'79 CB650 carb jetting

I have what seems to be a common problem with CB650s where you have to use
the choke for a good while before you can expect it to idle properly. I have
to choke it for about 6 miles before I can comfortably stop at an
intersection and be able to accelerate smoothly without choking it.

Awhile back somebody on a forum, likely this one, mentioned that they solved
that problem on this bike by increasing the jets size x 2. I live in
Minnesota where extreme elevations are of no influence, but changes in
temperature have some affect on length of time to warm up (this affect is
normal but the length of time/miles is still excessive).

If the poster who jetted his carbs up two sizes is among you, please
elaborate a little on how I should approach jetting it (I have no access to
a bike shop of any kind), i.e. would your solution apply to an environment
like mine. I would appreciate input from anybody with experience in this
area with this bike.

Thanks

frog

 

look up spanky's jetting guide...that should help a little. I've
missorientated my link to the page, fortunately I printed it out.

 

look up spanky's jetting guide...that should help a little. I've
missorientated my link to the page, fortunately I printed it out.


Thanks. I checked out Spanky's -- lots of stuff there, maybe too much for my
brain. I know there's a guy out there who solved my problem exactly. Guess I
will also try SOHC forum when I get time.

frog

 

That was me.

The bike was idling OK, and performing fine until about 7000rpm with the
throttle wide open, when it gave every sign of running lean.

So I did a plug chop - held it wide open under load at 7000 rpm, and ht
the kill switch. Then examined the plugs' colour. Yup, white. Not good.

The bike was running standard air filtration, standard exhaust and (it
turned out) standard jues. But Honda jetted the damn things *very* lean,
and added accelerator pumps to try and compensate. Could be there was a
problem with the pump...

Anyway, I went up not one, but two sizes on the jets, because it was
running so badly at the top end, and it was like magic. Sorted, and
clean lovely carburetion through the rev range.

I can't believe that it was so bad on standard jets, nor that it should
have been like that from the factory, nor that such a crude fix should
achieve the transformation it did, but the fact was: it worked.

I've stll got the old jets in a tin in the garage somewhere.....

 

The symptoms you describe are NOT solvable by the traditional methods
of "re-jetting."

When ignorant shed mechanics like Neil Murray or Keith Schiffner talk
about re-jetting a set of carburetors, they mean that they
experimentally throw larger *main jets* into the carbs.

But, as I mentioned above, this is NOT a main jet sizing problem.

Constant vacuum carbs such as those mounted on your CB650 are designed
to run on the slow jet most of them time and only use the main jet at
3/4 to wide open throttle.

If *any* jets are changed at all, the ones to be changed are the SLOW
jets.

And changing slow jets by 2 sizes is a far different proposition than
changing main jets by 2 sizes, because so much less gasoline passes
through the tiny orifices.

But, before I would make the decision to change slow jets at all, I
would experiment with the idle mixture screws, turning them in or out
1/4 of a turn at a time to see if I get better throttle response and
faster warm up.

 

I wouldn't expect the main jets to have any effect on idling, the idle
jets are controlled by the adjusting screw so to make the mixture richer
there would be an adjustment. I think the Honda's are made that way for
economy of operation.

 

Um, as my bike was runing fine at idle, fine in the mid-range, and weak
at high revs, this was *exactly* the right thing to do.

Doh.

And it worked

 

That may have been true for *your* particular problem, but frogliver
described the
common lean idle mixture problem that plagues *all* carbureted
motorcycles, and is aggravated by using gasohol.

The tiny idle jets, ports, and passages get increasingly plugged up
and the rider finds that the engine will only run on the choke.

The quick non-mechanical solution to frogliver's lean idle mixture
problem would be to mix 4 ounces of Berryman B-12 Fuel System Cleaner
with a full tank of gasoline and go for a ride to clean out the
carburetor internals.

B-12 contains powerful solvents like acteone, toluene, and methyl
ethyl ketone which quickly dissolve gum and varnish from the internal
ports and passages and jets.

If mixing B-12 with the gasoline and using up the whole tank of
gasoline doesn't work, the next step is to remove the idle mixture
screws and squirt *aerosol* B-12 through the holes the idle mixture
screws came out of.

 

Yes.

I.

Know.

Which was why I referred specifically to my problem right at the start,
and emphasised that it was a top-end problem.

I did this because I am 100% certain it was me he was referring to,
unless you know someone else who has posted in this ng about curing a
CB650 problem by going up on the jets.

You're not being bright here. No surprise.

 

"Wrong, Do it again!"

"If you don't eat yer meat, you can't have any pudding. How can you
have any pudding if you don't eat yer meat?"

"You! Yes, you behind the bikesheds, stand still laddy!"

 

Ah, good. The standard Krusty response when it realises it's fucked up
)

 

You never get *me* behind the bikeshed.

Pervert.

 

Krusty, stfu you plagerizing cock biting fatherless cur.

 

oh **** off you little shit. You don't know your ass from a hole in
the ground and you don't have a clue.

 

that's were you are busy raping little mexican kids you loser.

 

So, hows that T500 Suzuki project coming along?

Hmmmm?

 

better than you rapidly shortening life you child rapist. 8^) Now go
drink a bottle of lye and lay in teh street...or you can will me
everything and I promise you wont feel a thing.

 

"That was me."

Yes, I remember now. What I did not remember correctly was the reasons you
did that jetting. Sorry I didn't reply before you and Gus got heated about
it, but it appears like if it wasn't that issue there would be something
else to have your fun about.

From the SOHC forum I found the solution to the cold staring problem I have.
It is an ignition issue that another brilliant 1979 CB650 owner solved by
using a General Motors HEI Ignition Module and coils from a Chevy Cavalier
http://home.comcast.net/~loudgpz/GPZweb/Ignition/CBheiModCavCoil.html. It is
a project worth reading over for anybody.

As for the slow speed jetting, this guy says the problem with those carbs is
that those jets are pressed in, and the carbs lack adjustment capabilities
even if that "cold bloodedness" was a fuel issue.

I intend to check out the availability of the GM HEI parts (supposed to be
cheap, esp. if salvaged) and hopefully my brain will be able to handle it
given the instructions, illustrations, and photos.

Thanks for replying.

frog

 

That may have been true for *your* particular problem, but frogliver
described the
common lean idle mixture problem that plagues *all* carbureted
motorcycles, and is aggravated by using gasohol.

The tiny idle jets, ports, and passages get increasingly plugged up
and the rider finds that the engine will only run on the choke.

The quick non-mechanical solution to frogliver's lean idle mixture
problem would be to mix 4 ounces of Berryman B-12 Fuel System Cleaner
with a full tank of gasoline and go for a ride to clean out the
carburetor internals.

B-12 contains powerful solvents like acteone, toluene, and methyl
ethyl ketone which quickly dissolve gum and varnish from the internal
ports and passages and jets.

If mixing B-12 with the gasoline and using up the whole tank of
gasoline doesn't work, the next step is to remove the idle mixture
screws and squirt *aerosol* B-12 through the holes the idle mixture
screws came out of.


Thanks. I have tried your suggestions already to clean out the works with
little affect on the warm-up issue. It turns out these carbs lack adjustment
capabilities and the slow-speed jets are pressed in, which is all moot since
the solution lies in hacking the ignition (see my reply to the Older
Gentleman about using HEI modules from a Chevy Cavalier). The link there is
worth checking out just for the beauty of the project.

frog

 

You are right. The thing is the idle adjustment has no affect on the warm-up
period, it must be choked even to start. I wasn't sure which jets the Older
Gentleman had changed, just that he bumped up the jets x 2 which it turns
out was for totally different reasons than my problem.

It turns out to be an ignition issue according to a very astute and
imaginative CB650 owner who solved it by hacking the ignition with HEI parts
from a GM Chevy Cavalier. In my reply to The Older Gentleman there is a link
to this project that is worth looking for anyone interested in Hondas.

Thanks

frog