HondaCB650

Hi everyone! Wow, am I glad Iv found you lot! Been trawling through the site for a while, decided to take the plunge and join in! I'll post some pics as soon as I take 'em - but briefly, my interest in CB's took off about two years ago. I rescued a '79 650Z from a friend who'd had the old girl locked away in his shed for a while. He was in the middle of turning it into a “streetfighter” - IE – no chrome, square plastic headlight, mini indicators, cut front mudguard – cut rear mudguard etc. He’d also lowered the front end by sliding the fork tubes further through the headstock at the top by about 2cm – dropping the front end a bit. Apart from that it was standard. She ran but had no paint – and was, to be frank, a mess.
However, even in that state I could see that the bike is beautifully proportioned! It’s a fantastic looking machine from every angle and I love her!

Since then it’s had a yearly service and MOT’s – oh – and has been completely re-built! Not a ground-up job, but a bit-by-bit rolling restoration and improvement exercise. I won’t bore you too much, but she’s had paint, chrome, exhausts and wiring. She was off the road a good while because of an electrical problem I encountered – bad earth – dodgy OKI boxes and firing on two cylinders sound familiar to you guys!
The reason for her being off the road for so long, was…my enthusiasm! Let this be a lesson to you all! One job at a time. I know, I know, I learned the hard way.
You see the electrical problem surfaced DURING a carb cleaning, re-jetting K&N filter fitting exercise (that I wish I hadn’t started to be honest) So, what I thought was a CARB problem – was in fact an ELECTRICAL one. DOH!!
I ended up chasing my tail for months, not getting anywhere. I was so convinced it was my mucking about with the carbs that was the root of the problem; I bought two complete sets from e-bay! (including a set that were wrongly advertised and were too small) I was a broken man…
Then, a guy who I later learned has a CB 750 – asked me if I’d done the “pipe” test? I said “No” of course, not knowing what he was on about. He simply touched all four down pipes and told me that the bike was only running on TWO cylinders! I didn’t know to laugh or cry. Anyway, the problem turned out to be, like I said a bad earth under the petrol tank at the coil mounts and a non functioning OKI box under the seat.

Still got running problems, won’t go anywhere unless she’s got the choke on and is good for about 80mph only. Round town up to 60 she’s great. I’ll service the old girl again and have another go at balancing the carbs and looking again at the jetting and mixture issue. I have a vacuum gauge (though not the e-bay one everyone is raving about) Try £55 quid at my local bike dealer (sob sob!!)


Question: I put in a set of mixture screws in the CB’s carbs from another CB, as mine had (as is usual) turned to mush where once a screwdriver should have lived. The set I put in are the same length and to the untrained eye are identical in every way. EXCEPT there’s more thread at the bottom and these replacements and they also have a knurled or machined at the top bit so you can use your fingers to adjust them.
Are these OK to use on the bike? Or are these the reason for my un-even running bike? The bike has four “cone” K&N style filters attached and has the next size up jet.
Because of this needing choke issue, that tells me she’s running lean. Though the plugs tell me she’s OK. Low revs are fine, but after 6,000 revs she splutters and is taking in too much air. If I push her she stalls. Applying choke improves matters, but she feels laboured at anything over 70mph. Oh, she’s also running a non standard exhaust setup. They LOOK like the standard pipes that you see in Haynes manual and all the brochures, but I believe are Italian. There is no BS kite mark on them at all. They are a brilliant, two times two into one system. Makes the CB sound lovely. This isn’t helping matters either I bet. Thing is, I’m addicted to the induction noise the bike makes when ridden in anger and I have to see this through now!!!

Welcome, and good luck sorting things out. I'm sorry I don't have any advice myself, but you've definately come to the right place. I'm looking forward to your pics.

Thank you for that. yeah, I'll get some pics onto the site ASAP. Unfortuneatly I've got no "pre" restoration pics. Got a few other machines as well. a honda CBX 750 DOHC (83 vintage) an 81 Honda CB 25 RS, a suzuki GSX 600F, and a Triumph hardtail chop from 1965. a 650 with twin carbs. Not enough hours in the day though eh?

Welcome. That's quite a story. I hear you about the one task at a time with the bikes.

It sounds like she might be running lean. A fair number of guys put those pod filters on these Honda SOHC-4's and seem to have problems getting the kinks worked out. Not sure what the recommended jet size if for those with the carbs on your bike (which are on there now? you said you've swapped some in and out). She will go well over 80 once you get the fuel/air mixture worked out. Some one of this forum recently proved that by testing his bike out in front of a police radar gun

Welcome to the forum.

Yes, there is a company in Italia that are making replica systems for many bike models, but I can't remember their name right now. I even think David Silver Parts in England might sell them. The stock Honda system will have HM, or Honda Motors, and a serial number stamped on the mufflers.

It's really hard to tell about the pilot screws though.
But I think you can get the correct screws from www.mrcycles.com , or from David Silver Spares.

Stock main jet size are 90, but the carbs were jetted very lean from the factory, so it was common to go up to 98 with the stock air filter and exhaust on the Europen Z model. With mods, you might have to go up even more.

Thanks for everyone's advice on the carb set up on my CB. As far as i can remember, the bike is running 90's as a jet size. The annoying thing is, she's almost there. Just a bit of fine tuning to go I think. The induction noise produced by not having the stock air box is sexy though, but I don't want to damage the bike either, so I'll go back to the drawing board and start again I think. The mixture screw issue is still perplexing me a bit. If the threads on my replacement set go further down the body of the screw, it just means that they have the POTENTIAL to be turned in more than the originals? But, because I'm using the "turn in till stop - then two and a half turns out" method, it could be why its not working properly as the "stop" on the originals is at a different level than the "stop" on the replacements? Phew - does this make sense? Better rich than lean, so I'll try and work my way round this.
The pipes came from e-bay (what doesnt) and are made in Italy. they are not stamped - but instead are stencilled in black somethingI can't make out because of wear and the numbers 2938. As I say, they look EXACTLY like the stock items. Its just me, I grew up with twin piped Triumphs in the family. Having a 4-1 system just looked...wrong to me. Much prefer pipes hung from BOTH sides of the bike! saying that my Suzuki is a 4-1 sustem and that looks ok. Weird innit?

Oh, as a weird link to my signing onto this forum - my throttle cable broke this morning while out on a run. Three miles from home, and with the bike still going at 30mph, I pulled out the choke and felt for the throttle idle control knob - cranked her up and rode home doing a steady 45mph! This exact same thing was logged on the forum - I rememberd it and acted accordingly, where as before I probably would have just pulled over and pushed her home!

Thanks again guys!

Welcome to the forum!
Good to hear you saved a CB650 from ending up as a plastic monstrosity. If there is some medal to earn for that, I will propose you for it .
It seems that you already got to know a lot of your CB. The original airbox/carbs/exhaust combination is quite allright and not so easy to reach or even to exceed. I would agree that the mixture screw settings should orientate rather to originals than to thread counting on a different thread. If the additional thread is really used could be checked by turning in the two types you have into the same hole while counting the turns until it stops. If there should be a difference, you can try to add it to the original turns out. However I am not sure if I really understood how the screws look like.
During all the optimization work, don't forget to go riding from time to time .

bandwidth69 wrote:Round town up to 60 she’s great.

Welcome to the forum; you'll find it a great source of information.

But 60 around town.....??? You mean kph, of course!

Um..yeah. of course 60kph! Strange that, must be a glitch or something. I posted a longer sentence than that actually!