

 

Then it wasn't much of a rebuild.

 

Thats what I was thinking, shouldn't they have cleaned the carbs when
they rebuilt them?

But who knows, I don't really trust them.

 

That I have no idea. The guy I bought it from turned out to be a
complete con-man. He told me (and said in the auction) that the carbs
had been "gone through and were better than new" although they leaked
gas like no other and one had a punctured float.

He said he had it only for a year, only had ridden in a couple of times,
but who knows.

 

Well, a lot of guys work on cars and they don't realize how tiny the
idle passages and ports are. They've heard about "throwing a kit into
it" all their shade tree mechanic lives, so they think a kit is some
kind of magical cure, that a one of bunch of a bunch parts is bound to
cure the problem.

I looked at the carburetor drawing for the 1980 750 LTD-2 on
www.buykawasaki.com

You can look at the parts diagrams there without registering if you
click on the link that talks about ordering owners manuals. It leads
you to a series of screens where you can select whether you want to
look at motorcycles, watercraft, atv's, whatever. Once you get to the
motorcycle list, you can select the year 1980, and the engine size,
750. The list led me to the 1980 750 LTD-2 which has two carburetors,
so it must gots to be a twin, no?

Anyway, on the carburetors diagram, you can see #79, called Plug, Pilot
Screw on the left side of the lefthand carburetor. If you look at your
motorbike and you see that plug has been removed, you know somebody has
tampered with the carburetors, that's the anti-tamper plug.

What you need to do if the plug has not been drilled out is to follow
the instructions I wrote up about that. Google for "kaybearjr@aol +EPA
plug". You can follow the instructions for how to remove the pilot
screws and not lose the original settings.

The whole object of adjusting those pilot mixture screws is to get the
engine to idle at the specified RPM without stumbling and idling
erratically at the lean end, or sooting up the plugs and making the
exhaust smell like unburned gasoline at the rich end...

You go for the lean end to save gasoline, make your engine easy to
start and warm up and have good acceleration just as you open the
throttle.

What most shadetree tuners do is open the screws to far, they think the
engine should speed up when they turn the screws counterclockwise, and
the engine doesn't do that. They turn the screws so far and the exhaust
sound starts thudding instead of having a crisp sound to it and maybe
they even smell raw unburned gasoline coming out the exhaust.

The idea of adjusting the pilot screws is to turn them *clockwise* a
little and watch the RPM rise. Then you turn down the RPM using the
master idle knob and turn the pilot screws clockwise a little more and
turn the master idle knob down a little more, and so on.

Eventually, you'll find a point where the pilot mixture screws are
making the mixture so lean the engine idle starts stumbling. Then you
can turn the pilot mixture screw counterclockwise 1/4 of a turn and
that should be good enough.

Then you test ride the motorbike and see if it has good acceleration as
you just open the throttle. It shouldn't spit back out the carbs,
that's too lean. It shouldn't pop out the exhaust, that's too lean
also, you need to open the pilot mixture screws about another 1/4 of a
turn...

 

Sorry to hear you have been taken. I hate liars.

On the plus side, if you are careful and have some patience and some
mechanical abilities, you are likely to be able to get the bike running
well for just a few dollars additional expense.

You might as well plan on doing all the routine maintenance. Valves,
brakes, etc.

pierce

 

Thanks for the info, krusty. I will try that. The plugs were gone.

Should the screws necessarily be in the same position between the carbs?
I noticed one is turned more than the other.

 

I hope so. I did some maintenance already, it's shaping up pretty well
overall. I can tell the bike was neglected though - the chain is crusty,
the oil was low and god knows how old, the brake fluid was black syrup,
indicator lights burned out...

I did notice after I bought it that the head gasket is leaking oil,
though That one won't be fun.

 

What about acceleration from a stop when cold? If it stalls, I would
suspect bad plugs.

 

I would do a compression test (buy or borrow a $20 gauge) and replace
the plugs if you haven't.

I would pull off one or more of the bowls and inspect the idle jets,
maybe before and after the carb-cleaner-in-the-fuel routine. Maybe you
need a stubby phillips screwdriver or maybe one of those S-shaped
lever-handled screwdrivers ($2 at Sears). That way you don't have to
remove the carbs. I expect you can get a look at the idle jets in five
minutes each. You can get low-cost replacement idle jets (aka pilot
jets) at denniskirk.com or siriusconinc.com.

 

I would expect the screws to be pretty close to each other in the
number of turns out from lightly bottomed.

The carburetors are precision instruments, some of the parts are made
to fantastically close tolerances. When the carbs were brand new, the
only reason there was for setting the pilot screws any different from
each other was they were using an exhaust gas analyzer to adjust the
screws to get the emissions within spec.

With separate exhaust pipes, you can run the probe right up each pipe,
but watch out for systems with equalizer pipes between the two main
pipes, one cylinder will be feeding excessively rich mixture while
you're turning the other screw tighter to lean up one carb...

The average shade tree mechanic doesn't have an exhaust gas analyzer or
any other instrument except his screwdriver and his ears and nose and
his brain.

If you start out with both of the screws about 1.0 turns out and start
up the engine from cold and the idle RPM runs away and you have to keep
fiddling with the choke to get the RPM down, you know that the pilot
mixture screws are too far open and the idle speed is set too high to
compensate for the rich idle mixture, so you turn the idle RPM down and
screw the two pilot mixture screws in about half a turn and try
again...

If the engine RPM doesn't run away when the engine is cold and you warm
the engine up and twist the throttle quickly and release it, if the
idle RPM hangs up and takes too long to come down to the specified
idle, you turn the pilot mixture screws in another 1/4 of a turn and go
ride the motorcycle. By this time, if you've been paying attention, the
pilot mixture screws are only open about 1/4 of a turn...

If you go for a ride and the exhaust doesn't go fartPOP! when you roll
off the throttle and the idle RPM doesn't run away when the engine is
hot, the
pilot mixture screws are adjusted about right...

 

Hope the jets are removable. The ones on my 1981 are not. They are
pressed in.

pierce

 

wow