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R90/6 bogging down up to 1/4 throttle.

Mark Creek
(@mark-creek)
Active Member

Hello

My bike has been running well but recently on a ride it started bogging down as I would leave from a stop.  I had to give it more than 1/4 throttle to get it going and then above that it ran great. My thinking was to check the pilot screws and clean those out. That didnt solve it at all. Still behaves the same. Starts fine but definitely rough running at idle but then fine above that.  Hoping to save time can you direct me as to were to start?  My thought was its not points or it would run poorly at all speeds? If that correct thinking? Where else should I start?

 

Thanks.

Quote
Topic starter Posted : 05/18/2022 08:44
Richard Whatley
(@wobbly)
Member Moderator

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.

This post was modified 2 months ago 2 times by Richard Whatley

[color=blue]Don't hide 'em, Ride 'em !!
#15150[/color]

ReplyQuote
Posted : 05/19/2022 12:18
David Elkow
(@4949)
Estimable Member

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

ReplyQuote
Posted : 05/22/2022 04:08
Joe Hall liked
Mark Creek
(@mark-creek)
Active Member

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!!!!

ReplyQuote
Topic starter Posted : 05/23/2022 13:05
David Elkow
(@4949)
Estimable Member

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

ReplyQuote
Posted : 05/23/2022 13:28
Richard Whatley
(@wobbly)
Member Moderator
Posted by: @4949

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

I'm always thankful to only be embarrassed ! ? 

[color=blue]Don't hide 'em, Ride 'em !!
#15150[/color]

ReplyQuote
Posted : 05/24/2022 05:41
Mark Creek
(@mark-creek)
Active Member

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?

This post was modified 3 weeks ago by Mark Creek
ReplyQuote
Topic starter Posted : 06/17/2022 12:56
David Elkow
(@4949)
Estimable Member

Carb needles are loose fitting and jiggle about. 

ReplyQuote
Posted : 06/18/2022 04:02
Richard Whatley
(@wobbly)
Member Moderator

We need a year model.

[color=blue]Don't hide 'em, Ride 'em !!
#15150[/color]

ReplyQuote
Posted : 06/18/2022 06:01

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