NC30 losing power after 5 minutes of riding...

[ninja2]

Hello all!

I had my VFR off the road for a year and have just recently had it MOT'd and taxed and all that. It wasn't meant to be off the road for so long so I didn't do anything in particular to store it.

Coming back from the MOT a few weeks back and about 2 miles from home (bike had been running for 10 miles at this point) it lost power, and the revs slowly dropped. It had been running perfectly up to this point, but after this it felt like it was running on 3 or 2 cylinders. It seemed to clear itself just pulling into my estate, so I thought it was maybe a bit of fuel gumming up the carbs that had managed to blow through. I got some redex to clean the fuel system and dumped that in in preparation for my next run.

Today was the first run since (a month later... sorting out registration!) I took it out for a run and it started ok, but after 2 miles the same thing happened. I limped home and let it sit for a few minutes, then started it up (again, started no problem) and let it idle for a while. It then cleared again and so I went for another run. The same thing happened twice more (2 miles, limp home, let it sit, 2 more miles, limp home) before I gave up.

Does anyone have any ideas as to what could be up? I'm confused that it runs really clean the whole way up the rev range for a few miles, and then it runs into problems. I thought it was dropping a cylinder, but all four pipes are hot when I get back so I am pretty sure its firing on all of them... I'm not sure what to check next. All advice appreciated!!!!

[CMSMJ1]

I would chekc your vacuum pipe from tank to no 3 cylinder - it could be split or old and knackered and once it gets hot it collapses and the fuel tap is turned off...

If you want the bike sorted and you think there may be crap in the carbs then the only way is to get them off and clean them.. this is the likely issues if not the vacuum..for my 10p anyways!

[ninja2]

Thanks for the reply! I took the tank off last night and checked the breather, the vacuum line and the fuel tap (which I took apart). I didn't really find anything in particular wrong, but when I put it all back together it seems to work ok. Very bizarre!!! The only thing I noticed was that the fuel seemed to come through the tap a bit better after I had taken the back off and put it back on again. Maybe the diaphragm was sticking?

[TonyB]

Although the vacuum tube may look alright it gets very soft with age and when hot it collapses and gives the symptoms you describe.
Get some 3mm vacuum tube from Halfords or similar and replace it, then try again. A simple and cheap cure. Always the best sort.

[KD88]

My daughters bike has developed a similar fault. After riding or just running for about 15 mins it starts to die as if it is running out of fuel, then stops, and will not start again for ages. The vacuum pipe from the tap joins the one which goes to the top tank breather but I can't see where it goes too then, it sort of disappears down the left side between the two banks of cylinders. Is this right and if so do you think it is the pipe collapsing, as it appears to be in fair condition.