Air Bubbles in Fuel Line !?!?!?!?

rfriesen · #1 10-07-2009, 07:38 AM

I've had my bike running for about 4 days. It's been running great until last night. Every 3 minutes air bubbles would start to form in the fuel line. They appeared to be coming from the engine up to the tank. The Engine would rev way up and then die. I would take off the tank cap to relieve pressure and fuel would flow back down into the fuel line.

I called dax and he thinks that it's percolating. That my engine's running hot and percolating the fuel I guess. He said add more oil to the mixture. It was 46 degrees outside though. That should have helped keep it cool.

Although, this last tank I used Premium gasoline rather than regular. Maybe that would be part of the problem too?? Only thing is, I ran through 1/3 of that premium tank before the problem started.

Should the tank caps be letting more air in or are they supposed to do that at all? Maybe I have gas in my cap that is stuck in there and blocking the air???

Any ideas????

**BarelyAWake** · #2 10-07-2009, 07:49 AM

Premium gasoline is a waste of yer monies on these sloppy lil motors - don't bother. It seems like a good idea but there's really nothing to be gained.

Insofar as the fuel bubbling problem, I had a very similar problem - but it was back when my engine was new so maybe this isn't it... I like so many others, has tried "soft" motor mounts and in addition had a length of fuel line not securely connected to the frame. As a result of line vibration my fuel would "froth" and gave me symptoms very similar to what you describe. It was also somewhat intermittent for added amusement o.O

Have you messed with yer fuel line/filter recently?

Yet I'd listen to Dax, he's forgotten more than I'll prolly ever know about these motors lol if yer vapor locking due to heat that could explain it... but it does seem a little odd.

To get a good idea about yer fuel/air/oil mix - have you checked your spark plug's coloration? Is you engine/exhaust making excessive/more cool-down noises (the ting/pop as metal contracts) than it used to?

Bet there's someone here that'll nail yer prob lol, good luck

rfriesen · #3 10-07-2009, 08:01 AM

I love this forum. Replys are quick and people want to help!

I'm curious about the fuel line vibration. You can see how I have it set up in this picture.
Picasa Web Albums - Rick
It is touching the frame which could vibrate it or be said to help keep it from vibrating as much. I do have a few little black things in my fuel filter. but not enough to look like it's clogging it. Oh I do have the fuel filter installed in the Opposite way as shown in the picture so that the cone is up.

I have not taken my spark plug off in 3 days. Last time I took it off it was black except for the 2 spots where the spark arcs across.

I don't seem to be having excessive tings or pops from cool down or warm up. None at all for now I think.

Bikedad1 · #4 10-07-2009, 08:15 AM

I had this same exact problem and it turned out to be a combo of two things. First my gas cap wasn't venting properly so only a small amount of fuel was running down the line and that small amount would just foam up because of the second problem...vibration! So I took the black seal off of my gas cap and vented it better(with a drill bit) and insulated the fuel line from the frames vibration and magically it stopped doing it. Good luck

**BarelyAWake** · #5 10-07-2009, 08:16 AM

Hmm... while that setup could be prone to excessive vibration, given it's an almost straight shot down I'd imagine that'd not be an issue, but I'd still consider rerouting, strapping it to the frame w/stand offs if possible, tho on yer bike that could be tricky... I also forgot to mention it was the carb shaking too that made the prob so bad for me.

A few black specks aren't going to effect the filter, tho it's good you flipped it around

If in doubt, jus pull the carb end and drain into a bucket/gas can for at least 30sec to a full min, this will show if yer not flowing/venting properly.

Black isa lil dark for these motors, even during break-in, I'd shoot for a milk chocolate brown, cooking chocolate if yer paranoid

edit: Oh and BTW that's a sweet build ya got there!

rfriesen · #6 10-07-2009, 08:23 AM

Thanks Guys! Hey BikeDad. You're saying you drilled some holes in the black seal right? Maybe I'm not visualizing it correctly right now, but won't those holes seal up as soon as you screw the cap back on? Or maybe you drilled the holes from the side? I'm definately not getting the air flow through the cap. How could that have all of the sudden stopped?
I will try to mod my fuel line too.

Thanks for the compliment B.A.!

**BarelyAWake** · #7 10-07-2009, 08:32 AM

erm... tho it could very well be yer prob, I would try a flow test before drilling the cap - these caps are junk to begin with and gettin' gas all over yerself when you ride causes an odd "itching burning sensation" lol

Another flow test is to simply take the cap off/loosen it as you're riding and experience the prob, but be sure to have a 1/2 tank or less... see "itching burning sensation" again

I've been ona quest for a cap that seals tight, then drill it in the center and add a fitting & hose, much like you see on dirtbikes & 4wheelers. A far superior design for our bike application (doesn't spill as much/at all if knocked over), I've just not found a cap that fits/looks right yet.

rfriesen · #8 10-07-2009, 09:31 AM

Last night when I was riding my bike started to Rev up really high so I quickly opened the gas cap and the reving slowed down and everything went back to normal. If I hadn't opened the cap it would have reved up and shut down. So flow from my cap definately seems to be a problem.

**BarelyAWake** · #9 10-07-2009, 09:34 AM

Ah - gotta love an easy fix! Nice

rfriesen · #10 10-07-2009, 09:44 AM

Well, I'd rather not have to open the gas cap every 2-3 minutes. There has to be a better way.