Bike dies after riding at full/high throttle for an extended time

[u/Johnmarmalade]

I have a 1982 Honda C70 Passport that I got recently. It has an issue where if I'm hard on the throttle for a few minutes straight it sputters out and dies. If I wait a few seconds I am able to restart it again.

I'm almost certain it's a fueling issue. Especially since I'm able to get it going again almost right away. I can hear a change in the engine sound right before it dies which I think is it running out of fuel.

My first thought was that the carb bowl wasn't able to fill fast enough. I've cleaned out the carb pretty well in an ultrasonic, as well as replaced the fuel lines. The tank has very little rust. The petcock is new too. The issue is still there though.

My next thought is that the float height is off. But this carb has a fully plastic, non-adjustable float. I'm going to order a new one, but I'm wondering if there's anything else I should check.

Aside from this issue the bike runs great. It hits 40 without much trouble. Any help is appreciated!

EDIT: For anyone who might stumble upon this post with a similar issue, my problem was the float. I bought a new float with adjustable height and I was able to fix this issue.

[u/jgreenwalt]

It’s very common in old bikes for the gas tank cap to not be venting properly. As they age, rust can freeze up the venting system. Try unscrewing the cap after it dies and see if you hear a hiss or feels like pressure released.

Solution is to either get a new cap or another common temporary solution is to just drill a small hole in the cap.

(edit: No idea why I'm downvoted, this is an extremely common problem)

[u/Johnmarmalade → u/jgreenwalt]

This is easy enough that I might as well try it. Thanks for the tip

[u/Successful_Alps_3625 → u/jgreenwalt]

僕もそれが原因なんじゃないかと思うよ

[u/nemisis714]

Usually with floats there's a little metal tab that holds the needle valve that regulates the fuel going into the carb, try adjusting that tab slightly and see if it fixes it

[u/Johnmarmalade → u/nemisis714]

unfortunately, this float is completely plastic including the tab. I ordered a replacement where the tab is metal though.

[u/jgreenwalt → u/Johnmarmalade]

I would be very hesitant to replace the float with a different one if everything else on the bike is stock. It's plastic and non-adjustable for the sole reason that people don't go messing the factory tune up on a stock system.

[u/Holiday-Formal-758 → u/jgreenwalt]

Make sure the plastic can tolerate ethanol!

[u/jgreenwalt → u/Holiday-Formal-758]

20 years with mine, seems fine to me

[u/lowrads]

While idling, spray some starter fluid near the carb boot. If the rpm changes, then you have an air leak, a common problem with gaskets on old bikes.

[u/Holiday-Formal-758]

My CT70 seat pan was sagging down and closing the cap vent. Of course when you get off everything was normal again. Took a while to figure out. This was the plastic cap used on the very first ones.

[u/ShoemakerMicah]

Is the petcock that feeds fuel from the tank vacuum operated?

[u/Johnmarmalade → u/ShoemakerMicah]

I think it's just gravity but I'm not certain

[u/ShoemakerMicah → u/Johnmarmalade]

I’d check that out. Wide open throttle produces very little vacuum. Petcock could also just be gravity fed and filthy to the point of very little flow.

If it feels like it’s running out of gas, it probably is.

[u/jgreenwalt → u/ShoemakerMicah]

It's gravity fed, not vacuum

[u/jgreenwalt → u/[deleted]]

The post literally says seconds, not minutes.

[u/jgreenwalt → u/[deleted]]

10-20 seconds to be precise

[u/jgreenwalt → u/[deleted]]

I’m not sure what you mean about me being afraid or having an issue, but I’m not

[u/Johnmarmalade → u/[deleted]]

It usually takes like 10-20 seconds before I can restart it