is the vacume from petcock to manifold needed?

I just picked up my bike, did the whole carb cleaning thing, but she's still backfiring through the pipes and carbs - sometimes (airbox still disconnected). the bike only runs with the pilot 4 to 4.5 Out (unlike the standard 2.5 that i've seens somewhere). I just read in another chat that there is a vacume tube that goes from the petcock to the intake manifold. Could this be part of my problem? The carbs 1 & 2 have a syncronizing tube and 3 & 4 are capped off. It runs for about 4 mins then stalls...
Also, I can't find the 'fine tuning how to' Please add your 2 cents here :?: THANKS :!:

 

Parts of the problem could include:

-airbox still disconnected

-pilot 4 to 4.5 Out

-vaccum tube that goes from the petcock to the intake manifold.....yes, that creates a vacuum leak in that intake.


Many other possible problems exist, beginning with you may not have really cleaned and assembled the carbs correctly to begin with:

http://xjbikes.com/Forums/viewforum/f=23.html

You'll want to read the articles entitled THE SECRET LIFE OF CARBURETORS and IN THE CHURCH OF CLEAN for more perspectives.

 

The air box and air filter are needed to restrict the air going into the carbs since they are calabrated for that to be in place.
You should check your valve clearances and do a compression test to make sure you don't have a mechanical problem. Sticking or Burn't valve, bad rings / piston ect.
The next thing after all that is mostlikely needed is carb sync. That will straighten out any idle roughness and boging when you rev it up. Take your time and give it a once over.
The vacuum line can be off as long as you have the petcock set to prime and the vacuum port is plugged. If you leave it on prime but the gas won't shut off and could fill your crankcase with gas if the carb float levels are not set right or needles and seats are leaking by. Be Careful & Good Luck

 

What is a sync tube between 1 and 2 for? This smells like the work of the PO just playing with the hoses.

 

The vacuum hose from the petcock should go to the carb boot of the number 3 carb. this hose opens the petcock diaphragm when vacuum is applied to allow fuel to flow.

the caps should be on 1,2 and 4, with no hoses connecting them.

As was said above, you need the airbox and filter inplace in order to set the carbs correctly, your 4.5 turns is mostlikely directly related to the freeflow of air thats coming through.

Hope it helps.

 

AAH HA! i turned it to Prim and it started right up! so if I have to use this method, i need to turn the gas off immediately so the engine doesn't fill up w/ fuel right? First thing tomorrow I'm goin to get the right length hose to manifold 3 and the caps for the rest. For now could I get vacume from 1? the bike sounds pretty damn good except for the backfiring. I don't have an air filter but I'm sure when I get one the pilots will go back in like Ace said. THANKS GUYS!!!!

 

You can hook the vacuum line for the petcock up to any of the cylinders. #3 is traditional on a lot of XJs, the Seca 550s have it on #2. Doesn't matter but the other three should be securely capped off.

 

Thanks bigfitz. All 3 are capped and vacume is coming from #1 and it starts RIGHT UP, 1/4 second on the start button. Now all i need is an airfilter and I should be good. Hopefully Yamaha stocks them for 82's and hopefully the backfire stops. I'm going to post a separte topic now, the oil fill window is gunked and I can't see how much oil to put in. I just put a little less than 3 liters with a new filter...