Honda '72 CB450 Rough Idle After Carb Rebuild

This is my first carb rebuild. It will start out idling high around
3-4k RPM then gradually decrease in RPM's until it dies unless I rev it
up again. I felt the exhaust pressure which feels equal per side but
the right exhaust feels cold like no gas is burning in the cylinder but
there is spark at both plugs. The plugs are new NGK B8ES & gapped
according to the manual. I adjusted the idle mixture screws 1&1/4 turns
out. Thanks for any help.
Mark

 

<VBG>

Here we go again.....

 

This is my first carb rebuild.

Don't ya just love threads like these??? lol

 

This is my first carb rebuild. It will start out idling high around

OK....... if you feel a cold exhaust pipe.....guess what?? that
cylinder ISN"T igniting.... which means either your plug wire isn't getting
a spark thru the plug, or you screwed up your carb rebuild (prob not the
case).....I remember doing mine.... (both changing plugs AND doing a carb
rebuild)... mine turned out to be the plug wire. Hopefully your case will
be the same....
How old are the wires? maybe they've become old, dried out, and maybe
even frail for being from 1972? Maybe the wire(s) decided to stop
conducting spark because of a break somewhere.....sooo..I would check for
spark first.....

Let us know how you make out..

Fwed

 

Thanks for your reply. Yes I am getting spark & I did mention this in
my post but it might not be enough spark so I'll take out the coil &
have it tested. The plug wires are part of the coil & the resistance
from cap to ground was around 23K ohms per wire. Since I'm totally
inexperienced with carb rebuilds I'll check over the right-side carb to
see if I didn't do something correctly. I just bought this bike as is
so I don't know the history of it. The seller said that "all it needs
is a carb rebuild".

 

Bwaahahahaha! Yep, been there, done that. The answer is: "If it's so
simple, why don't you sort it out and sell it for a higher price?" and
the true answer to that is "I've tried, it's driven me mad, I'm
unloading it...."

Those early Honda CV carbs are horrible things. IIRC, you can't even
alter the needle position on them unless you engineer a system yourself.

Posted before but worth the repeat - three or four years ago I rebuilt a
CB500T which, as I'm sure you know, is a stroked version of the CB450
without any of the charm.

Would it run right? Would it hell. It kept sooting up carbs, despite
running stock carbs, needles, jets, exhausts and air filters. I had a
spare set of carbs (complete donor bike) and swapped over slide
assemblies, needles, jets and everything else to sort it out. No joy.

I re-checked ignition timing (discovering on the way that Honda's points
set up on this model, amazingly, doesn't allow for truly independent
setting of right and left pots), and swapped over coils, condensors,
plug caps etc with the donor bike.

In the end, I consulted with a guy in the UK who drag races the things
(!) and he said that the carbs were such horrors that when they got
gummed up, no amount of cleaning would sort them properly. Maybe
ultrasound. A replacement set of guaranteed-running carbs was the only
real solution.

Well, he had a set of such carbs, and I had a spare engine he wanted, so
the deal was done, and yes, it ran perfectly after that.

Still a horrible bike, mind. The 450 is much better.

But if it's been standing for a long while, this may be your only real
cure. Or stick a pair of Amals on it, which apparently works wonders.
However, like I said, those early Honda CV carbs are pure shite.

 

Indeed.

(Just for the record, Haynes do a book specifically on carbs which
covers slides, CVs and weirdos pretty well. Worth the price of
admission just for the photos showing where that bit which just fell out
came from. There's a good website or two out there as well.)

-----sharks

 

That's worth knowing.

Have you seen their manuals on the human body?

 

Ah.... saw the part that said spark at both plugs, but was unclear as to if
it was at the wire, or at the bottom of the plug....
Anyway, I had an instance where I had spark at the tip of the wire, but
when I attached it to the plug, the cylinder ran cold.... scratching my
head, tearing my hair out, thinking it was a coil etc...it turned out to be
a bad connection at the plug....once I cut the wire a little and reattached
it to the wire end, the bike was back up and running.... (this is a 79 honda
and the wires I think are original, or close to it still)

Fwed

 

(Disclaimer for the ubiquitous trolls in rec.motorcycles.tech: I
emphatically deny having ever owned a 1972 Honda CB-450, but I know how
to fix Mark's problem, so I am addressing this post to Mark.No trollish
comments are required.)

Mark, I wouldn't worry too much about the ignition system. The problem
is probably with your carburetor cleanliness and adjustment. It just
doesn't take a lot of spark to fire a mixture that is close to being
chemically correct, i.e., within the range of combustibility as regards
fuel/air ratio.

The cold exhaust pipe tells me that your pilot (idle) jet or idle
mixture passages and ports are plugged up, and you've probably
overcompensated by turning the idle speed up too high.

Constant vacuum carbs have a vacuum piston (like 1970's CB450's) or a
diaphragm-operated slide. Whichever the carb has, it is pushed up by
air pressure underneath as the engine vacuum pumps the air out of the
space above the piston of slide.

The piston or slide pulls a tapered needle out of an orfice called the
"needle jet". That part of the carburetor begins operating when the
throttle is open half way or more.

When the throttle is open fully, the piston or slide pulls the needle
all the way out of the needle jet and then the fuel getting to the
engine is controlled by the fixed orifice of the main jet.

But, you are never getting to the midrange or the wide open throttle
range of your engine because the engine is starving for fuel in the
idle range (sometimes called "slow running") of the carburetors.

Since it is starving for fuel on the one cylinder, that cylinder can't
make enough vacuum to lift the piston or slide.

There are two ways for the idle fuel and idle air to mix, but they
start out the same way.

Air comes in through a pilot air jet in the intake of the carb.
Sometimes the pilot air jet is concealed inside the carb though.

Idle fuel comes through the pilot jet (sometimes called "idle jet").
That's in the float bowl. The tiny hole in the idle jet has to be
spiffy clean. Sometimes the idle jet has cross-drilled air emulsion
holes that need to be cleaned out. If you have to remove the idle jet,
don't even try taking it out if you don't have a small slot screwdriver
that fits the jet exactly.

The idle air and idle fuel meet and mix. In early constant vacuum
carburetors (like Hondas used) , there is usually a pilot air screw
(sometimes called "idle air screw") which is upstream of the vacuum
slide. That's what you have probably been adjusting to 1.5 turns out.
Since that screw adjusts the idle air before it mixes with the idle
gasoline turning it clockwise reduces the amount of air and makes the
idle mixture richer.

Maybe turning that screw half a turn clockwise will fix your problem,
IF the idle passages and idle jet and idle ports are clean.

On later model constant vacuum carburetors, the idle air and idle gas
mix and the idle mixture screw actually does control mixture. So,
opening those screws by turning them counterclockwise make the mixture
richer.

That type of carb would have the idle mixture screws downstream of the
throttle butterflies.

Then there is the issue of transition ports. CV carburetors usually
have a pattern of idle mixture discharge passages just downstream of
the throttle butterflies.

The edge of the throttle butterfly will barely cover one of the
transition ports, and, as the throttle butterfly is opened, extra idle
mixture will pass into the intake tract of the engine, helping it speed
up.

Your described symptom of the engine running too fast at first and then
slowing down and stalling suggests that the pilot (idle) mixture
passages and ports are plugged up on the one carburetor and that you
have turned the idle speed up too high to compensate.

The whole idea of turning carburetors is to get the engine to idle
smoothly at the specified RPM with the leanest idle mixture possible.

If you have a plugged up idle mixture system and have compensated by
adjusting the idle speed too high, you wind up with one carb that is
giving the engine too much gasoline, while the other carb isn't giving
it any gas at all.

 

The edition I've got here is "Motorcycle Carburettor Manual" by Pete
Shoemark, Haynes #603, ISBN 0 85696 603 7. Apparently, minutes after I
bought it they brought out a new edition ("Fuel Systems Techbook", by
John Robinson) which covers some basic EFI stuff as well.

*checks www.haynes.co.uk* ... my ghod, they do too! But if I ever wake
up in hospital and the doctor is consulting a Haynes manual ...

-----sharks ("fabricate this simple tool")

 

"It says here, nurse, that reassembly is...."

<fx: turns page>

"....the reverse of disassembly."

<fx: pause>

"Oh shit."

 

FB wrote:

Take it to email, then.

None made.

 

Thanks for your input. I just had a similar problem with this bike. One
of the plugs was dead with no spark but I only found that out after
assuming the worst and troubleshooting it for way too long of a time
thinking that one side of the coil was probably bad so after this when
encountered with ignition problems the first thing I'll do is put in
new plugs to rule them out as suspects.

 

Thanks for your input. After also working with diaphragm carbs on a
GS850L I'm not to fond (at this time) of carbs in general but at least
I'm learning something!

 

I very much appreciate your detailed post and will print it out and
study it.
Thanks for your concern and for taking the time to share your
knowledge.
Mark

 

Spark plugs sometimes seem to just mysteriously "go bad", and the only
thing I
can imagine that may be happening is moisture condensing on the
porcelin insulator.

There were several times that I couldn't get an engine started and I
heated up the spark plug and reinstalled it and got the engine running.

Old style steel core spark plugs used to be prone to overheating if the
engine was run hard and the exhaust would start "shooting ducks". It
sounded like a shot gun. Then the spark plugs were ruined, they would
never work right again.

More modern spark plugs have a copper core and aren't so sensitive to
overheating.

And, motorcycles that have ignition points need a lot of points
cleaning and
re-adjustment, because the resistance across pitted points is higher
and it reduces current through the coils, so the spark is weaker.

I must have bought 100 sets of points for the last two motorcycles I
owned that used points, but I haven't had any trouble at all with
electronic ignition systems that use a pickup coil to replace the
points.

You might want to look at the www.denniskirk.com online catalog to see
if there is an aftermarket conversion kit for your machine. Electronic
ignition conversions were made for many popular Hondas by the Dyna
company.

 

That strongly suggests to me that you had a duff condensor on (all of)
them, because lots of the bikes I own use points (OK, only one of the
current stable does), and I generally reckon to get a minimum of 10k
miles out of them and usually something like 15k.

50 sets of points for one bike is a minimum of 500,000 milers....

Unless they're strokers, of course, but even the points-fed strokers
I've had could be relied on to deliver a minimum of 5k miles a set.

Or you were using cheap pattern points, which is never a good idea.

Or maybe you added a zero ;-)

There are aftermarket electronic ignition kits for the CB450 - Boyer
Bransden, for one - and they're supposed to make a helluva difference.

 

I was working on it today & it is running much better and firing on
both cylinders. I'll get it liscensed & take it for a road test.
Thanks again for your help