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59' R50 - Fuel Vacuum
- sripley1632@gmail.com
- Posts: 4
- Joined: Tue Jun 28, 2022 8:22 am
- Location: Annapolis, MD
59' R50 - Fuel Vacuum
When riding, bike would become "fuel starved" and cut out. Intermittent, but more pronounced when opening the throttle.
Originally I thought it was clogged fuel line so I would park the bike, pull the fuel line off the carb and blow backward into the tank. That seemed to correct the issue temporarily. I assumed dirty tank so I pulled it and did a full cleaning. Reinstalled and same issue. Noticed that the enamel coating on the inside of the tank was flaking off and a little rust, so I pulled the tank cleaned and lined with Red Kote, then installed two fuel filters.
Just went for a test ride and problem started recurring. When running, fuel was crystal clear, no bubbles and no dirt debris in the filter. Then I noticed one filter was bone dry. I hit the plunger on the top of the carb and no fuel. So I opened the gas cap and boom, fuel flowed into the filter.
1. I'm thinking the gas cap isn't venting properly creating a vacuum? Any other theories?
2. If it is the gas cap anyone know if it can be disassembled and repaired or do I just buy a new cap? Also my cap has a cork gasket. Could this be the issue?
Thanks!
- schrader7032
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Re: 59' R50 - Fuel Vacuum
Cork gasket shouldn't be an issue. I just took my stock cap off and didn't see an obvious way to take it apart...the cork gasket is in place, so I can't see it all. Looks like the cap breathes through a bunch of vents/holes around the outer edge. I suppose if you removed the gasket and soaked the cap in some kind of cleaner, or maybe spray some brake cleaner into those holes, that might cut through some corrosion.
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.
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Re: 59' R50 - Fuel Vacuum
R69S - R60/2 - R67/2 - R51/3 - R69
- vechorik1373
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Re: 59' R50 - Fuel Vacuum
There are cheapo after market reproduction caps out there, and then there is the original mfg Blau cap that is available from BMW. The good Blau brand isn't cheap, but it is better made.
Part number 51 25 0 035 314 is the Blau cap.
Technical Adviser, Former owner, Bench Mark Works
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- vechorik1373
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Re: 59' R50 - Fuel Vacuum
So look and see what you currently have. If you buy a new one, if it is a real one, it will have Blau stamped on the bottom.
Technical Adviser, Former owner, Bench Mark Works
662 312 2838 cell 9 am to 4pm CST PLEASE!
- sripley1632@gmail.com
- Posts: 4
- Joined: Tue Jun 28, 2022 8:22 am
- Location: Annapolis, MD
Re: 59' R50 - Fuel Vacuum
A. I am not crazy and that the vacuum may actually be a source of the problem.
B. All the great advice.
I did loosen up the cap while riding and it did help - though it is a little tricky driving with one hand.
I also attempted to disassemble the cap I had. Not smart. For those of you wondering it doesn't come apart, and you will ruin it . I ordered a new one and vechorik1373 I did see there's a big difference between the reproductions and the original. I am going to try the reproduction and probably invest in the OEM.
Still curious about how these things actually work. When the replacement cap comes I might do a full autopsy on the the old one. I think there's a spring inside the cap that keeps the vent closed until the vacuum pulls it open? I will keep you posted.
Thanks again! Love the knowledge on this forum
- schrader7032
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Re: 59' R50 - Fuel Vacuum
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.
Re: 59' R50 - Fuel Vacuum
I do like having clear fuel filters even when i want to start and tickle my carbs i can see the fuel starting to run through.
Shane
Wollongong, Australia
Tooo many projects —— oh well
53 R25/3 Project
57 R26 Project
62 R50/2 Restored
64 R50/2 Project
67 R69S Original
71 R75/5 Original
77 R100RS Project