low rpm vibrations

[thessler]

Thanks Bob

Numbers are (64/32/335 left side)
                    (64/32/336 right side)

I just remembered one of the needles that sticks out of the slide is new, I don't remember which side but I remember the new one wiggled and the old one was tight.
Not sure if it is set up right , or if this could be the cause just thinking out loud.
 Thanks,Tom

[Bob_Roller]

When I get home from work in about 3 hours, I'll post the info from the Bing manual, unless someone esle has it handy .

I know it may not be of any help, just trying to eliminate anything that is a 'quick' fix .

[georgesgiralt]

have a look here :
http://www.bingpower.de/en/service/tuning-manuals/
Dial in your carb number (it is far down) and note the values.
What you report can be caused by some non symmetrical conditions between two carbs.
IMHO you should buy a set of jets, needle jet, and needles and replace them in both carbs (including the rubber).
It will pay in the long run by increased mpg

[Barry]

I just remembered one of the needles that sticks out of the slide is new, I don't remember which side but I remember the new one wiggled and the old one was tight.
Not sure if it is set up right , or if this could be the cause just thinking out loud.

The needles fit in one of four positions ( It should be position 3 in your case).  Is it possible the needles are in different positions ?

If you measure the exposed length of needle that will tell you what position they are in.  The needles being 1 position out wouldn't normally cause very severe vibration but it's got to be worth having them correct.
 

          Length mm
1           42.37
2           40.57
3           38.79
4           37.10

[Bob_Roller]

The carbs came from Bing, with 138 mainjets, needle jet 2.64 if you can find a number on it, idle jet 45 and has been mentioned, needle position 3, this is numbered from the top .

On the /335 and /336 carbs you should have the upgraded parts where you remove a threaded plug from the top of the slide and the needle comes out from the top .

[thessler]

Ok thanks guys
Carbs are coming back off tomorrow, I know they are spotless but I really didn't check jet sizes or needle position.
I suppose if one needle was high and one low that might be problematic.
Otherwise I will post my findings.

  Thanks, Tom

[thessler]

Hi
Just been through the carbs , I even used a magnifieing glass.
Idle , main and needle jets are what they are supposed to be. Number wise. Needles in third slot. Everything looks good and nothing out of place.
I noticed the cap on top of one carb a little loose ( I mean the little 3/4 inch cap ) not the big one. I put a little silicone on it but even that couldn't have had much of an air leak.
Ran it synced it and I would say runs 10 percent better, but really not good enough. Not sure why it runs better ,must have cleaned something somewhere. Or just an optimistic imagination.
During the test ride I really zeroed in on the tach. The worst problems are around 2500 rpm. It vibrates unacceptable from about 2000 to 3000 .
At around 3000 it settles down to a typical  buzz then smoothes out real nice toward 4000.
Someone earlier mentioned timing and I'm thinking the timing should affect both cylinders at the same relative time and not cause excessive vibration so I really didn't go there, besides I don't have a light. Just thinking out loud, don't really know where to go from here.
Carbs have been off three times, that should be enough but my gut keeps going back to them.
Last thought change the throttle cable, maybe it's doing something weird while ridding I hate to just start changing parts, that is an expensive road to go down.
 That's where it's at !  Thanks, Tom

[flybot]

It sounds very much like a carb balance issue to me- one side working harder that the other. I would start over with the cable tension and idle setting.

[montmil]

... I have been through the carbs twice and feel there is nothing left to do there...

Does this mean you completed a total strip down of both carbs? new Bing carb kits installed?

Did you run a single strand of copper wire -from, say, an 18-gauge wire- through every passage; then flush and blow them clear? Replace the multiple o-rings with new? Confirm fuel rate of flow and float bowl condition plus adjustment/alignment? Confirm condition of diaphragms? Needle position in each carb slide? How about those tiny holes in the venturi adjacent the slides -are they clear?

Unless the carbs were totally disassembled, there's still the possibility of a blocked/clogged fuel or air passageway. Doesn't take much to disrupt proper operations.

Also, are the fuel lines new-ish and not deteriorating internally and restricting fuel flow?

Are the vacuum take offs sealed after the carb balance?

There's carburetor overhauls and then there's carburetor overhauls.

[Barry]

During the test ride I really zeroed in on the tach. The worst problems are around 2500 rpm. It vibrates unacceptable from about 2000 to 3000 .
At around 3000 it settles down to a typical buzz then smoothes out real nice toward 4000.

I was actually thinking about your problem on the way to work this morning and watching the tacho in the same way.  What you describe here is typical airhead as far as the characteristics at the RPM bands are concerned. They all rumble a bit and smooth out progressively above 3000. The only thing untypical about yours seems to be the magnitude.  

On the assumption that vacuum balance isn't giving the best result I'd be inclined to try a plug shorting balance to see if there is any difference. If you lock the throttle at 1500RPM and then short each cylinder in turn, observing the fall off in revs, it should demonstrate if both cylinders are producing the same output. You can do the same at idle except the engine will only run for a few strokes so you are comparing how quickly it stops or in practice just listening to the difference for a couple of strokes and not actually letting it stop.

Another option after a vacuum balance is to mark the cable adjusters at each carb with a dab of paint. You can then experiment by turning one adjuster 1/4 turn in either direction to see if the vibration get better or worse.

[Air4Life]

I hope I will be corrected if I'm spewing bad information.


With your throttle set at at the notorious rpm that will produce the vibes: Reach down with your free hand and pull slightly up on the cable housing just above the carb.  Do this one side at a time.  If the engine vibes smooth out momentarily when the rpm increase (assuming it will increase) then you LIKELY have a sync problem; but if it doesn't, its something else to move on to.

[Luca]

Keep in mind that carb imbalances are often more noticeable at lower revs.  A slight leak at 5% throttle is a greater percentage of the total air/fuel mixture going into the cylinder than at 90% throttle.  It's one of the reasons we dont synch carbs at high revs.

Like Monte asked, did you overhaul the carbs or overhaul them?  Often the butterfly shaft o-rings are neglected, and those can leak air.  Also, make sure the butterflies are properly installed.  They can be put in backwards or installed so as to not seat evenly.  Both instances will give you more trouble at low RPM's than higher.  Same with the dome cap that you siliconed.  If the cap leaks air, the diaphragm won't have the proper vacuum on it and the slide will not lift enough.  It doesn't need to be jiggly to leak air, so you might consider gooping up the other one as well.  It's also possible that one of your new o-rings was damaged during installation.

One easy way to identify air/exhaust (a bad exhaust leak can also cause you trouble) leaks is to hold a lit cigarette/incense/whatever near various joints (not just the carb boots) and give the engine some revs.  Look for a disturbance in the smoke.  You can also use the old brake clean/wd-40/starting fluid trick on the intake tract.  Spray that stuff at the joints and listen for an rpm change.  Usually the engine will bog down and then the revs will gently rise as the spray stuff burns.  Keep a fire extinguisher handy   [smiley=thumbup.gif]

Also, confirm that your enrichener (choke) levers are fully returning at the carbs.  If the spring is weak or the arm sticky, that can mess up your mixture too.  Are they installed correctly?  I'm not sure if installing the discs backwards can mess with the carb balance... hopefully someone who knows can chime in

Does your bike still have the EPA emissions system installed?  As Monte asked about the carb vacuum ports... there could be a leak in one of the vacuum lines between a carb and the EPA valve.  Also, less likely, one of the EPA pipes to one of the cylinders could be blocked up, have a leak, or have a loose fitting.

Are your plugs, plug ends, and wires the same and in good shape?  Sometimes resistor plugs make their way into non resistor boxes.  You can verify this with a multimeter.  Same with your plug wires/caps.  You could try reading the plugs at the most problematic rpm to see if there is an obvious difference in mixture.

Do you have much oil consumption?  Are you using good, high octane gasoline?

While my experience has been that motor mount torque makes more difference on the upper rpm vibration (usually around 5K RPM), you could try experimenting with the motor mount torque.  5-10 ft-lbs can make a substantial difference.  55ish front 45ish rear seem to be popular figures

Sorry to go on at length... I just don't want you to give up  

[Luca]

And here I go again...

nhmaf suggested you check your timing at idle.  Considering you recently had a problem with the bean can, it would be worth it to check the timing.  A local auto store will probably loan a timing light to you for a refundable deposit.

IMHO you should buy a set of jets, needle jet, and needles and replace them in both carbs (including the rubber).

 +1  Believe it or not, needles and needle jets wear with use

[Air4Life]

+1  Believe it or not, needles and needle jets wear with use

True true, but they don't all of a sudden wear, or do they?  The rubber I can understand having a sudden rupture.

[Luca]

True true, but they don't all of a sudden wear, or do they?

no they dont, but he said

I just remembered one of the needles that sticks out of the slide is new, I don't remember which side but I remember the new one wiggled and the old one was tight.

so one is new and one is worn.  That could cause an imbalance.  IMO, probably most noticeable when the slides first start to do their work...