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BambooU

Pressure Building in Gas Tank (510)

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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. 

 

 

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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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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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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).

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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. 

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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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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.

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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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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. 

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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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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.

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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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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?

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So this was my solution. My logic was that bringing the opening of the hose above the height of the gas tank would stop leaks and allow venting. I did this on the 22nd, so after about a week of driving, I'm confident it has worked. 

 

image.jpg

unnamed.jpg

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A note; I did not want to open vent it above the gas tank at the T fitting because:

A. I wasn't 100% sure the return line from the bottom of the tank wasn't leading gas back up to the top of the tank (I've only done a few tests with water on a disconnected gas tank. 

B. I didn't want to be venting gas into the trunk because I generally keep things in the trunk.

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You may get gas smells this way. Why not connect it to the flow guide valve? Or get a charcoal canister from a newer vehicle and a couple of small hoses and.... fixed?

 

Line from tank. (you already have) Vacuum source from intake and a purge signal from the vacuum advance line. All of which you have.

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I don't think I have a flow guide valve to connect. The car was a swapped '69 SR. It's my understanding that the early models didnt have a flow guide. 

 

The charcoal canister was my plan, but I didn't have the time to find one. As it stands, im not getting gas smells. That being said, if I do, it'll drift through the firewall. I'll fix that if it becomes an issue.

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I found a generic charcoal canister a couple months ago, but I can't remember who made it. Any old canister will work, but the smaller, older ones look better in a vintage car. Something from a 1980's Toyota maybe.. They are metal and have steel connections so they look good and won't break, plus, they are pretty compact.

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Usually marked vent, vacuum and purge.

 

Vent to the gas tank and on newer 720s also to the carb vent to collect boiled off gas after shutdown.

Vacuum. Anywhere on the intake to suck fumes stored in the charcoal.

Purge is just a vacuum signal to say the engine is running  and allows intake vacuum to empty the canister. Usually from the ported vacuum advance so it only operates above idle and when the engine has warmed up.

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I found a generic charcoal canister a couple months ago, but I can't remember who made it. Any old canister will work, but the smaller, older ones look better in a vintage car. Something from a 1980's Toyota maybe.. They are metal and have steel connections so they look good and won't break, plus, they are pretty compact.

 

I have a canister lying around somewhere, i just need to find it. Still haven't noticed gas smell, despite having several holes in my firewall :3

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You may not smell anything but it is a 'loose end' and should be addressed. Vent open to gas tank inside the engine compartment where there are ignition sparks. The charcoal canister is totally benign in operation.

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You will be sucking air into the line so you won't smell fumes until its parked for a while.  Venting through charcoal will eventually smell like fuel as well.  You can run a vent hose high in the engine bay and install a fuel filter on the end of it to reduce potential contamination.  Tanksinc.com sells a fitting that also acts as an anti-rollover valve so if you tip the car fuel won't spill out.  

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I made the mistake of venting the fuel tank in my Land Rover (rock crawler) to the valve cover PCV breather hose (with a tee). Thinking it would help act as a sort of roll over protection. Now, when I'm on a steep downhill, it sucks fuel (or maybe vapor) into the system and chokes it out.

 

So you should be careful where you vent the tank to. If you vent it into the engine bay, think about getting it as far away from the exhaust as you can. Unless you run a charcoal canister, then it doesn't really matter.

 

Old Chevy trucks have a hose that runs all the way up to the radiator support and the hose is capped with a breather.

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