Topic: 81 CB650 carb tuning

[tomkimberly]

I started with a stock 81 CB650 Custom, carbs were VB44A's, original four into four exhaust. When I could get the idle passages open on these carbs, the bike ran great, reved right up and ran like a raped ape!

Fast forward, I had a head gasket leak (the "O" ring thing) so I pulled the head and cylinders, replaced the gaskets and while i was in there, did a ring job. As I always was fighting with the VB44A carbs, I located a set of VB44B carbs off of a 82 CB650 from a guy in Canada. The advantage to the VB44B's (and VB44C's) is that the idle jets are removable, making it easy to ensure the idle passages are clear.

Anyways, everything is back together and the bike starts right up (before with the VB44A carbs it was hit or miss if the bike started easy first time in the morning) with just a little choke applied. It idles nice and smoothly and revs right up until you attempt to ride it, then it will not go past 5k RPM.
I had set the fuel mixture screws to 2-1/4 turns out then moved them to 2-3/4,m then to 3-1/4 turns out. each adjustment made a little difference but it is still a struggle to get the RPM's over 6k while riding. In the driveway, all is good but actually riding it is no-go. In frustration, I removed the air filter and wow, this bike goes! It is like night and day between having the air filter in-place and having it removed.

I have verified the idle jets are # 35's and the mains are 120's, just like the old VB44A carbs were and the Honda specs. For all I can tell, these new (to me) VB44B carbs are set correctly and should act just the same as the old VB44A's.

The exhaust is in good condition, no leaks and it is not plugged up (it's not a 2 stroke after all).

Any ideas on what and why this condition exists?

I plan on getting a new air filter and fabricating up a ring to hold it in-place instead of using the stock cover and see if that helps.

BTW, Terry, those VB44A carbs should be there by tomorrow.

Tom

[cb650]

Have heard that same problem from several people.   I never ran into it myself.  But I have pods on 2 bikes and a K&N replacement on another.
Will be looking for the carbs thanks.



                  Terry

[tomkimberly]

Here is an update for everyone to ponder over.

I put the air box cover on the bike but left the air filter out. The bike ran like crap and would not go over 5K just as before. I then removed the cover and installed the air filter and retained it with some straps leaving the cover off. The bike now runs great and quickly revs right up to 10K. The problem seems to be cover or airbox related.

Is there a difference in the air box cover or air box it self between a 1981 CB650 and a 1982 CB650?

For now I am going to ride it without a cover but I sure would like to know why this is so.

Tom

BTW, Terry, did you receive the carbs?

[kettlesd]

If its running better without a filter, the you have a rich condition happening in your Mainjet and/or Needle circuits. Dirty airfilter,   overly large mainjets (if not stock) or needles raised too high will do it.

[tomkimberly]

If its running better without a filter, the you have a rich condition happening in your Mainjet and/or Needle circuits. Dirty airfilter,   overly large mainjets (if not stock) or needles raised too high will do it.

These are CV carbs. The carbs are clean and I have stock jets installed, # 35 idle (pilot) and # 120 mains. The needles are non-adjustable and there are no washers under them. Everything is stock. The problem I think is with the air box or cover, I suspect there is a difference between the 1981 and 1982 versions. Service Honda web site has been down all day so I have not been able to verify this on their microfich.

This is very baffling.

Tom

[kettlesd]

hmmmmm - now it IS interesting.

I find it incredibly hard to believe that there would be THAT much a difference in airflow between cover on/off - or between the two years design - if they BOTH fit your airbox just how much difference could there possibly be in airflow?? Are the wire mesh screens in the cover restricted in any way??? Drilling a few 1/2 inch holes in the cover and/or airbox would fix it, but findng the cause would certainly be better.

[Ibsen]

As far as I could see from the parts lists at MrCyles, the air box and the cover is the same for both year models.

Are the jet needles and needle jets new, or are they the old ones? As others said, it sounds like a problem with the carburators and not the stock air box, unlees there's a birds nest inside it. If I take the cover off the air filter box on my 650, I get a flat spot because the carbs will be too lean.

[cb650]

Yea the carbs came thurs.       Did you leave the vent "T's" off intentionally or use them on another set.   Not complaining just noticed they where gone.   I like that someone else replases the philips with alan head.   Use stainless though as the black will start to rust in a few years.



                    Terry

[tomkimberly]

Yea the carbs came thurs.       Did you leave the vent "T's" off intentionally or use them on another set.   Not complaining just noticed they where gone.   I like that someone else replases the philips with alan head.   Use stainless though as the black will start to rust in a few years.



                    Terry

There never were any "T" vents on those carbs and the replacements I have are the same, no "T" vents. I think that is normal for these early CV carbs. BTW, these carbs also do not have the air cut-off valve either, just the accelerator pump.

Tom

[cb650]

My 2 other 81's have them.





                 Terry

[tomkimberly]

I solved my problems! Although the carbs were part of it, I never considered that the plug wires and caps. After replacement, the idle shot way up over 5k. After backing is down to 1200 rpm, I was able to re-install the air cleaner cover. This bike now rocks! I would have never belived a simple thing such as new wires and caps could have such a dramatic effect.

To all who have rebuilt their carbs multiple times to little or no effect, I suggest you change wires and plugs, it just might help.

BTW, here in southern California, this weekend has been great! Temps in the high 80's low 90's.

Tom

[WJL75]

Glad you got it running well.  I miss SoCal (used to live in Costa Mesa) and am jealous of your weather.  It's 20 degrees here in Milwaukee.

[Heironymous Josh]

Glad you got it running well.  I miss SoCal (used to live in Costa Mesa) and am jealous of your weather.  It's 20 degrees here in Milwaukee.

Ditto.  I used to live in San Diego.  It's close to 60 in Denver today but this has been the snowiest season on record and we're supposed to get another big one by the end of the week.