Carb trouble

[Redboots]

Anyone know what cause/how to cure a "bouncing" carb piston?

I could hear this tick/knock and first thought a loose tappet, but using the screwdriver in the lughole method, traced it to the LH carb. Tubes off and its bouncing like a good 'un.

If it was an SU, I'd say it needs oil in it....

Carbs cleaned and all new bits, including diaphragms fitted recently.

Ideas please.

John

 

[John Armstrong]

Not missing the big spring are you?

 

[SJRoome]

Hi

I have never experienced this. I am also a bit short of information to diagnose it. What bike are we talking about? When does the noise occur?

I am going to assume it has 40mm CV Bing carbs as these are the beauties I know and love, and that this is a tapping noise at idle. As it has occurred after a rebuild it is probably a rebuild error. Did you refit the return spring on the slide? Are you sure that the slide is the right way around? If you lift the slide with your finger and let it go how does it return? You should hear a "sigh" as air moves into the chamber above the diagram and the movement should be damped. If not I suspect you ripped the diaphram as you fitted it or that the top of the carb (above the diaphram) is not air-tight. You may also try refitting the old diaphram.

I also found this on a well known web-site:

"Some very late model R100GS, POSSIBLY others, and probably only 40 mm Euro shipped models, may be found with a groove in the upper area of the slide, with an O-ring at that point. The purpose is to help eliminate a rattle these sometimes had. It isn't necessary to retrofit YOUR bike with these, unless you have a bad rattle at idle (often on right side), and dislike the rattle."

Hope this helps

Steve

 

[Redboots]

Not missing the big spring are you?

Nope,

Took the top off to make sure though
The only thing I can think of is that the "big spring" ain't big enough... lengthwise that is.
Any idea how long the should be?

John

 

[Beemerboff]

According to the instructions that came with my vacuum gauge you have either faulty carburator adjustment, worn valve guides weak valve springs or a leaking head gasket.

I'd go with faulty carb adjustment if it has just appeared after work on the carbs.

The part number for the spring fitted to the latter carbs is different , and the spring is a tad stiffer.
This increases the mixture in transition, and some people think it improves performance a bit . It would prabably let you drop the needle a notch which would also improve consumption.

If you really fell like experimenting with springs your local SU dealer will have a choice of around half a dozen to choose from.

When I upped my R75/7 to 900 cc the only difference I made to the carburation was to fit slightly stiffer SU springs , and then drop the needle one notch. Works well, pulls like a train and Ive had 460 km out of a tankfull.

 

[Redboots]

What bike are we talking about? When does the noise occur?

'95 R100GS. A little at tickover but most noticeable just as you open the throttle.

I am going to assume it has 40mm CV Bing carbs as these are the beauties I know and love

Steady!!

As it has occurred after a rebuild it is probably a rebuild error.

Agreed

I also found this on a well known web-site:

Do share

"Some very late model R100GS, POSSIBLY others, and probably only 40 mm Euro shipped models, may be found with a groove in the upper area of the slide, with an O-ring at that point. The purpose is to help eliminate a rattle these sometimes had

That is exactly it! I remember now that one of the overhaul kits did not have that O-ring in it...

Hope this helps

It did! Thanks to all the other contributors as well. I now "know"my carbs slightly better

Cheers,
John