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Pressure Building in Gas Tank (510)


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#1 BambooU

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Posted 10 October 2017 - 11:31 AM

So after having run the car for a while (starting at ten minutes or so) and letting it sit for a while, i believe that pressure begins to build in the gas tank. After waiting a while and coming back to start the car, it takes several seconds and lots of pumps into the throttle to get fired up. I've found that after opening the gas cap, the pressure in the gas tank is  released, even hearing the gas tank pop back into shape. After this pressure release, the startup is just fine. 

 

When this first occurred, I checked out the hose from and before the fuel filter, and found that NEITHER had gas flowing. I believe that it is an issue isolated at the gas tank. Is there something that I routed incorrectly about the gas tank? I currently have filler neck and vent hose hooked up fine, with the hose going to the crank case secured and fine. The only thing i was unsure about was the second hose coming from the bottom of the gas tank, which I routed back up to the L tip, located on the top left of the gas tank (when looking at the tank from the trunk. 

 

 



#2 Doctor510

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Posted 10 October 2017 - 12:00 PM

Tank needs to be ventilated or get a ventilated gas cap.



#3 banzai510(hainz)

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Posted 10 October 2017 - 01:01 PM

try a vented gas cap as mentioned above unless soemthing has changed and this worked before.


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#4 shacks510

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Posted 10 October 2017 - 03:11 PM

Are you sure it's being pressurized? Or is it under vacuum from the fuel level dropping? I ran a hose from the top of the tank to under the car.
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#5 datzenmike

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Posted 10 October 2017 - 04:42 PM

Probably vacuum. It should get air in from the flow guide valve when the engine is running. .

 

The flow guide has three connections. The top just goes to the air filter for filtered air. The middle goes back to the gas tank. The bottom goes to the crankcase vent pipe, the PCV is also connected to this.  

 

Engine off.... evaporated gas in the tank builds slight pressure and forces it's way past the flow guide valve and into the crank case vent pipe for storage.

 

Engine running... Fumes in the crank case are sucked into the intake and burned. As the fuel tank empties a slight negative pressure opens the flow guide valve and air from the air filter is pulled into the tank.

 

 

BTW hard starting cannot be caused by any vacuum in the tank. The carb has a cup of gas in it, more than enough for a dozen restarts.


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#6 BambooU

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Posted 10 October 2017 - 07:37 PM

Alright, I'll take a look at my setup and see what I can do to get ventilation.

BTW hard starting cannot be caused by any vacuum in the tank. The carb has a cup of gas in it, more than enough for a dozen restarts.


Hm. Could be an unrelated problem. I haven't done a lot of tuning on these carbs and they need to be taken a look at. I have had bad starting in the past (before the gas tank issue).

#7 BambooU

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Posted 12 October 2017 - 08:23 AM

So this was my solution;

 

unnamed.jpg

 

Hose on the right is  the return hose from the larger port at the bottom of the gas tank. Hose on the left is a venting hose, emptying out below car. 



#8 KELMO

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Posted 12 October 2017 - 09:05 AM

So, the question on every ones mind is; Did it cure your starting issue?

 

I have somewhat of the same issue on my 1200 but am pretty sure mine is a carb issue (crap Hitachi), but have been checking in here for ideas.  I have not had the tank release pressure upon removing the gas cap....I think. Guess I better check that eh?


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#9 BambooU

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Posted 13 October 2017 - 02:00 PM

So, the question on every ones mind is; Did it cure your starting issue?
 
I have somewhat of the same issue on my 1200 but am pretty sure mine is a carb issue (crap Hitachi), but have been checking in here for ideas.  I have not had the tank release pressure upon removing the gas cap....I think. Guess I better check that eh?


I'm not sure that I've given it enough time to be sure, but since fixing the venting issue (tank no longer releases pressure), I haven't noticed a startup issue when hot. I'll check back in in about a week to confirm, but I think it certainly helped.

#10 datzenmike

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Posted 13 October 2017 - 06:34 PM

I think pressure is the wrong word. Vacuum is much more likely. The flow guide is connected to the PCV valve in the intake that sees around 16 in Hg at idle and well into the 20s on deceleration. The only thing containing pressure would be the combustion chamber.


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#11 BambooU

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Posted 13 October 2017 - 06:43 PM

I think pressure is the wrong word. Vacuum is much more likely. The flow guide is connected to the PCV valve in the intake that sees around 16 in Hg at idle and well into the 20s on deceleration. The only thing containing pressure would be the combustion chamber.

 

Yeah, you are correct. Pressure is the wrong word, and would be much more alarming in the gas tank than vacuum. 



#12 datzenmike

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Posted 13 October 2017 - 06:48 PM

I had a similar thing happen on a trip across Canada. Every time I took the gas cap off there was a huge WHOOOSH. I thought air going out but 80 miles from home the truck quit on 1/4 tank. I got out and the tank had imploded. Looked like a squashed banana. Pick up tube was bent up and sucking air.


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#13 BambooU

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Posted 13 October 2017 - 06:53 PM

I had a similar thing happen on a trip across Canada. Every time I took the gas cap off there was a huge WHOOOSH. I thought air going out but 80 miles from home the truck quit on 1/4 tank. I got out and the tank had imploded. Looked like a squashed banana. Pick up tube was bent up and sucking air.

 

Wow. definitely glad I fixed the issue then.



#14 datzenmike

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Posted 13 October 2017 - 06:59 PM

Yes. I got enough gas to get to my Dad's but couldn't locate another gas tank. With nothing to loose I washed the gas out, sealed the holes and held my hand over the filler opening while using an air compressor to pressurize the tank. It popped a bunch of times but regained it's round shape. It worked.


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#15 BambooU

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Posted 15 October 2017 - 06:48 PM

unnamed.jpg

 

Slight problem. Bit of leaking from the new hose. Probably just slash out and not too serious. I plan on routing the hose up alongside the fuel line into the engine bay, that way the opening is above the height of the gas tank. My car doesn't have the return line setup in the carb that stock does. Thoughts?