59' R50 - Fuel Vacuum

[sripley1632@gmail.com]

Hoping the forum can confirm a suspicion and offer advice on a fuel problem with my 59' R50.

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]

Next time when you're riding and it feels starved, carefully open the gas cap enough to let air past it...probably not something to do with a full tank. That will tell you if it's the gas cap.

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.

[808Airhead]

I have had a new,quality "BMW" boxed reproduction gas cap do this! I had to drill a small 1mm hole in the center for it to work,until I ordered another one from a different supplier that worked. so YES,this can happen. Just drive with it "loose" not engaged in the tank and see if it changes. Stumped me for the first few rides of my newly restored R69S.

[vechorik1373]

The original polished aluminum domed gas caps vent through the lower edge where the aluminum is crimped on the serrated edge. The steel bottom piece will rust internally over the years, and restrict or eventually stop the flow of fuel, when the cap can no longer adequately vent.

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.

[Jim D 5112]

Maybe you could put the cap in an ultrasonic cleaner tank.

[vechorik1373]

I forgot to mention, that the the original mfg Blau cap, has "Blau" stamped in the steel bottom. Repop caps have no marks.
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.

[sripley1632@gmail.com]

Thank you for affirming:

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]

It will be interesting to see what you find. I don't think the cap is that complicated. I think it's just a circuitous route that the air has to follow. That way the dirt and rain can't get it but it's open all the time to maintain a common pressure with the outside.

[kingcowie]

My 62 R50/2 has a non standard cap and for a number of miles i had this miss occur, and starving. It took a while to work out what was the issue but one day i was riding on highway and the starving occured. I looked down onto the clear fuel filters i have on each side and i could see air in them. I cracked the fuel cap and it came good, got full fuel flow. I am going to get another fuel cap.

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