Topic: 1981 CB650C - CV carbs rich at Wide Open Throttle

[mzbk2l]

racing33, interesting to see that you have the same problem.  (And sorry to hear it!) 

Ibsen, thanks for the links, more good information there.  That book looks interesting, too.  I might have to pick it up and see if there's anything I haven't tried yet.

cb650, no shims under the needles - the carbs are assembled exactly as shown in the factory exploded parts diagrams.

750goes, I have several good parts brush sets for automotive engine and transmission work; all stiff-bristled nylon and plastic.  I didn't use any metal brushes to clean the carbs, and I did the cleaning with all the brass parts out, so no critical dimensions should have been altered by cleaning.

I sure would like to find out if anyone knows where to source a new set of slide piston springs; shimming those was about the only thing that has made a difference yet.  Of course, they're the one single part of these carbs that is not available any more.... 

[cb650]

Try streching then out a little.   Maybe a industrial supply might have something.  Check the dohc sites. maybe the springs are the same in the 750/900 cv carbs.   I also always ran a K&N in the stock box also.




                           Terry

[racing33]

If you were to shim the springs I would think that would be a nightmare. I mean how in the world would you ever know how many shims to put under each spring to get them all working in unison. I would assume that each spring would fatigue at a different rate, so the amount of shimming for each spring would be different. I see lots of trial and error doing the shim thing.
Bruce

[mzbk2l]

racing33, I agree about the total lack of accuracy in attempting to shim the springs.  I only did it as a troubleshooting step to try to determine whether my old springs were shot.  Since shimming the springs did make a difference, that led me in the direction of the springs.  Unfortunately that part number has been discontinued by Honda.  I don't know if Honday keeps the same number across platforms (i.e., if the CB650C springs would have the same Honda p/n if they are identical to the CB650 Nighthawk springs), but if they do, then those springs would seem to be no longer available. 

Those springs have about as much compression as a slinky, so it would be tough to measure the spring rate to use for replacement; I think they're probably rated in grams/cm, rather than lbs/inch.  I wish I had a selection of carbs to tear down to measure various replacement candidates.....

[t3rmin]

Please forgive an outsider butting in but I've got a friend with a CB650 who's having this same issue. ;-)

First off, he hasn't been in the carbs (or the ignition, or the valves, although the air filter element is clean and in good shape) yet, so it's quite possible an obvious fix will present itself once they're dismounted and disassembled, but I'll stay hypothetical and assume his condition is exactly the same as yours.

I gather that the bike falls on its face with sudden WOT but not so much with less harsh rolls of the throttle to WOT? With that symptom I'd be inclined to suspect the accelerator pump. My Suzuki doesn't have one, but as I understand it from my muscle car days, the accelerator pump is there to provide a quick boost of enrichment for sudden throttle openings. Of course it'd be somewhat difficult to distinguish between the affects of the accelerator pump and main jets, since there's a lot of overlap there; but I'd think that if the problem is most pronounced with sudden hard acceleration under load, that you might want to give the accelerator pump another look over, especially since you've tried everything else.

If the main jets and slides/needles are indeed in good order, I'd think that's the next logical step...? Especially since I'd be hard-pressed to come up with an explanation for a *rich* condition with the stock intake/exhaust, the stock jetting, and non-adjustable needle.

I read somewhere that during dyno tuning, they sometimes disable or remove the accelerator pump somehow, so as to be able to accurately assess the main jetting without interference. Is it possible on these carbs to get the accelerator pump out of the picture temporarily and see what kind of effect that has?

Anyway, just some thoughts. Best wishes on getting this problem licked! I've just finished with a long bout of frustrating ignition and fuel delivery issues on my old (new to me) Suzy and while it's sure satisfying to have a great-running machine now, the process nearly drove me insane! ;-)

[mzbk2l]

t3rmin,

In my situation, the condition happens slightly past 1/2 throttle, no matter whether I slam it wide open, or creep up on it.

I just got home today, and hope to play with the jet kit this week.  I'll post the results of my changes.....

[dudeman750]

I have an 82 CB650SC that does the exact same thing. I have been fighting it as well and have no idea what the problem is. Hope someone figures it out. Mine shuts down at about 5-7k rpm and begins to bog. As soon as you roll the throttle back a little it kicks back in. I've cleaned the carbs, checked timing and timing advance. Runs great as long as you stay under 7k rpm. My last nighthawk didn't even turn on until 7k so I miss the power up there on this one. So far I have read that valves, coils, fuel supply, CDI can cause this. I feel it is the rich problem you all describe.

[rfrench924]

Just wanted to add my bike to the list!

81 CB650C does the exact same thing. I have been through the carbs over and over. Best outcome has been removing the air filter. Of course, i would like to run with an air filter but, i would like to ride more! 

Still awaiting any definite fixes for this situation. It sure does seem like there is ALOT of SOHC guys that have this issue with no real fix! Weres all the "old timers" input???

-filter-less 650

[mzbk2l]

Bringing this one back to the top with a solution!

I thought I had tried everything, but last time I was home and had access to the CB650, I ran it with only an air filter and no air filter cover.  PROBLEM SOLVED.   The bike side cover made no difference whether it was removed or installed, but the actual air filter cover completely chokes the bike down above about 5,000 rpm. 

I cut out the entire center section so I just have an outer ring to keep the air filter tightly installed, and the bike works exactly as it should. 

I haven't re-read this entire thread, but I think I got that idea here somewhere.  Thanks to whoever mentioned it!