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In the never-ending fight for truth, justice, and efficiently running 4-cycle engines, the newest arch nemesis of mowers and small lawn equipment everywhere has reared its ugly head:  ethanol!  As crude oil prices rise, gas companies are trying to cut costs and encourage renewable energy sources by adding ethanol to their gasoline mixture — up to 30% by volume in some parts of the U.S.  For an easy way to combat the negative effects of ethanol in your small engine, you can install an inline fuel filter.  Follow the jump to learn how.

First, I’m sure many of you are wondering why ethanol is so bad for engines in the first place.  Well, it’s actually not the ethanol itself that’s the problem, it’s what the ethanol has a tendency to attract — water — which is a big no-no when it comes to small engines. On a molecular level, ethanol and water attract each other like magnets, causing the ethanol to suck water into the fuel lines of 4-cycle engines when there’s even the slightest humidity in the air.

You can easily remedy this by adding an inline fuel filter to your 4-cycle engines. Most lawn equipment uses a 1/4″ fuel line, and the standard size fuel filter from Briggs & Stratton will fit the majority of the models on the market. The fuel filter blocks all water as well as any impurities in the gas before it gets into the engine.

Installation is easy:  Simply find the fuel line running from your gas tank into your engine intake port, and cut it cleanly with a sharp utility cutter. Install the fuel filter between the cut, with the smaller-diameter side facing the gas tank and the larger-diameter side facing the intake port.  Secure it in place with small spring clamps on each side, and you’re done.  Of course, you want to empty all the gas out of the gas tank and fuel lines beforehand.

This $5 worth of parts and five minutes of your time can greatly increase the lifespan and efficiency of your 4-cycle engines.  You can find all the parts at home improvement stores or online.

Inline Fuel Filter [Briggs & Stratton]


23 Responses to How-To: Install Fuel Filters On 4-Cycle Engines

  1. Jim German says:

    There is no way that filter is going to be filtering out any water. Its a 150 micron filter, which will do a great job filtering out dirt and twigs that might get in when you’re filling the tank, but its not going to do anything about water.

    And really I find it incredibly hard to believe that ethanol is going to have any negative effects on a small displacement, properly maintained engine.

  2. Scott says:

    I thought it was just me. The link between this filter and water escapes me.

    If the filter did trap water, eventually it would reach a saturation point and either begin to pass water or block any liquid flow.

    If water contamination is an issue–and I am not saying it isn’t–I’d think it was a better idea to add a dose of fuel drier (isopropyl alcohol, mostly) to the tank on fill-up.

    If the problem is debris in the fuel, a filter funnel and this sort of filter on the engine may be a good solution.

    • JD says:

      “If water contamination is an issue–and I am not saying it isn’t–I’d think it was a better idea to add a dose of fuel drier (isopropyl alcohol, mostly) to the tank on fill-up.”

      the fuel already has alcohol in it- roughly 10% ethanol. It not only distributes the alcohol in the fuel, it attracts it to the fuel as well. And over time, you can get “phase separation” which will render your equipment useless.

      I do take exception with this article in one respect, a typical Briggs fuel filter will no filter water out. However, there are filters that will.

  3. Toolhearty says:

    I have to agree with Jim. I’m pretty sure you have to get down to the 10 or 20 micron range before you start filtering water, and that’s ony when it has separated from straight gasoline. Add more ethanol (an alchohol, keeps water in suspension) and even the water can pass through a very fine filter.

    I don’t see ethanol, or even small amounts of water, as being a problem for most small engines. A fuel filter can’t hurt, but this one won’t work for the intended purposes.

  4. Toolhearty says:

    Scott: I’m pretty sure adding fuel drier (another alchohol) is the same as adding more ethanol. You’re not getting rid of the water, just keeping it in suspension (preventing it from separating out). It’s when it is separated that it freezes, which is why most people use fuel drier (for winter).

  5. rick says:

    bah, its not pulling water out. My diesel car has a water seperator built into the filter, but thats a seperate issue (and i have never emptied it after 250k miles on two different diesels cars.

    but this would be great for dirt dust and other junk that easily gets into the tank.

    How many people have had problems with debris in engines. I am sure a lot, i am not one of them though. (yet)

    good idea, wrong explnation.

  6. Sean says:

    On another note, other than the water/ethanol/filter issues being correct/incorrect in this article, notice that the power washer pictured above this article about gas engine maintenance, is electric… kinda funny…

  7. Fred Boness says:

    I am fortunate that I can still get premium gas without ethanol. That works like the gas that was good enough for your father. I have been running all my yard equipment on that premium gas.

    Premium ran better in my Pathfinder, too. I’ve sidestepped the ethanol gas problem there by getting a Diesel Jeep Liberty. That has its own problems but, we will learn to cope.

    I don’t think I want to try a Diesel lawn mower.

  8. Toolhearty says:

    Fred Boness Says:
    …Premium ran better in my Pathfinder, too.

    …and my Ranger runs better after I wash it. 🙂

  9. Penumbra says:


    You really need to update this post. As others have pointed out, an inline fuel filter will only remove particulate matter. Fuel-water separators will do the trick, but you aren’t likely to want to put one on a small motor.

    Many older lawn mowers and tractors had a small glass bowl with a screen on top. Although not recommended anymore for safety reasons, they do work fine and do the job well. However, they don’t function unless upright, and hence aren’t a good idea for most small gas motors.

    Please update your post to reflect this information. You’ve given a good tip (fuel filters are rarely a bad idea), but this is misinformation.


  10. MeasureOnceCutTwice says:

    I’m not so sure it is misinformation. I’ve got a Coleman filter funnel for camp stove fuel, and it claims to remove water. I tried it, and sure enough the white gas goes through just fine but water will bead up on the top of the filter. I think it has something to do with not being able to wet the material.
    I sure wouldn’t bet against it working.

  11. Rob says:

    MeasureOnceCutTwice why do you think the two filters are comparable? Fuel/Water separating filters for aircraft and boat use are typically about 100 microns. The specs on this cheap B&S filter says it filters to 150 microns.

    I agree with everyone else here that says the author shouldn’t be making claims he can’t back up. Isn’t that the essence of good journalism?

  12. Yuppers says:

    Doesn’t STABIL fuel additive keep water from coming out of solution in your gas? I add it to all my gas.

  13. Kevin Pace says:

    Well, it’s very nice to see such an in depth conversation about my article. Makes me happy knowing my stuff is being read. I would like to address a few points.

    After the first few posts earlier today, I decided to call my contact at Briggs & Stratton to see if he could answer a few questions for me. The bulk of the information used in this article was pulled from a engine docket I received at work that was assembled by Briggs for us. My contact did confirm for me that all the information contained in the docket was factual. According to Briggs & Stratton, this 150 micron filter will indeed filter the water trapped in your gas tank before it hits the carburetor. While the technical aspects of it escape me, his explanation revolved around the joint workings of both the filter and the shape of the device. Either component alone would not be able to filter water, but the combined aspects of it will. Should you wish to further dispute this, I would recommend contacting Briggs.

    Secondly, thanks for pointing out my mistake on the picture Sean. I Googled pressure washer and saw a gas one but wouldn’t you know it, clicked on the wrong picture and ended up with an electric. I would change it but this brings me to the third point.

    Once an article is posted, the authors can’t edit it. Only the Toolmonger editorial staff can so if you’d like to see something changed they would be the ones to contact.

    Thank you all very much for the comments. I learned a few interesting things today I did not expect to find out when I woke up and I look forward to learning many more as I continue submitting articles for this wonderful resource.

    • Tony says:

      Hey Kevin,

      I normally don’t post comments, but I felt inclined in this instance. I know you posted this a decade ago, but I came across it after buying some aftermarket Briggs & Stratton clone red fuel filters. I was wondering which direction to install it due to no “arrow” present.

      I bought the 150 micron filter over the 75 micron filter due to concerns of early fuel filter constrictions due to contamination. I understand these are usually recommended for gravity feed motors compared to fuel injection engines.

      Your comment on the filter separating water intrigued me as it did others. Well I have 10 aftermarket filters sitting right in front of me. So, I decided to put it to the test. But, surely these aftermarket filters will probably fail by not being B & S specific. Before I proceeded with the water test, I looked in the filter in both directions and could see only a metal screen and then I blew in both ends to make sure of no restrictions. I took a medicine dropper full of water and squirted in the smaller diameter side. 1 squirt and no water came through, 2 squirts and shook it and could see the water in it and looked under in the exit and could see the water moving on top of the metal screen. After 3 squirts the water overflowed from the top of the fuel filter. No water came through.

      My conclusion is that it works for separating water like you originally stated. I installed these filters on 5 gravity feed engines with no problems. I can definitely say they work with these aftermarket fuel filters with separating water from fuel. All I can say is that it’s cheap to go down to your local parts store and buy one and test it for yourself before commenting further because I didn’t really believe it either but now, I am a believer.

      Thank you Kevin Pace for the information and knowledge about these seemingly overlooked small red fuel filters.

  14. Phil says:

    I won’t point out that the pressure washer shown in the lead-in photo would not benefit from this modification due to it’s being powered by an electric motor. 😉

  15. Jim German says:

    Sorry Kevin, the Briggs guy is feeding you a load of crap, or is talking about a different filter. Ignoring the fact that the filter has no “special shape”, and that I find it extremely hard to believe that a 150micron filter would pull water out of gas, whats going to happen too all this water? There is no way for it to drain off, or evaporate.

  16. Zathrus says:

    @Fred Boness:

    Putting premium gas in your 2 or 4 cycle small home engines is a bad idea. They simply don’t have the combustion ratios needed to fully burn the fuel, which leads to numerous problems.

    As for your Pathfinder — unless the engine was designed to use premium fuel, it didn’t run any better either. You just convinced yourself that paying more was worth it.

    My Maxima recommends premium fuel, but it runs just fine on 87 octane regular. No difference in mileage either. Sure, I sacrificed some power, but since I’m not running it at 110+ mph anyway, or flooring it off the line, I really don’t care.

  17. FredP says:

    Higher octanes allow an engine to run higher compression ratios without knocking occurring. Period. That’s it. Nothing else. The only way you can get more power out of higher octane gas is if you run your engine at at a higher compression ratio, and that’s not the way 99.999% of the cars made today run their engines.

    With the exception of truly high performance (Lamborghini, Maserati , etc) cars, you won’t see any change at all in the performance of your vehicle with higher octane gas. The Pathfinder didn’t get any stronger on high octane and the Maxima didn’t sacrifice any power on lower octane.

    Only a sucker or the ignorant purchase high octane gas for normal vehicles.

    Here’s some light reading for those seeking to educate themselves. In particular, note the government website at the bottom. It’s the Federal Trade Commission, and among other things they say, “Unless your engine is knocking, buying higher octane gasoline is a waste of money, too.”:




  18. Bama Cracker says:

    Some of you guys are obviously very smart, but you seem to miss the concept of a fuel filter and water. . . the larger diameter of the filter allows the fuel to move more slowly and the water can fall out. Toyota used to have a small, clear fuel filter mounted on the firewall. You could see the water dollected in it, and when the water got deep enough, the car would hesitate or die going up hills. Yes, there are filters that will actually “seive” water from fuel, but they are expensive and clog easily. But a small bulb-type filter will capture enough water to prevent the engine from sucking water into the cylinder.

  19. Tim W says:

    Guys water separation is not a function of filter micron rating. for the most part until the extreme are separate independent functions. I actually have worked in the diesel industry and had countless hours in tech discussions with top filter tec from a number of the top filter makers.

    Just think about it for a second first off. Filter mesh size is to catch debris only nothing else. I can push water thru a .02 micron filter. Look how small that is and it will not stop a lick of water. Think of reverse osmosis water filters that will take out all kinds of small matter but let water pass thru. In fact these are used in the pharma industry as they will catch all bacteria etc but water oil solvents pass right thru. So lets end the misinformation about filter size having anything to do with stropping water as it does not.

    If you need the correct info go to a filter site and such as Donaldson, Luber Finer (champion), Baldwin, Cummins and read the tech papers on how water sep works ( diesel is where you will find the most info)

    It works via a coating on the filter material creating a hydrophobic barrier ie repels water. Now sure I guess you can get to a filter size so large it would pass thru but this is even higher than 150 mic. BTW I just picked up said B&D filter from Lowes and its rated @ 75 mic according to the packaging. I will not even get into how the rating work today as its not what 99% of you think. A 150 mic filter does mot necessarily mean it will stop 100% of anything that size or larger. It usually is a mean average per pass thru the filter thus anything where from 2%-50% at the rating may actually pass thru. You want to do some real reading on that if you looking for filters for say your aircraft or truck diesel engine. You want the absolute rating which is 98% or better.

    Anyways back to the water filter and statements made. You think that filter size is way to small. Really!?! For what use exactly. Take a look at the inline filters on cars how big are they now take a look at the filters on diesel engine in small cars. Now consider how much fuel in ratio is used ( flows thru) those in a given period of time versus a small little single cylinder 1 or 2 hp engine. You may use 2-3 gallons in a all day job. How much fuel goes thru a car engine in that time? Its all in ratio. SO you have a car diesel engine water filter that has a 2 cup volume and is suppose to be drained or replaced every 25K say 30 mpg or every 800-850 gals of fuel used that passes thru it in a single pass ( many have recirculation so the fuel passes over and over and it sits bring more water in so its actually way more depending on driving habits vs time.)

    Now little pressure washer pump that gets on average really likely at most 5-30 hours of use per year ( these are home not commercial models after all) and a filter that should be changed out either when it no longer flows or best every season ( its not exactly $$$ ya know) So every year as part of the prep you change the filter blah blah blah. So 5-30 gallons or 0.5 % ie 0.005 ratio. Now that the ratio and apply it to the filter volume and size. That little disc is not so small after all is it!?!

    You have to be careful about applied basic level logic to technical areas as unless you do the research to understand at least the a rudimentary level you are commenting from a place of ignorance. That is not meant as a insult as I and all are guilty of it plenty of the time. After all ignorance means not knowing or more correctly being uninformed. It is actually a very fitting word that has just been used as a insult its really meaning is somewhat lost on most.

  20. Tim W says:

    Guys on the premium fuel I think the OP used that word incorrectly as he was describing fuel without ethanol added not a fuel with tech a higher octane rating how effect is basically what has been posted. Ethanol other than cold winters basically is a way for them to make cheaper fuel and not have to maintain it as well. Just a FYI here in the USA we have some of the worst quality of fuel in the 1st world countries. This is especially true of diesel. Its worth it to the oil corps to just pay the fine. But in terms of ethanol it has a lower btu or energy (heat) and thus takes more more so mpg suffers. Its hard on all the hoses and thus they now have to be made of more $$ materials. But its dirt cheap compared to gasoline and the normal additives. In fact its only because of computer controlled engine systems they can get away with it above 10%. See how bad a carb run engine would work setup for no ethanol and then given say a 25-30% mixture. Its going to run weak and have to be rejetted to all more flow etc. You can see this with vehicle down i south america where the main fuel source is pure ethanol. They have to flow much more fuel but its cost makes it much much cheaper as they get it from their crops.

    Point being if we are talking reg premium gas ie higher octane rating unless you are running a higher compression and to the same effect certain turbo engines its basically worthless. But if its about ethanol as I think the OP was meaning what he says can be very true. We have 1 or 2 stations that still offer ethanol free gas and I always use they from all my small engines. IF you check Stihl (the top chainsaw manf) they actually do not want any ethanol fuel and even sell bottle fuel (way $$$) if you can not get it. BTW ethanol actually has a higher octane rating or predet rating and thus says them even more as a cheap octane booster of sorts thus allowing them to use even slower grade base stock.

  21. butch says:

    the heck with the fuel filter,how do you replace the fuel line on a 10hp tech motor? it goes behind the flywheel,i am about ready to go buy a new one.i just cannot get the new hose behind the flywheel can any one help pleae

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