Author Topic: Preventing pump overpressuring carb needle/Fuel return  (Read 190 times)

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Offline happul3

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Preventing pump overpressuring carb needle/Fuel return
« on: March 22, 2021, 09:05:09 AM »
As I was preparing my v1 Eskape for new season, I noticed that after 10 min idling there is some gasoline at  the bottom of air intake. It can be easily seen after taking aircleaner element out. Motor starts and runs just fine, but i decided to troublshoot anyway. Took carb out and apart, but nothing wrong: bowl is pristine inside, no gunk or water or residue, the needle appears perfect, the seat is clean and looks polished (it was actually polished a year ago). The only element that can use replacing is packing - the gasket that's between aircleaner and carb body. That gasket is all sticky and gummy, especially the lower half, which is not surpising after being exposed to liquid gasoline. But the state of that gasket not the reason for gasoline being out of place, it is a consequence. And another consequence is the smell of gasoline coming out of the airbox, something that has been sometimes bothering  me for years.

So the actual problem isn't new or uncommon and has been discussed before: carb needle is overpowered by pulse fuel pump under certain conditions and fuel overflows the bowl. Poor design by Mokai to supply fuel to the motor that is supposed to be gravity fed. This is more problematic at idle because the fuel demand is lower, yet the pump may be at its peak. Tom has experimented with pump modifications, but AFAIK his current recommendation is to clean/polish needle seat. Apparently that works for the many boats he services. Well, it does not seem to work for me, at least not always reliably. I also suspect that it may not work for some others, but because the motor can start and run fine (just rich at low rpms), it can go unnoticed.  Be that as it may, checking for this condition is quite easy: idle for 10 min without load (I remove jet pump too), take out aircleaner element, check down the very bottom of aircleaner for wetness/presence of liquid gasoline.

But if that's what one finds, how does one fix it? Adding a fuel return line seem like a preferred solution that won't depend on bunch of factors that might change at any moment. I am thinking adding a T after stock fuel pump, maybe a fuel filter as some resistance, and connecting to the tank. That would require making another hole in a fuel tank and quick connect couplers. Alternative is another T on the tank side of fuel pump and "return" line connecting two Ts, which must incorporate some resistance to prevent fuel pump "short circuiting". Thoughts?
   

Offline Painlesstom

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Re: Preventing pump overpressuring carb needle/Fuel return
« Reply #1 on: March 22, 2021, 02:09:38 PM »
The needle seat is defective so the needle won't seat properly. I have been using a tiny ball bearing (don't recall the size) I tap into the seat to form a perfect circle and seating area. 90% of the time it works. There is no alternative that is consistently reliable, I went down that path and it was all just band aid fixes that didn't last. Polishing doesn't help unless you get really lucky, very rarely is that all that's needed. If you would like me to see if I can do something with it feel free to send it.
Tom

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Offline George

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Re: Preventing pump overpressuring carb needle/Fuel return
« Reply #2 on: March 22, 2021, 02:21:34 PM »
Adjustable 1 to 5 lb fuel pressure regulators for carburetor car engines go for about 30 bucks.  might work.
Airplane George

Offline Painlesstom

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Re: Preventing pump overpressuring carb needle/Fuel return
« Reply #3 on: March 22, 2021, 02:35:37 PM »
Most of the time the amount of pressure reduction to stop flooding will also cause starving at full throttle and loss of power from running lean.
Tom

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Offline happul3

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Re: Preventing pump overpressuring carb needle/Fuel return
« Reply #4 on: March 22, 2021, 05:39:17 PM »
The needle seat is defective so the needle won't seat properly. I have been using a tiny ball bearing (don't recall the size) I tap into the seat to form a perfect circle and seating area. 90% of the time it works. There is no alternative that is consistently reliable, I went down that path and it was all just band aid fixes that didn't last. Polishing doesn't help unless you get really lucky, very rarely is that all that's needed. If you would like me to see if I can do something with it feel free to send it.

Thank you for clarifying and offering to help, Tom. I might take up up on that offer, but first I am looking forward to experimenting with fuel return options. I assume that if the motor is designed to function with gravity feed, it does not actually require extra pressure to run even at full throttle. If you have reasons to believe that is not so, please share.

Offline happul3

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Re: Preventing pump overpressuring carb needle/Fuel return
« Reply #5 on: March 22, 2021, 05:53:29 PM »
Adjustable 1 to 5 lb fuel pressure regulators for carburetor car engines go for about 30 bucks.  might work.

Thanks! Something like this, perhaps?
https://www.ebay.com/itm/Metal-Adjustable-Fuel-Pressure-Regulator-Kit-for-Carburetor-Engine/224205625276?_trkparms=aid%3D1110006%26algo%3DHOMESPLICE.SIM%26ao%3D1%26asc%3D231410%26meid%3D7c4a99917ffc4699a268053723562fb4%26pid%3D101195%26rk%3D1%26rkt%3D12%26mehot%3Dco%26sd%3D402314986691%26itm%3D224205625276%26pmt%3D1%26noa%3D0%26pg%3D2047675%26algv%3DSimplAMLv5PairwiseWebWithBBEV2bDarwoXgbV1%26brand%3DUnbranded&_trksid=p2047675.c101195.m1851

Tom's point about starving at full throttle is worrisome. I certainly don't want to run lean. Have seen some nasty pictures of holes in pistons and other  scary stuff. Maybe I can use it in return line instead? Will prevent excessive pressure at idle by returning fuel and will do nothing at full throttle when motor eats all the fule pump can supply.

Offline Painlesstom

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Re: Preventing pump overpressuring carb needle/Fuel return
« Reply #6 on: March 22, 2021, 05:54:35 PM »
You need volume, and if you're going to go gravity feed then it needs to be from a reservoir above the the carb that the fuel pump feeds, and then that has to have a return that exits it's full point back to the tank. There's no guarantee that gravity fed it won't still leak. I understand your need to experiment, I've just been there done that and had absolutely zero luck with any of it. Fixing the problem is much easier than creating band aids to mask it. If I can't get a carb to hold fuel pump pressure at idle, I do the needle seat trick I described, if that doesn't work I strip the carb of any reusable parts and melt it down to be part of a steering nozzle..... for real. Maybe you'll get lucky.
Tom

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Offline happul3

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Re: Preventing pump overpressuring carb needle/Fuel return
« Reply #7 on: March 24, 2021, 07:53:40 PM »
While waiting for the regulator and gasket to arrive, I've set up a simple pressure test, something inspired by pop off pressure test commonly done for diaphragm carburetors found in old jetskies.   Basically, I removed the bowl, flipped carb upside down and connected controlled pressure source and gauge to fuel inlet. It is normally done after spaying WD40 or something similar, but not being sure how compatible that is with the needle material I decided to do it dry. Doing it dry wouldn't increase the pop off pressure anyway. So, the long story short, the needle remained sealed all the way to the top pressure I dared - 7 psi. Pressure kept steady until I'd lift the float with my finger, then I heard a slight hiss and pressure gauge went to zero. The upside down position means that needle gets somewhat different force from float's weight compared to when the float is lifted by by gas displacement, but it should be comparable, I think. So the bottom line is that the needle works quite well and I now have a quantitative way of measuring whether there is a problem with the needle even without taking carb out. In hindsight, I should've done this before cleaning carb and polishing the needle seat, but there is always next time (or so I hope).

The question remains as to the why is there flooding in the first place if needle can overcome pressures in ballpark of 7 psi or even above that.  Maybe it is really super-sensitive to dirt and seat imprefections, which is along the line of Tom's previous recommendation to polish the seat. It may even be that his current way - reshaping the seat with a bb - is to reduce said sensitivity to dirt. It is also possible that my pressure measurement is misleading because it is done outside of vibrating motor. Perhaps the flooding is fundamentally the result of combining pump pressure with vibration making needle to to open and close rapidly. We can't do anything about vibration, so the only thing is to reduce input pressure. That's what I will experiment with next - still have time before starting my Mokai season when temperatures will get pleasurable.

Offline Painlesstom

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Re: Preventing pump overpressuring carb needle/Fuel return
« Reply #8 on: March 24, 2021, 08:39:13 PM »
You're going down the same path I did. The motor shaking can make the problem happen or make it worse, so I'd say it's more about that than dirt. So you might find the problem only happens out of the water, the hull is supported in the water and the water acts as a damper of sorts. I don't see dirt as any of the problem. You should do a pop off test while upright with the bowl on and full of gas, I'd be curious to know what pressure is required to overcome that. The fuel pump puts out around 4-6 psi depending on pump and pulse signal.
Tom

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Offline happul3

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Re: Preventing pump overpressuring carb needle/Fuel return
« Reply #9 on: March 24, 2021, 10:01:52 PM »
I haven't thought about differences in vibration on/off water, thanks for pointing that out.

 I will do pop off test upright/with gas when I put everything back together.

Offline happul3

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Re: Preventing pump overpressuring carb needle/Fuel return
« Reply #10 on: March 28, 2021, 06:15:18 PM »
Found something surprising. Recall the I started this thread after finding that there is gasoline at the bottom of airbox, right where it is attached to carburater. I speculated that the problem is with the needle leaking because of pump (over) pressure. Bowl overfills and floods and gasoline ends up where it should not be. Seemed like a sound theory and Tom recommended fixing the needle seat.

I started with a thorough carb cleaning (it has been awhile anyway) and replacing deteriorated gaskets. After putting everything together, filling with fresh gas, motor started nicely and run for few minutes until I shut it down and examined the bottom of airbox. It was perfectly dry. I had to do something unrelated for a while and when I came back to the motor I examined airbox again and was stupefied by finding  liquid gasoline there. Not just wetness, but a minute amount of actual liquid gas. OK, I thought, maybe I missed it before. Difficult to believe but still. So I dried it out, started motor again, run it for few minutes, and saw no liquid gas there afterwards. BUT AFTER FEW MINUTES JUST STANDING, THE LIQUID GAS REAPPEARED. Waited 20 min, dried it, and if few minutes it is back...

The motor was completely off all that time, not moved, not even shaken, yet liquid gasoline somehow finds its way to the bottom of airbox. Incidentally, I also checked the "pop-off pressure" for the needle at that time - needle did not open at 7 psi pressure, so everything seems to be working just fine. The only explanation I can come up with is that the venturi is wet with liquid gasoline when I shut motor off and that gasoline slowly oozes into airbox. But that would mean that the fuel ingress in airbox is perfectly normal and not an indication of some internal problem. Thoughts?

Offline Painlesstom

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Re: Preventing pump overpressuring carb needle/Fuel return
« Reply #11 on: March 28, 2021, 06:25:09 PM »
If the engine is hot enough that can happen, and does quite often, due to the fuel in the bowl continuing to heat up for a few minutes after shutdown. The bowl acts like a coffee pot, the main well is the percolator that fuel travels up and into the carb throat. This is what causes the hot start issues and why improving the cooling is so important.
Tom

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