Tank Vacuum issues.

Hey all! This question is for a 1993 CBR 1000 F
Ive have a problem with my bike but didn't know how to ask it until now.
I have narrowed the issue to fuel supply and how it makes its way to the carbs.

Question: Could the petcock valve cause this symptom.

My tank has vacuum which is starving the carbs for gas at high speed.
It rides and runs great until I try to get fancy.

If you know a short cut like compressed air or what not... give me some details.


Edit: I took apart the gas cap and found a track / airway inside the top part. There was a pile of lint in there. I don't know if that could have affected the vacuum problem. (it did not help cleaning it)

Just one more dumb question: Does the gas cap itself have a relief vent to dissipate pressure from the tank?
I've been reading quite a bit on this and there seems to be some the believe there is a vent on it but damned it I can find it or figure out how to unblock it.
Now this sorta looks more like a novel.

I found this network of passage ways in the gas cap. Could it be the relief valve?

Edit: I bypassed the inline fuel filter to test (its going right back on).

 

Try riding with the gas cap loose, if that doesn't change things perhaps you have a blocked filter ?

 

Um, Im just thinking out loud here.
1st, that sounds dangerous ( I guess it would be fine in a parking lot but reaching high RPMs in a parking lot with the gas cap open is even more dangerous. ).
2nd, I dont need to drive with the cap loose to know if the pressure will release, of course it will.

Others feel like jumping in would be great.

 

I believe that tank has a vent hose which is under the tank. It may be plugged or pinched.

 

Bit snippy ain't ya Bill. I didn't mean wide open, just not closed tight.

 

there has to be a vent somewhere, otherwise when you parked it in the sun pressure would build up in the tank or at night you'd get a vacuum. keep looking

 

Sorry dude, I really mean that.
Iv'e been searching google, cbr forums and figured this was my last hope.

I did however do some modifying on the cap. On the underside I drilled some relief holes to the side of the Key/Tumbler hole to see if I could remedy this problem. I think it did somewhat but only if I leave the key latch up off the lid.

 

The manual shows a vent hose that runs from the tank to the bottom side of the breather. I looked at the bike from all angles and do not have that hose.
Thank you for the suggestion though. I think its a crusty fuel cap issue now.

 

I am looking at new fuel caps now. Thank you guys for helping me trouble shoot!!!!

 

I think it may not be a fuel tank issue. You would have problems at any speed if it was having too much vacuum in the tank. I don't know if that bike has a setting on the petcock for it to be open with out needing vacuum but that would be a way to rule that out. Could it be a bad vac sync on the carbs? Or a clogged carb? If it's bogging at high rpm's a clogged pilot would cause some fuel starvation and some bogging. I'm still a newb to motorcycles but it makes some sense to me.

 

It runs PERFECT until I get up and around 9000 rpm.
I did a modified run with the fuel cap key cover up... no problems at all.
Its the gas cap I suppose.

Again, thanks for the suggestions.

 

How strange glad you found the problem.

 

Not strange at all. XJs often have a similar problem; if the gas cap can't "vent" then air can't get in to replace the gas that's trying to get out. This causes a condition us old-timers call "vapor lock." Which causes fuel starvation.

 

I had been "googling" vapor lock at one point and all the symptoms were identical to mine.
Thanks Fitz.

 

I have suggested to shut the cap on 2 strands of "speaker" wire, toy size.
This keeps you safe from spills, and vents just enough.

Good luck BillB

 

Ok, the problem still occurs with the cap cracked getting plenty of air.

I read in another forum that a gas line that is overtly too long may cause fueling issues (heat was another issue mentioned too).

I guess I'll trim the gas line down tomorrow to only what is necessary.
(if this works the petcocks vacuum valve may be weak causing sporadic fueling?)

 

Just installed the rebuild kit for the petcock.
Testing 1,2,3