Scavenge Tube/Fuel Pressure Gauge

[joe_db]

The scavenger tube is a safety device that is a last-ditch backup. If you routinely have gasoline pooling in the carb, you need to fix that ASAP

[ndutton]

Perhaps you're overthinking this Warren. The only thing the two systems have in common is they both use available intake vacuum.

Fuel may pool in the intake horn but it's not like there's a quantity there all the time and if there were you'd know it because it would screw with the fuel/air mix. The inevitable vacuum loss is minimized by using a very small tube. The intake air horn is part of the scavenge system as it's designed to collect and hold pooled fuel so the tube can draw it into the intake the next time the engine is started. Point being, pooled fuel is contained rather than dribbling into the bilge.

The PCV system extracts blowby vapor from the crankcase and as you can see uses a much larger tube. The significant effect it could have on intake vacuum and therefore fuel/air mixture is supposed to be managed by the valve. Without the valve the engine would either not run or would run like shi-shi.

[Dave Neptune]

The "scavenge tube" is a MUST!!! It is a safety factor. I have run with a PCV valve for at least 20 years with never a bit of an issue. If the engine and carb are tuned properly the additional air is actually a plus as the carb stock carb tends to be richer as you start from base through the idle circuit.

Dave Neptune

[JOHN COOKSON]

I didn't articulate it very well. Why shouldn't I put a tee at the bottom of the carb and run one scavenge tube to the manifold, in addition to the one run to the throat plate as part of the Indigo PCV installation? Would that redundancy be bad, or counterproductive?

It seems to me that they're both part of the same vacuum 'loop,' so to speak. I'm speculating that one might pull more strongly than the other under certain conditions, and so might dominate. Whatever is pulled into the throat plate would seem to be more uniformly distributed, dumping into all four cylinders, while the manifold fitting seems closer to just the rear two cylinders, but maybe that proximity has some sort of effect?
.

What did Indigo say when you asked them this question?
If you're are the inquisitive type give it a try.

TRUE GRIT

[BunnyPlanet169]

I agree with Neil - don't over think the purpose/effect of the scavenging tube.

It's very, very low volume, and only there for safety, to sip up any pooled fuel (if present) that may have accumulated during rest. Probably while you're choked anyway. During normal operation, there will be no fuel in the air horn, ever. Just a lot of moving air.

I would not expect to find any operating bias because of the location of the scavenging tube fitting on the manifold. A 1/16" ID tube has an area of about 0.003 in^2. The carb, with a nominal throttle bore of 1.00" has an area of 0.79 in^2, or 260 times larger. Just not plausible.

PCV valves are pollution control devices, venting crankcase back into manifold instead of atmosphere. The claims of 'improved performance' are just that, claims. There is no data to show that the introduction of a PCV valve on an Atomic 4 has any measurable effect on engine performance. It should reduce gasses vented to the cabin through the oil fill, that's it. That's all it was designed for.

I don't have a PCV, and can only report what I see on the Forum. Some have them and like them, some don't. They may or may not cause carb tuning issues because the larger vacuum leak.

[ndutton]

A 1/16" ID tube has an area of about 0.003 in^2. The carb, with a nominal throttle bore of 1.00" has an area of 0.79 in^2, or 260 times larger.

FYI and off topic for those of us with PC's, superscript for exponential annotation is achieved by holding the Alt key and typing 0178 on the numeric keypad thusly: in²

0178 is for squared, 0179 is for cubed. In², In³
Thanks again to Shawn for enlightening me many years ago.

[joe_db]

I never heard of PCV valves being performance devices. What they do is make the boat smell better

I agree with Neil - don't over think the purpose/effect of the scavenging tube.

It's very, very low volume, and only there for safety, to sip up any pooled fuel (if present) that may have accumulated during rest. Probably while you're choked anyway. During normal operation, there will be no fuel in the air horn, ever. Just a lot of moving air.

I would not expect to find any operating bias because of the location of the scavenging tube fitting on the manifold. A 1/16" ID tube has an area of about 0.003 in^2. The carb, with a nominal throttle bore of 1.00" has an area of 0.79 in^2, or 260 times larger. Just not plausible.

PCV valves are pollution control devices, venting crankcase back into manifold instead of atmosphere. The claims of 'improved performance' are just that, claims. There is no data to show that the introduction of a PCV valve on an Atomic 4 has any measurable effect on engine performance. It should reduce gasses vented to the cabin through the oil fill, that's it. That's all it was designed for.

I don't have a PCV, and can only report what I see on the Forum. Some have them and like them, some don't. They may or may not cause carb tuning issues because the larger vacuum leak.

[BunnyPlanet169]

I never heard of PCV valves being performance devices. What they do is make the boat smell better

The 'other' vendor has some pretty fluffy marketing. "Improved performance" is one of the claims, along with the idea that "The automotive industry recognized the need for a different approach to the problem in the '60s and cured it by harnessing the suction available in the intake manifold."

Reality: The automotive industry never vented crankcase to the passenger compartment! The automotive industry went kicking and screaming against their will to introduce anti-smog devices. They were perfectly happy with a plain old vent.

[joe_db]

It would "improve performance" I guess in the sense of not fouling the carb with oily gunk

The 'other' vendor has some pretty fluffy marketing. "Improved performance" is one of the claims, along with the idea that "The automotive industry recognized the need for a different approach to the problem in the '60s and cured it by harnessing the suction available in the intake manifold."

Reality: The automotive industry never vented crankcase to the passenger compartment! The automotive industry went kicking and screaming against their will to introduce anti-smog devices. They were perfectly happy with a plain old vent.

[BunnyPlanet169]

I'm not trying to say the other vendor is bad - I will almost certainly end up with one of their thermostat housings.

The engineer in me over-reacts to marketing....

[Clucas]

Please make sure your life insurance is paid up with that 5psi gauge if you plumb it into your fuel system. It's an air pressure gauge, not a fuel pressure gauge.

https://www.amazon.com/Kodiak-Controls-KC25-5-Pressure-Gauge/dp/B00H9ZW7F0

I don't think 5psi fuel pressure gauges exist at the A4-level price point. While I hate the idea of wasting all that angular space on the gauge, I've found the stock 0-15psi gauge is adequate. We're typically looking for the difference between 3-4psi and 0-1psi, the additional resolution is not really very useful.

This is the fuel pressure gauge that I use: Mr Gasket #1561 (recommended by others on this forum). Available on Amazon at https://smile.amazon.com/dp/B000BWCFLO.

[Wrsteinesq]

Could someone please explain to me why the gauge I have selected (and received) could be in any way dangerous? As indicated, the manufacturer says I'm fine using it for fuel, but of course, he/she will not be on board at the time something bad might happen. I have asked if anyone thinks I have been misled with that information, but no one has suggested that I have been, and no one has offered an explanation for why it is thought to be dangerous.

Obviously, if it is dangerous I would not want to use it. But the mere fact that it is different from the gauges that others have used, doesn't make it dangerous.

The manufacturer site says that it is a capsule type gauge with brass internals.

Assuming the truth of those statements (capsule type with brass internals), what indications of dangerousness or unsuitability remain? Are there other questions I should ask the manufacturer? Thanks!

[joe_db]

I doubt the gauge would be dangerous. It looks like any other pressure gauge with brass parts. You could ask the manufacturer if there are any parts inside of it that would be damaged by gasoline.
My issue with it would be that it may get damaged in use. When I had a gauge like that, I had a 15 PSI liquid filled gauge (I still have it in my spares box). I found that undamped gauges near the fuel pump could jump around quite a bit with fuel pump pulsations. The liquid filled gauge with the damped needle was very smooth and it was even correct after I opened the vent
So......if that gauge ends up being jumpy and hits the 5 PSI limit enough times, it will likely get damaged. If that happens, I would suggest a snubber or a different gauge. If not, you should be good to go. My gauge was a generic 15 PSI glycerin filled gauge from FleaBay, not an official "fuel pressure gauge".
I really like the electric one I have now because I can see it underway and it seems to electronically damp any pulsations

Could someone please explain to me why the gauge I have selected (and received) could be in any way dangerous? As indicated, the manufacturer says I'm fine using it for fuel, but of course, he/she will not be on board at the time something bad might happen. I have asked if anyone thinks I have been misled with that information, but no one has suggested that I have been, and no one has offered an explanation for why it is thought to be dangerous.

Obviously, if it is dangerous I would not want to use it. But the mere fact that it is different from the gauges that others have used, doesn't make it dangerous.

The manufacturer site says that it is a capsule type gauge with brass internals.

Assuming the truth of those statements (capsule type with brass internals), what indications of dangerousness or unsuitability remain? Are there other questions I should ask the manufacturer? Thanks!

[toddster]

FWIW, while struggling to install the EWDS fuel pressure sensor (my engine has zero clearance on the carb side) I discovered that the scavenger tube was only inserted loosely at either end and had never, during my ownership, and probably the previous, ever been functional. Tightening it down doesn't seem to have made any discernible difference to the function of the engine.

[romantic comedy]

Indigo uses a spacer between the carb and the manifold. This spacer has two fittings connected. One a hose barb for the PCv, the other a fitting to connect the scavenge tube.
The PCV valve hose is connected via hose barb screwed into the spacer.

The stock set up has the scavenge tube, from the carb bottom, connected to a fitting on the manifold. The Indigo system moves this connection to the spacer. This leaves the manifold connection in need of a plug.

My thoughts are that the spacer is not needed at all. One can just install a tee fitting on the manifold. Hook up the PCV and the scavenge tube. Cost is a few dollars. This fitting is where one connects a vacuum gauge also. So now one needs another tee for the vacuum gauge.