1978 XS400 Carb/Airbox Help please!

Hello,
I just bought one that had been sitting, it has no fuel lines-im confused!!!!
-There are two fittings on the petcock, where does the second one go?
-Anybody know what size these hose are? i cant seem to find them at the auto parts stores?
Thanks!!
DG

 

Smaller port goes to a vacuum reference from the intake manifold (one of the balance ports), larger port to the fuel feed for the carbs.

 

The large hose is simply 1/4"ID fuel line.

The smaller hose is a vacuum line that "controls" the petcock. You won't generally find the right size vac line at an auto parts; you can get some that will work but it will be a bit "chubby." But is has to be VACUUM line; conventional hose will sometimes collapse under vacuum.

Look in the "XJ4Ever Supporting Vendor" forum, click the link at upper right, or PM member chacal. He carries the correct size and type line for the XJs and it will work for your bike too.

 

Thanks for your help! Im going to try to find that port, its near the where the carbs attach to the engine?
i had a "chubby" fuel line hooked up to the carb, and left the vaccuum port empty. When I did this the petcock was leaking, however much I tightened the pipe clamp around the fuel hose going into the petcock it would leak there, is that because it needs the vacuum line hooked up?

 

The petcock itself might be leaking. It has a vent from one side of the vacuum chamber to the outside, and if the diapragm has a hole or is cracked it will leak.

Petcocks 101: http://xjbikes.com/Forums/viewtopic/t=25058.html

Here's how the whole thing is supposed to work:

There is a fuel line from the petcock to the carbs.

There is a vacuum "signal" line from the petcock to a vac nipple on one of the intake manifolds (yes, where one of the carbs attaches to the engine.)

When the petcock is in the "ON" or "RES" positions, the petcock should only flow fuel when the engine is running and it is getting a vacuum pulse from the motor to activate it. With the motor off, the petcock should also effectively be "off" in either ON or RES.

When the petcock is in the "PRI" position, it flows fuel all the time.

If it's passing fuel all by itself when in the "ON" or "RES" position, there is a problem that you need to correct.

 

Thanks, so theoretically i could just run it on prime, and turn to "ON" when i park it until I get the right kind of vacum hose?

 

Theoretically. However you still have an open vacuum port on one of your intake manifolds which will make that cylinder run lean.

 

thanks!!!!
im still not sure how the vacuum line form the petcock hooks up to the engine,

 

Take off one of those short hoses with the screws in it, get a 6-12" length of new hose, and run it from there to the petcock.

 

Thanks! Is it supposed to have the screws in?
Which one should I remove and connect to the petcock?
Should I leave the screw in the other one?

 

The screws are to stop the vacuum. Only take off one hose with a screw in it. Leave other one in. It doesn't matter which one you remove or which side. Connect hose to vacuum port on petcock. (Smaller one) U need to test your petcock though. Suck on vac line and see if gas flows out of fuel line. If it.doesn't, which it prolly wont, then don't remove either lines with screws in em. In that case does.gas flow freely when gas is on" ON oreserve."?

 

NO NO NO.

Those hoses with the screws in them are incorrect, period. There should be a rubber CAP on one (held on with a spring-type hose clamp) and the vacuum line to the petcock on the other one.

Those hoses with the screws are PO tracks. BAD PO. (OK for now since one is going away, replace the other one with a cap as described.) But they gotta go. Screws screwed into hoses don't seal all that well.

Chacal carries the correct little rubber caps, clamps, etc. You can also get the rubber cap/plug at an auto parts store, and use a screw-type clamp to hold it on if you insist.

A little rubber cap like I'm referring to is the very bottom object in this pic, below the green gasket toward the lower left. A clamp of the proper type is pictured next to it.

 

*nod*

Listen to Fitz,,,,, bad PO tracks for sure......

 

thanks, you guys are the best!