Boxerworks.com

Hi, I think I have a problem, but perhaps it is better to talk about a discomfort during the ride:
when the engine is warmed up if I travel at constant speed between 2000 and 3500 rpm I noticed that bike has a limping pace,
I'm oriented to think about ignition problem but the sparks are clean, cables and bobbins the same...
mimble mumble! maybe the diode box?

please unleash your expert minds

I don't think it's the diode board...the diode board and charging system just does that...charges the battery. You can run the bike with just a battery.

So after 3500, bike runs well?? If so, then it would seem you need some mid-range carb work.

Kurt

Kurt in S.A. wrote: ↑Wed Aug 02, 2023 10:35 am I don't think it's the diode board...the diode board and charging system just does that...charges the battery. You can run the bike with just a battery.

So after 3500, bike runs well?? If so, then it would seem you need some mid-range carb work.

Kurt

Or a sticking advance mechanism

Rob

Could be right, Rob. I looked at some of the older posts...sounds like this is the first chance to actually get the bike running. So no indication of having checked the timing and looking for the F-mark in the timing window.

Kurt

If limping pace means poor response from the engine which then recovers above 3500 rpm then those are the revs when the carb needles have lifted. As an experiment to confirm or otherwise that's it's a mixture strength problem, I would lift the needles one position and see what effect that has. That's a big change but if it works you can always put the needles back where they were and go up one needle jet size which is a smaller change.

Another easier option to confirm a weak mixture is to add a bit of choke when you are at those revs and see if that improves response.

barryh wrote: ↑Thu Aug 03, 2023 5:25 am If limping pace means poor response from the engine which then recovers above 3500 rpm then those are the revs when the carb needles have lifted. As an experiment to confirm or otherwise that's it's a mixture strength problem, I would lift the needles one position and see what effect that has. That's a big change but if it works you can always put the needles back where they were and go up one needle jet size which is a smaller change.

Another easier option to confirm a weak mixture is to add a bit of choke when you are at those revs and see if that improves response.

True enough but I would check the advance first... mainly because it's easier to do and involves less variables. You don't even need to take the front cover off, simply rig up a strobe and watch the antics of the advance as the engine advances. If the advance is OK, the you can tear into the carbs in the knowledge that you're more than likely looking in the right place.

Rob

thank you all,
Kurt I already cheked the timing and valves clearance

Barryh I will try for sure the tests you suggested, for the needles you mean the minimum regulation?

and Rob, what you mean when you say check the advance? to verify the correct oem advance, to verify the movement of mechanism or vary the original advance?

pioppiny_81 wrote: ↑Fri Aug 18, 2023 8:24 am
and Rob, what you mean when you say check the advance? to verify the correct oem advance, to verify the movement of mechanism or vary the original advance?

Well, all of the above really.... however, without fairly sophisticated equipment you can't be absolutely accurate with advance angles and timing. What you can do, though, is to view the various events using a strobe timing lamp and the vehicle tacho.

Put simply, the strobe should show the timing marks at the idle setting up to around 1500rpm (800rpm for /5 models) and full advance setting after 3000 rpm. The advance between those two engine speeds should be smooth and continuous. That tells you all you need to know at this point, the engine should run perfectly acceptably unless you are tuning for peak output. An advance that 'hangs' or is sticking might well cause the situation you describe.

If the timing points aren't seen when they should, you need to service the advance mechanism by lubricating it, checking the weight pivot points aren't worn, checking the stops which prevent the weights from moving too far out and, if the rev/advance specs aren't right, replace the advance springs. Exact procedures to do this depend on which year engine you have and how bad the readings seem.

My real point, though, is that checking with a strobe will give you a pretty good idea whether this is the source of your problem and may render a lot of dismantling unnecessary (or rather change the nature of the work you end up doing).

Rob

ok, very clear explanation, i'll try,
I think one of my mate have the strobe tool..
Anyway everything takes me in the direction of an electronic ignition ,
but not for the moment or at least until I discover the tree of banknotes

pioppiny_81 wrote: ↑Mon Aug 21, 2023 9:27 am ok, very clear explanation, i'll try,
I think one of my mate have the strobe tool..
Anyway everything takes me in the direction of an electronic ignition ,
but not for the moment or at least until I discover the tree of banknotes

If it's your '81 R100, it already has electronic ignition. BMW started fitting it in 1981. The advance mechanism is still mechanical though.

Rob