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TECHNICAL MOTOBRICK WRENCHING In Remembrance of Inge K. => The Motobrick Workshop => Topic started by: Doodle on August 25, 2025, 01:20:42 PM
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Hi and hello,
I kind of recently bought my K75 (1990) and the guy I bought it from said that I shouldn't fill the tank all the way up, just around 10-15L max. (The tank holds 21L I think). Otherwise there wouldn't be any space left in the tank for the fuel to go around and the tank would become hot.
I haven't filled it up more than that, but I have been searching the forums for an similar problem but haven't really found any. I found something about the ECU, but it did seam to be the same problem.
Now it didn't really sound like from the seller that it was a big problem, but he said it twice that I shouldn't fill it up, so I am wondering if there really is a problem or not. I can try to fill up all the way, but I rather not take any chances in case something breaks from it.
Dose anyone know if there is such a know problem or if I should try to fill it up all the way?
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Bogus "advice" as far as I am concerned. 40 years with my K100RS and I fill it up to the brim when riding. The exception is when you are going to park it afterwards. The engine heat expands the fuel and it could overflow onto the ground. I wait to fill it until I start the next ride. These bikes don't have evaporative cannisters that can be damaged by over-fueling. Just my 2 cents worth.
Frank
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The tank will get hot from engine heat regardless of whether fuel is sloshing around in it. Ask any Brick owner. The previous owner might be referring to vapor lock which is a real condition. When overheated gasoline turns from liquid to vapor in fuel lines, the increased volume of vapor can interfere with fuel flow and performance.
Another condition: When heated, fuel and fuel vapor expand regardless of whether they are heated inside the tank by the sun shining on it or by the engine when its running. When fuel expands too much without enough room to accommodate expansion, it can sometimes leak out of the cap onto the tank and out of the vapor hose on the ceiling of the tank onto the ground behind the right foot peg. That can be a nuisance, a fire hazard or both. In the States, and maybe in some other counties, there is a flapper valve at the bottom of a short funnel attached to the fuel cap. If used correctly, that prevents overfilling. If your tank doesn't have one of those, you can fill the tank right up to its opening with gasoline and find out what happens while riding, or keep the fuel level a centimeter or two below the cap and see if that's okay.
Limiting filling to 10 or 15 liters seems too cautious. You'll find out. Consider it practice in implementing the scientific method.
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No. In forty years of riding K75s filling the tank has only caused an issue for me once.
It was a hot day and I filled up before stopping for lunch and then left my K75RT in the sun on the side stand while I had lunch. When I came out there was a slight dribble of gas from the gas cap down the left side of the tank. Gas at gas stations comes from underground tanks that are often a lot cooler than the ambient air temperature on a hot day. Heating of the gas in the gas tank can cause it to expand a little which was what happened to me and it's only happened to me that one time. (With flap installed.)
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'87 K75S, fill up to the flapper every time, no issues or worries.
PO superstition is like when I spilled Cenex gas station coffee on my camouflage, and harvested a big buck that morning. Hunting buddies started wanting to go to Cenex for coffee 😆
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And to drop in a dime worth nothing. I have all my electronics underneath my gastank and removed the overflow hose for it. And also i fill up to almost max and have no issues with overflow/drops.
Just dont max it and park it when filled to the brim underneath a blazing sun
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Thank you all, I will try it soon. I was going to do it this morning, but the bike wouldn't start. Hopefully it is just the battery since I was trying to fix the turn signal and had it on, but not started. So it probably drained the battery since it was on quite a while until I figured out what the problem was, its recharging now :D
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... I figured out why he said to not fill it up more then 12-14 liters, because it can't hold more then around 13-14 liters xD
There is a custom hole in the tank, it is welded, for some kind of police equipment I guess. He mentioned the hole but not that it reduced the tank capacity. I should have figured it out sooner.
Well I guess I might be looking for a replacement tank. (It is a hole that goes straight through the tank, it dose not leak, it is there for a purpose)
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How about posting photos here showing us this beast and its black hole?
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From the other thread, when I saw that I thought wow that sticks out like you know what.
Do you remember what was said about it or why it was done?
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Why, what, and most of all, why does your gastank have a phone holder or a pocket.
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Why, what, and most of all, why does your gastank have a phone holder or a pocket.
A wooden stake with a sign mounted on it reading, "For Sale" or "Mustache Rides $5" would fit right into that opening. All it would need is a collar to keep it from slipping too low—an altogether attractive advertising venue when parked.
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'87 K75S, fill up to the flapper every time, no issues or worries.
Take the flapper out, you can get more gas in and as a bonus you can see in the tank better. Maybe the OP's bike has a hole 2/3rds of the way up the tank :laughing1:
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A wooden stake with a sign mounted on it reading, "For Sale" or "Mustache Rides $5" would fit right into that opening. All it would need is a collar to keep it from slipping too low—an altogether attractive advertising venue when parked.
For the banner men!
Bit non practical having it on the throttle side
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For the banner men!
Bit non practical having it on the throttle side
It isn't meant to be used when mobile; it's a means to post advertisements when the owner is in a café, restaurant, jail or otherwise preoccupied and the Brick is parked. Clearly, craftsmanship was used in its design and execution.
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Sorry for the delay, here comes some more pictures. You can see from the second picture that the hole goes all the way through.
All I remember the seller saying was that it probably was for some type of police equipment, he didn't know for sure. I was able to find some old pictures of Swedish police bikes. I am not sure if they are K75 or maybe K1000/K1100.
(https://motobrick.com/gallery/4/8457-020925024650.jpeg)
(https://motobrick.com/gallery/4/8457-020925024719.jpeg)
(https://motobrick.com/gallery/4/8457-020925024738.jpeg)
(https://motobrick.com/gallery/4/8457-020925024754.jpeg)
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The police museum in Stockholm (https://www.hemmings.com/stories/off-duty-touring-swedens-police-museum-historic-vehicles-collection/) would be the place to go for information about that tank because Bengt Svensson, its motorcycle caretaker, rode a Motobrick during his career.; however, the museum is undergoing renovations and will be closed until 2026. Perhaps Svensson could be contacted.
In the meantime you could request information by sending the photo that shows the two openings in your Brick's tank and another photo of the side profile of your Brick to the headquarters of two or three large police precincts in Sweden like those in Stockholm, Gothenburg and Malmö, and ask if their chief motorcycle technicians know what use those openings had on a police motorcycle.
One thing is certain about that tunnel in the tank. If somebody ran a chain through it, around a streetlamp pole and locked it, stealing the motorcycle by riding off with it could get complicated.
:laughing4-giggles: