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Sears SS/16 Onan Carb

6944 Views 13 Replies 7 Participants Last post by  Tractor-Holic
I have a 1973 Sears SS/16 with a ONAN 16 HP twin. I'm having trouble getting enough fuel to the carb. I'm not sure if it's a week pump or if the float is adjusted to low. It's the carb with the intergrated fuel pump, vacume operated.
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Have you changed the fuel lines in the last few years? The ethanol gas will age neoprene lines faster than you'd believe. It's also rough on the fuel pump diaphragms, making the 'rubber' flake off the fabric. Then there's always the ills that it works on the carb itself... The float adjustment would be low on my list of possibilities, the needle & seat would rank higher. You might want to catch any fuel you can & see if you have neoprene granules in it. Good luck!
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:Welcome1: IMO a good way to test it is to hook up a remote tank, set up so it can gravity feed. Just becarful removing the fuel line from the carb. The plastic nipple the line goes on can break pretty easy.
I've replaced some of the lines and the vacume line. I also replaced the filter. The filter won't completely fill, I've tried the original filter and a slightly larger filter both you can see through. At first I thought I was getting vapor lock. I have to keep the chock at half to keep it running thats why I'm thinking float.
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You may also be having trouble because of the filter. These machines didn't have one installed from the factory. My tractor has a B43M Onan engine, with the same diaphragm fuel pump, and after installing a fuel filter, I couldn't get it to run after it sat for a while. I removed the filter and it ran just fine. The restriction of the filter is just more than the Onan's pump is designed to pull, especially with the long line to the gas tank under the seat.

I'd try removing the fuel filter and joining the hoses with a union. If it runs fine, you've solved the problem. Please keep us posted on what you find.
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+1 with Loremaster. There are different filters used on applications with gravity feed or pumps due to their restriction. All manufacturers I am aware of don't recommend ANY filter between the pump & tank, only on the pressure side of the pump. I guess if your pump is integral with the carb, that's out too. A length of stocking stretched over your fueling funnel is about as good as you're likely to get. Crude, but effective.
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Loremaster and H-D,
Sears parts breakdown for 917 25950 shows a filter.
Fuel filter
Don't quite know what to say, other than my Dad bought our SS16 new in 1973, and it never had any external fuel filter. We also did the engine swap after the first engine burned up. I just replaced the factory fuel line a few years ago when I brought it out of storage to start using it. Maybe they added it on later models. I'd still try removing it as a diagnostic tool at the very least.
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What I'm going to try as well is pull the tank and check for dirt blocking the hose connection and replace the rest of the hose. I'm also going to re-check the float and the needle valve. If that don't work I'll replace the pump with a 12 volt electric low pressure pump.
It could be you have a vacuum leak at an intake gasket or a crack in the intake,etc..that'll make it lean and need the choke closed up some to compenstate for the additional air getting in the engine without being drawn in thru the carb..

I would think a fuel filter with a screen rather than a paper element wont present any excessive restriction no matter if its located upstream or downstream from the fuel pump,,I've had issues with paper elemnt "car" filters not letting sufficent gas flow by gravity feed or with feeble diaphram fuel pumps..with an electric one they work fine though..(and one will help reduce the pressure a bit and make things easier for the needle & seat valve).
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Ok, I learn something new again. I think you're on the right track changing out ALL of the fuel line. The auto manufacturers have had to switch from neoprene to a fluoro-elastomer to handle the newer fuel & it's expen$ive. Any fuel line that is immersed in the E-gas should be of that material. Aged parent's tow tractor is behaving just like yours; requires choke to run. It's been down for months (transaxle) & we're just beginning to seek the fueling issue. The E-gas WILL have detrimental effects on the pump diaphragm as well. Small Fry's suggestion would take out a lot of the possibilities & quicken the trouble shooting.
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Mount a 12 volt two p.s.i. Mr. Gasket fuel pump in line up under the frame, make sure to mount a fuel filter in line BEFORE the pump so no dirt accidentally ends up in the pump. Then remove the diaphragm from the pump on the carb, plug the vacuum port on the engine and carb, wire the pump to the RUN terminal on the key switch, and your done. Plus, this will eliminate hard starting if you let the tractor sit up for a while; because when you turn on the key, it will "prime" the fuel bowl. Just remember, electric pumps are much better at pushing gas uphill, not pulling it, so try to make the pump the lowest point of the system.

Good Luck,
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Old thread but this answers my issue with low pump pressure in my Sears / Onan . Great info , Thanks .
I highly discourage the use of the Mr.Gasket electric pumps,they are junk in my opinion..
I have had two go sour on me in short order,also a few friends who tried them had them fail very quickly...I suspect the plastic coating they appear to be dipped in to seal out moisture makes them run hot and burn out inside ..

These types of pumps I've used work better and last--one of the round ones has been on my diesel pickup since I bought in in '03 and was there before that--however,I bought an identical unit to use on my Sears Suburban's HH120 and it lasted one summer,then after I let it sit a few months it died a few minutes after I started it--no gas coming out,and it felt hot,like it had shorted out inside--I think the round pumps have a rubber bellows for the pump that doesn't like ethanol--the Facet square looking pumps have a nylon ball operated by a solenoid to do the pumping,which is much more resistant to ethanol,dry gas and fuel additives.


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