Uncontrolled Revving in XS400

[akamor]

Hello All,

Thanks for all of the ongoing support. In its current state the bike is very close to being rideable. The motorcycle will idle with the air screw turned in completely and the idle screw turned out 2.5 turns. I am now having a different problem however. The motorcycle idles around 1500 rpm currently, which is too high, and as soon as you touch the throttle the engine will rev up, seemingly without bound, until you throw the kill switch. I cannot think about what would be causing this. Does anyone have any ideas? I was thinking about an air leak, however, I do not believe there are any leaks from the airbox->carburetor->intake manifolds OR from petcock->intake manifold nipple. Could other leaks be present?

Thanks as always,

Adam

[drewpy]

Hello All,

Thanks for all of the ongoing support. In its current state the bike is very close to being rideable. The motorcycle will idle with the air screw turned in completely and the idle screw turned out 2.5 turns. I am now having a different problem however. The motorcycle idles around 1500 rpm currently, which is too high, and as soon as you touch the throttle the engine will rev up, seemingly without bound, until you throw the kill switch. I cannot think about what would be causing this. Does anyone have any ideas? I was thinking about an air leak, however, I do not believe there are any leaks from the airbox->carburetor->intake manifolds OR from petcock->intake manifold nipple. Could other leaks be present?

Thanks as always,

Adam

pilot needs cleaning, check the pilot adjusting scerws tips havn't snapped off!

you have a weak mixture

[akamor]

Hello All,

Thanks for all of the ongoing support. In its current state the bike is very close to being rideable. The motorcycle will idle with the air screw turned in completely and the idle screw turned out 2.5 turns. I am now having a different problem however. The motorcycle idles around 1500 rpm currently, which is too high, and as soon as you touch the throttle the engine will rev up, seemingly without bound, until you throw the kill switch. I cannot think about what would be causing this. Does anyone have any ideas? I was thinking about an air leak, however, I do not believe there are any leaks from the airbox->carburetor->intake manifolds OR from petcock->intake manifold nipple. Could other leaks be present?

Thanks as always,

Adam

That is what I initially thought so I should have mentioned it, but I have already taken the carburetors apart twice and cleaned them thoroughly. The pilot is definitely clean inside and out. Is it weird that my air screw must be completely seated for the bike to run? Does this mean I should unscrew the pilot even more?

Thanks,

Adam

[Paulwhite]

adjust the jet needle in your carb as the mixture is week, should have a black plastic bit on the end (pointy bit facing down), if you put that down a notch then that will make the mixture richer might take 2-3 attempts to get it right but should sort it out then check your spark plugs, white = week mixture, black = rich mixture, orange/browny = spot on

[akamor]

adjust the jet needle in your carb as the mixture is week, should have a black plastic bit on the end (pointy bit facing down), if you put that down a notch then that will make the mixture richer might take 2-3 attempts to get it right but should sort it out then check your spark plugs, white = week mixture, black = rich mixture, orange/browny = spot on

Thanks. That is something I have not yet considered. The problem is only with idling however. I was under the impression that the jet needle only affects the mixture when the throttle is above 15%. Can it also affect the idling circuit? Once I make the mixture richer by adjusting the jet do I need to then mess with the idle settings again?

Thanks a lot!

Adam

[Paulwhite]

i think you might need to change the idle settings again but do a plug check 1st you never know might b ok . but the way i understand it is the needle jet is used throughout the throttle range, the pilot is used for idle to 1/4 throttle then the needle jet takes over then the main jet kicks in for top end, but every time i have the problem you have i just change the needle jet position and the air screw and idle settings and it sorts it out lovely.

[drewpy]

pilot mixture controls the engine up to 4 k revs!!

anticlockwise (out) enrichens the mix

[jebb]

pilot mixture controls the engine up to 4 k revs!!

anticlockwise (out) enrichens the mix

Hi,

just a silly thought: have you got the air and idle screws mixed up ?

If not I retract the possibility of and offer my apologies.

[drewpy]

Hi,

just a silly thought: have you got the air and idle screws mixed up ?

If not I retract the possibility of and offer my apologies.

there is no air screw, that is metred by the air jet situated inside the intake mouth of the carb.

the pilot screw on the body near the engine regulates the fuel.

the idle screw just generally raises or lowers the revs like a throttle!

[akamor]

there is no air screw, that is metred by the air jet situated inside the intake mouth of the carb.

the pilot screw on the body near the engine regulates the fuel.

the idle screw just generally raises or lowers the revs like a throttle!

Ok, I think I got it now. The pilot screw is located outside of the carburetor on the mouth on the engine intake side. It should be turned out ~1.5 turns. When you guys have been talking about the idle screw, I thought you meant the idle jet which is inside of the float bowl. What you call the idle screw, I call the throttle stop screw. Does all of this make sense?

Adam

[armyofda12mnkeys]

Ok, I think I got it now. The pilot screw is located outside of the carburetor on the mouth on the engine intake side. It should be turned out ~1.5 turns. When you guys have been talking about the idle screw, I thought you meant the idle jet which is inside of the float bowl. What you call the idle screw, I call the throttle stop screw. Does all of this make sense?

Adam

> It should be turned out ~1.5 turns

Some peeps here recommend 2.5 turns on that idle-mixture (the screw towards engine side... on 1978 model at least) to get non-factory but proper performing air-fuel mixture ... Lemme tell you, it can be real finicky, at least it was for me. Sometimes i thought i had it fine, but riding it in 2nd gear at 3k revs bike would stumble. I redid it more carefully and turned out better. I then re-did it with a 'Colortune' spark plug and it worked out even better.

Read the plugs after each half a turn to get more accurate result (.25 of a turn too hard to notice difference in colors on plug). Start off more lean as its hard cleaning the black each time if you too rich to re-read the plugs.

I had a side question i didnt take into account when setting my mixture ... Should the 'throttle stop' screw be turned all the way down (aka revs go down, or up?) and then readjusted after idle-mixure screw is done being adjusted... or it doesnt matter where it is when adjusting mixture?

[HoughMade]

Get the mixture right (mine are between 3 and 3.5 turns out), then just set the idle speed. Also, it would not hurt to balance the carbs (and after that, set the idle speed).