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Discussion Starter · #1 · (Edited)
In the span of 1,5 years i had experienced surging problems 3 times. The last time it happend was this monday the 22th of february after i just got my bike back from the mechanic where it had its yearly checkup and checked also for my issue. He couldn't reproduce it or find any issue (He also checked the fuel cock for correct fuel delivery).
I did check the F650 FAQ but had no success finding any similar problems.

What happend?
Bike starts surging and i feel it is losing power and is going to stall.

When does it happen?
I recently noticed all these 3 occurrences happend on a highway going slightly downhill at steadily ~105km/h in 5th gear making around 4k-4.5k RPM with under 8 liters of fuel in the tank.

What did i do when it happend?
The first time it happend i was shocked, moved to the right lane a.s.a.p. where the bike stalled. I rolled into to the emergency lane and came to a stop. After about 5 mins by putting the bike on reserve fuel and shaking the bike, it started again. I went to the nearest petrol station and filled the bike up, i could only add 10 liters of fuel.

The 2nd occurrence i immediately switched my fuel cock to reserve (after doing this the surging is gone) and drove to the next tank station where i could tank again only 10 liters.

The last time this happend i was on the exact same road and place as the first occurance (daily commute) and switched my tank immediately to reserve. When the road was not in a decline again(flat) i switched the fuel tap back to normal without any issues. I havent filled up my tank since, i think the bike now has around 6 liters of fuel.

It sounds like you were just out of fuel mate!
After 2 of the 3 occurances i went to the nearest petrol station and i would only be able to fill it up with 10 liters. This would mean that there is still 7 liters of fuel in the tank...

My question;
Is it possible that the fuel in my fuel tank goes under the upper straw from the fuel cock when the petrol level is under ~8 liters and leaning to the front because of a slight downhill? Because to me this seems highly likely but is the only thing i can think about with these occurances.
38065

For the super interested people, this is the place where it happend twice (Google maps), the decline is minimal.

Any tips or advice is welcome!
 

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Discussion Starter · #2 ·
Update: I scraped the lack of fuel as an option.

Last night on the highway it happened again twice. Last night the bike started surging again so i put the fuel cock on reserve, unfortunately the surging didnt stop and the bike lost complete power while i coasted onto the emergency lane. The oil light lighted up and the bike didnt want to start again. After waiting 5 min the bike started again and i revved it slowly till 4.5k RPM and kept it there for a while checking the RPM for drops. These didnt occur so i moved on. I think 15 min later the surging started happening again (SHIT!) this time it was a bit more abrupt and the bike quickly stalled after 3 loud pops (this is new!). Stopped again, started it again after another 5 min of cooling down. When i got home i checked the coolant, oil, fuel and the airbox.
The airbox was a bit dirty with some sand/dirt so will clean this out.
There was too much oil in the system so i will have to remove some (mechanic filled 2.1. liters so it should be fine.... But i will remove oil until it is in the right spot).
Fuel was ok, plenty left in the tank, will add some fuel treatment/cleaner in the tank.
I will check the VR with a multimeter.
I will check the spark plugs.

If someone has any other ideas/suggestions let me know :cool:
 

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Tank vent blocked , causing vacuum ?
Do you have an in line fuel filter ? I had issues with one restricting fuel flow.
 

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Discussion Starter · #4 ·
Tank vent blocked , causing vacuum ?
Do you have an in line fuel filter ? I had issues with one restricting fuel flow.
Thanks for your reply, I do indeed have an inline fuel filter after my fuel cock, i use a "mahle kl15".
How can i check that this is indeed the problem or should i just remove the filter and see if the problem still occurs?
 

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But it was not the same filter as yours , so may not be the issue but worth a try.
Also turn fuel off and drain the float bowls and catch what comes out in a container check for water and debris , you may need some small tube for the drain tube at the bottom of the float bowl to do this.
 

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I have a 1996 carbed funduro with a very similar problem. Oddly enough the dealer I bought it from fitted an in line fuel filter (for no particular reason as far as I could determine)
As I only ride bikes and am not mechanically minded I have it booked in at my local dealership. It has recently had a new rectifier (apparently a faulty rectifier gives similar symptoms when it is on the way out) to no avail.
The spluttering and on occasions stalling happens when I am proceeding slowly, thankfully it has not happened at speed, overtaking etc!
The bike restarts (sometime after cooling down) and then proceeds as normal until the end of the journey. Sometimes I can complete a 30 odd mile journey with no probs.
The fuel filter suggestion looks interesting and I will mention to the dealership when I take it in.
I will keep a lookout on this forum for any other suggestions, obviously if the dealership can fix I will post on here.
Intermittent problems are always the worst!
 

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Discussion Starter · #9 · (Edited)
Small update; emptied the fuel tank and checked for debris. I couldn't find the tank vent... Can someone show me where this could be found? Couldn't find anything on the internet nor my manual. Checked fuel pet cock/tap (was surprisingly clean, and almost looked factory new) cleaned it a bit with some pressurized air and a cloth. Checked the fuel hose. In this hose i found a weird bend and very thin filter of which i am not sure if it belongs there (could this be it!?). Is this supposed to be there if so, should i buy a new (not bend) one? Or just clean it and put the it back.
38107
38108

I also drained float bowls with a rubber tube, a tiny amount of sand came out (nothing too special).

When talking to my really handy neighbour about what i was doing he offered to check my carbs so i pulled them out and will check them with him later.

After the carbs will be back i will check the spark plugs just to be sure and go back to driving it daily. If for some reason the issue still occurs, i think it has to be an electrical problem (probably VR related me thinks), electronics ugh.
 

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Vent built into petrol cap , if it does not vent creates vacuum and reduces or no fuel flow.
When the bike is running , and engine is warm-hot --- when the problem shows up open the cap the problem may go away do not do this with full tank.
 

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Discussion Starter · #11 ·
Vent built into petrol cap , if it does not vent creates vacuum and reduces or no fuel flow.
When the bike is running , and engine is warm-hot --- when the problem shows up open the cap the problem may go away do not do this with full tank.
I will remember this. If the vent in the petrol cap is indeed the problem, how would one fix this?
 

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Discussion Starter · #12 ·
Well well well my fellow inmates upon dismantling my mikuni bst33 carb the top cap just crumbled apart, i do not know how or what this caused it but its no good (the crumbling part isnt even near a screw which could have been over tightend).
The inside of the carb was a bit dirty including the main jet and especially the float bowl. The sealant was ok but is better to replace. The rubbers and floats were in good state.
I have ordered a new carb cap from england for 30 euros since i couldnt find any in the netherlands (will probably have to pay import taxes wohoooo 😤). I also ordered 2 revision sets so me and my neighbour will almost replace the whole damn thing after a thorough clean. I dont know how long the delivery from england will take but i think i'll come back with an new update after a month or so. I think i will have to see if it occurs again and then open my gas cap.
38109
 

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Discussion Starter · #13 · (Edited)
Okay the carb top came incredibly fast and arrived 2 weeks ago. Installed the carbs and set the mixture screws to the factory default (3.5 turns). After installation drove the bike for a while and took some highway which did not show any signs of failure, yay! Because of a busy schedule i didn't ride much but today i took the bike out for work. Filled her up with some nice superPlus fuel and guess what... After 15 mins the bike surged again 😤, stopped besides the road and opened my gas cap. After that bike started instantly and drove away without any problems. Drove back from work without my gas cap without any problems. So now i am 99% sure it is the vent in the patrol cap which is malfunctioning.
 

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Discussion Starter · #14 ·
Okay i cleaned my gas cap which was actually pretty easy (one of my two left hands are slowly turning right) . Since i could find absolutely nothing about the funduro gas cap i will post some pictures so people will know how it looks and have an idea how it operates. If this did not fix my issue i will check the ignition coils.
 

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