Engine dies when choke is closed

[jeffcycles]

I've got this '78 xs 400. I have logged near 800 miles since I purchased it a couple weeks ago with no issues. It was running wonderful last night, but then today after startup, it began acting up. I started it with the choke open cause it was around 60 degrees farenheit, but after warming up and closing the choke it was idling rough. It would sputter and slowly die off unless the I gave it some throttle. It would backfire slightly (quietly) and as I continued to ride the problem worsened.

I'm in the process of learning how to maintain this bike so any more info would help. A neighbor said that draining the float bowls is a good place to start.

When cruising the bike has great power, so I'm hoping its a simpler carb issue.

[jeffcycles]

I've begun messing with the carbs (for better or for worse). I"ve got the bike to idle fine with the choke off, and it seems to run with good power. I set the mixture setting by dialing it in until it gently seated and then backing it of 1 1/2 turns.

It is running well, though it does have some a subtle backfire. My biggest concern at this point is that the engine rpms don't settle down like they should.

Any help from this point would be great!

[jrhendryx]

I've begun messing with the carbs (for better or for worse). I"ve got the bike to idle fine with the choke off, and it seems to run with good power. I set the mixture setting by dialing it in until it gently seated and then backing it of 1 1/2 turns.

It is running well, though it does have some a subtle backfire. My biggest concern at this point is that the engine rpms don't settle down like they should.

Any help from this point would be great!

have you checked your plugs/wires/points?

that was part of my problem, and i still have a bit of a backfire when i downshift and hit 4000 rpm.

[jeffcycles]

Both the plugs and wires are solid. I'm assuming that the points are fine, though I haven't checked them closely.

Is there a chance that the points could go from perfect to shitty overnight? I assumed that wouldn't be the case and therefore have focused my attention on the carbs. perhaps I'm wrong?

[drewpy]

Both the plugs and wires are solid. I'm assuming that the points are fine, though I haven't checked them closely.

Is there a chance that the points could go from perfect to shitty overnight? I assumed that wouldn't be the case and therefore have focused my attention on the carbs. perhaps I'm wrong?

its your pilot jet, this is the smallest drillings in the carbs and will clog up first. You also need to fit a fuel filter as your tank will have rust particles from gas stations and in the tank itself.

Try some of the proprietry carb cleaner you put in the tank and see if it unblocks. You could also open the screw another 1/2 turn and see what happens.

mine always runs the roughest at 4k revs. this is the point where the main jet starts to take over from the pilot system. Just age and 30 year old technology really.

[jeffcycles]

its your pilot jet, this is the smallest drillings in the carbs and will clog up first. You also need to fit a fuel filter as your tank will have rust particles from gas stations and in the tank itself.

Try some of the proprietry carb cleaner you put in the tank and see if it unblocks. You could also open the screw another 1/2 turn and see what happens.

mine always runs the roughest at 4k revs. this is the point where the main jet starts to take over from the pilot system. Just age and 30 year old technology really.

From what I've read over the last day I've learned that pilot jets control idle, correct? Will unblocking also help with the slow dropping rpm's, or could that be symptomatic of another issue?

Also, will any small fuel filter do the job?

Thanks again!

[YamaHead]

Try an inline fuel filter between the petcock & the carbs......after cleaning those pilot jets.

Should make a noticable difference.

[drewpy]

From what I've read over the last day I've learned that pilot jets control idle, correct? Will unblocking also help with the slow dropping rpm's, or could that be symptomatic of another issue?

Also, will any small fuel filter do the job?

Thanks again!

yep they control idle to 4 - 41/2 K revs. my bike used to rev on idle which means too lean. just richen up on the pilot screw.(after you have unblocked it)

[jeffcycles]

Try an inline fuel filter between the petcock & the carbs......after cleaning those pilot jets.

Should make a noticable difference.

Hey Yamahead, we're practically neighbors!

As a result of our damp Pacific NW air, should I be concerned with day-to-day carb changes like this?

[jrhendryx]

Hey Yamahead, we're practically neighbors!

As a result of our damp Pacific NW air, should I be concerned with day-to-day carb changes like this?

sorry to kind of horn in on this one, but on that inline fuel filter, what kind of filter would you suggest? the hose from my petcocks to the carbs is very short (since there is one for each side, it doesnt have to go very far). how big are they, because size will be an issue since the hoses are so short.

thanks.

[drewpy]

sorry to kind of horn in on this one, but on that inline fuel filter, what kind of filter would you suggest? the hose from my petcocks to the carbs is very short (since there is one for each side, it doesnt have to go very far). how big are they, because size will be an issue since the hoses are so short.

thanks.

I use the smallest i could find with a see thru bit so I know I can check for fuel (if the bike starts mis behaving) on the move.