The fact that the engine runs good at higher RPM points to a limited set of causes. If it was ignition, valves or air filter, then the poor running would typically be consistently poor across the whole RPM spectrum. Only 2 cause typically segregate the performance by RPM:

• Auto Advance Unit. If you are still running the OEM "points ignition", then the auto advance changes the timing significantly for running at speeds under ~2000 RPM. You should remove the AAU to 1) lubricate it thoroughly, and 2) replace the tension springs which control the rate of ignition advance. Finally, re-set the ignition using a strobe timing lamp at the full advance mark (F) at a very high engine speed somewhere around 4000 RPM.

Even with the AAU in perfect condition, the engine won't be running its best. This is because the ignition advance curve built into the mechanical advance is tuned for fuels that haven't been produced in 35 years. If you find that the AAU is seriously worn, you may find that a drop-in, fully Electronic Ignition may be in the same price range as a new AAU. In the best EI units, the advance curve is built into the software and all reliance upon regular physical servicing of the AAU is totally eliminated. And the newer EI units have the proper advance curves for use with modern fuels.

• Idle (Slow Speed) Jet. It is Springtime. You most proabably have just wheeled your Airhead out of the garage after some type/length of Winter storage. When ethanol fuels sit, the alcohol "comes out of solution" (separates) from the gasoline. The alcohol attracts water and the water starts to react with the metals of the various carb parts. One notable and universal reaction is a green or brown varnish that coats anything submerged in the float bowl liquid. This coating covers not only the exterior, but also the interior of the jets. A coating on the inside of a jet tends to close it up. Large jets, like the Main Jet, start to run "leaner", but the smaller jets, like the pilot/ idle/ slow speed jets, tend to clog up to 100% closed. 

Sorry, squirting a little carb cleaner down the pilot screw hole isn't going to do anything to help. ALL parts which were at one time submerged in fuel need to be removed from the carb body and each fuel passage scrupulously cleaned and then re-assembled with NEW jets. "New" because the metering though the orifice is so finally tuned that use of any type probe to poke into the jet simply distorts the orifice and upsets the fuel metering. If this is the issue, then this is often a painful and time consuming task that could have been completely avoided if the float bowls had simply been removed after the last Autumn ride. No float bowl = No fuel submersion = No fuel related clogging.

The R90S uses a special Delorto pumper carb. Information about these carbs is scarce on BMW sites. It is my understanding that information is much more plentiful on Ducati and Moto Guzzi web forums. Good luck.

? 

I’d also check the carb diaphragms. If it was fine, then suddenly not, could be a troubled diaphragm, perhaps. 

Thanks Richard and David.

i ride all winter (as long as no snow) and run ethanol free fuel exclusively. I even put 4 oz of aviation fuel in each take for my lead.

embarrassingly, the issue was leaking compliance fittings. I had initially sprayed them with carb cleaning and didn’t notice a change but today I was able to put many turns on the screws holding them on? Maybe cheap clamps?

 Thanks again!!!!

We are always thankful for simple solutions, even if they are embarrassing. 

Posted by: @4949

We are always thankful for simple solutions, even if they are embarrassing. 

I'm always thankful to only be embarrassed ! ? 

Ok I guess I spoke too soon and now the problem seems worse, happening consistent from idle including idle till I rev it high enough to get it past the “bog”? 

advance unit cleaned with light grease added, diaphragms good, timing spot on, valves checked. 

it kinda behaves like it starts out with the breaks on but they’re working fine. Also it sputters while idling as if there’s air getting in?

one thing I did notice in a high speed Frey run I didn’t have the slow speed issues for a bit?

any thoughts? I’m lost.

 Thanks 

p.s. does it matter if the carb needle has lateral play or does it need to be firm without movement?

Carb needles are loose fitting and jiggle about. 

We need a year model.

Model is in the title of the post, R90/6. Its a 1975.

i believes I have finally found the culprit an fixed that particular issue (float needle valve replaced) and problem seems to have gone away.

My new

prob is that the idle is high, 1200-1300. Throttle stop screws backed all the way off, slack in cables, no air leaks, timing spot on, choke off, valves spot on, new o rings on pilot jets. 

any ideas? One thing I did notice is that while one pilot jet is flush with the carb casing, the other sits below the casing a bit?

I recently put new needles in and the diaphragms are good??

signed; lost and confused

thanks

Mark

Posted by: @mark-creek

Model is in the title of the post, R90/6. Its a 1975.

I missed that. Thanks.

My new problem is that the idle is high, 1200-1300.

Classic case of worn out ignition Auto Advance Unit. Either the springs are stretched and/or the mechanism pivots are so worn they mechanically allow the ignition cam to advance the timing. When the timing creeps forward, the idle speed increases that little bit. 

This is one of the better reasons to select and fit an electronic ignition to an older model. You will likely find that the cost of a new mechanical advance is a huge proportion of the cost of a whole electronic ignition. But, select an ignition that does not depend upon springs and mechanical movement. Buy a modern EI (like the Boyer) that generates all the ignition advance in software. 

Hope this helps.

Thanks Richard, that helps a bunch. You mentioned that issue several weeks ago and the advance was cleaned and “it appeared” to be functioning but then all at once? 

Is there an electronic unit you would recommend? Seems there lots of talk about that new Australian unit? Is it easy for a green horn like me to install?

I just read a post you made elsewhere on this forum. Sounds like a Boyer is recommended, thanks for all of your help!

I'm not familiar with every EI on the market. Therefore, I'm in no position to recommend anything. 

My story is this.... I had a 1978 RT which had minor ignition issues. I knew from long years of work on British bikes that 40 year old Auto Advance Units would proabably be at the heart of my problems. If not that day, then not too much further down the road. At that time I worked at a British bike shop and we only installed Boyer (and later Pazon). As an employee, I could get things at dealer cost, so I bought the Boyer and installed it. The installation was straight forward and all the necessary parts were included, right down to the cable ties. The job took about 2.5 hours, mainly because the tank and engine covers had to come off, then the bike had to be strobe timed.

I ran that bike for nearly 40,000 miles in all sorts of weather conditions and it always performed flawlessly. I'm in touch with the current owner, and ignition is not on his "can you tell me about this" list. 

The older (black) Boyers for British bikes have a small issue with not cranking at battery voltages below 12.1V. I knew that the Airhead's electric starter would drop the battery voltage, and that concerned me. But my Boyer turned out to be the newer red "digital" model, and I never had a starting issue, even after a night out in sub-zero temps. 

I have since installed several Pazons on some British bikes, and I believe the Pazons are tiny bit better. This is based on looking at the overall quality of the hardware. (My day job for ~20 years was military electronics.)

The only thing I would not recommend is a unit built in China. But that may be more personal than anecdotal.

► If I was starting today, with zero knowledge and no discounts, then I would proabably look first at the offerings from Motorrad Elektrik. This simply based on my dealings with Rick Jones. I have NEVER gotten a product from him that wasn't first rate, did what he promised and it was delivered on time. ME is more than a business, the guy actually "lives" Airheads. 

Hope this helps.