Factory RPM's for lawnmower engines

[Engineer]

### Husqvarna Lawn Mower with B&S 160cc Engine Issues and Fixes

I have a Husqvarna lawn mower with a B&S 160cc engine. Recently, it stopped moving and then the engine quit with a puff of smoke from the air filter!

After 9 years of heavy use, I had no patience and time to fix it and bought a Honda lawn mower, which is working fine. However, when I posted in my neighborhood about giving away the old mower for free or as metal scrap, I got lots of calls saying I needed to pay them to take it off my property. Its a great engine and lawn mower that has served me well for the last 9 years actually. Its a Husqvarna LC 221A.

Intrigued, I decided to work on it. Here’s what I found and did:

- **Issues Found**:
- Idler wheels jammed due to bearing failure.
- Belts melted the plastic idler wheels due to friction.
- Blade adaptor spins freely as the key is gone.
- Carb flooded with oil and dirt.
- Flywheel pin crushed but okay.

- **Replacements Made**:
- Replaced the carb, spark plug, flywheel key, blade adaptor, wheels, cleaned air filter and replaced fresh oil. I have always taken care of the lawn mower with regular oil replacements and clean filters. But the jammed idler wheels caused a whole series of issues.

- Checked and adjusted the valves for the first time and it was within the spec of .004". Same engine with different serial numbers but same capacity have different valve clearances. Had to get the QR code on my engine to get the exact specs.

Everything is back to normal except for the engine RPM, which slowed down after replacing the carb. The replacement carb was a cheap one from Amazon; I still have the original, which I plan to clean later.

I adjusted the RPM by pressing the tab that holds the governor spring, but I’d like to know the factory RPM setting. I just got an RPM meter to check it and measure it on my Honda for reference.

If anyone knows the factory RPM for these engines, please let me know. If not, what would be the ideal factory RPM?

Another strange thing in my B&S manual is the oil spec: SAE 30 preferred, 10W-30 for varied temp zones, and 15W-50 Vanguard synthetic for ultimate performance. That’s quite a leap from SAE 30 to 50! I’ve always used Shell Rotella SAE 30, FYI.

Any insights or advice would be appreciated!

 

[GRUMPYDUDE2]

I beleive the factory rpm setting is 3200-3600, I have set mine at 3000 without issue going on 12 years.
I use syn 5-30 or 0-40 oil.
I get great mow mileage!

 

[CarLuver]

i set my BS engines to around 3200 rpm.

 

[atikovi]

I'm no engineer, but doesn't the operator vary the engine speed based on the load? If cutting tall wet grass it's higher so it doesn't bog down?

 

[mk378]

3200 is a good number. As you go higher it may seem to work a little better in tall grass, but it will also guzzle gas and reduce engine lifetime.

 

[thunderfog]

Looks like around 3000rpm or so...

 

[Engineer]

I'm no engineer, but doesn't the operator vary the engine speed based on the load? If cutting tall wet grass it's higher so it doesn't bog down?

Me neither, couldn't come up with a good name.

There is a RPM flywheel inside the engine, which no matter what will try to keep the engine RPM constant. Engine under load, more fuel to the carb, less load less fuel etc.

 

[thastinger]

3400 should be the factory RPM. Your RPMs likely dropped due the to new carb design, the throat on the new carb is probably not as large as your old one was.

 

[Propflux01]

I'm no engineer, but doesn't the operator vary the engine speed based on the load? If cutting tall wet grass it's higher so it doesn't bog down?

Blade length determines engine rpm. It’s a blade tip speed safety thing.

 

[oil pan 4]

I checked the rpm on my trash mower, a Husqvarna 5521 series and I only know that because I was just ordering a part for it like 5 minutes ago. It's rpm appears to be factory set at 2,800rpm. I probably need to turn that up a bit.
I'm going to crank mine up to like 3,400rpm.
Those self propelled Husqvarna mowers are trash I got a free one in 2020 and found all the same stuff you did.
I run whatever oil I have that's open at the moment that ends with 40 or 50.
Lately I have been using Walmart brand "low emissions Diesel" 15w-40 that was on clearance last year for $3 a qt.
The winter grade is irrelevant since I don't mow grass in the winter so a 15w or 20w is fine.
Mowers run hot so they need that thicker oil.

 

[oil pan 4]

I'm no engineer, but doesn't the operator vary the engine speed based on the load? If cutting tall wet grass it's higher so it doesn't bog down?

Most mowers these days run a fixed speed.

 

[Engineer]

Blade length determines engine rpm. It’s a blade tip speed safety thing.

True, but in a 21' push mower who is experimenting with different blade lengths?

 

[Engineer]

3200 is a good number. As you go higher it may seem to work a little better in tall grass, but it will also guzzle gas and reduce engine lifetime.

I did bend the tab roughly and the engine RPM was a little high. I could tell by the sound after using it for 9 years. It mowed and cut grass like a champ and didn't seem to be bogged down. But, I still want it at factory spec.

People say lawn mower engine lifetime is around 7 years and some here have had one for 20 years so I don't know what that means. But now that I put in so much work and time on it, I intend to keep it and will check the RPM today evening and adjust it to 3200 RPM like many said here.

I need to replace wheel bearing next, one of the wheel is sagging to the side lightly. Then wash, ceramic coat it, coat under the deck with slip plate and call it a day.

Just picked up Super Tech HD 30 from Walmart. Looks like its now API SP rated. Will try that and see how that works. Plus need to change oil on my Cub Cadet riding mower anyways.

 

[atikovi]

Blade length determines engine rpm. It’s a blade tip speed safety thing.

If a blade is meant to spin at 3200 rpm, will it explode at 4000 rpm? I mean, the engine will blow up well before a thin piece of solid steel, no?

 

[Engineer]

If a blade is meant to spin at 3200 rpm, will it explode at 4000 rpm? I mean, the engine will blow up well before a thin piece of solid steel, no?

Unless you are talking about a propellor on a plane these low RPM motors cant shred a blade due to RPM. The centrifugal force needed to rip apart a steel blade is very high. Unless you have a crappy blade, and it hits a stone or rock at the right moment where it can shatter.

 

[oil pan 4]

True, but in a 21' push mower who is experimenting with different blade lengths?

I have. An old weak 3.5hp Briggs and Stratton it had a 20 or 21 inch blade. It needed a new blade and I put a 19 inch on there it was both cheaper and thinner than the appropriate length blades available at Lowes.
I say put the thinnest cheapest blade on your push mower so if you hit something it's more likely to bend the blade and not the crankshaft.
Also the smaller blade made that mower a little more difficult to stall out.

 

[oil pan 4]

If a blade is meant to spin at 3200 rpm, will it explode at 4000 rpm? I mean, the engine will blow up well before a thin piece of solid steel, no?

I had one mower the engine ran wide open. I would control the speed by mowing quickly or shutting it off. It ran like that for 3 years till the engine exploded. I'm sure unloaded it was hitting well over 4000rpm and valve float limited the rpm.
The blades on my riding mower are slightly over driven and are hitting about 4,200rpm and they're 23 inch blades.

 

[Engineer]

I checked the rpm on my trash mower, a Husqvarna 5521 series and I only know that because I was just ordering a part for it like 5 minutes ago. It's rpm appears to be factory set at 2,800rpm. I probably need to turn that up a bit.
I'm going to crank mine up to like 3,400rpm.
Those self propelled Husqvarna mowers are trash I got a free one in 2020 and found all the same stuff you did.
I run whatever oil I have that's open at the moment that ends with 40 or 50.
Lately I have been using Walmart brand "low emissions Diesel" 15w-40 that was on clearance last year for $3 a qt.
The winter grade is irrelevant since I don't mow grass in the winter so a 15w or 20w is fine.
Mowers run hot so they need that thicker oil.

I don't know why you call them trash. Mine has served me well for the last 9 years. Picked up on sale in Lowes for $175 back then. Later on it retailed for 450 I think and now Husqvarna has completely stopped making push mowers. Only one they make is 1200 dollars and has a Kawasaki engine. So for 175 bucks? Mine did its job.

 

[Engineer]

I have. An old weak 3.5hp Briggs and Stratton it had a 20 or 21 inch blade. It needed a new blade and I put a 19 inch on there it was both cheaper and thinner than the appropriate length blades available at Lowes.
I say put the thinnest cheapest blade on your push mower so if you hit something it's more likely to bend the blade and not the crankshaft.
Also the smaller blade made that mower a little more difficult to stall out.I

Usually there are two safety mechanisms to bypass the shock. One is a key in the blade adaptor and the other one is in the flywheel.Always buy factory keys when replacing as the softness of the key (Metal) is calibrated for shear. But I have known people who have hit a tree root and bent their crank.

 

[oil pan 4]

Unless you are talking about a propellor on a plane these low RPM motors cant shred a blade due to RPM. The centrifugal force needed to rip apart a steel blade is very high. Unless you have a crappy blade, and it hits a stone or rock at the right moment where it can shatter.

I have ran blades untill they break on my riding mower. They embedded them selves in the dirt or get knocked around and come out the discharge.
I made sure to run the ones that looked like they were about to break on the inside.