low rpm vibrations

[thessler]

Hi
I have been fooling with this bike for a while now and can't seem to get it to run smooth.
When you give it gas from 1,000 rpm up to 4,000 rpm it will rattle your teeth. At 4,000 it smooths out real nice, anything below that it's just not a pleasure to ride.
I have been through the carbs twice and feel the is nothing left to do there, new plugs.
I used a home made clear plastic tube with colored liquid in it to balance the carbs and feel they are pretty good. I don't mind a little vibration it is a motorcycle after all, but this is unacceptable. I do own several other bikes and this by far vibrates the most.
Any thoughts on what to do next ??
 Thank you, Tom

[Barry]

I think you'll find that they all have a slight roughness around 3000rpm. I'd hardly call it vibration though, more torque pulsing and only really noticeable in contrast to the super smoothness at 4000rpm.

Yours sound worse than normal so something must be wrong. How about other variables like valve clearances and compression ?

Try this as an indicator that vacuum balance is not producing the best results.  

Lock the throttle open just a enough to lift the throttle off the stops by 10 - 20 thou. If you don't have a throttle lock you can tie or tape the grip to the brake lever. Now using feeler gauges compare the gaps under the throttle stop screws to see if they are equal.  If they are not equal within a thou or two that suggest there is some mechanical reason why vacuum balance is being achieved with unequal throttle openings.  If the difference is very small It can be something simple like unequal idle settings.

If everything checks out equal - valves, compression, mechanical and vacuum carb balancing then your are left with engine or transmission balance factors. Does it run as rough on no load or light load compared to wider throttle openings ?  If it does (which would be unusual) that would indicate something mechanically out of balance.

[thessler]

Thanks Barry
valves are spot on, compression never checked.
I'll try that feeler guage test right now.

 Tom

[nhmaf]

How many miles on the bike.   Maybe a worn timing chain or semi-working timing advance (beancan) would cause unsteady ignition timing at idle and through the advance curve.  By the time you get up to around 3500-4000 RPM, the ignition is fully advanced and no further advance is forthcoming, so it would be smooth then.

[Bob_Roller]

With the bike not moving and the transmission in neutral, when you increase the rpm's, does the vibration  occur, or is it just when the bike is moving ?

When you have the manometer connected to the carbs, do you get any imbalance when the rpm' are increased into the range of rpm's that the vibration occurs

[thessler]

I set the balance at around 1500 or maybe a little more so it should be good.
This thing is mostly vibrating under a load while accelerating, if that helps.
  Thanks, Tom

[Bob_Roller]

My first thought, is a cracked  or torn carb diaphragm .

It doesn't take much of a crack/tear to cause an issue .

Another less likely condition, is the metal plug on the dome top of the carb, they on occasion come loose and may cause a similar condition to a diaphragm crack/tear .

[Bob_Roller]

Here's a few other things that may be present, there are aftermarket carb diaphragms that are a thicker material than the Bing parts, they are too thick and cause problems with the engine running .

Loose or missing screws that hold the round metal retaining ring on the diaphragm .

On one of my LS's, I found one screw that was too long and 'bottomed out' in the threaded hole, but there was still a gap between the screw head and the retaining ring, so it wasn't clamping the ring to the diaphragm and sealing .

[thessler]

Thanks Bob
Negative on the cracked diaframs. I have been through these real good and checked condition of them. No weird screws or anything out of the ordinary. No air leaks all clamps tight and boots in good shape.
 Tom

[montmil]

Confirmed motor mount bolts torqued to spec?

[nhmaf]

Do you have a timing light?  How steady is the timing at idle (~1000 rpm)

[thessler]

Thanks guys

Motor mount bolts are tight not sure about to spec.
This is an engine running problem.

No I do not have a timing light to check that.
Not sure weather to buy a light or sell the bike ! I am running out of patience with this thing. If I can't get it to run smooth ill sell it cheap and let someone more knowledgeable straighten it out.

Thanks, Tom

Will fool with it again today, I needed to walk away from this for a day or so.

[Barry]

I needed to walk away from this for a day or so.  

Usually a good plan. Sometimes gives a new insight.


There is that story on the net of the guy that sells his vibratory R65 cheap and the new owner discovers that one cylinder and piston is from an R65 and the other is from an R45 hence the gross vibration. Not wishing to cause alarm. It would be near impossible for that to happen in a market that didn't get R45's but it goes to show that just about anything is possible.

Did yours ever run smooth ?

[thessler]

Barry
I don't know if it ran smooth, I got it a year or so ago with a swallowed exhost valve. I did get it running last year but never got serious about it .
Now it's registered and I'm ridding it around, I have done a lot of work messing with the carbs and lubeing up the advance weights. It has improved significantly since last month when I started fooling with it.

But this vibration under load is unacceptable, I am just starting to ride one of my other bikes and let this sit. Which is not a good plan, but the way this vibrates it's just not enjoyable.

Once I get it up around 4000 rpm it's smooth as silk, so I'm thinking its not a mechanical balance problem. More like fuel or electric but I could be way off .

Thanks for the interest, Tom

[Bob_Roller]

Why don't you look at the carbs for their Bing numbers, it's on the forward part of the carb on a vertical flange .

The number should read something like this : 64/32/325 .

Post the numbers here and we can give you the  jets, etc. that the carb should have, no telling what previous owner(s) may have done in the past .