[78 xs 400]

you would have to measure the bore wear, could be just worn rings and a bore hone or at worse case, a rebore and new pistons/rings.

its complicated if you don't know what you are doing. I suggest you get a haynes manual and read it, then you'll know what's involed.

It's not an engine out job, just top end, new gaskets required (£20for top end). pistons will be he hardest to source I think +.75mm is the only ones you can get now and a specialist to rebore and hone the engine (who will need the pistons for measurement)

I woudn't recommend running the bike like that as you'll blow seals in the crankcase, Get a second opinion off someone who can actually see the bike in person, because I can only go off what you tell me on here.

Wow i guess i would probably have to get someone to help me on this.man i was really exited with this caferacer project when i saw the show on discovery channel the worst thing is that i already invested over $1,000 o well it is what it is.

i check compression today again and the left side stay on 0 the right goes up to 70 so i will do both sides. thanks for your help Drewpy.

[78 xs 400]

sounds like the rings are worn and air is being compressed in the crankcase.

yes someone talked me the same thing since i get low compression on that side and when i disconnect the crank case hose to the carbs the bike idles but it shuts off when i connect it,also the spark plug on that side always look smoked. how hard is it to fix? i really want to be able to ride it at its full potential or can it be driven like that at least until the winter that is if is to complicated a job to do for someone with no experience but mechanically incline? i need your opinion my friends.

[78 xs400]

I know on my bike if I screw up the light wiring the little relay box that is supposed to make them flash will click like crazy even tho the lights are solid. do you hear that box making any noise?

no i haven't check if they are led lights only and no the relay box doesn't make a noise. thanks....

[78 xs 400]

why is there oily smoke coming out of the crank case by the hose going to the carbs. im trying to used pod filters! .

when i leave the carb connections the original way there are traces of oil on the left side filter,now remember i mention there is very low compression on the left side.

anybody can help?

[78 xs400]

yes im sorry,i mean my cables are black and green their cables are 3 red,black,and black and white. isn't my battery a 12 volts?cause i try all connections and all it does is steady light.it doesn't blink.

[1980 XS400 leaking oil.. pictures .. how screwed am I? lol]

Hi guys! I just replace my starter and after riding around the block the bike was leaking oil. your comments were really helpful, i will check Ace hardware for the O ring and if anybody knows what the part# is for gasket for the cover where the starter chain is i will appreciated.

thanks...

thank you Drewpy. this is taking forever if is not one thing is another. but i getting to know the bike i guess. Thanks..

[78 xs400]

i bought aftermarket signal lights that i don't know how to connect to my 78 xs400. they have 3 cables red,bloke and bloke with white stripe.

my original only 2 green and bloke. when i connect the new light stays on don't blink. where should the cables go.

[1980 XS400 leaking oil.. pictures .. how screwed am I? lol]

Hi guys! I just replace my starter and after riding around the block the bike was leaking oil. your comments were really helpful, i will check Ace hardware for the O ring and if anybody knows what the part# is for gasket for the cover where the starter chain is i will appreciated.

thanks...

[1978 xs 400]

Like Wow you have a problem that made me some money in the 70's !!! ... The main fuse getting hot .. this indicates a short within the 12v DC system, this may be the ignition switch, reg/rec or just a bad/broken wire in the loom .... Talking of points if you have access to a single cylinder dwell meter set the dwell angle to 22.5 degrees (rather than gapping) ... the motor will run well !!! but if you do not have access to a dwell meter you may find setting the ignition timing statically is better is better than setting the point gap & using a strobe to set the timing. if you need info on statically setting the timing PM me I'll knock something together for you

ok so it seems that it was the starter motor. it was blown,i got a new used one from Ebay. put the bike together today,but still wont idle.i put those pod filters on the carbs and im waiting for a 12mm for the vent under the the carbs,now let me ask you something.Why is there oily smoke coming out of that vent under the carbs,if i use the pod filters and leave that vent open(not connected to the carbs)the bike idles but o lot of oily smoke comes out, if i put the stock filter and connect the hose from that vent , it wont idle and when i shut it off oily smoke comes out the left carb or filter and muffler ,this is the side where i get no compression.

What do you think??????..

[1978 xs 400]

It would be nice to know if you get the bike to run !!!

so i check the valves and they were wrong too close, so i did it right this time but now im looking to see where i can find the right gap for the points. last time just by turning the inanition to the ON position the Main fuse will get really hot.i got a new charge battery and when i try to start it it only does half a spin to the engine and the fuse will get even hotter. as we speak i'm taking the starter apart to see if there is something burn. they said the starter is what causes the fuse to get hot.What should i look for on the starter and how do i fix it if this is the problem?

Thanks again JimR.

[1978 xs 400]

To Check the valve clearances do the following

1) remove the valve covers

2) remove the l/h engine cover (shiny one with Yamaha written on it)

3) using the 17mm bolt you can see turn the motor over until LT shows in the little window and both valves on the l/h cylinder are closed

4) adjust the intake valve to 0.1 mm clearance

5) adjust the exhaust valve to 0.15 mm clearance

6) turn the motor over until RT shows in the little window and both valves on the r/h cylinder are closed

7) adjust the valves to the same as the l/h cylinder

but from reading this thread I guess your problem does not lie there ...

Do your compression tests (both wet & dry) this will give you an idea on which part (if any) of the motor is failing

if all is well turn your attention to the carbs & ignition timing

Thanks o Lot JimR, i will as soon as i get a chance, will let you know how it went.Cant wait to ride

[1978 xs 400]

You need to get the valve clearances right, get them wrong and you will have reduced compression.

The question is...how is this done properly?

ok yes, how do you do the valve adjustment? where do you start? Thanks..

[1978 xs 400]

I know i don't even make sense any more. Ok why do the spark plug look smoke after a few minutes of riding? i heard of oil on the tip but not smoke.

[1978 xs 400]

Hi guys i have already introduce my self a couple of weeks ago and since then i haven't been able to ride.

first it didn't idle and i also noticed low compression on the right just by putting my hand on the exhaust,i went and adjust it the valves but i think i may be a little bit off not much.However compression was the same (hand feel),the only thing that change some how the left sound it aggressive and louder still didn't idle,one more thing i just got a new battery because i had to jump start it with my car all the time,i don't know if is ok to jump it with the car but i did until it wont even turn any more with the jump so i got a new battery. I'M A MESS

I also took the carbs apart and spray cleaned but i notice the floats were not level so i try to even them.Now wont even start,i finally check the compression took spark plug off and attach the tool, kick 4 times.Left 60-rigth 0000

I put little oil on left went up to 90-Right up to 30. Been reading o lot and everybody says piston rings but i will adjust the valves better and also would check the points. AHH ALMOST FORGOT THE LEFT THAT SHOWS NO COMPRESSION, ITS SPARK PLUG ALWAYS LOOK SMOKE SO IM THINKING IS NOT FIRING PROPERLY. or running lean?HOW DO I CHECK SPARK? PLEASE HELP SUMMER IS ALREADY HERE I WANT TO RIDE!

[1978 xs 400]

thank you guys is really good to b here

[1978 xs 400]

i guys my name is luis im riding a 1978 xs 400. used to ride a 2004 cbr1000 and a 2007 999s ducati. i lost my job and had to sell them.now i found this one for $500 and trying to make it into a caferacer,i tell you what i like the challenge and i'm enjoying it. i live in long island New York.

thanks guys you really help.

[1978 xs400]

can anybody direct me to the right website for a carb rebuild kit for my 1978 xs 400, just bought it but it does not idle

[XS400 idle problems]

It seems the air filters have a mesh cage connected to a metal cylinder. The metal cylinder part sticks out of a hole in the filter box, and this is the part that connects to the hose leading into the carbs. The rubber between the carbs and the engine and between the carbs and the air filters looks brand new, so I assume they were recently replaced. The air filters look home made, but with the original cage. The metal cylinder on the cage was wrapped with black electrical tape, so there was no way in hell they were going to fit inside the rubber. I removed the tape, but I still have a hard time pushing the metal into the rubber. It seems to be an extremely tight fit, and the metal part of the air filter retreats into the filter box when pushed making it even harder to connect. I think the rubber intakes may be a size too small? Did the intakes and air filters I described sound normal, or has something been 'ghetto rigged'?

I unscrewed the battery box between the two air filter boxes to make this task easier, but that was rather annoying. I got the left air box connected solidly, and the right one still lets air in at the connection so I will work on it tomorrow. It ran great today after I fixed this. It idled around 1600 which is high (should be 1200 I read). I ran it around the block for about 30 minutes and I had no loss of power all the way up to 8000RPM. The idle didn't stick when I made it to a stop sign, so its some improvement I think.

There 'was' an inline fuel filter, but I managed to break it yesterday :-/ I will get another one, it looks the K&N one I saw on a previous thread. I am trying to avoid removing the carbs because I live in an apartment and management gives me dirty looks when I work on the bike, but if I have to I have to.

The idle screws still have the metal plug from the factory, so I will have to drill those out to access the screws. If the bike is running, what are the downsides of having it idle a little high?

Thanks for your suggestions, I am not only new to the XS400, but this is my first motorcycle.

Hi guys are just bought the same bike xs400 1978 and is weird im having the same problem bike starts but wont idle,as we speak i'm looking for a carburator repair kit,my friend saids we should repair it, the bike was sitting for a while.