Topic: CB650 Stumbles Above 5000 RPM in gear...among other things

[aircraftgrade]

I have been working on an '82 CB 650 for some time now. I picked up the bike after it had sat for (allegedly) two years, having ran (allegedly) just fine when it was parked. I've since:

--meticulously cleaned the carbs, which have stock jets (and a stock airbox to go with them). I had all the jets I could find pulled and soaked in carb cleaner, and used 2 cans of carb cleaner to clean every orifice I could find.
--Installed a new battery
--Rebuilt the top end, with all new gaskets, rings, and intake/exhaust valves. The cam chain tension was adjusted and valves set to tolerance when I did that. The engine doesn't leak any more oil, which is an improvement! I decided to do this because the compressions were 30/30/60/60, but that was based on a cheap HF compression tester so they may have been higher. Either way, they must be well within spec now. I'll check them again once I break in the engine more.

But, here's the problems I have had since I got the bike to run after cleaning the carbs, and these problems remain after rebuilding the top end:

--The air coming out of the left side exhaust (cylinders 1 and 2) is cool--the headers are too hot to touch, but the air coming out of the muffler is pretty cool--almost ambient temperature. The right side is much warmer, and feels like what I imagine the exhaust should feel like.
--The bike falls on its face above 5000 RPM in gear. In third gear that brings me to about 50 MPH top out speed. Above that, it stumbles and bucks forward and backwards. It won't let me do anything above about half throttle.
--In neutral, I can blip the throttle but the RPM will "hang" before it decreases again.
--The bike pops and spits real bad out of the exhaust. It's not a nice smooth deceleration by any means.
--Once the bike is warmed up, it dies at idle. Which is fun, riding around trying to keep the RPMs up as you decelerate...

Whether or not these are related, I'm not sure. But I don't know where to look next.

Also, I have good spark on all 4 plugs. And I have found that depending on the day, I can remove either the #1 or #2 spark plug lead and the bike will continue to idle. Which is probably good for fuel savings, I would imagine.

[scottly]

Also, I have good spark on all 4 plugs. And I have found that depending on the day, I can remove either the #1 or #2 spark plug lead and the bike will continue to idle.

Try swapping the #1 spark-plug lead with the #2 lead. The left side coil feeds cylinders 1-4, and the right side feeds 2-3.

[aircraftgrade]

Also, I have good spark on all 4 plugs. And I have found that depending on the day, I can remove either the #1 or #2 spark plug lead and the bike will continue to idle.

Try swapping the #1 spark-plug lead with the #2 lead. The left side coil feeds cylinders 1-4, and the right side feeds 2-3.

I have not tried this yet, but I have swapped the right and left coils with no fix. I'll see about swapping those leads next time I'm working on it. Do you mean just swapping the leads at the plugs, or completely moving the #1 plug lead to be on the right side coil  and feeding the #2 cylinder, and vice versa for the #2 plug lead?

[1976ssport]

I have had mufflers packed with seeds from mice.  Do both pipes blow the same amount of air.

[aircraftgrade]

I have had mufflers packed with seeds from mice.  Do both pipes blow the same amount of air.

Yes, and I've looked down them as far as I can see with a flashlight to make sure nothing is in there. Good idea though.

[aircraftgrade]

As an update to this, if anybody has a similar problem:

When I cleaned the carbs, I 100% forgot to clean the interconnecting tubes BETWEEN the carbs 

So all that rust made it downstream and contaminated my nice clean carbs the second I turned the fuel back on. I've since removed the entire fuel system to remove all the rust I can before reassembly. Update coming soon...

[scottly]

Do you mean just swapping the leads at the plugs

Yes, just swap the leads from the #1 plug and #2 plug.

[aircraftgrade]

Alright guys, I'm at a loss for words on this one.

I've fixed some of the issues mentioned above. The #1 and #2 carbs weren't synced with 3 and 4. After syncing both, the exhausts are equal temperature and the bike runs a little better.

The bike still backfires out of the intake at idle. It's very random...but the carb boots randomly pop pretty loud and smoke. I feel like this may just be dirty fuel...I've cleaned the gas tank twice but am still working through what I imagine is very small rust particles and a small amount of water from cleaning it. There is a fuel filter installed on the bike. I honestly think this is just dirty fuel (because it is so very random) but I wanted to mention it if it could be a symptom of something else.

The BIGGEST problem I have is at about half throttle, which works out to be around 5000 RPM in third gear, the bike falls on its face. The best way I've heard it described is the bike feels like it's going to throw up...it kind of rocks forward and back and looses all ability to accelerate.

I don't think this is related to ignition advance. I've lubed the advancer and I can see it move outward when I rev the bike. Also, it will stumble at the same throttle setting but lower RPMs if you shift into a higher gear.

Because of this, I thought maybe the slides in the carb weren't lifting. I noticed they were kind of stiff when I cleaned the carbs (which I've done 3 times now...), so I polished the interface between the slides and the bore with 2000 grit sandpaper. I also put a couple drops of multipurpose oil on the inside bore where the piston is received as it slides upward, and they all slide like butter now. This made no difference though.

In response to above: I am not able to swap #1 and #2 spark plug leads because the #2 doesn't reach far enough. But I have tested the resistance of both leads and they both are about equal. I've also swapped ignition coils from side to side with no difference.

Whoever figures this out gets a beer. Or seven.

[MJL]

I found, eons ago, that when I replaced my stock 4-4 exhaust with a 4-1 that I had to remove the air filter cover. (My bike was a 650Custom, which had 4-4 pipes. I forgot non Customs had 4-2.) It did the same thing, wouldn't rev much past 5000 and the best I could do on the high way was 55mph. Try that.

Also... just throwing out ideas... but possibly valve lash or cam timing.

[dazemc]

And I have found that depending on the day, I can remove either the #1 or #2 spark plug lead and the bike will continue to idle. Which is probably good for fuel savings, I would imagine.

I cannot possibly see a scenario, in which you are dumping unburned fuel in and out of the cylinders, end in a healthy way for the motor.


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[dazemc]

If you are getting spark on all 4 cylinders, all 4 exhaust headers are getting equally hot, idles fine but hangs like you are describing, synced your carbs, and have good compression. Then what you are describing sounds like it could possibly be a lean condition. How is your exhaust, intake (pods, box, stacks), and jet sizes configured? Checked for vacuum leaks yet?

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[aircraftgrade]

I found, eons ago, that when I replaced my stock 4-4 exhaust with a 4-1 that I had to remove the air filter cover. (My bike was a 650Custom, which had 4-4 pipes. I forgot non Customs had 4-2.) It did the same thing, wouldn't rev much past 5000 and the best I could do on the high way was 55mph. Try that.

Also... just throwing out ideas... but possibly valve lash or cam timing.

So I have the stock 4 into 2 exhaust that comes on the standard '82 CB650. BUT the one component I have yet to change (which honestly sounds almost comical at this point) is the air filter. I'm not sure how old it is, but I'm starting to realize it's the last piece of the puzzle I haven't tried. I've looked at it multiple times; it isn't clogged with leaves or anything, but it definitely looks old and yellow.

Re: dazemc, I have a stock airbox with stock jets in all 4 carbs as well. Everything is stock. I even have new intake boots, so that eliminates what was probably an intake leak from the original rock-hard boots.

An air filter is on order. Let's see what happens. 

[jsmith800]

I have this very same issue.  Wonder if and how this post was resolved.  I noticed a few valves need lashing, i'll do that tonight and check compression.  other then that i'm at a loss.  the carbs have been disassembled & cleaned and still stumbles around 5k.  also one of the carbs must be shooting fuel into the airbox, notice it dripping out the bottom.  perhaps i'll pull the airbox and see who the culprit is.  i've installed new coils & wires, swapped the the old coils and wires to verify the new ones are not faulty......i get the same issue with old coils.  also sounds like its governed at 5k but i know thats not whats happening.

[RAFster122s]

Have you checked your carburetor fuel level to see if the floats are at the right height. Running low on fuel from carb float height set wrong will affect the ability to keep the fuel atomized correctly.

[fizzlebottom]

This sounds like fuel starvation. Have you opened the petcock to see if that nasty original fuel filter is still in there? It very well might be clogged to high heaven. Also, have you confirmed that your floats are all holding air and still leveled correctly? They're non-adjustable, and aftermarket units suck on these carbs.

Lastly, even though this bike doesn't use points it will still need timing adjustments. Use the static timing method in the manual and see if that helps at all. You may need to do a valve adjustment again afterwards.

[jsmith800]

thanks guys!

last night i adjusted the float heights.  all 4 were not to spec........they were at about 5/8" and spec is 1/2".  i'm running the IV bottle fuel since in the shop. the filter and petcock look fine.  i'm running an after market filter...perhaps i should remove it.  Tonight i'll get it out and see if the float adjustments help.  i'll check timing again also, i checked awhile back and it was perfect.  i'll also sync the carbs but the vaccuum tools are at my dads place at the moment.
lastly, i ordered a new air filter, the original one looks like its 44 years old, perhaps i've got an air issue.  (i'm using the OEM airbox and filter...not pods)

[fizzlebottom]

Unless you have floats from some other bike that just happen to fit, yours are non-adjustable. 100% plastic, not nice handy metal tangs. That means new floats if they're out of spec.

I DO think that there's a possibility that some CBX floats would work, but I haven't taken the risk of slapping them into my carbs to find out.

[grcamna2]

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[jsmith800]

carb sync was my problem.  now revs high and very responsive.  thanks folks!