'80 XS400 not starting - backfires through carb

[xa-mont]

Hi everyone,

I Have an 1980 XS400 that i'm restoring/modifying a little for my mother in law (yes, i am going to make sure the brakes work... come on guys ) - I bought the bike off a mate of mine who had rebuilt the whole thing (not very well, but had done it. I helped him with the new piston and rings part and a few other little bits) and never got it running after putting back together. I have pulled it all down and painted everything and started putting it all back together. i've pulled the carbs apart and cleaned them out (everything in there was new looking) and put them back together properly.

We're at the point of getting it all back together and we can't get it to start.

first of all we couldn't get anything.

• Checked spark, all good (battery all charged up and good) - still nothing
• Removed LHS filter and sprayed in some "start-ya-bastard" - best we could get was some backfiring and flames out of the carby.
• Checked and reset ignition timing to where it should be (it was basically there anyway, but just made sure) - still just backfires out the carby
• Checked valve timing, and looked a lot like it was 180 out, pulled the rocker cover off and YUP about 180 out, so sorted that out - still the same with maybe a little bit of a pop through the exhaust as well
• Checked valve clearance ~6mm is right isn't it? so I adjusted those... and they were all at about 6mm. - still the best I can get is backfiring and a bit of a pop now and then
• Now my leg is really tired (starter relay or starter is stuffed)

Any suggestions on what to look at next?

Cheers,
Trav

 

[rshutchinson]

Are you absolutely positive that you're setting the valves on the compression and not exhaust stroke?

It sounds like a valve issue (either cam timing or way out of spec clearances).

Did you make sure the dot on the cam lined up with the mark/arrow on the timing sprocket and that both those were perpendicular to the head?

You can also bypass the solenoid for the starter by jumping the two cables together if its just the solenoid that's busted.

 

[bcware]

Back in the day I was getting "backfiring" through my carbs before I replaced all of the intake parts: I believe the carb boots in particular. It was more of a pop and the carbs actually popped right out of the boots a few times!

Did you check for intake leaks using gas of some kind? I would check for intake leaks in particular. Regardless, if the carb boots are original they need to go.

Also, your issues may not be one of low fuel. Spraying more fuel in the carbs may be making it worse. Just keep that in mind