gpz500 lost almost all engine power!

Havent used my bike for a while (had flu). I did do a quick 5 mile run up the road about 2 weeks ago and it was fine then.

This morning I go to set off, idles fine, started ok. Soon as I reach about 20mph its struggling to accelerate. So badly that I've got a top speed of about 35!

It sounds ok but the throttle just doesn't seem to do anything much past about 1/4 throttle. It will rev all the way in neutral, but sluggishly.

I thought maybe the battery was low on power causing spark issues, so I've put it on charge just in case but its already charged.

help?

Does it live outside? Start it up and check it is running on both pots. Sounds like bad fuel maybe water has got it. Lots of other possibilities but your decription is limited.

It lives in an underground car park, sheltered from the elements.

Forgive me for being daft but how would I tell for sure if its only running on one cylinder? As I said it sounds the same as it always does at idle, and on the road except for not wanting to rev.

Forgot to mention it has done this kind of thing a couple of times before, but both those times opening and closing the fuel cap seems to have resolved it, unless it just went away on its own and that was a coincidence. Its done a couple of thousand miles without incident since the last time that happened. This time I even tried riding it up and down the car park with the cap open and it made no difference.

I've tried moving the fuel tap from on to reserve, back to off and on again. no difference.

Pull the fuel line off and put a container under the tap. Turn it on and see if you are getting a good supply. May well be blocked and restricting the fuel supply. It will tickover as it gets enough fuel but revving it needs more fuel and it is not getting enough. I would drain the tank and remove the fuel tap and clean the filters.

you can check to see if both sides are firing by checking the exhaust pipes are getting hot. You may have a blocked carb jet, but its more likely a blocked fuel filter.

You have a partial vacum which restricts the flow of fuel. Take off your filler cap and have a look for the air vent. There'll be a small hole there somewhere which is designed to let air in to displace the fuel as it's used. If this is blocked or partially blocked then the fuel cannot flow.

Thanks, but I've tried running it with the fuel cap totally open today and it wasn't any better, so I think we can discount that on this occasion. Also, the times when this has happened before, it then totally ceased to be a problem for 1000s of miles. Does that make sense? There is a whistling from the tank sometimes which goes away on opening the flap, mind. I've looked at the cap and tried to clean it in the past, but it never seems to make much difference. Hard to tell if its fixed or not because its usually fine for ages, even shortly after an episode of this (usually straight after I open and close the cap, just not this time).

Now it gets weird. After charging the battery for an hour (despite the charger showing a green "charged" light within 5 mins, plus the fact that it started and idled perfectly fine beforehand) its now running fine!... Is this likely to indicate a reg/rec problem? It did do 5 miles in a month, so I expected a flat battery, and was surprised to find that it wasn't apparently showing any signs of being flat, tbh.

I suppose there is a fuel breather hose down by the battery compartment that was perhaps disturbed in removing the battery to charge it, but then again, why would it suddenly have become blocked while the bike was left standing?

meh, bike works again now. Ironically I had planned a nice long ride today, and after all the faffing about thought its a bit late for that trip now. So I popped over to my mum's instead, for a nice short test ride and a safe haven in case of further probs. Turns out I was expected for Sunday lunch today and had totally forgotten about it - so the bike has done me a favour!

Thanks for all the input guys.

Time for a new battery. If it's weak anyway and then not used for a week or more, while it's cold, that can be enough for it to give up. Should only be £20ish for a cheap one and that should sort you for about the next 2 winters.

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I used to be BlessMyCG, long ago in a dark place.
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I'd follow maccecht's advice - it sounds like a fuel flow issue. It could be a partially blocked filter or a split in the vacuum hose to the fuel tap. Either will restict fuel flow.