$$!??#%%! Fuel starvation when it's raining

[smoke]

THAT, is creepy. Is the only vent on that tank the cap?

[datzenmike]

Only way the filter can be empty is the tank went dry, air was sucked up the pick up tube or there is an air leak in the lines and the pump sucks in air.

[mtngoat]

Yes. And I've driven it this way for 20+ years

No problem until the last three months

[datzenmike]

Maybe a rock hit the fuel line or it took 19 years and 9 months to rust through. Hell I don't run as well as I did 20 years ago.

[smoke]

Filter empty, is the gauge reading no pressure as well?

[smoke]

Out of nothing more than curiosity, I would love to see what happens if when this thing was sputtering, you dumped hot water on the intake and carbs.

[smoke]

The reason I am beating this carb icing thing into the ground is that I am out of ideas, but this makes the most since to me given your wideband reading.

Icing relates directly to dew point. A higher dew point will make a carburetor more likely to ice up at warmer temperatures. The dew point represents the temperature when water fully saturates air and coalesces as dew. Greater humidity reduces the temperature at which dew forms.

Here’s the cool thing about dew point: it’s the measure of absolute humidity. Air can’t hold more than 100 percent humidity. So rain falling from the sky indicates that the dew point is 100 percent at that particular temperature. Like if it starts to rain at 55 degrees F it means that the dew point is 55 degrees F.

Where that causes problems with carburetors is the pressure differential. Speeding air up decreases its dynamic pressure. Take a wing for example. The top of the wing has a hump in it. Because the topside of the wing has more surface area, air has to move faster over it than it does the bottom side. That reduces the dynamic pressure on top of the wing. Nature always tries to correct an imbalance. If the negative pressure on top of the wing exceeds the weight of the wing then the wing will rise. That’s how a venturi in a carburetor works: if the negative pressure overcomes the gravity of the fuel then the atmospheric pressure exerted on the fuel in the bowl will push the fuel over the wall and into the negative-pressure area.

But pressure has another dynamic: temperature. Reducing dynamic pressure also reduces temperature and vise-versa. Take a compressor for example. When you fill a compressor the tank gets hot. A little bit of that is from the friction that the air encounters in the pump. But that pales compared to the energy stored in the air. When you compress air into a smaller space you also condense the energy into that smaller space. So the energy stored in three cubic feet of air is now confined in one cubic foot. The tank gets hot.

But hot for only so long. The heat eventually radiates from the tank until it reaches temperature equilibrium with the atmosphere. Releasing the air from that tank reduces its dynamic pressure. At the same time the energy confined in that small area now expands to its original displacement. Because the energy dissipated when it was in the tank the air gets super-fuckin' cold as it leaves the nozzle.

The same thing happens to a lesser degree at the tail end of a wing or venturi. And that’s what you experienced with the super-lean condition. When enough ice accumulates along the low-pressure area it eliminates the pressure differential. When that happens on a wing the plane falls from the sky. When it happens in a venturi then it can’t develop a low-enough pressure area for the atmospheric pressure to push fuel into.

So that’s why an engine goes lean when the carburetor ices up and why it makes sense to me with what's happening to you.

[datzenmike]

Next time it happens, as quickly and safely as possible, stop and get the air filter off. Have a fleshlght handy. Look into the primary barrel (closest the valve cover) The venturi in the middle will have frost all over it if carb icing. Mine was plugged solid. I flicked it off with my finger and away I went.... for about 4 blocks and it did it again. Sometimes just waiting a few minutes and it will melt from engine warmth.

[tr8er]

Next time it happens, as quickly and safely as possible, stop and get the air filter off. Have a fleshlght handy. Look into the primary barrel (.

I can't find the air filter on my fleshlight, but I think I've found the primary barrel...

 

Good job mike. I'm loving that you've typed fleshlight enough times that spell check didn't catch that.

[datzenmike]

Yeah, next time you have carb icing and are stalled on the side of the road, a fleshlight is always handy to have.

[hang_510]

Have a fleshlght handy. Look into the primary barrel (closest the valve cover). Mine was plugged solid. I flicked it off with my finger and away I went..

cant beat that

[mtngoat]

It's still pulling the same crap and now all the time. i went the lines one more time very, very closely...nothing. 

 

At this point I guess all that's left is the tank. Maybe there's crud slopping around the bottom. I should have washed it out when I had it dropped before and seen if anything came out. The other thing I was thinking about is the number of rock hits on the bottom...since the pickup must be near the bottom, and one of the largest hits is on the lowest point, perhaps it's bent or pinched the pickup and gunk is finally plugging it. 

[Just Joel]

The pickup is at the lowest point towards the front...

[mtngoat]

That's where there's a pretty good dent an inch deep

[mtngoat]

Pulled the tank again

The dent is huge, more like 2" deep and covers the width of that part of the tank. I pulled the sender so I could peer in there with a tiny led flashlight.

 

Most of the inside of the tank is very clean and appears gray and galvanized. The area of the dent, including the drain plug, has a rusted appearance.

 

The pickup tube is just that ... A simple tube, which sure enough ends right down in the area of the dent. If it wasn't in contact it was super close.

 

I hit the tube from the outside with the air compressor ( which I did last time), then did a suck n blow on the pipe, flow seems fine.

 

I could hear crud rattling around in there so I tipped it up float sensor port down and got a small pile of crud, some in larger flakes slightly larger than a BB. I reached in there with the shop vac tip and sucked it out, then got the hose and out about a gallon in. Swished this around, poured it in a bucket, no crud.

 

I reached down into the tank using a broom handle and gently tweaked the pickup away from the bottom a bit and towards the deepest remaining portion of the dented area. It looks to be about 1/2" off the bottom now.

 

We'll see if this does anything. There was gunk, and the tube was suspiciously close to the bottom.

[.sunlover]

Maybe a fuel pressure gauge would help. Is your return bad?

[ggzilla]

A little of the gunk always gets past the filter and can cause intermittent running or idling problems. Let's hope your cleanout has fixed it. But be prepared to change the filter a couple more times.

 

Does your tank not have a drain plug?

[Sealik]

How much gas is in the tank when you're on these treks in the rain?

Low?....Pick up could be sucking in crap,  and or.

If not .....you're tank is compromised and water is somehow finding a way in

 

I'd consider swapping the tank out.....being that is has a "huge" dent in it.

Can't see carb icing being of issue

 

[datzenmike]

Getting too warm for icing now but any time it's very damp and close to freezing it is a possibility.

[mtngoat]

Well it's still running like ****. I tanked it up to see if it might be a crack in the pickup from being bent, but the tank is full to the filler neck and it made no difference. I let it sit overnight and crawled under, no leaks anywhere. Tried driving it with the cap off yesterday before filling it up on the chance that venting was an issue, no luck. 

 

Today I'm going to have one more look in the inlet strainers on the float bowls just because, but the filter is still running mostly empty. At idle the fuel pressure gauge is showing 4, when I rev it, 4, which is consistent with the regulator I put in the system....but I don't trust this *@!! new pump either. I had trouble years ago with one of those crimp top fake OEM's instead of the stock looking ones. You don't get nearly the demand even revving at idle as you do with your foot into it under load, so i'm still suspicious of the rev it in the driveway fuel pressure test. 

 

If I just crank with outlet hose off how much fuel should I see in the jar after X seconds? 

 

I've been pricing NOS Datsun pumps and electrics. There are a couple low pressure electric pumps out there at reasonable prices, the PEFP P11K and Airtex E8106S show up in searches for carbed Datsuns of this era. 15 GPH at 4 PSI they claim. Would I have t put one of those in back, or do you put it up front? I'd put a relay on the power with the relay run from the IGN switch power, I know how this is done at least!

[mtngoat]

And yes, I'm considering swapping the tank. The thing is, after my thorough cleaning on Friday I'm pretty sure it's not the tank. That pickup felt perfect during the suck n blow. I even did the same with a new 1/4" hose just to compare, and they felt the same. 

[Sealik]

Just find a Nikki fuel pump for a 2 liter...no wiring, no relay needed.

The question is...how is the rain factoring into the scheme of things

 

Elec pump would be mounted near the tank, low pressure 720 pump would suffice.

[mtngoat]

I've been looking and unless I'm a lousy googler, which is possible, all I see is a lot of those stamped sheet metal cap crap 'OEM' pumps

 

Oh, and a Nikki from some Ebay guy from Thailand or someplace who wants 40 bucks plus 30 for shipping. November Foxtrot Whiskey on that deal with a non CONUS seller. 

 

Electric is looking more attractive all the time. It's reliable, yes? 

[Sealik]

http://www.ebay.ca/itm/NEW-1973-1974-1975-Nissan-610-Nikki-Fuel-Pump-17010-U6027-/370733685069?pt=Vintage_Car_Truck_Parts_Accessories&hash=item565173194d&vxp=mtr&_uhb=1

 

 

11 bucks for shipping

[ggzilla]

Dont "trust" the pump - test it. You're already getting too much pressure. Use a pop bottle and a stopwatch. It it pumps at least 500ml in 60 secs, the pump is good.

 

The fuel bowls will need to be cleaned over and over until clean fuel is available.