Topic: CB650 carbs

[rudy]

Well, first post and I'm already asking questions!  I'm getting an 82' 650 engine running for a guy.  I have CV carbs from an 81' that I've taken apart and dipped overnight and cleaned with aerosol carb cleaner and compressed air.  All looked good when I was putting them back together.  The carbs weren't in bad shape, but where fairly dirty.  Bought the new "T" vent fittings between 1 & 2 and 3 & 4 for an arm and a leg from Honda, but don't really have anything to connect them to, so they sit in the air pointing up (as per part fiche).

Anyway, on to the problem.  First of all, all four carbs overflow right out the throats as soon as I crank the engine and the vacuum operated fuel valve opens (amazingly doesn't leak!).  Ok, I can deal with that.  Just fill the bowls and then shut off the fuel to see if it will run.  Well, no dice.  Fires here and there a little, backfires out the carbs and pops pretty nasty randomly out the exhaust.  All four pipes will get warm if you crank long enough.  Spark is great, as I have ignition directly wired to a switch on my engine stand.  Seems to have pretty good compression, but I don't have the 12mm adapter for my compression tester or leakdown tester (waiting for them to arrive).  I've checked valve clearances as well, all are good.  Tried adjusting ignition timing while cranking, but this changed nothing from retard to advanced.  Tried adjusting idle screws as well, and playing with the throttle while cranking.  Throttles are bench synced.

How picky are these carbs?  Do they really need to be cleaned out with wire in all the little holes even if it's soaked overnight in full strength kleen-flo?  I should get some kind of firing with the main air box, but no air filter, right?  I've tried putting my hand over the inlet to the air box to provide some restriction, but no change.  I've tried choke, I've tried partial to full throttle.  Tried pumping the throttle to squirt fuel with the accel pump.  One or two fires, then nothing.  Even if I keep pumping.

Fires a bit if I spray aerosol carb cleaner in the airbox opening, so that leads me to believe that the carbs aren't putting fuel in the air at all.  Plugs smell a little like fuel, but are dry for the most part, but sometime randomly damp and never soaking wet.

Now to address the overflow problem.  All floats (plastic ones, non-adjustable) are perfect.  No holes or damage in any way.  Tested in a bucket of water, no air bubbles and they float forever.  Tried squishing a bit under water, still no leaks.  Inspected float valves.  Needles look new, and are viton tipped, so they must seal pretty nice.  Not cracked or worn.  Checked to see if the valves will actually close when float is up.  They all do.  EVERYTHING moves freely and is clean.  Is there some other place that the gas would be flowing from and into the carb throats?  It's not just a little leaky, it's full bore POURING out of all 4!  These are my first Keihin's, and I can't really say I'm impressed yet.

One other thought.  Would it make a difference that my auxiliary tank is maybe two feet above the engine?  Could this create enough head pressure to flow past the float valves?


Well, that's one helluva first post.  Anybody have suggestions?

Rudy

[martino1972]

did you by any change install the floats upside down? would be an honest mistake that many people made...

doesnt matter how high the tank sits,thats not ur problem...

[rudy]

From what I can tell, floats only fit one way.  The needle only fits in one spot.  Believe you me, I checked as it was my first guess.

[martino1972]

how about float hight..?? just trying to think along with ya..

[rudy]

Float height not adjustable on plastic floats.

[martino1972]

well,theres a saying,always look for the stupid little things...
like,are the plug wires hooked up to the right cylinder....
how new are the plugs....
etc..
you have to pull the carbs anyway to fix ur leaking,inspect them a million times,full bore overflowing is not normal,should be easy to find whats up....

i would say,fix carbs first,get that out of the way...maybe try new plugs...
then when the most ovious things are iliminated,its time to start checking timing etc..
like on brakecleaner it should fire up...so im leaning towards plugs or timing...
a plug showing spark outside the engine doesnt mean that it also fires under compression,i got fooled one too many times by bad plugs..

[Hush]

Owning a CB650 and having rebuilt all 4 carbs and managing to fit all 4 floats in upside down I'll vouch it's easy done especially if you are used to other Jap bikes, the floats look correct when upside down.
But......if you do that nothing flows, no gas no way so I don't think that is your problem.
These carbs are notorious for blocking up the little galleys and tubes especially if the bike has been left for any length of time with old gas in them.
My best guess.....the float valves...replace them as a set...cheap fix and almost guaranteed to fix your problem.
Last ones I got cost $48 for all 4.
They may look good but old ones develop a bad seal, change em and get on with running the bike.
Last piece of advice for someone not used to CB650's is you need to hold choke right out physically as you press starter, these machines are choke junkies esp when cold.
Happy highways.........Hush

[rudy]

I know what you mean about them looking upside down.  I took a second, and third look thinking it was wrong, but it's right.  Obviously fuel does flow.  I know these bikes LOVE the choke, and I've held it on for quite a while.  These carbs sat for a number of years in a box with no gas in them.  Probably sat for a while before that with some gas in them, but I don't know for sure.

Will tear them all apart again tonight and wire all the holes and check everything again and again.  Make sure that everything is good to go.

It should at least start with stock jetting, right?  It has stock airbox (without filter, but put my hand over opening for some restriction) and a 4-1 MAC with a Micron can from an R6.  I know it's a little lean, but I think the 81 stock carbs came a little richer than the 82 carbs did.  I seem to remember that from somewhere, don't remember where.

Will let you all know how things go.

[eurban]

If you immersed your friends carbs (either the whole rack or individual carbs) in strong cleaner overnight and failed to remove various plastic and rubber pieces first then you will have likely damaged / destroyed parts. Hopefully you were careful in your technique?  Regardless, the very tiny passages (like those in the pressed in pilot jets, which need to be removed to be properly cleaned) in the idle circuit will not often come clean if badly clogged without a good soaking and a manual chasing with a strand of copper wire and carb spray. . . You should not expect a set of carbs that are overflowing fuel to start the motor.  Fix this problem.  . . .You can easily bypass the vacuum fuel safety device by running your fuel line straight to the intake on the carb.  This will take it out of the equation, even though it appears to be functioning by your description.  In general, these engines will start more quickly with the safety device out of the circuit. . . . If you (or Hush) need to hold the choke knob up by hand to keep it from self closing the you can adjust the knurled tensioning nut located underneath the rubber cover that the knob seats on when the choke is closed.  Tightening this nut increases tension on the choke knobs shaft and keeps the knob from auto closing. . . . . While you seem to have gotten a good number of responses here, this type of post properly belongs in the "SOHC4/bikes"  section rather than in the "Project Shop".  Much greater readership in that section BTW.

[rudy]

When I dipped them, I did take everything off.  All parts that were screwed on, came off.  Didn't take the pressed in jets out, how do you do this?  Not that it really matters too much, as I got the pilot circuits cleaned last night.  Only one was plugged solid.

Anyway, took the carbs apart again last night. Stripped everything down and ran a wire in to unplug everything except for one of the three tubes that stick down into the float bowl (one has main jet, other has pilot, last is choke? This last one is the one I couldn't unplug). Don't know for sure where it goes to, or what it does, but it's plugged SOLID in all 4 carbs past where it takes a turn to the outside of the body. After I ran wire in everything (was all pretty clean still) I boiled each carb body in plain water at a RAPID boil for about 10 minutes each. I've read that this is good to soften up the varnish. Well, it worked! I got all the passageways for the idle circuit to blow completely clean. Then blew some carb cleaner through them just for fun. Put them all back together, no more flowing from bad floats, but no starting either.

Put brand new plugs in. Closest I got to starting was all four cylinders fired consecutively....... once. Then back to sputtering.

Original coils were about 2.5 ohms on the primary and about 12k ohms on the secondary. I don't have anything that will hook up to those coils though, so I'm using the original ones from the GS as they have boots that are in one piece and attached. These are 4.5 and 22k ohms. They still spark decently, but should I be using the original ones? I REALLY don't want to pull out my nice new green Dynas and wires for this POS.

I noticed that all four pipes were getting hot after cranking for a while. This is good. When I pulled the plugs afterwards, 1 and 2 were wetter than the others. This says to me a problem with spark. But I've plugged an extra plug into all wires and it never misses a fire.

[Hush]

Can only be the timing then, logical conclusion as you have followed the hallowed path of carbs/plugs, next must be timing.
Careful to remember that these bikes have 2 sets of timing marks, one is for firing the plugs ("F") the other is for timing the valves ("T").
Drove me nuts when I started trying to time mine.
Oh yeah forgot about them, valve clearances are pretty critical for these bikes......better do them too.

[eurban]

Yes, seems like time to move on to the ignition.  That said, the pressed in pilots have both the central tiny metering orifice as well as a number of cross running holes.  You won't be able to completely clean them with out pulling them out.  Soft jaw plyers or regular plyers padded with a leather strap works fine.  Twist and pull; tap back in place.  Also the crud that you dislodge when you clear them (when they are left in place) doesn't get flushed from the idle circuits.  Removing them for cleaning really is an important step but it does seem like something else is going on . . .  .

[rudy]

So..... how exactly would I get a pair of pliers down into that tube where the pilot jet sits?  It's way down in the bottom of the tube.  If it was just in the top of the tube, I'd have taken them out already long ago.

I've tried setting the timing too.  Used the timing light to make it good and even tried retard and advanced a little as well.

Will check the clearances again.

[eurban]

Pilot jet otherwise known as the slow jet is a small brass cylindrical object (about 3/16" diameter and about 1/2" tall when installed in the carb body) pressed in or screwed in near where the tower for the main jet / emulsion tube is located.  Remove bowl, you should see float mechanism, emulsion tube with main jet, pilot jet.  It is possible that your 82 has screwed in pilots in which case you may have already cleaned them; some of the parts diagrams I have seen show a screwed in pilot in 82.  My experience is with an 81 CB650 custom that has essentially identical CV carbs but Honda may have wised up and used screw in pilots on the 82s.  If yours is pressed in then remove as I described previously.   If you are referring to the needle jet (bushing that the main jet needle slides in and out of) it should most certainly be removed in carb cleaning.  Remove the carb tops and main jet needle and push it out (down) from above with some sort of sturdy plastic object.  You might also be talking about an air jet in which case you can't remove em. . . .I couldn't find a good pic of the CV carbs on the 650 but if you need more info, take a pic of the carb body undersides with the bowls removed and I can point it out . . .

[wbm3640b]

Hello: I've been reading your carburetor saga with great interest. I, too, am working on a friend's 1981 CB-650.  Which is impossible to start from cold.  With starter fluid it will start with full choke and when it warms up is actually ridable.
However, I observed the same thing you did re: the rear jet tower orifices being totally plugged- that is the drilling that angles up from the jet tower to the main chamber. Are you saying that you actually cleared these by boiling?
And,,, what was the final solution to your no start situation??
Many thanks,

Bill Mitchell
661-978-5828 (tel. call welcome)

[cb650]

Hush the CV carbs are near impossible to get the floats upsidedown inlike the mech.    Make sure the "non removable" idle jet is clean.  You can remove it if your carefull.

[wbm3640b]

Oh by the way, the cb650 carbs leaking profusely when the float needles appear in good condition is caused by the accelerator pump adjustment.  There must be 004" + clearance between the pump shaft and the actuator linkage.
Bill