engine will not draw full into cylinders

[David Houston]

hi there
i have a 1979 xs400 , it has a non vacumm fuel tap on fuel tank and both vacumm pipes on carbs have been sealed. the compression is 145psi each cylinder.the timing is 100% spot on and replaced both HT coils so there is a great spark on each.
i cant get the bike to start , the only way it will start is by injecting a small amount of fuel in to each cylinder
(wet start) then it only runs for about 30 seconds.
it is like the engine is not sucking fuel through the carbs to the cylinders.
(the carbs have new needles and jets fitted)
can any body help.
cheers dave

 

[simon_t153]

Hey Dave!

Yep sounds like you are not getting enough (or maybe any) fuel into your carbs. What have you done so far to figure it out?

You can start by checking if the floats are stuck (or if there is an obstruction in the the fuel passages to the carb) by trying to drain the carb bowls. The bowls should be full to the parting line so there should be a good amount of fuel that comes out when you try to drain them.

If there is fuel in the bowls it's likely a completely clogged pilot jet. Just take the carbes appart and blast all the passages you can see with carb cleaner untill you see the stuff come out the other side. (you really want to get the thing surgically clean).

Once you do that you can back the idle mixture screws (on the top engine side) 1 and a half turns from fully closed. That should give you a starting point.

Hope this helps!