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post #1 of 5 Old 09-18-2019, 05:10 PM Thread Starter
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What's a safe top RPM for a Briggs 9S502?

So I know it's not a riding mower, my Snapper is running fine for now. I picked up a throw away Weedeater 20" with a Briggs 300 series engine for a couple dollars. I've so far cleaned the fuel tank, rebuilt the carburetor, changed the oil with 5W-30 Synthetic oil, spark plug, cleaned the combustion chamber deposits, replaced the governor springs. It has a manual control lever on the front. You have to bend tangs to adjust the RPM. At idle I've got it running right at around 2000 RPM, midway it runs around 2400 RPM, and full throttle it varies between 2500-3100 RPM. Now the blower housing cover says it's a 9L602 but the original mower manual says it should have a 9S502. So I know the blower housing has been changed because the 9L602 didn't have the pneumatic air vane, this engine on this mower does have a pneumatic air vane. In my Briggs L Head repair manual it says for a 20" blade mower top no load is 3400 RPM. The parts list for the 9L602 and 9S502 says 3100 RPM. Now if it sometimes hit's 3300 or 3400 RPM the way I have it set now is that going to be detrimental to the rods/rod bearings or do I need to drop it a little. I'm guessing it's not possible to keep it at a perfect 3100 RPM. I tried to set it where it maxed out at about 2900 RPM but varies. The engine sounds strong and doesn't miss a beat.

Snapper 3312X6S w/ 12.5HP Briggs engine
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post #2 of 5 Old 09-18-2019, 10:50 PM
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Re: What's a safe top RPM for a Briggs 9S502?

Quote:
Originally Posted by wh23g3g View Post
So I know it's not a riding mower, my Snapper is running fine for now. I picked up a throw away Weedeater 20" with a Briggs 300 series engine for a couple dollars. I've so far cleaned the fuel tank, rebuilt the carburetor, changed the oil with 5W-30 Synthetic oil, spark plug, cleaned the combustion chamber deposits, replaced the governor springs. It has a manual control lever on the front. You have to bend tangs to adjust the RPM. At idle I've got it running right at around 2000 RPM, midway it runs around 2400 RPM, and full throttle it varies between 2500-3100 RPM. Now the blower housing cover says it's a 9L602 but the original mower manual says it should have a 9S502. So I know the blower housing has been changed because the 9L602 didn't have the pneumatic air vane, this engine on this mower does have a pneumatic air vane. In my Briggs L Head repair manual it says for a 20" blade mower top no load is 3400 RPM. The parts list for the 9L602 and 9S502 says 3100 RPM. Now if it sometimes hit's 3300 or 3400 RPM the way I have it set now is that going to be detrimental to the rods/rod bearings or do I need to drop it a little. I'm guessing it's not possible to keep it at a perfect 3100 RPM. I tried to set it where it maxed out at about 2900 RPM but varies. The engine sounds strong and doesn't miss a beat.
Sounds like you might just be getting into tinkering with small engines...
For years, 3600 was the rpms. HP rated at this often too but also a lot ot to with the mowers they were on.
19 and 20 inch blades were common place. Then 21 took over. Now many are 22.
I think blade tip speed and limit of about 19200 feet per minute were more of a concern than rpms or engine.
2 inches make a pretty good deal of difference in FPM at those kinds or rpms.

A small engine like that is fine to run at 3600 all day if you wish or need more up front power or speed and inertia. They don't normally damage themselves until 5800-6200 rpms. I would never run one long term over 3800 but even that in unnecessary. They governor does the work for you and it really doesn't cut much better at 3700 vs 3200.
Most mowers in the past 10 or so years are spec'd at 3150-3350 rpms.

I don't like 5w30 syn in mowers. Even though Briggs "recommends" it in many. I would rather see you use rotella T6 5w-40 diesel, but I prefer standard 30 wt or HD 30 (same thing) or for one you really love...Valvoline VR1 racing 30 with its high zinc.

You really don't need to bother taking heads off and cleaning carbon off.
Makes no difference. You can also run it at higher rpms and keep finger on gov to rev up and run 4-8 ounces of carb spray or berryman's B12 through it OR even water steam cleaner trick to cmean the heck out of the combustion chamber and, head, valve, and top of piston but it really isn't needed.

Jsut run some Berryman's, Seafoam, MMO though with the gas over a longer period of time.

Not sure about the low medium and high speeds.
Must be older than 2000-2001 to have a throttle control.
Most that did only had slow and fast.
Slow is typically about 1200-1500 maybe faster and fast is the full 3300 or so.
Running a small engine like this at slow speeds is more of a risk than it is worth.
Many have lubrication issues at slow speeds.
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post #3 of 5 Old 09-19-2019, 04:30 PM Thread Starter
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Re: What's a safe top RPM for a Briggs 9S502?

Not really new to working on small engines just haven't had many to work on. I usually work on vehicles. I sold all couple of hobby vehicles so now I use my spare time to work on these couple mowers, which I actually get some use out of too. They don't just sit around. Lot easier to work on than a vehicle engine bay. But it sounds like I've got the RPM set right then on this little 300 series. I just always take the head off and clean the carbon off. This one had the most built up carbon between all the mower's I've ever messed with the past few years. I was just wondering about the top RPM because I'm not exactly sure what model number this engine is because the blower housing has been changed. So it is not a 9L602 because that didn't have the pneumatic governor. In the Weed Eater manual it says the original engine model was a 9S502 which according to Briggs parts list did in fact come with the pneumatic governor. It's parts list is exactly like the engine I have. The parts list for the 9L602 was similar but without the pneumatic governor and some other different parts. Any way it's running good. Actually better than my Snapper. I haven't tested it mowing because my grass is already cut so I may try it this weekend over where my Snapper is stored.

Snapper 3312X6S w/ 12.5HP Briggs engine
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post #4 of 5 Old 09-19-2019, 09:46 PM
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Re: What's a safe top RPM for a Briggs 9S502?

If I understand correctly the one you're running now has an air vane Governor. Those are good little engines and I like the simplicity of the governor but I do like the other style Flathead or Briggs calls it an L head engine better. It dominated mowers for about 20 years and has a very, very good durable engine.
I was just commenting about removing the head to remove the carbon that if you do these often it's really just a waste of time and possibly a head gasket to remove the head and clean it. The head gaskets used to be $6.48 but some of the plain old arrowheads are now 15 to $20. Unless you order them online because prices have just started to come down this season. But still the time involved is really just wasted because it makes no difference in the running or the overall life of the engine.
You should, however, always check or Torque the head bolts as a bottom right corner 3 will always be loose.
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post #5 of 5 Old 09-20-2019, 10:29 PM
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Re: What's a safe top RPM for a Briggs 9S502?

Just dose it good with seafoam. Run. Let sit. Repeat. Rpeat.repeat.
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