Bike won't idle and won't start sometimes

[februarysnake]

So my 1980 xs400 is just being super weird. I've had the carb completely replaced a month ago and a few other things done (spark plugs, the fuel line, the rubber circles at the front of the carb because they were rotting). Battery is fully charged. I don't know much about bikes but I have a friend who is a mechanic and has helped me out with a lot of stuff. However, the bike isn't starting often and when it does start- it won't idle. It just dies immediately after starting up unless I roll on the throttle. Two weeks ago, it was idling just fine. This thing seems to have a life of it's own- every time I go to ride it, there's a new problem.

Sorry I can't give more specific information but if anyone could offer just a general list of things that could be causing this, I could give him some direction and avoid paying for extra labor to diagnose. I know there's some rust in the exhaust that's clanking around, and I have not replaced the air filter. Could either of these be the problem? Thanks!

 

[DARKMIDKNIGHTWARRIOR]

Rust in the pipes shouldn't be your issue unless it is enough to completely plug it up. If you can post as much info on any modifications and pics would help!

 

[Lou Ranger]

The go-to response on the Forum regarding any inconsistencies is dirty carbs. People often have to clean them 3 or 4 times.

Another common problem is the need to re-jet any time something is modified which changes the air flow, e.g., installing pod filters or less restrictive exhausts. Any chance not replacing the air filter could have had the same effect - namely increased air flow so the mix is now too lean? Try putting it back in.

A 3rd problem could be air leaking into the air/fuel stream trough a crack or an improperly sealed connection. Try the carb cleaner spray test when you can get it idling - any change means a leak.

Hope like hell it's #2 or #3

 

[willem]

hope it's #1, you can usually fix that with a bit of work and no new parts!

 

[Lou Ranger]

hope it's #1, you can usually fix that with a bit of work and no new parts!

You'd rather boil carbs over putting the air filter back on? lol
It further occurred to me that if the bike had an old clogged air filter and a leak in the carb rubbers, the 2 might have offset each other. Putting on the new rubbers could have sealed up any leaks and maybe now the bike is starved for air ... or it's the carbs

 

[Requiem]

He might just mean he hasn't replaced it, as in bought a new one, instead of your interpretation of 're-placed' (if that makes sense)

 

[februarysnake]

I feel like it absolutely cannot be the carb or carb connection because every bit of that has been replaced within the last month. Brand new carb, brand new rubber connectors.... I've done practically everything regarding the fuel. Hmmm

 

[februarysnake]

You'd rather boil carbs over putting the air filter back on? lol
It further occurred to me that if the bike had an old clogged air filter and a leak in the carb rubbers, the 2 might have offset each other. Putting on the new rubbers could have sealed up any leaks and maybe now the bike is starved for air ... or it's the carbs

I really think the air filter could be the problem, like you said, they might have offset each other. I'll look into that ASAP.

 

[februarysnake]

He might just mean he hasn't replaced it, as in bought a new one, instead of your interpretation of 're-placed' (if that makes sense)

Correct, sorry about the confusion: I haven't purchased a new air filter.

 

[HoughMade]

Brand new carbs? 2 of them? What did that cost and where did you get them?

 

[februarysnake]

Brand new carbs? 2 of them? What did that cost and where did you get them?

I'm sorry for the confusion, I know very little about the mechanics of a bike so I'm doing a terrible job explaining. My mechanic bought a "carb kit" for less than $70. Cleaned my entire carb, replaced the floats with these nice new plastic ones, and replaced the other parts inside the carb (like that tiny metal piece with the rubber at the end). One of the floats had a leak in it.

 

[HoughMade]

Well then, I'm back to the "your carbs (2) are still dirty" camp. Your mechanic may have had the best of intentions, but these things do not like to come clean with one cleaning. Even then, they take some tweaking to get right, but nothing a person with some basic DIY skills who is willing to learn can't do at home.

After much trial and error, here is what I came up with.

http://www.xs400.com/forum/showpost.php?p=64401&postcount=17

 

[februarysnake]

Well then, I'm back to the "your carbs (2) are still dirty" camp. Your mechanic may have had the best of intentions, but these things do not like to come clean with one cleaning. Even then, they take some tweaking to get right, but nothing a person with some basic DIY skills who is willing to learn can't do at home.

After much trial and error, here is what I came up with.

http://www.xs400.com/forum/showpost.php?p=64401&postcount=17

Hahaha love the shirt. Thanks for the link to that post... that's really helpful.

 

[JezXS420]

hey mate! have you tried adjusting your valve clearances?? I have found bikes that wouldnt run or were running rough would fire straight up after adjusting valves.. if that combustion chamber isnt sealing properly, it doesnt matter how many fixes you throw at it, it'll run like a pig.. pretty easy job to do, let me know if you need any tips!!

 

[februarysnake]

hey mate! have you tried adjusting your valve clearances?? I have found bikes that wouldnt run or were running rough would fire straight up after adjusting valves.. if that combustion chamber isnt sealing properly, it doesnt matter how many fixes you throw at it, it'll run like a pig.. pretty easy job to do, let me know if you need any tips!!

You are 100% correct! Turns out one of the spark plugs had been jammed in at the wrong angle too many times (I think possibly using some sort of mechanized/air pump to get it in quickly) and the interior of the holes were totally crossthreaded. As a result, the combustion chamber wasn't sealing. I ordered a new engine head off of ebay and everything is running great now. Bums me out that someone could be so careless.