Topic: 1979 CB650 - Hanging Throttle

[IainC]

Hi all,

Troubleshooting my 1979 CB650 with PD carbs. I’ve had this bike for years, but it came out of storage last summer after a winter of rebuilding/cleaning and TLC after 10 years packed away.
Since getting it back on the road, I’ve had a consistent problem with the throttle hanging at 3000rpm+ and slowly returning down to idle while waiting at stoplights etc. If I put any load on the engine by releasing the clutch it settles down. I’ve thrown a lot of new parts at this trying to solve this issue and tried all the usual suspects.

Current configuration:
•   Stock air box with “Made in Japan” air filter (also tried K&N air filter)
•   Original stock jetting and needle (have tried other combinations but back to OEM spec and still an issue)
•   4-to-1 exhaust with Laser K2 muffler (original exhaust is toast and full of holes, have a second 4-1 I could try but haven’t)
•   New intake boots on carbs, best 4 airbox boots from two sets
•   Brand new choke and throttle cables, checked routing multiple times, no binding or issuing with throttle when turning handlebars

Troubleshooting tried:
•   Slow jets: #35 stock, #38 and #40 (starts amazing with #40 but still hangs)
•   Main jet: #90 stock, #95, #102
•   Advance mechanism clean and greased, tried spare mechanism, tried shortening spring by 1 loop
•   Fogged down carbs and intakes with lots of carb clean and MAP gas looking for intake leak, never any surging.
•   Carbs vacuumed synced multiple times w/ Morgan Carbtune (all very close)
•   Timing static and dynamic (timing light with digital rpm)
•   Air fuel mixture adjusted a million times (have special tool to adjust carb air screws, followed instructions from both Clymer manual and official honda service manual after every jet change)
•   Tried only throttle pull cable with return disconnected
•   Carbs snaps shut hard and “forcing” it closed with throttle doesn’t lower idle
•   Carbs have been completely rebuilt with new floats, float needles, air cut off, accelerator pump and orings (have a few aftermarket carb kits, but using OEM brass needles and jets currently)
•   Carbs are super clean, ultrasonic multiple times, compressed air. Arguably never been cleaner and runs better than ever except for the hanging idle.

At this point I’m feeling discouraged and am grasping for potential issues. Symptoms point to air leak or other fueling issue. I replaced the felt seals with new ones I punched by hand, and have new machine cut felt seals for the carb shafts (choke and throttle), but if they were leaking air I would anticipate fogging them down with the carb clean would have indicated an issue. Potentially the slides are sticking open ever so slightly from my replacement felt seals, but this is difficult to confirm, and the throttle snaps shut with a lot of force.

[Ozzybud]

When bench synchroniing  the carburetors you likely set the slides too high. This does not.allow them to close far enough when it returns to idle. And likely keeps the idle screw non effective. Lower the adjustment screws to.lower the slides 1/2 turn and re vacuum sync the carbs. This is a common problem and there is lots of information on here about hanging idle. It has happened to me as well as most of the.member's on here at least once.

[HondaMan]

Four years ago, I reassembled a CB650 that an owner had seriously mis-assembled (top end was a mess) and got it running for the owner, who rode it for 16 months all over the Rockies. Then it got parked for about 5 months and wouldn't run: the owner took it to a local "vintage bike" shop who said they "rebuilt the carbs" for her, and if it would start at all, it idled at 3000+ RPM. After warmup it was even higher. She brought it back to me for another fix-up.

The "shop" (whoever they are) had 4 widely different settings on the idle-mixture screws (ranging from 4.5 turns to less than 1/2 turn) and 4 different float levels inside the carbs. The slides at idle were also set to 4 widely different openings at idle, and one of the air cutoff valves was entirely missing its gasket. Three of the 4 idle-mix screws had the components on them mis-assembled (wrong sequence), in 2 different ways, and one of the slide needles was in a different 'notch' from the other three. In short, a total screw-up of reassembly and settings in that case.

When I finally got them all re-assembled the right way, the floats all set the same, and the throttles all synced to zero at the same positions, I put the carbs back in and opened the idle-mix screw 1 turn from full closed. It fired at the touch of the starter and idled a little low (600 RPM-ish), but that was all it took! What I discovered along the way was: those carbs are real touchy buggers.