Welcome to Netrider ... Connecting Riders!

Interested in talking motorbikes with a terrific community of riders?
Signup (it's quick and free) to join the discussions and access the full suite of tools and information that Netrider has to offer.

Air in fuel filter - Problem? Solution?

Discussion in 'Technical and Troubleshooting Torque' started by Bravus, Dec 7, 2007.

  1. Just picked up my GSXF 400 from the shop after a top end rebuild. Delighted to have it back after months, but it's running a little rough and feels as though it's not getting enough fuel (or spark?) I noticed air in the top of the inline fuel filter, and in fact when I got home noticed it was mostly full of air with fuel just flowing across the bottom. Is it normal to have air in the fuel hose/filter? Or does it need to be 'bled' or got rid of in some other way. I guess I can take it back to them, but if it's a 5 min fix I can do that's even better.

  2. Is it a factory fitment (few bikes are), if its a gravity fed system, which I'm almost certain it will be then gravity will have a hard time pusshing enough fuel in one side while the engine drains the other side.

    Did the shop who did the rebuild just decide it needs a filter? If so piss it off, will probably solve your problem.
  3. Not seen an in-line filter yet that didn't have some air in it.

    Whilst I'd agree with Tweet that a filter could be causing a restriction, having nearly died, in part, because of a carby full of crud, I'd be reluctant to give it the flick altogether.

    I'd check whether it's a problem by checking the flow with and without it.

    Make sure the fuel tap's OFF (or RUN if it's a vacuum tap), get yourself a small, clean jam jar, disconnect the fuel line going INTO the filter and stick the end in the jar. Turn your fuel tap ON (or PRIME or PR for a vac tap) and time how long it takes for the jar to fill. Turn off the tap. Tip the petrol back into the tank.

    Now take the filter off the line to the carbs, stick it in the line from the tap (making sure that any arrow points in the direction of fuel flow) and do the timed jar fill exercise again.

    If there's a difference between the two times, the filter is restrictive. Whether this is significant depends on how big a difference there is. Off the top of my head, I wouldn't worry about anything less than maybe 20%.

    If there is a restriction, I'd hunt around for a physically bigger filter (more area available) if there's room to fit it in.

    I've had cheopo car accessory shop filters on quite a few gravity feed bikes and not had a problem.
  4. I'd be checking to see if the filter has been put in back to front,
    there is a one way valve in 99.9% of them.
    Air in the fuel filter can be 'normal' sometimes
  5. Excellent, thanks all. Will have a play tomorrow and see what I can learn.
  6. Gravity-fed, inline fuel filters fitted by well-meaning mechanics can cause your engine to run lean.

    Unless they are specifically designed for that purpose and contsructed of the same sort of fine nylon mesh just like the ones in fuel taps, they will tend to restrict the flow of fuel. They should be mounted vertically, not on their side.

    If you have a dirty/rusty tank you should have that cleaned out by using a handfull of nuts and bolts or "blue metal" (a type of insoluble gravel) in either water or diesel, and shaking the tank vigourously. You MUST make sure there are no petrol fumes inside the tank when you do this - both items could create a spark in there with brisk agitation...


    Trevor G
  7. Yeah, I dunno whether there was a filter there before or not - it would have been under the fairing and I hadn't had the bike long. It's still piss-farting around a lot (went for a bit of a ride today) but I haven't had time to work on it - that's tomorrow.

    it seems to be much worse going uphill, and to bog down and do things like stall at uphill lights, and be happier going downhill. Filter is horizontal and still mostly empty.

    The fuel tap does have a 'PRI' setting. What does that mean and how is it used and....?
  8. That's common to most (all?) Suzukis. The fuel tap normally uses the vacuum created by the engine to switch the fuel on/off - so engine off = no fuel flow. The PRI, or PRIME, setting overrides this and forces the fuel on. It's useful to fill (prime) the carbs if you've drained them or if the bike's been sitting for a while and the fuel's evaporated. Can also be useful for checking the fuel system - open the drain tap on one of the carbs and let it drain. Then set the tap to PRI. If more fuel doesn't start coming out of the drain tap then there's a blockage in the fuel system somewhere (nothing really to do with your current problem but might come in useful in future ;)).
  9. Thanks for that. I wondered whether maybe the vacuum line that opens the fuel tap in 'On' position might not have been refitted, or might have a hole in it. That might have this effect. Will check it when it gets light in the morning.
  10. I guess the simplest way to test that is to go for a ride with it on Pri and see if the problem is fixed.
  11. Good deduction.

    If the fuel filter is below the top of the float bowl mounting surface of the carb it most definitely is wrongly fitted. I would also bypass the filter and see what happens.

    I once cooked a 250 Honda engine that way, broke a ring through overheating through running lean. Mind you it took a few thousand kms to do it...


    Trevor G
  12. Since it's just had new rings the last thing I want to do is cook the rings! Took it out for a ride just now on 'Pri' and it ran beautifully (and I finally got to see how much more go it had with actual, functional rings!) So clearly the problem is not the filter itself but something in the vacuum setup - either a hole in the hose, a blockage in one end or something in the petcock. Can chase it down now, anyway.

    Thanks again all for the great suggestions.
  13. Here's my simple (but possibly dangerous) method to check the vacuum operated fuel tap.

    The vacuum is applied via a rubber line from the inlet manifold. Remove this line at the manifold and, as long as no fuel is dripping out of it, suck on it.

    As you suck, hold the end of the line against your tongue to block it - if the diaphragm is still good it will maintain the vacuum pressure against your tongue. If the vacuum pressure drops away you have a hole, possibly an extremely minute hole which is invisible to the naked eye, in the diaphragm.

    Apart from stopping the tap from working correctly it can potentially lead to a hydraulic lock in the engine, or even a sump full of fuel. Petrol can pass through the hole, down the tube and into the inlet manifold in this way.

    All the best

    Trevor G

    PS Please don't blame me if you get a mouthful of fuel by using this method - I have done it heaps of times on cars and bikes without problem, altho only bikes really use vacuum lines in this way.
  14. Thanks, yeah, I'd kind of imagined trying something like that, so someone with experience suggesting it is encouraging. Will give it a go without sucking too hard (darling!) and see what happens... and I've had the odd mouthful of fuel in the past from syphoning so it's not that terrifying. The possibility of a vapor (liquid) lock is a nasty I hadn't previously considered, but it makes sense.