Rd 400 misfires or one cyl.

[daves1977rd]

  Have a 1977 RD 400 it cuts out, runs on one cyl and will not revout.It is all rebuilt and I have cleaned the carbs 3 times.Could this all be caused by a weak battery? shows 14.4 volts when you rev it so its not the stator.Has a cheap lead acid battery in it?Might be two years old. Thinking of ordering a new agm battery for it,what is the best one? and where can i get it. 

[2steve]

When that happened to me, it was a coil gone bad. Coils were relatively new but one decided to die.

[m in sc]

if you replaced the coils with new ones, did you remove the stock ballast resistor the 400s have? it under/behind the steering neck. if not you need to. if its the stock coils on the bike, they are probably toast.

[Dvsrd]

Voltage drop through the fuse/ ignition lock/ kill switch/ multiple connectors?

[daves1977rd]

My bike had giant accel auto coils in it.Had to take them out to make room for a yboot air cleaner.The stock coils were still their so they just rewired them.Said the 400 coils in 1977 were good ones.Guess testing them is the next step? 

[m in sc]

stock 400 coils are 1.2 ohm or so and need the external resistor. the car coils were more than likely 3-4 ohm and can be wired directly to 12v. IF you have th stock 400 coils wired directly to 12v, that's not going to work.

If it were mine, id never rely on 40+ year old coils and replace them with 3ohm coils. fwiw, 1st gen sv650 coils will bolt on the stock location with little muss/fuss, there's a thread int he tech library about it.

[daves1977rd]

I paid an expert to work on my bike.Shipped the whole bike to him because i8 didn't want to mess with wiring.When I got it back it ran good and had over 40hp on his dyno.it's just acting up this year.I it continues my manuals give me # hope they are correct.

[m in sc]

I stand by what i said. 'experts' make mistakes all. the. time. (like the rest of us).

[daves1977rd]

More info on coil test.The side that has no spark reads 15.75 oms not the five it should have.the side that runs gives me nothing on the meter?Replace the left with oem.coil or replace it all? 

[m in sc]

yeah 15 is shot. def replace it. tbh, id replace them as a pair.