Jump to content

Pressure buildup in Clarke Tank

Recommended Posts

Well, I had a learning experience this week.  I was riding tough back country trails with friends, and in the most difficult section I ran into engine problems.

I have a black Clarke 13.9 plastic tank, with a screw on cap with vent tube that extends straight up for 2 inches and has an aluminum valve on the end.  I have a raptor petcock.  At the time the tank was down to a few litres of fuel, the air temp was over 30 celcius, and we were making slow progress in very tough terrain.  Engine temp was around 100F in the second rad, with the fan cutting in and out.  It was HOT out.

At one point I noticed fuel dripping out between the petcock and the tank, and at about the same time started having problems starting it, or getting it to run.  Felt to me like it was flooding.  Tightened up the two fasteners on the petcock mount which did little to stop the leak.  After repeated stalls and very tough times to get it to start, and a number of good ideas and choice swear words, I finally unscrewed the cap to check for gas...and the cap nearly flew off as the tank vented a bunch of air out! 

I pulled the vent tube/dongle off the fuel tank cap, and the bike started and ran absolutely normally for the rest of the ride.  The fuel leak immediately stopped.

So....I checked the dongle and it currently functions one way only: air can get into the tank....but nothing is getting out!  So...all that heat on a tank full of mostly air, with no way of venting the pressure out, led to the fuel being pressurized out past the oring at the petcock mount (my leak), and obviously overfilling the carb despite the best efforts of the float valve to stop the fuel, which caused the flooding and hard starting.

My quick research indicates that these valves on short stems are designed to be one way...."so that fuel will not be vented out or splash out".... This means that any tank that is sealed with one of these has no method of venting pressure out of the tank when heated.  

What have you done to fix this?  I am not too kene on putting on a long tube and directing it down into the engine or frame area.  Has anyone modified the "dongle" to allow venting out of the tank as well as in?

Share this post


Link to post
Share on other sites

Long plain rubber over the tops yoke and down through the wiring behind headlight so any petrol goes on front mudguard

Edited by GuyGraham

Share this post


Link to post
Share on other sites

Have a look around.  As I recall I purchased a clever little blue valve quite a few years ago, it would pass air but not liquid in both directions.  It sat on the tank vent via a piece of hose.  The Safari tank that I have uses a diaphragm type in the tank seal and has two small hole that can pass air but get sealed to the cap with the weight of fuel.  As @GuyGraham suggests a long tube down the steering head.

Share this post


Link to post
Share on other sites

I think they're supposed to work like Roleyrev described, I haven't had any issues with mine but have had stuck check vaves in both open and closed position which leads me to believe they are prone to failure.

Maybe soaking it in some type of cleaner will have it working properly.

Share this post


Link to post
Share on other sites

If you are using the original Clarke black plastic cap,they are junk.The rubber vent diaphram inside bunches up and deteriorates and then the venting doesn't work.I also used the stubby vent with it too on my Clarke 3.9 gal tank but it made little difference because the cap kept cracking and the rubber gasket and venting diaphram kept going bad.I fixed it for good by replacing the cap with a new one from a Honda XR650R.It comes with a vent hose attached as well.It's been on that tank for at least 10yrs now and I've never had any trouble with it.It's black and matches the tank perfectly.

Share this post


Link to post
Share on other sites

I know you didnt want to do this but i had the same problem..  90% of those little vent valves are junk and dont flow properly.  I ended up removing mine and just ran a longer hose up and down the steering stem.     And i also second that you should toss the original clarke cap if you are still using it they themselves are junk and dont vent properly and crack easily 

Share this post


Link to post
Share on other sites
18 hours ago, adkbiker said:

I recently put a Race-Driven cap breather hose on my acerbis fuel cap.  I just checked it and it does vent both ways.

https://www.amazon.com/gp/product/B0758KRW8B/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1

 

I went to this link, and the add, like all of the "dongle" vent devices, says "one way" meaning it allows air in but nothing out.  I have a billet aluminum screwon cap that came with such a valve, and when I check it....it really does allow airflow in, and absolutely nothing out.  So...I am becoming convinced that the hose to the frame will be the solution: allow air in and out. 

Share this post


Link to post
Share on other sites

Thanks for all the input gang.  For now, I have put a hose on the billet aluminum cap and directed it down the front.  My S has a fork nut that is completely capped, so I will order the nut that has no cap, so that I can insert the hose down the centre of the fork pivot tube as recommended by mmiguy.  In any case, it will be important to avoid overfilling the tank with cold fuel from the gas station underground tanks, and then immediately leaving the bike in the hot sun while I go for lunch.....as the fuel expansion will certainly cause liquid fuel to flow out the hose and all over.  

Should I have any concern about dust/moisture getting in the tank through this open ended hose?  I thought of the small plug in filters that one uses for trimmers etc. but I wonder if moisture or dust would block them and create a worse problem: no venting.

Share this post


Link to post
Share on other sites

In addition to dumping the one-way check valve, I would suggest insulating the bottom of the gas tank with shiny reflective aluminum foil type heat shield.  This is especially helpful with 4-strokes to keep the hot exhaust and engine from heating up the gas tank too much. 

Share this post


Link to post
Share on other sites

Okay, for posterity here is my final (hopefully) solution.  I was a bit nervous having the fuel tank vent line directed down the front of the engine (between the rad guard and the rad) and after thinking about it was not too kene on having the vent line down the steering stem either: in both cases, if I tip with a reasonably full tank the vent line will likely pass liquid fuel, and I was very uncomfortable with the idea of venting this down the front of a hot engine.

I decided to run a vent line down the frame to the carb area, and have a vent in the same place as the carb float bowl vent.  I went further: in that I do water crossings, I did not want the possibility that the end of the vent line would be submerged and not provide air, so I used a "T" as I have done with the carb float chamber vent line, and ran an additional line up under the seat, so there is both a place for liquid fuel to more safely vent out, and there is always an air source to prevent the tank building a vacuum pressure during water crossings.  

Here are the photos.  Should I have any issues, I will return and edit this further. 

image.thumb.png.c2088dbe72642110cbf4ed70b8bd1539.pngIMG_8397_LI.thumb.jpg.a8db811f146a5374a1803361193a15f4.jpgIMG_8396_LI.thumb.jpg.410d1b18bfd9364452aa5cad0f08927d.jpg

Share this post


Link to post
Share on other sites

That works.  Back in the early 1970's it was cool to route gas cap tubing through the nut in the steering stem.  If the steering stem nut didn't have a hole in it, then we'd drill it.  

Share this post


Link to post
Share on other sites
5 hours ago, dad2u said:

Okay, for posterity here is my final (hopefully) solution.  I was a bit nervous having the fuel tank vent line directed down the front of the engine (between the rad guard and the rad) and after thinking about it was not too kene on having the vent line down the steering stem either: in both cases, if I tip with a reasonably full tank the vent line will likely pass liquid fuel, and I was very uncomfortable with the idea of venting this down the front of a hot engine.

I decided to run a vent line down the frame to the carb area, and have a vent in the same place as the carb float bowl vent.  I went further: in that I do water crossings, I did not want the possibility that the end of the vent line would be submerged and not provide air, so I used a "T" as I have done with the carb float chamber vent line, and ran an additional line up under the seat, so there is both a place for liquid fuel to more safely vent out, and there is always an air source to prevent the tank building a vacuum pressure during water crossings.  

Here are the photos.  Should I have any issues, I will return and edit this further. 

image.thumb.png.c2088dbe72642110cbf4ed70b8bd1539.pngIMG_8397_LI.thumb.jpg.a8db811f146a5374a1803361193a15f4.jpgIMG_8396_LI.thumb.jpg.410d1b18bfd9364452aa5cad0f08927d.jpg

You are way over thinking this, never the less, you now have a very have a very robust set-up

Share this post


Link to post
Share on other sites
7 hours ago, billie_morini said:

That works.  Back in the early 1970's it was cool to route gas cap tubing through the nut in the steering stem.  If the steering stem nut didn't have a hole in it, then we'd drill it.  

I thought of that too....but when I had a look where the fuel would be dumped.....it would end up vapourized and spraying back.  In any event, with the clear line I should see if I end up with liquid flow or not.  I like the "way overthinking" which should mean that this pressure issue is safely and permanently licked.  Now to get back to riding.

Share this post


Link to post
Share on other sites

I've been riding almost 40 years now, and in all that time I've had my tanks vented by simple hose looped over the bars or pushed into the steering head like billie_morini said. Now, I've crashed a lot in those 40 years and I don't ever recall fuel actually flowing out the vent.

Share this post


Link to post
Share on other sites

You are way overthinking this. Millions of dirt bikes are running a simple vent hose with no problem whatsoever, hardcore riding, enduro and mx, they're even lawyer approved.

If you tip over and a small amount of fuel drips out it will always head down to the ground not up to your engine.

I've tipped over and no fuel has ever come out the cap vent with the aluminum valve. There always is a small fuel leak in the dirt, not sure where it came from but not from the cap.

Share this post


Link to post
Share on other sites

@dad2u  Absolutely right, rechecking the valve I mentioned above it is one way.  I've gone back to a vent tube over the bars.  You mentioned a filter at the end of the tube that got me thinking; I added a RC Model Airplane fuel filter I had laying around to the end of the tube, something like this:

https://www.horizonhobby.com/pro-fuel-filter%3A-gas--glow-han143

I ran the tube between the bars and computer and down through the speedo cable hanger on the fork.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:


×
×
  • Create New...