I promise I will post pics as soon as my hands are clean enough to manipulate my camera. (I don't care about my keybord)
So, my 1975 400f is back on the road after 5 years of neglect. Well, not that neglected actually. I got a 1982 silver CB750 Super Sport a while back and the 400 got less and less used. It was not going well anymore despite the work I put in to it. I cleaned the carbs maybe 4 times, changed all the internal carb components, syncroed the carbs as often I removed the carbs, new rocker adjustment screws, new rectifier, piston wrings, new valve seals and valve lapping, coil swap, the list goes on and on... I could just not get it to run well. The sparkplugs were all equally powdery black. D8ea, d7ea, no difference. Finally, I stored it.
Sold the 750 to a friend 2 years ago and somehow managed to live a complete summer without a motorcycle.
So this spring, I was determined to solve the problem. I found this fine site and read and searched for any clue susceptible to help. I had never really dug into the electrical stuff since I know very little on the subject. I am slowly learning. I now understand why my batteries got frequently discharged due to the anemic charging system these bikes have. Tested every charging component and every thing seems OK.
Something I never checked was the voltage to the coils. I read that a low voltage at the coils could give a weak spark and foul the plugs. With a brand new battery, charged and stable at 13.00 Volts, I got 9.18 volts at the coils. I read that cleaning the connectors and switches could cure this kind of voltage drop so I cleaned every single one of the connectors until they shined put dielectric grease and assured they were tight. I cleaned the kill switch, the starter relay, the fuse box, every electrical component on the bike got the royal treatment. After all that, the coils still only got under 10 volts. I checked with my voltmeter where could be the voltage drop in the ignition circuit and found that the kill switch robed 1.2 Volts and some connectors, even though very clean, had voltage drops in the .3 / .5 volts.
Still, I tested the bike with new plugs to see if the slight gain in Voltage could have solved the plug fouling problem... not.
So I decided to try the modification that has been debated on this site. I am talking of the modification where you create a separate circuit with a relay, feeding Voltage directly from the battery. I got all the components (cost under $10) and hooked it up. This installation did not require any molesting or cutting of the original wiring and it would maybe take me 30 seconds to put it back the way it was.
I cleaned the best I could the new spark plugs and started the bike. Immediate difference! Once warmed up, I had to lower the idle as it was now RPMing in the 3K, It used to be a little under 2K before the mod. It could hardly run at the suggested idle of 1200 RPMs, now it can and there is more power overall. But the best part is that my plugs are now clean with a nice coffee color. It is not perfect yet. I think I need to check the carbs syncro again since I had to sync them at 2000 rpm and the motor was stumbling maybe too much for a correct result.
It idles fine. When I take off, it buckles for a second but once the RPMs go higher, it's very smooth. Maybe adjust the idle screws?
Anyways, just wanted to share my recent findings here. I eventually would like to find a new kill switch that does not rob the coils voltage and go back to the stock ignition, but for now, I'm happy.
Simon
