Author Topic: cb650c problems.. (Read 2184 times)

weltzing · « **on:** January 02, 2011, 10:09:04 AM »

So I have a 1980 cb650 custom in pretty good shape. Have it running, but doesnt want to idle and never seems to fire on all 4 cylinders all the time always have to use choke to keep it running. I've had the carbs off soo many times that im just sick of it now and dont know what to do besides taking it to a local shop. The two left carbs seem to work great the left side pipes get real hot, then the right side is only warm and the exhaust is cold. All cylinders have compression and good spark so what would be best...would like to get it running great because plan on selling it as its too small for me. thanks for any ideas or help!

kslrr · « **Reply #1 on:** January 02, 2011, 11:03:05 AM »

If you've got good spark (like you mentioned), good compression and the valves are setup right, then most likely it is still the idle circuits in the carbs. On my CB650, I took the carbs apart 6 times before it all worked right. Taking it to just any shop wont solve your problems: most shops now-a-days don't know how to work on carbs. Search for a member in near by.

phil71 · « **Reply #2 on:** January 02, 2011, 11:18:39 AM »

I assume you know which cylinders aren't firing. You don't say if these carbs are CVs or not, but I've got to assume they are based on the behaviors your talking about.
First, and foremost.. don't assume you haven't got a bad plug somewhere. Testing spark at the boot is only half the story.. so take the dead cylinders out and, with the plug in the boot, and the plug base held to ground, crank the bike over and look for arcing.
If you've never really done a proper clean and rebuild on the carbs, there's a couple low-buck tricks that have served me well over the years.
Carefully strip the carb bodies of all their soft parts, and boil some lemon juice outdoors, on a bbq, in a big pot. Dunk the carbs for about 20 mins. (it'll be a stinky process.)
That'll free all the deposits in the circuitry.. but it's still wise to chase what you can with compressed air (even the kind from a staples for cleaning computer keyboards). Also, get some guitar strings and run it thru the circuits, then blow again.
Now, on to the brass parts.. They can gunk up pretty good, but years of varnish will come off with extreme heat. Get yourself a nice needle-nose and a pair of gloves, and hold the jets needle and seat, one by one over your stove's open flame. It'll turn a bluish color while the deposits burn off, and then back to brass, only a bit brighter. Keep a glass of water around to quench or drop in after.
That can be a little stinky too, so keep the windows open.
I'm sure the previous poster is right about your idle circuits, but a thorough cleaning will stop the guesswork and multiple disassemblys to chase it down.
Good luck!

Hush · « **Reply #3 on:** January 02, 2011, 11:20:47 AM »

Exactly what Kslrr suggested, your idle circuit in your carbs is not clear and allowing good gas to get to all cylinders.
I had the same model as yours and it was a PITA until I sorted the carbs out, one other thing that happened on my 650C was the choke bracket that runs across the carbs was loose and the bike only ran with the choke out (if it started at all) and once I had that nailed down many of my problems were solved.
One thing to check is where the choke cable is secured, make sure the carb butterflies are wide open when the choke cable is at rest, mine moved and the chokes were half cocked all the time until I found it out.
I do agree taking the carbs off these bikes takes some determined effort but once you get it right and the bike is in constant use they seem to stay good.
Look at the problem annalytically, start with removing the gas tank and checking for loose crud, clean it out and dissasemble the petrol cock to make sure that is clean.
Once you can guarantee fuel supply from the tank you move to the carbs and ensure the emullsion tubes tiny holes are clear, unfortunately on the "C" model the floats are preset and if they are failing it will require a new set to cure float issues.
At the end of this journey you will need to have the carbs synchronised to get maximum efficiency from them...good luck...Hush.

Mainerider · « **Reply #4 on:** January 02, 2011, 12:22:42 PM »

Hi,

I have the same bike in my stable. By the way, they are mechanical slide piston carbs, they went to vacuum-operated slides in '81. A few questions and assumptions...

I assume you are running with fresh and correct spark plugs. What is the condition of the plugs in the near-dead cylinders; Do they look like they are fuel-starved or are they wet-fouled? That will help to ascertain whether the cylinders are receiving fuel and not burning it or not receiving enough fuel to begin with. I know you said you have spark but sometimes it is difficult to judge the spark quality by sight alone.

I assume that the idle screw is set properly? Does the bike idle when warmed up? If not, have you tried to turn up the idle a bit?

I also assume that you have checked/adjusted the valves; When I bought mine I had 2 cylinders not doing the job properly and I found both in and ex. valves way out of spec.

Does the bike run okay other than at idle or just off-idle? Does it accelerate fine past that point? How many miles on the bike and did it sit for an extended period? If all of the above checks out, it probably is your low-speed circuit.
However, mine had 4,600 miles on it and had sat for ages; ran like crap initially. Did all the adjustments/maintenace, worked the seafoam through the tank a few times, shot the carbs with carb cleaner and runs great now. BUT... these bikes are notoriously lean-running; I still need the choke for the first mile or two.

Hush · « **Reply #5 on:** January 02, 2011, 01:06:24 PM »

Try this for the 650 start-up
I stole it from Kit but she won't mind cos she's on holiday somewhere.

Turn on the ignition, turn on the gas tap (mine goes off the minute I shut the bike down) pull the choke lever right out to full.
Give the throttle 3 good full (right around to stop) twists, now hit the starter.
As soon as the bike fires cut the choke right off and adjust your idle up a bit to compensate, when the bike is warm readjust the idle wheel back to just over 1000 rpm.
This is from dead cold and once the bike has been started like this it is good to go all day with just a normal press of the strter button.
I never ride my bike from dead cold, causes too much damage internally, these old bikes are "cold blooded" but once going really do the job.
This works first time everytime for me and even works on my partner's 400.

Seriously try it exactly as I described and if you still need the choke on for a few miles then something else must be wrong.

weltzing · « **Reply #6 on:** January 02, 2011, 01:13:14 PM »

Thanks for the help, they are new plugs and fresh gas. The two plugs on the righ side seem starved of fuel. I have also cleaned the tank which had bits of rust settled to the bottom and also cleaned the petcock valve. The bike has around 14k miles on it and the guy I bought it from had it sitting in his garage for quite some time. I've used heat and seafoam and other carb cleaners to clean the jets. The two idle jets were clogged on the right side so i was happy when i found this and got them cleaned, but after I put the carbs back on still had the problems. So I took them off again and cleaned the jets out again and still have the same problem..I have not checked the valves and kind of uncertain of how to do that. Going down the road with it it doesnt have power and have to use about half choke for it to run its best sometimes (for like a second) it will run on all cylinders and I can feel it get power and then it goes back to the same problems...

Mainerider · « **Reply #7 on:** January 02, 2011, 01:16:47 PM »

Given that, I would do a valve adjust; probably will be its first. Its very easy to do on these bikes. Do you have the service manual?

Mainerider · « **Reply #8 on:** January 02, 2011, 01:19:50 PM »

PS
If you don't have the manual I can walk you through it. It is actually easy on these SOHC motors

Mainerider · « **Reply #9 on:** January 02, 2011, 01:24:34 PM »

PPS
If after the valve adjust you still have issues I would rule out the carbs the low speed circuit shouldn't affect the main jet performance. Next would check spark and timing. Again very easy process on these bikes. Let's do valves first. Should be done regardless...

Mainerider · « **Reply #10 on:** January 02, 2011, 01:36:00 PM »

Just one more thought...are your carb float levels in spec? Set too low and that could also cause a fuel starvation issue, both at idle and in main jet performance. FWIW I also have a spare carb rack off of an 80 that had 9K on it when put in dry storage.

weltzing · « **Reply #11 on:** January 02, 2011, 03:50:18 PM »

I checked them and they seem to be correct did the simple check of putting the hose on the drain and holding it up and the gas filled to the top of the bowl. Thats correct right?

Mainerider · « **Reply #12 on:** January 02, 2011, 04:18:41 PM »

They sound okay. I would go to valves and eliminate that variable, since they can affect performance considerably. On these older, high-maintenance bikes, crossing all the Ts and dotting all the Is is critical.

phil71 · « **Reply #13 on:** January 02, 2011, 04:45:56 PM »

dumb question... WHICH cylinders don't fire. If I'm not mistaken, the petcock has 2 feeds, which "T" off to two carbs each. If both of the weak or dead cylinders are sequential, (1&2, or 3&4) you may have something blocking one petcock outlet. Swap 'em and see what happens.

weltzing · « **Reply #14 on:** January 02, 2011, 04:52:59 PM »

Its the 3&4 and no there is only one petcock that feeds all of the carbs

Mainerider · « **Reply #15 on:** January 02, 2011, 05:08:06 PM »

When you have performance issues across a wide rpm range it usually isn't primarily a carb issue. I still think ignition spark / timing and/or valves are likely. .

Hush · « **Reply #16 on:** January 02, 2011, 07:29:52 PM »

True there is only one fuel feed but it runs right across all the carbs through connecting tubes, I got some old fuel line rubber trapped in this connecting tube and it shut the gas off to #3 and #4 until I figured it out.

weltzing · « **Reply #17 on:** January 02, 2011, 08:07:08 PM »

I know that it doesnt cut the fuel off to my 3 &4 carbs because it fills the bowls. ive had it running and opened the drain on these carbs also to make sure that the bowls keep filling and nothing is slowing the fuel from getting to the carb and it seemed to be fine. I really do think the valves need to be looked at..

Hush · « **Reply #18 on:** January 02, 2011, 09:09:25 PM »

Yep if you have checked gas flow as far as the bowls then you probably have extremely badly adjusted valves, some P.O.'s have no idea how to adjust these and get the timing all wrong for them.

dave500 · « **Reply #19 on:** January 02, 2011, 10:05:10 PM »

have you or a friend got a multi meter?check the plug caps resistance,they unscrew off the wire,if you go to an auto electrician with them in your hand he might do a quick test for free or a few bucks,you want 5kohm,if they are closer to 8 or 9 bin them,they are pretty cheap to replace.

Mainerider · « **Reply #20 on:** January 03, 2011, 04:03:26 AM »

Just one final comment. On these older bikes that have sat around and maintenance-ignored its often a combination of issues. You prob do have well out of spec valves AND weak spark AND less than ideal carburetion. Its the nature of the beast Do a valve check, check your spark (and yes, w a multimeter) and you should find the major issues haunting you. Then sell it! Quickly

Mainerider · « **Reply #21 on:** January 03, 2011, 04:06:19 AM »

Forgot to add. If you need any parts (coil,wires etc) I have a nearly complete spare bike.

Hush · « **Reply #22 on:** January 03, 2011, 11:05:55 AM »

I'd also do the plug lead swap trick to test to see if it is actually plug related.
Just swap the plug leads from the #1 plugs onto the #2 & #3 pligs, if the problem follws the swap then the leads or coils and possibly the condensers are suspect......condensers are cheap and if they start to fail can cause you all sorts of funny results.

weltzing · « **Reply #23 on:** January 03, 2011, 03:40:03 PM »

Now thats a pretty good idea, i have unscrewed them and the ends of a couple wires were not even showing and kind of corroted. I will have to check that out soon.

Hush · « **Reply #24 on:** January 03, 2011, 03:50:10 PM »

I think you just self diagnosed your problem mate.

weltzing · « **Reply #25 on:** January 04, 2011, 07:59:45 AM »

but as I saw this as a problem before i cut away some insultation from the wire and exposed them, didnt seem to help before.. but I will have to swap wires once.

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Author Topic: cb650c problems.. (Read 2184 times)