XS400 Loses Power when Hot

[akamor]

Hello All,

My 1980 XS400 starts up and runs great for the first 15 minutes of driving. After that the bike will randomly lose power when in motion and will sputter and eventually die unless I put the clutch in and come to a stop. The problem is especially bad when going up hill.

Since it only happens after the bike is warmed up I suspect an electrical failing because I figure the electrical components are more sensitive to heat. Does anyone know where I can start on this? My initial thought is something is wrong with the timing.

Thanks,

Adam

[SgtPepper519]

Check and make sure you are using the correct spark plugs. My car use to misfire all the time when going up hills and it turned out to be the plugs.

[drewpy]

I suggest you do a proper service first, timing, valves, carb balancing etc

then have a look at fuel delivery, as you may have reduced flow starving the bike after 5 mins!

[akamor]

I suggest you do a proper service first, timing, valves, carb balancing etc

then have a look at fuel delivery, as you may have reduced flow starving the bike after 5 mins!

Thanks. That sounds like a great place to start.

1) I have already tuned/synced the carburetors, so that is out of the way.

2) The valves are definitely on my to-do list, I just need to muster the courage.

3) In regards to timing, however. According to my manual my model (1980 XS400G) has transistorized timing as opposed to break point ignition, which means timing need only be checked if the unit is removed from the motorcycle, which has not happened. What is your opinion on this matter?

4) I do not believe fuel delivery to be the problem. I should have given more details when I first made this post. Let me explain further.

After the bike has warmed up it ONLY has trouble going up hills. If I start at the bottom of the hill I cannot get up the hill unless I keep the rpms high. I have no problems on flat ground. Do you still believe it could be a fuel delivery problem?

The mechanic that sold me the bike says it could be an electrical failing that only manifests itself once it becomes hot. What is your opinion on this statement?

Thanks,

Adam

[KirriePete]

A few random thoughts:

1. Balancing the carbs without first setting the valve clearances is a waste of time - once you've done the valves you'll need to recheck the carb sync again.

b. An electrical breakdown when the mill gets hot would surely affect the bike regardless of uphill, downhill or whatever, so I'm dubious about that.

iii. Fuelling - could be some crap in the fuel tank blocking the outflow when you go uphill? Or something blocking the cap vent? Try running with the cap loose/open & see what happens.

x. Check everything that could move when going uphill - HT leads, LT wires, fuel pipes etc to see if something may be shorting out, or kinking a fuel pipe or anything like that. Loose choke flap? Bit of rag or similar under the seat covering the air intake?

Like I say, just some random thoughts .....

[drewpy]

my money's on fuel delivery, either carb float or blockage