Topic: '79 CB650 Carb Issues

[John C]

A friend has the bike with 10,000 miles on it.  They bought it used about six years ago and it sat for four years as it never ran well.  The carbs have been through carb cleaner by the friend and I've been trying to help him get it to run better.  It won't run except by the choke below 3,000 RPM.  The idle passages and jets seem open by blowing through them.  I have the carbs on my bench and they are stamped '50A RC7'  They are unlike any in the Clymer manual as they have manual slides (non-CV) but have air shutoff diaphrams.  The only thing I've found is the small o-ring on the air shutoff cover to carb body is missing on the #1 carb.  I haven't disassembeld the block to check the rest of the carbs.

If the choke is off the bike instantly dies below 3,000 RPM.  It dosen't seem to hit on any cylinders.  Timing is OK per my timing light and the engine sounds normal.  My history is with the mid-60s bikes when I worked my way through college on Hondas, Nortons, & BMWs so air cutoff valves and other emissions ilk are new to me.

Does anyone have any ideas, or am I on the right track.  I have heard rumours of something else being the issue, but no clue as to what.

I've searched all the '650' threads and havn't found anything similar to this.

Thanks,

John C

[cb650PK]

Did you pull the pilot jets out? I like to clean them with thin cooper wire in addition to carb cleaner and compressed air. What are the idle mixture screws set to?

[John C]

Didn't pull the idle jets; air screws set at 2 1/2 turns.  We are at 5300' altitude - Denver, CO area.  I have subsequently found out the owner used carb cleaner and destroyed the # 1 cylinder air shutoff diaphram.  I asked about the other three..."OH, there is one for each carb?"  Looks like the next step is to disassemble the carbs from the brackets and see how bad the remaining three are.  He's to call his source and see if they have more pending my disassembly work.  A project for tomorrow...

John C

[trueblue]

Probably not much help to you but my 79 650 has the casting on the side of the carbs for the air shut off diaphram on the side but there is nothing in there, there isn't even a hole into the carb from memory and it runs fine.

[w1sa]

The air-cut-off valves are not strictly necessary for running. They are used to richen the mixture when the throttle is closed quickly (from higher revs).

They should (however) be maintained in proper working order or,  disabled.  Diaphrams and o rings that are torn or ineffective will also likely allow air leaks into the carb(s) to the manifold side (at o ring) and/or the air/fuel metering  line from the cut-off valve (thru a torn diaphram) to the low speed jetting outlet(s) in the carb throat.

What you describe (engine wont run below 3000rpm without choke) could be caused by the above.....It could also be caused by low fuel level in the carb bowls, blocked/restricted jetting/passages, mixture screw spec etc. or a combination of all the above.

[cb650]

I'm in denver to if you want some hands on with it.  Did u clean the idle jet out good?

[John C]

Carbs are separated and disassembled.  The other three diaphrams were stuck and gooey from the carb cleaner.  Float height OK at .500'.  I'll pull the idle jets and check passages although I can blow through them.  Man, how I like jets that screw in!  Thanks for all the suggestions.  CB, I may take you up on an additonal set of eyes.  I'm in Broomfield at 136th north.

John C

[cb650]

Are u aware the idle jets are "pressed" in?    They and be removed with care.  I actually just rode up by 136 and I-25 today.

[John C]

Yep, pulled the idle jets a while ago and looked through them and they seemed OK.  I also put one strand from a 20 gauge flexible wire through them.  Wire measured .0125"D so doubt it screwed up the jets.  Got three new diaphram kits on order from PartsNMore as well an oil filter.  Owner had replaced the #4 diaphram when cleaning the carbs.

Meanwhile I'll begin cleaning the rest of the carb parts.  UPS ground may take a while, but 2-day was out of sight.

John C

[John C]

More to the story.  Got the parts OK and rebuilt the carbs and set the idle screws to 2 1/2 turns.  Owner reinstalled today while I ran the valves; didn't find anything interesting there.  Got the bike started and still have the dying at below 3000 RPM when the choke is turned off.  Opened the idle screws to 4 turns and things were a little better, but not much.  Squirted carb cleaner around the rubber manifolds.  When well flooded, the bike idled at low RPMs, but with the choke on.  Finally, it died and would not restart, I'm guessing it's flooded.  Our next step is to remove the carbs and grease the connections and see if this will stop any air leaks, at least long enough to get it to run properly.  All the passages in the carbs look good, and the engine internally was very clean.

Anyone have any other suggestions?

Thanks,

John C

[w1sa]

I'm not familiar with that carb description, but........is the 'air screw' between the carb and the engine or between the carb and air filter?

Is there a fast idle cam set-up on the carbs/choke mechanism

Can you connect a clear tube to the carb bowls to verify fuel level in bowls? (especially while running the engine)

Did you bench/static sinc the carbs to establish minimum slide settings?

Did you verify carb cleaner flows  thru (from) the air passage/jetting, and atomises out the slow speed  fuel jet as well as the mixture entry/metering hole into the carb bore.

[John C]

The idle mixture screw is at the engine side of the float bowl and is shown in the Clymer manual.  Yes to all of w1sa's other questions except checking float bowl level while running.

I had a brainstorm last night (or earlier this am  )  I'm wondering if the manifold between the carbs and head has o-rings to seal to the head.  If so, these could be the source of a vaccuum leak rather than at the carb attachment side.  There are some about 1" diameter o-rings that came with the carb rebuild kits that haven't been used.   Has anyone else run into this as a vaccuum leak source?

John C

[trueblue]

I'm pretty certain that the snouts on the head the carb boots slip onto are part of the head and not screwed in, the only possibility for a vacuum leak in the intake runners is the carb boots themselves.  Just a question, have you checked the cam timing, the reason I ask is my 650 had a horrible idle, didn't want to idle below 2500 and on the rare occasion it did idle it would all of a sudden just cut out as if I had turned off the key, I blamed the carbs to start with, then I checked the ignition advancer, and I checked the cam chain chasing a bad cam chain noise when I found it was out a tooth.  I replaced the cam chain and retimed it correctly and all my idle problems went away instantly.

[juntjoo]

so what did you ever discover?