Total Motorcycle Community Forums

First off, i did my first compression test on my bike, both cylinders are at 60psi.......... im pretty sure thats extremely low, but wat do you guys think? its a 78xs400, with an 81 motor.

Second, as im preparing the bike for winter, im gonan start ripping things apart for cleaning (carbs mainly) but sumthing Id like to know, is what the PROPER/BEST way is to clean the gas tank?

can anyone point me in the rite direction? thanks

2 stroke should be in the range of 120psi
4 stroke should be 160+

alright so my motor is toast...... another thing, my fuel petcock valve has 3 settings, prime, res, and on, when i want to make sure theres NO fuel going into the chamber, how do i close the valve? Sum bikes have an OFF position, mine doesnt, what do i do for my bike? (For when im doing compression test, or coating cylinder walls with oil)

Disconnect the fuel line, or just remove the tank. I think you'd be alright with just turning the petcock to "on" and disconnecting the line from the carb. Have a rag handy. I'm not 100% sure about the vacuum operated petcocks, so someone jump in if I'm wrong.

On and Res (Reserve) are activated with engine vacuum. Engine off, no flow.

Pri (Prime) bypasses the vacuum operated valve and allows fuel to flow with the engine off.

Prime is generally used when the bike is off a long time (days, weeks, etc) and allows the carbs to fill with gas before you start.

Don't condemn your engine on the basis of just a compression test. Wet test it, adjust your valves and re-test it. There is a good chance all you need is a valve job, and that's much cheaper than a full rebuild.

It may turn out that you do need a full rebuild, but you ought to check it out more fully first, before you throw all that cash at it.

BuzZz wrote:Don't condemn your engine on the basis of just a compression test. Wet test it, adjust your valves and re-test it. There is a good chance all you need is a valve job, and that's much cheaper than a full rebuild.

It may turn out that you do need a full rebuild, but you ought to check it out more fully first, before you throw all that cash at it.

And if all that don't work let me know. I have an extra xs400 engine.

if the tank looks rusty inside and most bikes from 1981 will be rusted. Empty the fuel and then get yourself a POR-15 kit, this works well at cleaning and sealing older tanks. I have used this product many times.