Topic: Pros & Cons of the CB650 engine

[Pinhead]

I didin't really want to. It scared the hell out of me the first time!  Got the bike good and warm, put it down in 1st gear, and slowly took off. Got the revs up to about 4k rpm... Opened up the throttle and OMG better let off! After that first experience, I thought I'd get a quarter-mile time. I didn't finish the quarter-mile because I was too shaken when the front tire came off the ground in first (I let off because I'm not that crazy of a driver). I hit second and opened it up again and the front came up about 2 inches.

All of this with I think stock gearing. 70mph is 5500rpm. With all of the extra power, I'm thinking of getting a little taller gears to help fuel economy a bit. Before the jet change I was getting 40mpg. Now I'm getting 50. Hopefully with the change in gearing I'll be getting 65+. 75 is my finishing goal.

[markymark]

Question was asked about 750 carbs on a 550.  Well I have done it.  I read a web page of some guy back in the 90's that used to do it.  He said it was the easiest best way to get a significant amount of more Horses out of your 550.  All I did was get a set of rebuilt 750 kehein carbs from ebay and put 120  main jets on. I also changed slow jets to i thinnk 35's.  Anyway, my 550 screams now.  I also had to change the needle position with the C clip to the 2nd positoin from the bottom of the needle.  I first tried the 3rd position and that didnt work to well.  Then I went to the top and had to run the choke almost fully engaged.  Then I went to the Second from the bottom position on the needle and wallla, she purs now even when she is cold it only takes a little while to warm up and shut off choke.  Oh yeah, I went to a hotter plug.  Just one step hotter.  Hope this helps. 

[Pinhead]

Question was asked about 750 carbs on a 550.  Well I have done it.  I read a web page of some guy back in the 90's that used to do it.  He said it was the easiest best way to get a significant amount of more Horses out of your 550.  All I did was get a set of rebuilt 750 kehein carbs from ebay and put 120  main jets on. I also changed slow jets to i thinnk 35's.  Anyway, my 550 screams now.  I also had to change the needle position with the C clip to the 2nd positoin from the bottom of the needle.  I first tried the 3rd position and that didnt work to well.  Then I went to the top and had to run the choke almost fully engaged.  Then I went to the Second from the bottom position on the needle and wallla, she purs now even when she is cold it only takes a little while to warm up and shut off choke.  Oh yeah, I went to a hotter plug.  Just one step hotter.  Hope this helps. 

Very nice! And the 750/550 is the same as the '79 650??? I wonder what the difference is between the 750 and 650 carbs... Did it hurt your gas mileage??

[markymark]

Yes I am sure it hurt my gas milage.  I pretty much did this swap as soon as I got the bike 4 months ago.  In fact when I first did the swap I had terrible gas milage until i re-jetted the carbs.  I was dumping gas like you wouldnt believe with a 3 gallon tank.  Thought I had a leak.  But it seems pretty good now. 
The 550 carb I believe is a 24mm carb.  The 750 carb is a 28mm or 29mm.  So yes it is a much different carb.  Problem with a 550 is that you can't go to a bigger valve because there is absoultly no room. 
I don't know what the specs are on the cb650 carbs, but by looking at pictures of them they look almost about the same as the 750.  But, I don't know.  Anyone have specs on the cb650 carbs?  I think I will just put the 750 carbs on when I purchase a 650 motor. 

In an ideal world, the 750 carb would do my 76' 550 more good if I had a different cam and an overbore kit.  Anyway, I do want my cb550 to be stock looking but with more power.  So, hopefully I can do the 650 swap.  I don't really care much about the kick start not being on a 650.  But I would hate it if the charching system is not that good.  i am not in great shape to push start my bike all over the place.  LOL

[cb650]

Remember in all this carb swaping the 81/82 650 went to cv's that had wider spacing.  Along with the head.




                                        Terry

[markymark]

Ok, I'll try and keep this short. I did the 550 650 swap.  OMG love it.  The 650 motor fit right into the 550 frame with little effort.  Had to take the oil pan off of course and also the mechanical thing inside the pan(spring and bracket).  Ok now for the good part.  Since it is a 80' cb650 I was able to put the same 750 carbs I was using on my 550 onto the 650 motor.  Lucky for me, I had NO re-jetting to do.  I did this swap about 6 months ago and have no problems until about 3 weeks ago.  My charching system failed.  Kind of a bummer because I did what most people do and replaced the Regulator/Rectifier thinking that was the problem.  But it wasn't  Turned out to be the problem that most 650 owners encounter -  a bad rotor.  I have done some reading and found that it is common because of the way the copper(color) wires on the coil spin with the engine.  Wish they had done the same set up as a 550 charging system.  When working properly, the 650 runs bright(lights) and strong on the electrical system.  But since my copper wires looked burnt and are coming loose, I now have to replace or rebuild it.  BUT<  The 650 puts out a significant amount more power than the 550.  Since my charging system did go out, I do wish I had a kick start now.  But the power gains make it worth it.  I could keep going on this swap but I will just keep reading the post if anyone has any questions.  Peace:)

[markymark]

Oh yeah, forgot to add that with my 750 kehein carbs on the 650 I can almost beat my friends stock 72 cb750 off the line.  I am right next to him in the drag race. Just an inch or 2 off his tail.  He has only a little on me when racing.  I don't race often, just had to see how much different our bikes are.  Of course I don't know how warn his engine is but he as a 4 into 1 exhaust and some other mods I'm not sure about.  My buddy with and 836 in his cb750 can still pull ahead of me with little problems but for the stockers, watch out.  LOL.  I have heard in some other posts that some of the guys with the 650 can pull wheelies in 1 and sometimes 2 gear.  Well that is due in part to the fact of the way the stock 650 is geared.  The rear sprocket on a 650 is pretty big compared to the 550 rear sprocket.  So I put in a sprocket that is 1" (inch) smaller than the 650 and 1" bigger than the 550 and it does very well.  The 650 clutch is much better than the 550 and easier to use and much smoother.  Anyway, I am not that technical and don't no much about specs and all that.  I just do stuff and see if it works. Yeah, I'm a clean cut grease monkey but I love working on this bike.  I love ridiing it even more.  Love the old timers and young guys at the bike nights asking us vintage guys all the questions.  We still steal the show.   

[bill440cars]

          Sure getting some cool info and experiences posted on this one. I've got an 80' 650C that I picked up a while back (minus body work, and front brake system) and motor turns over nicely too. Also has a 4 into 4 exhaust, that doesn't look bad at all. The more I see and heat about this one, the more I'm kinda wanting to do something with it. One main thing I'm missing on it is the alternator rotor. Anybody got one that's not too badly priced?

                                                            Later on, Bill

[bryanj]

Bill, that good,reasonably priced rotor is the "holy Grail" of 650 parts, was a discushion a while back about fitting a 550 genny, dont know if it was ever resolved!

[c_kyle]

The '79 US CB650Z came with a hotter cam than the others, does anyone know if the later 650s were the same, or detuned?

[MRieck]

The ports are decent....better than the 750's thats for sure.

[kslrr]

Very interesting topic.  My wife and I picked up an '81 custom 2 weeks ago (you might have seen the topic I started).  A '79 engine is in the frame right now and the '81 engine is waiting for a rebuild.  The stock air box is gone and I have just ordered some pod filters.  I plan on keeping the stock pipes.  Sounds like going to 98 mains might be a good idea, especially since one day we might put a sidecar on it.  This bike is going to be my wife's daily ride.

[bill440cars]

         bryanj,

               I think it was decided that a fair amount of work and expense would be involved to change the alternators on the 650s.


         wulfshrunting,

                I don't have the answer about the other 650s being the same as the 79' 650Z or if they were detuned.


         MRjeck,

                That's good to know about the ports.

                  Thanks guys, all the info about the possibilities with the 650, will be helpful to all concerned.


         kslrr,

                  Way to go there guy. Lots of luck with that one.

                                                    Later on, Bill

[kslrr]

So the generator rotor is wound.  There are companies that rebuild automotive alternators and others that rewind electric motors of ail types.  I would think that a wound rotor would not be too much different.  I might find out from my Diesel Engine Forman at the railway museum about the motor rebuild companies.

[bill440cars]

So the generator rotor is wound.  There are companies that rebuild automotive alternators and others that rewind electric motors of ail types.  I would think that a wound rotor would not be too much different.  I might find out from my Diesel Engine Forman at the railway museum about the motor rebuild companies.

           Hey, that sounds like a possibility.   Good luck and let us know the results, okay?

                                                         Later on, Bill

[kslrr]

Here's a place that sells used stators and also rewinds your stator for $150.00.

http://www.rmstator.com/index.php/Motorcycles

[Caferacernoc]

"Still reading through old posts........

So here's a question. Why would a person want to take a 650 motor and put it in a 500/550 frame? Or a 750 motor, for that matter? Just curious."

    Because it's a bigger engine and the 550 frame is smaller and lighter.