Topic: Two cylinders running weak - CB650 Help!

[DonSoares]

Hey Guys, Happy Thanksgiving!
I've been working on my 81 Custom 650 and getting it tuned up as I can. The engine runs great, starts up first turn, pulls hard through the RPMs, idles right at 1050. But always trying to get it better...so this AM:

- Checked plugs - 1, 2, 4 all the same: fine, light brown. 3 was a little black and maybe a bit wet
- Shot the header pipes with a laser temp: 1, 2: 200*, 3, 4: 100*
- Shot the exhaust pipes: all the same, around 75*
- 3, 4 pipes seems to be blowing out some light whitish smoke

I looked under the carbs and what I think is the air/fuel screw is straight ahead for 1,2,4 while 3 is turned about 1/4 off.

Need to do cam chain and valves again I guess, but I feel like I'm doing the damn valves every week. Hate to think that they get out of adjustment that quickly.

What do you guys think? What are the next steps to try? I've been a car guy for years and this is my first bike so still learning the intricacies of tuning these 4's. I'm hoping that it's not the rings on 3 and 4 and a compression issue...

Thanks!

[kslrr]

Are those Temp readings taken while the engine running?  You should see north of 400 *F on each header while the engine is running.  Looks like 3,4 may not be firing consistently.

You shouldn't need to adjust the valves more than every 3,000 miles.  Check the carb float bowls for fuel by loosening the bowl drain screws.

[DonSoares]

Yes, at idle after startup. After a ride around the block they were hotter...but still a big difference between the 1,2 and 3,4.

What are the next tests I should be doing? I'll check the floats tomorrow but I imagine there's fuel in them...otherwise how would the engine run as well as it does?

Thanks

[scottly]

3 was a little black and maybe a bit wet

#3 maybe a bit fouled. Was #4 wet as well? If so, perhaps at some point the spark-plug wires got switched. If #4 was dry, I would try replacing the #3 plug with a new one.

[DonSoares]

#4 was all good. The plug wires haven't been switched, though why would that affect fouling the #3 plug?

Thanks.

[trueblue]

1 and 4 fire off the same coil, and 2 and 3 fire off the other coil, so for now I would assume you ignition system is OK.  My best guess would be that you have a carb issue.  This normally seems to be the case when 3 and 4 play up, especially if the bike has sat a while.  Another assumption from the fact you have an '81 650 is it should have CV carbs.  Make sure the slides are free to move, because unlike the PD carbs the earlier bikes had the CV's slides are operated by vacuum not mechanically.  Also if the bike has sat for some time, a less likely scenario, the rings may be stuck and causing the cylinders to be down on compression a little.  If this is the case the best cure is go give the bike a good fang.  This forces the rings out against the cylinder walls and beds them back in.

[DonSoares]

Hey

Just spent the last two days cleaning and resetting the carbs. Good news is the the carbs looked OK, just a little gunky inside. Bad news is now that they're back on the bike it still runs eehh. I tried to play with the pilot screws to tune the idle, and none of the 4 screws has any effect on idle whether they're all the way in or almost completely out. That doesn't make sense.

3 and 4 are still running cold, and I'm not sure what to do next. Haven't done a compression check cause I don't have a tester...maybe that'll be the next thing to try.

Thoughts?

[Mainerider]

How did you clean the carbs? If the idle screws have no effect, it sounds as though your idle circuit passageways may be clogged or restricted.  I'm not going to regurgitate some of my other posts concerning carb cleaning; suffice to say that until you give your carbs a true cleaning using a quality ultrasonic cleaner and proper chemicals you can't be sure that the carbs are truly clean. And if you don't know that for sure you can't eliminate contaminated  carbs as the source of your problem. I have a set in my garage right now, shipped to me from a Texas forum member, who tried cleaning them by hand three times and still had issues. I'm going to clean them using my commercial-grade ultrasonic cleaner and carb-specific chemicals.  If his bike still runs poorly afterwards, it won't be due to dirty carbs.

The problem with any other type of cleaning is that you can't look inside the carb bodies to vertify all is clean; only the ultrasonic cavitation process (along with high heat and proper chemicals) can achieve this. And the idle/slow speed circuits are so tiny that even a little shellac buildup constitutes a substantial percentage drop in fuel flow. I don't think most people realize just how clean these carbs need to be in order to operate properly; you can fiddle with these carbs until the end of time but if they aren't meticulously clean you are just chasing your own tail.

PS
The whitish smoke that you are seeing may be from the valve stem seals; I replaced them on an '80 650 a while back that was exhibiting the same symptom, took care of the smoking. The seals can leak even on low mileage bikes; they just dry up and contract from sitting dry for long periods.

[Teatimetim]

My one comment, would be, make sure the fuel flow from the petcock is good on that side.  I don't know much about the 650, but this turned out to be my problem for very similar issue.  one side was clogged inside the petcock and was not feeding enough fuel.

Cheers.

[grasscutter]

Yeah...definitely should not have to adjust valves more than the usual 3-5K, unless you're taking it to track days!

1-Cam chain adjust.
2-Valve adjust.
3-Compression test.

I'm also on the side of carbs not being 'thoroughly' cleaned. 

Are you feeling the performance lacking @ low idle, low speed?  Mid-range?  Top end?
This will help narrow down what circuit to better attack.
Also...carb 'may' be clean, but o-rings have seen better days.  The CB650 CV carbs have connector tubes between the carbs.
These o-rings are often overlooked, and when they go bad, they suck air.

As if the CB650 wasn't lean enough from the factory!

(o-ring I'm taking about is #1 in the diagram)

[DonSoares]

Thanks guys. I'm thinking that the carbs are still busted too. Might just send the whole rack out to be soaked and cleaned and rebuilt professionally. But I'm also worried there's something internally wrong as well, rings or valve seals. Of course, the compression tester I bought doesn't fit the threads and the stores are all closed because it's Sunday, so can't even test it. Very frustrating. Hopefully tomorrow I can get the correct compression tester and then see where to go from here.

I love this damn thing but man it's frustrating.

[Mainerider]

I'm not fishing for business because between my shirt and tie day job and my nights/weekends carbs/mc repair work I'm busy enough but if you can't find anyone locally to clean them properly PM me. I clean all of the carbs for a local mc shop and I know what I'm doing.

Whomever you find to do it, make sure:

1. They have a true carburetor ultrasonic cleaner, not a $59 Harbor Freight jewelry cleaner or similar. A mid-size commercial-grade ultrasonic cleaner retails for around $900. There is a reason why they cost so much more than the crap that HF and others peddle.

2. They use the proper chemicals.  Ask what they use; If they are using a chemical such as Simple Green, pass. It's not optimized for ultrasonic cleaners and it is no more than a pre-wash.  You want them to use a true shellac and gum dissolver that is specifically made for ultrasonic cleaners.

3. Lastly, if they quote you a ridiculously low price, run away. It takes a fair amount of time to properly clean a 4 rack and the proper chemicals are expensive (typically, at least $75 per gallon for the ultrasonic shellac buster).

[Mainerider]

Thanks guys. I'm thinking that the carbs are still busted too. Might just send the whole rack out to be soaked and cleaned and rebuilt professionally. But I'm also worried there's something internally wrong as well, rings or valve seals. Of course, the compression tester I bought doesn't fit the threads and the stores are all closed because it's Sunday, so can't even test it. Very frustrating. Hopefully tomorrow I can get the correct compression tester and then see where to go from here.

I love this damn thing but man it's frustrating.

I missed it if you've mentioned already; what is the mileage? I'm guessing here but if you have relatively low miles, the white smoke is probably just your valve seals.  Not a hard job and really not a serious issue either, just time-consuming and a little tedious. Good opportunity to inspect the valves and lap the valve seats at the same time.
I also understand your frustration. I enjoy working on vintage bikes and I fully understand the attraction they hold ( I'm seasoned enough to have bought an '84 v65 Sabre...new) but  I don't own any of them any longer ( except for the  occasional winter project bike that I flip in the spring); I get my nostalgia blasts now when I see the look on customer's faces when I have to tell them bad news about their vintage rides.... It all comes flooding back to me at that point why I ride more modern bikes now... Love the looks of the early bikes but to live with one on a daily basis...there's only so many hours in a day...

[grasscutter]

<<<I love this damn thing but man it's frustrating.>>>

Yup.  Sure can feel that way, when you're starting out!

[DonSoares]

Got the compression test done. Good news, readings across 1 - 4 are 160/150/150/150 so the internals are all good. Big sigh of relief! Looks like the carbs are the issue.

Also, the battery was getting a little weak but did a quick plug test on all four wires and 3 and 4 looked like weak or no spark, so once I charge the battery back up I'll do some more tests. Might be an electrical issue too.

Anyway, I'll sleep better tonight knowing it's not rings or valves! Merry Christmas!