Topic: 650 sc Vb carb issue

[rmjam8540]

I have an 82 650sc that bogs badly at about 5k rpm.
I have ultra sonic cleaned jets. 118 main, 35 slow, all Keihin.

Bike idles fine (2 turns out on mixture screws), pull good until about 5 k, then bogs badly as if way too much fuel.  Ease off throttle and it snaps back to pulling again until you reach 5 k again.

I've done a couple of throttle chops during this and plugs confirm rich. ( black soot).  Seem ok at idle ( kinda tan color )

These are the tall silver-top vb carbs.  Needle is 64 A. ( no clip ). I'm wondering if the spring in vacuum chamber are weak, letting the Pistons pull off before the engine is ready for that much air?

Anyone have any thoughts, ideas, suggestions?

Thanks,
Mike

[rmjam8540]

3 days and no ideas?
While waiting and hoping someone would think of something I have re-cleaned and re-cleaned without un-racking.

I did find that the accel pump had a hole in the bellows.  Ordered a new one.

Anyone with an idea, no matter how outrageous or simple, please let me know.

Thanks,
Mike

[Hutch]

Try the Honda cafe racer fb group, might have some better luck there.  Might not be a carb issue.  Those have a vacuum assist petcock, which I eliminated on mine.  compression?  make sure the choke is working correctly.  main jets clean?

[DaveBarbier]

Did you remove the Jets for cleaning? If I remember correctly the VB carbs have pressed in pilot jets.

How about if you remove the air filter, can you then rev up higher?

I was going to suggest shutting off the fuel as you’re riding which would lower the level in the bowls and lean out the mixture (until they empty of course). But I’m not sure if the vacuum assisted petcock can be shut off like normal. Try that if you can and see if you’re able to rev up higher.

Do one of those suggestions at a time, not both.

Did you verify your advance ignition timing with a strobe?

[rmjam8540]

Hutch, Dave,
Thanks for getting back.  Vb carbs have screw-in jets.  They were removed, ultra sonically cleaned, passages were cleaned as well as I could with wire and pipe cleaner.  Jet openings were checked under magnifier (all were clean and of equal size).  They are Keihin 118 mains, 35 slow.  Last thing I did is to clean and polish the piston and vacumn chambers.  All lift and fall back nicely now.  Haven't been able to run it since I cleaned the chambers.

I don't have an adapter for my compression gauge so I haven't checked. Timing is on "F" with number 1 TDC.  Will check further as soon as I remount carbs.

Pretty sure this is carb related.  Started as soon as I pulled the carbs to check why it suddenly decided to pour gas while idling, plus it pulled to redline before this started, no issues.

I feel like I have done something that has caused this.  Just can't figure it out.

Mike

[rmjam8540]

This was a combination of several things. 
Cyl 1 float bowl had crud built up on the float needle lever.
The needle valve Pistons did not move as smoothly as they should.
One new in the box NGK plug will not fire (cyl 4). I have never in my 70 plus years seen a brand new plug that was bad in the box!
After everything else was corrected, cyl one plug fouled and was shorting out ( probably the result of the #1 float.

I still have a slight stumble at 4700 rpm under heavy load, but now a little twist and it's gone.

Thanks to all who offered ideas, and to all that have gone before and noted their fixes.

Mike

[fizzlebottom]

I've got the same bike and have had the carbs off multiple times for various reasons. The VB44C carbs are very finnicky sometimes. One thing I haven't seen mentioned yet is synchronizing them. When you had them off, did you bench sync at all? Have you vacuum sync'd since? Before doing this, make sure you've got the other bits covered: cam chain tension adjustment and valve adjustments.

These carbs were setup to run lean from the factory, so running rich sounds like your pilot screws are way off. You said your pilot screws are set to 2 turns out. The FSM states 2 3/4 (or 2 1/4) turns out from lightly seated. I can't remember exactly, so I'll check when I get home tonight. These carbs are weird, in that the pilot screws meter air even though they are on the engine side of the carbs. They really may be running too rich.