R100 carb adjustment

[Brickboy]

Ho hum. I'm sure I used to be able to do this with my eyes shut...

Full service completed prior to adjustment. New air filter, valve clearances set, throttle cable slack sufficient for range of adjustments. I used the Snowbum shorting method first of all to get me somewhere close. It's a twin plugged bike so quite fiddly to short both plugs out simultaneously but I made a special tool!

I am nearly there with the carbs but feel that the revs drop a little slower than I think they should when closing the throttle and that engine braking is thus reduced. Pretty sure my last R100 made you almost hit your teeth on the speedo if you slammed the throttle shut!

I've got the mixture screws 3/4 turn out but notice that if I screw one, or both, right in then revs rise a lot. For some reason, I imagined that screwing them right home would cut out that cylinder at tickover. I suspect my issue lies in this area.

Tickover is about 1000rpm on the tacho. It also sounds about right. Vacuum gauges suggest carbs are well balanced both at tickover and 1500rpm and above.

Any suggestions as to how to make the revs drop more sharply/decisively when the throttle is shut?

Many thanks, Simon

[Rob Frankham]

I've got the mixture screws 3/4 turn out but notice that if I screw one, or both, right in then revs rise a lot. For some reason, I imagined that screwing them right home would cut out that cylinder at tickover. I suspect my issue lies in this area.


Many thanks, Simon

Something not right there... Each screw should have a 'sweet spot' where revs are at their max and they should drop off as the screw is turned either way from that spot. I would be looking for air leaks at the carb to head tube or the choke housing.

Needless to say, once you have found the problem, the carbs will have to be tuned again from scratch... Whether it's an air leak or not, it will throw all of the adjuctments off once it is corrected...

Rob

[gspd]

....if I screw one, or both, right in then revs rise a lot.

You're idle circuit is blocked. Read all this but your problem is in part 3.
https://www.forum.boxerworks.com/viewtopic.php?t=11214

[Brickboy]

Thanks gspd - that's very helpful. I shall enjoy an hour or two investigating my carbs - usually a pleasure with Bings in my experience.

[Brickboy]

A great improvement thanks to GSPD's advice. One carb now has a fully functional idle circuit. The other still a work in progress but I now have engine braking, a decent strong tickover and the future prospect for perfect running looking hopeful!

[gspd]

Turning the mixture screw in all the way should stall that cylinder. On your carbs the mixture screw controls the fuel, on some other types of carbs, the mixture screw controls the air. Either way, all the way in should cause a stall or something is amiss. Usually, when dealing with these old relics, it's often more time and cost efficient to totally disassemble the carbs, clean everything, and replace all the 'soft' parts. Good idea to throw in some new needles and needle jets while you're in there.

In the mean time...Remove the idle jet and mixture screw and assure yourself that all the 'red' passage-ways in the pic below 'connect'. If you shoot brake cleaner into the small hole (idle air jet) at the mouth of the carb while blocking the idle jet and mixture screw holes, you should see it come out of the by-pass holes. Faulty or worn o-rings on the idle jet, on the mixture screw and on the main jet, are cheap and easy to replace, but are often neglected. ditto for 'choke' gaskets.

[Brickboy]

Thanks again GSPD, that's exactly my plan. I am in a rush because there's a classic show ride this Sunday. It's much better so it will do for that and then on Monday it's full strip and new soft bits, needles, jets etc.

It's such a lovely bike - I'm looking forward to it being even better!