Topic: Bike slowly cuts out and dies under slow speed, warmed up conditions

[tommychains]

Hi guys, my 78 CB750 runs very well after a cold start and during moderate to high driving conditions (above 20 mph). Under those speeds and when the bike has warmed up after about 5-10 min of riding, the bike slowly has trouble getting consistent power and eventually seems to choke and die. I would strongly assume this is a heat-related problem. This should eliminate air as an issue. The fuel I believe to be ok. I just took apart and cleaned out the carbs (this problem existed before that), changed fuel filter, checked lines, etc. Could the spark be an issue? I just changed spark plugs as well. I do have an electronic start which replaced the original points. Any ideas on how to diagnose this?

[70CB750]

Is your tank ventilating alright?

The fuel tank has to have access to the atmosphere, othervise a vacuum is created and it will eventually stop the fuel.

The vent hole is part of the tank cap.

[madmtnmotors]

Sounds like the vent in the cap is plugged, or the fuel filter is clogged. The fuel filter on the 78 is inside the tank. Removal of the fuel valve is required to get at this. When you remove the fuel valve the filter typically remains up inside the tank and looks like a plastic ring inside the connecting bung. They typically have never been removed and take some persuasion to get out. I usually pry one side in using an awl, then grasp it with needle nose pliers and twist. This usually deforms the filter beyond reuse, but you would probably do good to get a new one from Honda anyway. It comes with a new petcock seal, Honda P/N 16952-388-015.

You can inspect the filter through the fuel filler opening, just shine a flashlight down towards the petcock connection. 

Don't use a lighter or any other open flame!   

[2wheels]

I think you need to drive it (warm up and some time above 20 MPH)  Then drive around the parking lot, this is where you have problems, right?  Then stop removed the spark plugs and have a look, maybe take a picture.  Find out if it is rich or lean or carbon fouling.

Edit
And if you drive it as above, then go from the parking lot back out on the highway, is it fine?  If so I don't think it would be a heat problem.

Plus, did you sync the carbs?  Someone is going to ask might as well be me.