Topic: RPMs fluctuate at cruise

[Goldbug]

I bought a '79 CB650 that had the carbs "rebuilt" by a "reputable shop." The shop was selling the bike for a customer. The bike is very cold blooded, loves choke, loves to hesitate/die if you give it throttle at low temps and also has this problem where the RPMs bounce +/- 100-200 rpms just cruising down the road.

I kind of have this feeling like they didn't do the work they charged the customer for, just sprayed some carb cleaner at it hoping it would run well enough to get sold to the next sucker (me).

I'm pretty sure the carbs need synced and adjusted but the rpms bouncing around is a new one to me. On cars rpms would bounce if you lost/gained a cylinder due to ignition / fueling issues. Is that what I'm potentially facing here on the bike?

[Gordon]

Is the engine itself actually surging and slowing in conjunction with the indicated rpm fluctuation?  From your description it's hard to tell if it's a running issue or just a bouncing tachometer needle. 

[MCRider]

Hate to bring it up, and my fingers are crossed, do you have pods or stock air cleaner?

[Goldbug]

Stock airbox and filter.

Good point Gordon. I may be an idiot. If the RPMs fluctuated the speed would change... I don't think it did, but I can't remember. I want to say the speed was steady. I only rode the bike from the shop to my dad's house and mostly I was concentrating on the unbelievably stiff throttle that gave me a cramp in fifteen seconds. I already bought a Crampbuster but maybe some monkeying with the cables will help a little. If the speed was steady then I guess it's just the tach. I'm going to ride it again in a couple days when I have time.

I still think the bike could run a lot a better. I just have this feeling like the shop did not put much care into the bike. For $750 I still think it was a good deal.

[Gordon]

If the engine speed were actually fluctuating 100-200 rpm while cruising you would definitely notice it.  Probably just an old, jumpy tach cable. The throttle grip shouldn't be that hard to turn on a 650.  Check the cable routing for sharp bends, and check all three components of the throttle system separately.  Carbs, cables and throttle grip should all move smoothly and relatively easily by themselves.

[Goldbug]

The bike has inverted Clubmans. I wouldn't be surprised if something is amiss with the cables. I was going to either un-invert the Clubmans or run Euro bars I have laying around. Either way, I'll have the opportunity to get into the throttle problem and get it done right.

*sigh*

[Danno]

there is one thing I know of that can cause this try taking the tops off the slides and spraying cable lube in them put them back together and drive it if it does not do it as bad or stops then you have worn slides on old bikes it is fairly common i use cable lube because it leaves a slight residue of grease not enough to stick the slides in the bores but enough to keep them from sucking air
also check around the boots between the carbs and the motor it is less likely but could happen
 it sounds like you have an air leak at high rpm if it is not just the tach as already stated

[Pinhead]

there is one thing I know of that can cause this try taking the tops off the slides and spraying cable lube in them put them back together and drive it if it does not do it as bad or stops then you have worn slides on old bikes it is fairly common i use cable lube because it leaves a slight residue of grease not enough to stick the slides in the bores but enough to keep them from sucking air
also check around the boots between the carbs and the motor it is less likely but could happen
 it sounds like you have an air leak at high rpm if it is not just the tach as already stated

The '79 has mechanical carbs (hence the heavy throttle return spring).

If the engine were really stuttering by 200 rpm you'd feel it and hear it. Also, the 650s are notoriously cold-blooded from the factory; the choke and hesitation you're experiencing while cold probably isn't anything out of the ordinary...

[coldwave007]

1980 cb650 here and mine is definitely cold blooded too.  When mine is in good running order: 

 1) Petcock on for 30 seconds or so, two twists of the throttle, full choke.  Push the button and BAM!  Fires right up
 2) Let idle and after 30 seconds or so move to half choke.  RPM jumps up some. 
 3) Idle another minute, drop to 1/4 choke and adjust idle if needed.  I ride like that for a mile or two.  If I don't 1/4 choke it'll tend to surge some until warm.
 4) No Choke, bike's happy!

If your bike is warmed up and still stumbling, then you should do the list below.  Really, it's a 650 and you should do it ANYWAY.  The PD (mechanical carbs) are a little fickle and you'll be fighting it until you go through the list!
 1) Carbs need a basic cleaning.  Make sure all jet orifices are clear, gaskets are intact, and the accelerator pump diaphragm has no holes/works..  You don't HAVE to break the carb rack right now, but may have to eventually. 
 2) Make sure the floats are set right. 
 3) Set the pilots to stock setting, 1.5 turns out (some bikes like more, mine's happy at about 2ish)
 4) Sync the carbs!!  Made a night-and-day difference on my bike.  Way more power and much smoother. 

As for the RPM fluctuation on the gauge, if you're not "feeling" the change, then it's probably either a worn tach or the cable just needs lube. 

[Danno]

I am aware of that Pinhead they still have slides or pistons if they are mechanical however if they are cv type they operate by the vacuum dome and last I knew they were not
sorry if this sounds mean not my intention just had a long day bud

[Pinhead]

I am aware of that Pinhead they still have slides or pistons if they are mechanical however if they are cv type they operate by the vacuum dome and last I knew they were not
sorry if this sounds mean not my intention just had a long day bud

Ah, I see what you're saying about the leaky slides. I thought, when you mentioned sticking slides, that you were talking about CV catbs. No harm, no foul.

[Goldbug]

Good info. Thanks.

Unfortunately the choke on this bike is broken at the carb. The choke is now rigged with safety wire but must be held by hand. I'll have to look into this myself but basically they said there was no easy fix without getting a second set of carbs to get parts off of and tearing the carbs down. I was thinking of putting two hooks somewhere behind the headlight- full choke and half choke- as a temporary solution. Not the best but it sould work.