Topic: Electric issues: Zero throttle and popping

[mishrechberger]

I am a new owner of a 1978 Honda CB750K. I have just installed new velocity stacks and a full carb clean. The bike was leaking from the tachometer and that has been fixed. No major air, gas or oil leaks. Recently I have been having problems with the electrical system. For a while I had problem getting the light (oil) to come on upon engaging the key to "on". I would have to jiggle and mess around with the wiring behind the headlight a bit and the bike light would come on. I had recently have the wiring system a full spray down (with the battery out) with contact cleaner. The issue seemed to have been resolved, no problems when engaging the bike to "on". However, I have recently been caught in a rain storm and had to bike home. Two days later, I go to start the bike and I experience similar issues. I figure I could just jiggle the wire all day and survive. Halfway through the ride, suddenly, I lose all throttle response. My bike comes to a slow and I pull over, turn off the bike. I check the throttle cables. They're good. When I go to push the start, I suddenly hear a single and very loud POP. That should've been my sign to go home, but I was able to get the bike to start and off I went. Obviously, as soon as I turn the bike off and leave it for a couple minutes, I cannot, no matter the jiggling, to get the bike to start up again. I know it is mechanically sound, it must be a electrical problem. I managed to get it towed back home (thanks to a friends hitch and car) and will work on diagnosing the electrical issue this weekend. I was hoping to possibly fast track my diagnosis here. Anyone else experience sudden lose in throttle control? Anyone know what the loud pop could have been been I pushed the start? I am new to this. However, I am pretty certain it is electrical and not mechanical. The bike runs fine, low compression and all. I will work on using my multimeter to test each component of the electrical system this weekend. Thanks all!

[scottly]

Since the problem seemed to come and go with wiggling, look for a physical connection problem in that area. If everything, headlight etc loses power, the first suspect would be the ignition switch.

[Pielz]

When I first got my Dyna III on my 77 750, turning the key would sometimes cause a loud bang and backfire. But only after I attempted to start it but it never ran. Someone once told me that when those Dyna systems are turned off, sometimes they'll discharge whatever charge they're holding and it'll ignite whatever's leftover in the cylinder. If a valve is open, it causes a loud ass bang. Do you have an electronic ignition?
 
Also, you meant you were jiggling the key in the ignition things started to work, yeah? Or the whole of the wiring harness? If it's the first, Id check the connections to the wiring clip that plugs in to your ignition switch. I recently had a wire go bad up there and it took ages to figure out why my tail light wouldn't work. Lol. I buy wiring clips on ebay from 4into1. They're like 6 or 8 bucks I think, and they come with the connectors and whatnot that you need. Some day I'll get around to just replacing all of the wiring clips on the bike when I have some cash burning a hole in my pocket. Always helps to apply a little dielectric grease to the pins in those connectors too while you're in there!
 
Like scottly said, it's very possible its the switch too. They're not crazy expensive, may be worth checking out. Maybe a few shots of contact cleaner inside? That stuff is like miracle liquid for electronics, I swear. I always have multiple cans.
 
Unfortunately that's about the extent of my input when it comes to the few issues you have. I'm only about a year in to working on these old bikes so I'm a bit of a newb as well

[bryanj]

If that is the switch between the clocks the electrical part is a known failure point