Page 1 of 1

1980 xs400 special loss of power

Posted: Sun Oct 23, 2016 12:28 pm
by danimal72
I'm really hoping you guys can help me out. I got a 1980 Yamaha xs400 special this summer off a buddy who had it sitting for 6 years in a barn. I changed plugs, wires, new gas, fuel filter, fuel lines, and took off the mufflers because I didn't know if they would be clogged. I drove it and it responds amazing in 1st through 3rd gear but when I got it in 4th through 6th I would lose power. I tried cleaning carbs and ended up having to buy all new ones off of mikesxs.com and I just took it for a ride and still have the same problem. Amazing response until I hit 55mph then loss of power. Please help! I'm very mechanically inclined but motorcycles are a whole new animal from what I'm used to. Also I tested the coil packs and they are within the parameters so now I'm at a loss.

Re: 1980 xs400 special loss of power

Posted: Sun Oct 23, 2016 8:52 pm
by GS_in_CO
If the plugs are gapped too wide you may get misfiring when the cylinders are filling under full power load.
Air and gas is a better insulator than vacuum so the spark can't jump as far under full cylinder fill (full load).

Weak wires could give similar symptoms as energy can leak off thru deteriorated wires.

Re: 1980 xs400 special loss of power

Posted: Sun Oct 23, 2016 10:02 pm
by danimal72
Thsnk you for the quick reply! The plugs are gapped at the recommended .060 and I replaced the wires with silicon wires that are brand new. I even replaces the plug caps (the piece that goes on the sparkplug, not sure the technical term) and the problem was present before I changed the plugs and wires. I changed the oil and oil filter as well. Like I said I'm at a loss lol

Re: 1980 xs400 special loss of power

Posted: Sun Oct 23, 2016 10:05 pm
by danimal72
I should mention that when I do get around 55 I can't hear the exhaust at all on the right side and it kind of sputters randomly, almost like it wants to fire but doesn't. I don't know if there is something other than the coil pack that could be providing the signal to spark. I checked the timing and it seemed spot on. The problem only persists when I want to go on the highway, but not in town.

Re: 1980 xs400 special loss of power

Posted: Mon Oct 24, 2016 7:39 am
by GS_in_CO
The coil is just a voltage converter from 12V to 12,000V. It is excited by the ignition triggering system. Corroded connections or weak output from there would turn into lower spark voltage.

My old man who taught me a lot about motors had a saying " If it isn't running right- check the ignition. If it still isn't running right check the ignition again." (This in the days of points, condenser, rotor and distributor caps)

That it's highway and not city fits the theory of weak spark.

Re: 1980 xs400 special loss of power

Posted: Mon Oct 24, 2016 2:50 pm
by GS_in_CO
A subsequent thought. My 1st motorcycle was a 1979 Suzuki. It used a points type ignition system.
Your 1980 might also use points. If they are on the bike they should be checked or replaced. Certainly they should be examined for condition, dwell, and correct timing.