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Topic-icon HIGH SPEED FUEL STARVATION?

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2 weeks 2 hours ago #5464 by Johnb
Replied by Johnb on topic HIGH SPEED FUEL STARVATION?
Hi Wobbly
Air box and filter checked out OK, manometer plugs are in place, rubber hoses to and from carbs are fine. Fuel filters are clear and allowing proper amount of fuel to flow.

Based on the above, what is the most likely cause - diaphragms, main jet, or needle? Also, when you stated that at 70mph wind places the most load on the engine. Does that mean that the wind is making the engine work harder and requiring more fuel to maintain rpm ?
When rebuilding carbs, will I need to order all internals or only specific parts based upon visual. I am not sure if these carbs have been rebuilt as I am the 3rd or fourth owner. Thanks again for your help
JB

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2 weeks 1 hour ago #5465 by 8166
Replied by 8166 on topic HIGH SPEED FUEL STARVATION?
A bad diaphragm will cause the bike to not rev much above about 4000 RPM. Have you pulled the slides and held the diaphragms up to a strong light while stretching them to check for pin holes?

Regardless, you don't know when the carbs have been rebuilt, so you might want to just rebuild them so you have a known good basis from which to conduct further troubleshooting. It is very likely the rebuild will fix the problem.

There's a search field at the top right of these pages. Type in "carb rebuild" and you'll get some good reading and advice on how to do it. Mosey on over to the Tech Articles section, and you'll find detailed instructions with photos. I believe I contributed one such article about when I did it last time on an R100GS.

Whatever you do, only take one carb apart at a time so you can use the other one as a reference when you forget how something goes back together. Use a phone or digital camera to take photos before you start, and after you remove each part.

Finally, buy your parts at the BMW dealer; they will cost you less, and you'll have a much better chance of ending up with the correct parts back in your carbs.

$.02,

Scot

8166 Scot Marburger, This email address is being protected from spambots. You need JavaScript enabled to view it.
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1 week 6 days ago - 1 week 6 days ago #5469 by Wobbly
Replied by Wobbly on topic HIGH SPEED FUEL STARVATION?

Johnb wrote: Based on the above, what is the most likely cause - diaphragms, main jet, or needle?

JB - The most likely cause is hidden in the answer to my question #3: History of the bike. Has it been in constant service, or was it once stored. Have you just acquired it ? Have you owned it for years ? YOU have to supply the requested information because frankly my Lucas-powered crystal ball is on the fritz again.


Johnb wrote: Also, when you stated that at 70mph wind places the most load on the engine. Does that mean that the wind is making the engine work harder and requiring more fuel to maintain rpm ?

Yes. That's what "load" means. If it wasn't for the friction of moving through air, then we'd all be driving at Mach I and getting 900 mpg.

Don't hide 'em, Ride 'em !!
#15150
Last edit: 1 week 6 days ago by Wobbly.

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1 week 6 days ago #5470 by Johnb
Replied by Johnb on topic HIGH SPEED FUEL STARVATION?
Wobbly
Regarding question #3. I have owned the bike for two years. When I purchased the bike from the PO it needed a lot of mechanical work but it started and ran fine. I dont have any prior history regarding storage, rebuilds or general maintenance. During the two years that I have owned it , it had never been on the interstate above speeds of 65 until recently and was ridden mostly on back roads. Everything has been reviewed, renewed or replaced where needed except for the carbs. Also as of late, I am experiencing quiet popping through the mufflers when backing off the throttle when coming to a stop. I have tried adjusting the air mixture screw to no avail. any ideas?
Thanks again for your help
JB

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1 week 3 days ago - 1 week 3 days ago #5478 by Wobbly
Replied by Wobbly on topic HIGH SPEED FUEL STARVATION?

Johnb wrote: Also as of late, I am experiencing quiet popping through the mufflers when backing off the throttle when coming to a stop. I have tried adjusting the air mixture screw to no avail. Any ideas?

Lean on idle circuit. Carbs need cleaning.

Don't hide 'em, Ride 'em !!
#15150
Last edit: 1 week 3 days ago by Wobbly.

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