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Need a little help

Discussion in 'Technical and Troubleshooting Torque' started by thepumpkinslayer, Aug 26, 2007.

  1. hey guys, i joined the site only recently and was hoping someone could help me out. Im a new rider, just bought a yamaha 1979 xs400, and have only just started to ride the beautiful thing, but am having some toruble. The bike seems to have real trouble idling and accelerating. At first there was no trouble, i could leave it to idle for hours, and accelerate in 1st like a bat out of hell, but now it seems like there isnt enough fuel making its way to the engine. Ive tried switching to reserve, but alas same result. Ill get some fuel and top it up to see if thats why (its down to its last few litres, but still runs on both normal and reserve lines). Im just bummed cause its unridable like this, and i dont have money to get it repaired.

    You all seem to be pretty knowledgable and helpful, and id seriously appreciate any help someone can give

    cheers guys
  2. If you have let it idle for hours then try cleaning the plugs.

    Don't idle it for hours. Not good.
  3. if it's that old and maybe has been sitting for a while unused (?) then you may have gunk in the carburettors. As well the accellerator pumps in the carbies could be sticking.... A good carbie clean-out and flushing the fuel lines may solve the problem. Or, it could be crook spark plugs, in which case, replace :).
  4. Hey maybe it just is out of fuel like you say. I guess if it was fine before and you have very little fuel left maybe it could be fixed by filling it up.

    What fuel do people recommend? Would you have to fuel up with premium and valve saver because this bike would have been designed for leaded petrol?
  5. If you don';t have money to repair it, sell it.
    An older bike will need more money spent on it, so better get out of it now.

    Regards, Andrew.
  6. wow, talk about responses. lol, Ive never let it idle for hours, i was just conveying the fact that it idled okay before. I did replace the fuel line to start with, but i like your idea of cleaning the carborator (sisters boyfriend is pretty know how on that sort of thing so he may be the best bet). I got the bike for $1650AU, its done 13,000 km and looks beautiful. Id sooner let it sit around and wait until i can fix it then sell it. As far as fuel goes, ill have to use the lead addative, unleaded shouldnt hurt it but i dont want to take the chance.

    thanks again guys
  7. I've found bikes you have to muck around with lead additive on tend to be sensitive to plugs.

    Put some new ones in as a matter of course and the carbie clean can't hurt.

    Do you know about the head cracking problem with these bikes? Do a compression test when you get a chance.

    I'm a bit jealous BTW I wouldn't mind one of these to play with.
  8. well i filled it up this afternoon (no lead addative at present, but the caps should be hardened so ill use that later), and still had the same problem. I decided to change it across to PRI and voila! runs a dream. I rang the mechanic and he said he had the same problem when he did the checkup, the autoshutoff hose needs a look at. What are these plugs you speak of? Im not sure what the head cracking problem is, how would i do a compression test to make sure its ok. I feel like such a nublet asking all these questions, but once again, thanks
  9. The "plugs" are sparks plugs. They are easy to change and cheap. Just don't over tighten them.

    Most Japanese fuel taps are operated by a vacuum line that draws vacuum of the carbies or inlet stubs.

    This is your problem or the internals to the tap itself. so check the vacuum hose for good fit and make sure it doesn't have any cracks or is blocked.

    If all that is good, then you probably need to put a kit in the tap. the reason it works in prime is this setting doesn't rely on the vacuum to operate, but be careful because if left you can dump the contents of your fuel tank into your sump

    As to the head cracking, it's a crack in the aluminum casting. with an air cooled bike it's not the end of the world. Just pull the head (or get it pulled) and someone can weld and machine for you.

    To check the compression, you use a compression tester that screws into the spark plug holes. You will need a workshop manual to tell you what range it should sit in.

    Be careful when buying a compression tester as some testers are not small enough for some bike spark plug threads.
  10. ill take it to the mechanic and he can repair the hose for round $50, id rather do that than risk screwing it up. It should be okay if i use it on PRI for about a month id imagine? I change it across to normal when im not using it, no idea if that makes a difference. Ive got no idea what dumping the fuel into the sump is, but im guessing its not good, so ill try and avoid that. Ill look into some new spark plugs or a new battery aswell, cause its a biatch to start on push, i usually end up using the kick start.
  11. the hose that the mechanic is talking about sounds like a pice of $5 vacume hose and it just needs the old one taking off and the new one putting in it's place. a good bike shop or even better a wreckers will point you in the right direction. especuly if you are getting the hose and plugs from the (about $15 in total). but try a wrecker first. it's in there intrest for you to start doing your repairs you self. they'll sell you more stuff and you'll get free advice.

    where are you? some one here will be able to recomend a good bike shop or wreckers.
  12. i live in Tamworth, nsw. country music capital wooohooo. I replaced the fuel hos with the same system, blowing into the tank and catching the fuel, is it difficult to replace the tap?
  13. thanks heaps mate, this will prove valuable for years to come im sure. Youve been a great bunch and i owe this forum alot so cheers
  14. That’s OK mate, that is what it’s all about. Helping out each other. Any more questions get back on here.

    let us know how you get on with it.
  15. this 'PRI' mode you speak of. i thought this was used only to get the fuel to the float bowls or something. once that's achieved, you should switch it back to 'ON'.
  16. Quite right, but if the vacuum hose is knackered or there's a fault in the tap rendering "ON" permanently "OFF", it will do no harm to use "PRI" to get the bike rideable.

    All it represents is a manual "ON" position. The only problem is that the tap won't turn off automatically when you switch off (which is the object of vacuum taps) so you've got to remember to turn it off manually as we old buggers had to in the days before vacuum taps.

    Running and riding will be unaffected. Although you won't have any reserve either so you've got to keep an eye on fuel level.

  17. Oh, i rember comming back to my bike after work to fimd a full tank of fuel in a nice big smelly wet patch for meters afound my bike. O for the good old days........braking down.......you know i rember when it was all fields around here.......

    ok i'll go now shall i?