Fixing Briggs and Stratton 6 HP Quantum lawnmower engine

[Snagglefoot]

I recently solved the no-start issue on a 6 HP B&S mower engine. It’s 20 years old and was the series when they used a red bulb to prime the engine rather than have a real choke assembly. This engine is overly susceptible to a no-start condition due to old gas and the carb can get clogged up easily.

The red bulb actually pumps air to cause a venturi effect to bring gas from the bowl into the carb and uses a large plastic plate assembly to house the bulb and priming circuit.

You tube videos suggest doubling up the gaskets between the carb and plate, and removing the bulb assembly and cleaning it up. Those things did not work here. I also removed the bowl and sprayed the carb with carb cleaner with no improvement.

The easy way out was to order a carb kit made in China that came with a new carb, plate, fuel hose, shut-off valve, gas filter, air filter, governor springs, spark plug, blow by hose, clamps, heavy duty air filter cover all for $30 US. It runs fine now, and has the shut off valve and fly screen filter installed. Incidentally, this is for the widow of the LEO who has a place down the street. Here are a few shots. Enjoy.

 

[Sequoiasoon]

It is sad that it is just easier to replace than repair.

I had the same answer from local shop on maybe a snow blower or thatcher. I inquired about a rebuild kit and looked on online. Parts kit was like $50 with shipping, Shop said the same plus labor and that they pretty much just get new for less cost. Same deal about $30, came with everything including new lines, a couple bolts and ran great.

Nice job and good karma for you helping out.

 

[fantastic]

You're a real gem to put a fuel filter on this. Bugs me these never came with one to begin with. That baffle on the new one really helps to line up the gasket it appears. I honestly didn't know how long these Brute's have been made. I convinced my Dad to buy a self propelled push mower & these were being sold at our local Walmart that is a 6.25 HP. He put many hours on this & then gave it to my brother which knows how to ruin everything. Not pictured is a 2014 6.75 HP Brute I bought from Menards brand new & gave it to my son for his property a few years back. Plan on restoring this one below the best I can afford to eventually. ha! Again, I didn't know these were made 20 years ago. cool & good job!

 

[Gyro Gearloose]

I had the same problem with a barely used 25 year old mower my dad bought. A new carb made it run like a champ but it still required a spritz of ether to get started. I sprung for a complete new primer bulb plate for $20 and that fixed it even though I couldn't see any problem with the old one.

 

[Fattylocks]

The easiest fix is usually to clean the carb.
Five minute job.
Weren't you working on this mower last summer?

 

[Snagglefoot]

The easiest fix is usually to clean the carb.
Five minute job.
Weren't you working on this mower last summer?

Yup. The primer stopped working. A shot of starting fluid or carb cleaner ( for combustion) would get it started but the lady’s kids weren’t capable of that.

 

[Snagglefoot]

Looks like I’m getting some variation on the repeatability on the red bulb air pump. Some you tubers have mentioned to control the torque bolting the plastic plate to the carb, because flexing of the plastic plate ruins the seal agains the carb. I’m thinking of backing off the torque and using an in - lb torque wrench to make sure all three bolts get the same amount. Anyone know what’s the proper value?

 

[Gyro Gearloose]

Snug it up good with a nut driver seems to work well. Whatever in lb that translates to.

 

[FordFETruck]

I have had lots of those Briggs quantum primer/air filter bases warp resulting in no primer action. Sometimes doubling the gasket up will fix them and do NOT over torque the screws, you can ruin a brand new base plate right off the bat by doing so. I've had to use 2 gaskets on a brand new base plate as well to make it work. I much prefer the ready start auto choke on these. The very first quantums were manual choke and had a metal base plate, they should all be made of metal this problem wouldn't exist at all, but I guess if that's what it takes to sell parts.... A few years later they went to the primer style but it wasn't like this, it was a big primer button that mounted to the side of the fuel tank and had a small line that went to the port on the carb if I recall correctly, that style worked well.

 

[50yr Street Mech]

Had the same engine. All I did was double the backing plate gasket and it worked fine. Yes, those plates flex way too much causing air leaks. Crappy design. Take the breather tube off the backing plate and let it blow. Put a rubber cap on the backing plate. It will save you're air cleaner. The engine doesn't need it. Did it start rod rappin yet ????. Usually louder under load. That's the 1st thing it will do when the engine is on it's way out. Just add heavier oil or STP blue bottle to the 20 oz oil fill. 20W50 or 30wt with blue bottle. That will keep her going a few more years. Frequent spill and fills will dramatically increase life. Sharp blade will reduce drag on engine too.,,,adding to engine life.... Those flatheads can go a long time if you maintain them. Best of luck with the double gasket. they are a PITA to set just right. Run some stabil fix in it every so often to clean carb and head. Works well

 

[FordFETruck]

Had the same engine. All I did was double the backing plate gasket and it worked fine. Yes, those plates flex way too much causing air leaks. Crappy design. Take the breather tube off the backing plate and let it blow. Put a rubber cap on the backing plate. It will save you're air cleaner. The engine doesn't need it. Did it start rod rappin yet ????. Usually louder under load. That's the 1st thing it will do when the engine is on it's way out. Just add heavier oil or STP blue bottle to the 20 oz oil fill. 20W50 or 30wt with blue bottle. That will keep her going a few more years. Frequent spill and fills will dramatically increase life. Sharp blade will reduce drag on engine too.,,,adding to engine life.... Those flatheads can go a long time if you maintain them. Best of luck with the double gasket. they are a PITA to set just right. Run some stabil fix in it every so often to clean carb and head. Works well

I've heard lots of those quantums sound knocky from new, guess some are built looser than others. Never thought much about the PCV clogging up the air filter on those, they're always nasty packed full every one I work on. My 1967 2.5HP Briggs on my sidewalk edger's PCV is set up like that, just like a road draft tube. I'm not sure when they started plumbing it back into the carburetor. They're good engines, but still not as durable as the old 92000 series 3-3.5HP flatheads.

 

[50yr Street Mech]

Just threw away a 1972 3hp Briggs. push mower. Was running on pure STP blue bottle. Smoking / rappin / noisy thing. I beat the crap out of it, and it would not throw a rod. Neighbors hated it. Costs more in blue bottle than gas. So, I said " time to go bye bye". It was trash picked 10 yrs ago. Little 20" blade on it. Great for getting into tight spots ....Don't make em like that anymore. The engine color was white. Nothing plastic in that engine, guaranteed !

 

[97prizm]

I recently solved the no-start issue on a 6 HP B&S mower engine. It’s 20 years old and was the series when they used a red bulb to prime the engine rather than have a real choke assembly. This engine is overly susceptible to a no-start condition due to old gas and the carb can get clogged up easily.

The red bulb actually pumps air to cause a venturi effect to bring gas from the bowl into the carb and uses a large plastic plate assembly to house the bulb and priming circuit.

You tube videos suggest doubling up the gaskets between the carb and plate, and removing the bulb assembly and cleaning it up. Those things did not work here. I also removed the bowl and sprayed the carb with carb cleaner with no improvement.

The easy way out was to order a carb kit made in China that came with a new carb, plate, fuel hose, shut-off valve, gas filter, air filter, governor springs, spark plug, blow by hose, clamps, heavy duty air filter cover all for $30 US. It runs fine now, and has the shut off valve and fly screen filter installed. Incidentally, this is for the widow of the LEO who has a place down the street. Here are a few shots. Enjoy.

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On some Briggs&Strattons the carburetor jet has three jets. A starting jet low and high speed jets. A retired guy the next block over showed me that he cleans the jets with a welding tip cleaner tool. I've also had the gasket fall apart on the plastic housing that your primer bulb is attached to.

 

[50yr Street Mech]

Surprised doubling up gaskets didn't work. Maybe plate was too warped ...It happens. I got a few of those old quantum flathead mowers. They can be finicky to start cold. Gotta keep pushin the primer bulb till it warms up a bit. Had that bulb gasket fall apart on me also. Poor design. But they run, some rod rappin eventually, but they run well. Their a breeze to work on too.

 

[FordFETruck]

Surprised doubling up gaskets didn't work. Maybe plate was too warped ...It happens. I got a few of those old quantum flathead mowers. They can be finicky to start cold. Gotta keep pushin the primer bulb till it warms up a bit. Had that bulb gasket fall apart on me also. Poor design. But they run, some rod rappin eventually, but they run well. Their a breeze to work on too.

I've worked on several with the ready start auto choke, much better than the primer bulb system on them. Simple and they usually always start first pull and the primer issue doesn't exist. If you get one the primer doesn't want to work on, take one of your hands and push inward on the entire air filter cover while priming it will usually make it prime lol.

 

[FordFETruck]

On some Briggs&Strattons the carburetor jet has three jets. A starting jet low and high speed jets. A retired guy the next block over showed me that he cleans the jets with a welding tip cleaner tool. I've also had the gasket fall apart on the plastic housing that your primer bulb is attached to.

Those torch tip cleaners work great, and can also be used to enlarge the jets if you desire to. Although, you're not supposed to do that but I've had good luck with it on small engines, wouldn't do it to an automotive carburetor.

 

[50yr Street Mech]

Yes, the auto chokes work, for awhile anyway. Then the spring inside gets shot from moisture. Kinda like the old chokes on car carburetors back in the 70's. One way or another, I get them going. Once they warm up, their fine..I hit the primer bulb and just keep feeding it that way until it picks up on it's own, or starting fluid if it's really cold out and I want to mulch leaves...They get movin eventually.

 

[FordFETruck]

Yes, the auto chokes work, for awhile anyway. Then the spring inside gets shot from moisture. Kinda like the old chokes on car carburetors back in the 70's. One way or another, I get them going. Once they warm up, their fine..I hit the primer bulb and just keep feeding it that way until it picks up on it's own, or starting fluid if it's really cold out and I want to mulch leaves...They get movin eventually.

I've never really had a problem with one of the auto chokes, the one my sister is using is 12 years old now it still works perfectly. They say to not oil them, but I've loaded them up with ATF and cycled the thermostat a bunch by hand that always frees them up if I come across one that's a little stiff or gritty. The thermostat on them is only to hold the choke off when its warm, the small spring that stretches across the top of the carb to the air vain is what controls the choke on initial startup on the quantum's anyways.

 

[nbpt100]

The Quantum was really the best walk behind mower engine Briggs made. IMHO. Plenty of power and they are robust. Good ones are a desirable in demand engine today. I only seen them die when someone never changes the oil or runs it on low oil. Or someone just leaves it outside in the elements for years. The Primer issue people have talked about above was one of their flaws. Double or triple gasket can sometimes work. People have straightened them with a heat gun using very light heat. Some use an appropriate RTV around the primer air passage way and place the gasket on top to create a permanent seal. There are work around methods to buying a new housing. Yes, Auto choke carbs are a step up. The problem they can have is grass & debris build up or mice build nest inside them and then they can not work. People who think they know more than they do are the second biggest problem. I do not understand why some people think they can modify it to fix a no start issue. Just clean the darn carb and use better gas. If the spring is broken or lost, just replace the **** spring instead of trying to McGiever it. The thremostats are pretty robust if they are set up right and not abused. Again, a big problem is people, abusing their equipment. You can always start them cold with starter/brake/carb cleaner fluid if you are stuck. Warm starts are not a problem. Things are the way they are now. I just accept the newer Briggs engines are inferior and we have to live with that.

 

[FordFETruck]

The Quantum was really the best walk behind mower engine Briggs made. IMHO. Plenty of power and they are robust. Good ones are a desirable in demand engine today. I only seen them die when someone never changes the oil or runs it on low oil. Or someone just leaves it outside in the elements for years. The Primer issue people have talked about above was one of their flaws. Double or triple gasket can sometimes work. People have straightened them with a heat gun using very light heat. Some use an appropriate RTV around the primer air passage way and place the gasket on top to create a permanent seal. There are work around methods to buying a new housing. Yes, Auto choke carbs are a step up. The problem they can have is grass & debris build up or mice build nest inside them and then they can not work. People who think they know more than they do are the second biggest problem. I do not understand why some people think they can modify it to fix a no start issue. Just clean the darn carb and use better gas. If the spring is broken or lost, just replace the **** spring instead of trying to McGiever it. The thremostats are pretty robust if they are set up right and not abused. Again, a big problem is people, abusing their equipment. You can always start them cold with starter/brake/carb cleaner fluid if you are stuck. Warm starts are not a problem. Things are the way they are now. I just accept the newer Briggs engines are inferior and we have to live with that.

I've seen a lot of quantums die (my neighbor with a small yard has somehow killed off many) they start sounding more and more rattly every year until they kick the bucket. I like them, but they don't seem near as robust as an old 92000 series engine as far as being hard to kill off. I just finished working on a Toro with a 149CC Kohler, that's the second one I've worked on but the first one was older and had a vacuum pull off diaphragm and then the choke thermostat on the muffler took the choke off the rest of the way. That one worked great I remember that.

This newer one (2018) has a thin metal slotted flat bar that connects the choke shaft to the throttle shaft. Engine off, throttle WOT, choke fully closed. Engine starts, throttle closes and it pulls on that bar and opens the choke up a bit as a choke pull off then the thermostat on the muffler takes over. I gotta say I think that setup is better than the Briggs ready start auto choke and both are LIGHT YEARS ahead of the Honda auto choke, talk about garbage. I still never seen a better lawnmower carb than the Briggs pulsa jet auto choke though. I have several of those they never give any issues, stale gas no issue, full of debris no real issue. They only quit working when the carb and tank need sanded flat for the diaphragm to seal. The one my dad bought brand new in 1977 has only been rebuilt 1 time and that was 17 years ago. First pull just about dang near every time year after year, decade after decade.....

One thing I will say is those quantums especially the newer ones really seem to like 20W50 for some reason, they sound so smooth and quiet with it. Perhaps 15w50 would be even better. I've run the 20w50 in my sisters quantum for 5 or 6 years now never had a single complaint with it even bagging leaves in the cold weather.