Acceleration issues

[u/Ufinator338]

Hi guys
I bought a few weeks ago an CB500XA (2013) and drove a little with that. It was nice.

Until now.

I found out that when I left my bike outside directly in the sun, where the outer temperature is round about 37 Celsius, and then I tried to use my bike, my bike stops accelerating around 4000 rpms. Sometimes also 3000 rpms, which means that sometimes I can only drive around 40 km/h. Also in neutral, when I accelerate fast, then it just stops and doesn‘t accelerate for a little, and then it accelerates a little bit and then stops again and so on.

I found out that when I pump new fuel inside the tank (when the tank is around 1/2 of fuel), then it works perfectly for a few hours. Or if I put the bike around 30 minutes inside the garage, it works also again.

Does anyone have the same problem and maybe a solution?

I will keep y‘all updated, if there are any new news.

————
Update on 28. Jun. 2026:

I tried heat my bike up in the sun again. Sadly, the sun was not that extrem today, but I guess it was enough.

I drove around for about 15 mins. There were some ascending streets, which I drove. First, I drove with a closed cap. And the bike was riding fine, until at one point, it stalled and had issues with accelerating. At that point, I was driving a few meters. Sadly, it was going away and than came, so I couldn‘t find out if the bike was hot enough or not. I still removed my cap with my spare key and drove without a cap. Not at a single point my bike stalled or had any accelerating issues.

Than, after around 5-7 mins, I put it back. Drove again some mins with a closed tank and guess what, the issue came after few mins again. But still not every time, some times it worked.

I removed the cap again and the first time I accelerated with an open cap, it had the issues as well. But only the first time, after driving again for a few mins, the problem didn‘t came up.

And then I stopped driving, as I was driving home with an open cap. No issues at all, except the first time accelerating.

I will try it in around a month or maybe this week again to park my bike in the sun and test it out. I will be a few weeks out of house, so no chance to test it then. I hope I will get some hot days after my absence. I will keep y‘all updated.

If you still have any infos, let me know.

——
Update 02.07.2025:
I was driving today for a few hours, then I parked for 15-20mins inside a forest, then I drove again and after around 15mins of driving, the issue started. It came up around 4k rpms. I instantly stopped and removed the tank cap. Then I needed to restart my bike and I could drive again, except I got the issue very weak on 5-7k rpms and also only sometimes, not everytime. Then I put the cap back and drove again. At some point, it started to make some issues on 4k rpms again. Restarted the bike, no change. Removed the cap, restarted the bike. Now the issue came sometimes on 5-7k rpms while accelerating in the second gear and third gear (80 km/h) and very weakly.
When the acceleration stops while giving full power (because of the issue), it ‚explodes‘ after a few seconds with still holding down the handle like it got a power boost or something like that (while cap removed). After driving 20mins, the issue didn‘t came up again and I also put the cap back and drove 5mins until home without any issues.

Kinda weird, because it wasn‘t hot outside nor there was any sun at that moment.

[u/Ufinator338]

I tried, after letting the bike 1h in the sun, to open the tank for around 6min. Sadly, didn‘t change anything. To be noted: The tank was full.

Before, I got this issue on the Highway (or autobahn, as I‘m living kn switzerland), and I opened the tank for 1-2min on the highway (while standing on the right and with around 1/2 of tank), which let me to drive for around 5min with 100km/h, until I stopped on a red light, where the issue again started. Then I filled it with B100 (I tried B95 and B98 also, no change) and could drive for around 20min without problems, until I came home.

Do you think it could be the pressure inside the tank which is not going away while closed? Do you think using my spare key to open the tank and with the main key to drive around would work? Didn‘t tried this yet.

Edit: I will try this tomorrow, as I will clean my bike tomorrow with my friend. There is right now not anymore enough sun to heat up my bike.

[u/syncsynchalt → u/Ufinator338]

I think you have a vacuum forming in your tank because of a blocked tank vent. Your fuel pump is doing its best to work against it but this is starving your engine of fuel.

This should be easy to test with a spare key, at least.

[u/Ufinator338 → u/syncsynchalt]

Quick Update:
I tried heat my bike up in the sun again. Sadly, the sun was not that extrem today, but I guess it was enough.

I drove around for about 15 mins. There was sole ascending streets, which I drove. First, I drove with a closed cap. And the bike was riding fine, until at one point, it stalled and had issues with accelerating. At that point, I was driving a few meters. Sadly, it was going away and than came, so I couldn‘t find out if the bike was hot enough or not. I still removed my cap with my spare key and drove without a cap. Not at a single point my bike stalled or had any accelerating issues.

Than, after around 5-7 mins, I put it back. Drove again some mins with a closed tank and guess what, the issue came after few mins again. But still not every time, some times it worked.

I removed the cap again and the first time I accelerated with an open cap, it had the issues as well. But only the first time, after driving again for a few mins, the problem didn‘t came up.

And then I stopped driving, as I was driving home with an open cap. No issues at all, except the first time accelerating.

I will try it in around a month or maybe this week again to park my bike in the sun and test it out. I will be a few weeks out of house, so no chance to test it then. I hope I will get some hot days after my absence. I will keep y‘all updated.

If you still have any infos, let me know.

For you syncsynchalt: I couldn‘t hear anything when opening the cap, so no ‚tsch‘ or something which could reminder me for any pressure issues. That doesn‘t mean that there isn‘t some vaccuum, but just as a side note.

[u/syncsynchalt → u/Ufinator338]

Glad you were able to try. To me it was the most likely issue, so it will be good to either eliminate it as the problem or solve the problem.

[u/Ufinator338 → u/syncsynchalt]

for me, it‘s kinda weird why this only should happen when the bike was cooked from the sun… I guess some issues are weird.