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Background: I recently acquired a new 78 750k project bike and made some engine changes. Added cam, 4 into 1 pipe, and new electronic ignition. Upped main jet one size, left airbox on.
Now I was out riding for about 15 minus and all of a sudden it started bogging bad until I pulled over and it died. Every time i started it back up rpm's wouldn't go past idle or it would bog and die. If I left it idleing for more than a minute or two it would bog and die. When I got it back home I pulled plugs and they looked a bit lean (light tan to almost white). Any ideas why it would run great and then just die all of a sudden? I really dont want to break the whole damn thing down again. If one jet got clogged, or one float valve got stuck closed, would it kill the entire bike that that?
If your plugs looked lean then it could be fuel starvation. When I went to pod filters I had to go up two sizes AND move the clip on the jet needle up a notch to get enough fuel.
I've got a couple of CB 750's myself, so here's what I'd check out if you think the jetting is correct:
1. Do you have an in line fuel filter? The 750 feeds from a single line from the petcock and goes to a "Y" connector. One side feeds carbs 1&2 the other side carbs 3&4. If you have a filer it's usually in the line from the petcock upstream of the "Y". Check for rust in the filter.
2. If you don't have a filter then you could have sucked some junk from the tank into the carbs. What does the inside of the tank look like? You may be up for a carb cleaning.
3. You can also pull the petcock and look at the filter screen. If you have a really dirty tank that filter might be clogged. Unlikely but not impossible.
It could just be running out of fuel. In addition to the things already suggested, check the filler cap to be sure that the breather hole is open and breathing.
did you check if it still had spark? coils heat up when they are near the end of their life and the bike dies after 15 minutes or so of riding.
fuel starvation is possible, but unlikely after working for 10 minutes without issue. either way, check that fuel lines arent clogged and everything is clear. could be something like a piece of rust in main jet if you dont have fuel filter...
Thanks for the advice. I got motivated and pulled the carbs. Petcock works fine. New in line fuel filter. Jets clear. Float valves all working fine, carb diaphragm good. Really didnt see anything out of the ordinary. I put it all back together and it ran awesome. 10 miles down the road... bog died. Its like someone just flipped a switch and it decides not to work anymore. Sucks. It is funny though seeing the wife pull up with my truck and ramps all irritated lol.
As for spark, I put in a brand new ignition system. Its definitely getting spark. I tried pulling each cylinder while running to see if it did anything and am getting zapped when I pull the plug. Is this normal or am I not grounded properly? Thats prob the only thing "off" with the bike that I see.
getting zapped is pretty normal in my experience. have to time it justtt right
so you replaced coils? just want to confirm...
we need some more info... around what rpm does it die at? if its not fuel starvation, could be overfueling... i know a guy whos bike had a pretty loose choke, and the choke would rattle open as he rode around, causing a bog and die. did you try fiddling with the choke when it dies?
hmm. if you replaced your ignition system... check that alll connections are tight. something might be rattling loose as you ride, such as the connection at the coil,plug, or otherwise.
I'll say it again, did you open the gas cap when it dies? What you are describing could be an electrical fault but sounds like it's run out of gas. As fuel is pulled from the tank, it has to be replaced by air which comes in through a tiny breather hole - usually in the filler cap.
When that tiny hole is blocked there comes a time after about 10 minutes when it can no longer draw fuel; and it dies. Leave it for a while and the bowls will fill up and it will run fine for another 10 minutes or so.
If it's electrical, it could be anything from the battery losing charge to a short of some sort or a bad coil or cdi or pickup. If you have points, it could be the condensers breaking down when they get hot.
The fuel cap sounded like a winner, but I checked and its open and clear. I blew right through it just to make sure. I just unloaded the bike off truck (left it there overnight) and fired it up and guess what... it hauls ass again. So apparently there's no fuel blockage. The only thing that changed from last night to today is that the bike is cold. Something happens after riding for a while and then bam, no go. Cant even get a 1mm twist on the throttle or it will die. It wont even idle for more than 10 secs or it will die.
I rode the bike before changing the cam and ignition, and it was fine, so logical thinking puts the problem somewhere there. Im going to call the guys who I bought it from and see if they can offer any insight.
Update: Spoke to the guys at cyclexchange and they said it might be a faulty ingition due to heat. I took the sidecover off the motor where the old points used to be and rode with no cover. I pulled a 30 minute ride with no prob. Once I got home, I put the side cover back on and let it sit for 5 mins (to heat up the inside of the cover) and bam. Didnt make it to the end of the street. I guess the heat inside the side cover was causing the new ignition to fail. Lame. Hope a new ignition will fix it.
Glad that you found the problem. Heat does funny things to electronics and they do fail sometimes. We had a DYNA do that once when we sat it on teh end of a cam where it gets much hotter than on the end of the crank - strange as that might sound.
At least is wasn't those nice new coils.
Nice looking 4-2-1 pipe BTW. They are hands down the best configuration
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