Briggs & Stratton Mower Service

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rockcrawler

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Feb 25, 2009
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Melbourne, Australia
My old mower needed a bit of attention again so I busted it apart and did a service on it. Thought I'd share this.

The mower is an '83 Supa Swift with an 110000 Briggs & Stratton motor. Last major service I did was the rings about 5 years ago.

Problems before this last service were;

- Hard to start, had to fiddle with the mixture to get it going and then re-adjust it when warm.

- Once warmed up I had to be very quick in emptying the catcher otherwise it would not start again until cold.

- Stall easily under load.

Ok so on with the show, here it is with the cowl off and then with the head off.
 
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Next thing to check is the valve clearances, you can do this by removing the valve cover. I found my exhaust valve to be too tight, upon removal the valve seat was all sooted up confirming that once the motor was warm and running it was not closing properly.

To remove the valves you compress the springs back and drop the keyhole clips. Easier said then done if you don't have the right tool but it can be done with a couple of screwdrivers.

For the 60000-120000 series the clearances are:
Intake Valve min/max - .005/.007"
Exhaust Valve min/max - .007/.009"
 
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Next I cleaned up the head and removed the valves.

Checking with bearing blue the intake valve was ok but the exhaust valve needed some major lapping.
 
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Got a bit tired with the traditional lapping method after some time so got the drill onto it, this got it done in a couple of minutes. Make sure you oil the valve stem if doing this.
 
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I gave the intake valve a light lap as well and then it was on to getting the clearances right. A little sanding of the ends of the valve stems on emery paper got them both back to spec. This pic shows how the valve spring case gets flooded with oil if these mower are tilted over. It's ok to tilt them but you have to wait 5 minutes for the oil to drain. If you try starting them before that the oil will get sucked through the PCV valve in the cover and into the carb.
 
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Another common problem is the intake tube. This loosens up on the fitting and you get a vacuum leak. Sure enough mine was loose too.
You have to remove the magneto to get this out.

To fix this clean it up, line it up on the cylinder and mark its position. Then put some retaining compound on the tube and tap it home, making sure it is aligned correctly.
 
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The carby will need a service too. Clean it all up and replace the diaphragm.

Pic shows where the dreaded spring and washer lives - on the carby not the tank. Put this on the tank and the mower will run, but not correctly.
 
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Here's a couple of pics of the completed job.

I also did do an oil change after I warmed it up. You should do this yearly.

The trick to starting these older Briggs & Strattons from cold in one pull is to tilt the mower towards the spark plug for a second or so. That primes the carby. There won't be enough oil spilling into the valve spring case to cause a problem if you do this for a second or two. This method does not apply to models with the priming bulb.

To set the mixture, turn it all the way home and then two turns out. This will start it. Then lean it out to max smooth rpm and lean it maybe 1/8 turn more. Do this with the filter off. Once you put the filter back on it will richen up a tad. Make sure you have the air filter screw in the carby when running it without the filter.

The difference between before and after this service was dramatic.
No starting problems - started on one pull.
I got through knee high grass without the motor even straining.
edit: Cold start mixture problem resolved.
Not bad for a 26 year old mower.
 
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Wow rockcrawler
Man this post was like a blast from the past . I remember doing all this stuff when I was in small engine repair class in HS .
Makes me want to go out and start picking up some curbside sale lawnmowers to fix again

thanks for the post
 
Koffer said:
Wow rockcrawler
Man this post was like a blast from the past . I remember doing all this stuff when I was in small engine repair class in HS .
Makes me want to go out and start picking up some curbside sale lawnmowers to fix again

thanks for the post
Click to expand...

Glad you enjoyed Koffer. The only problem with the job was it didn't last long enough. Volunteered to do my mate's B&S next which is out of action while his lawns are turning into a jungle. Definitely fun :)
 
When trying to start it without an air filter, you have to put the air filter bolt back in the carburetor, as you did. Leaving the bolt out bypasses the autochoke.

Do you still have to mess with the mixture screw when starting?
 
Started with two turns out on first pull though running a bit rich of course. Leaned it out a bit as it ran then did the final adjustment when it had warmed up.

edit: just realized what you asked. No I no longer need to fiddle with it anymore, starts all the time without a hassle now.
 
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Briggs Valve job

Rockcrawler, I've been looking for information like your post for a long time. Not only did you have excellent information, the pictures really tell the story and I THANK YOU A TON! I have a 1996 Generac 2400 generator with a Briggs 5 hp just like the one you show. Of course mine has a fixed rpm ( really hate to start an engine at full throttle) and no adjustments.
I tried to start it yesterday and with the air filter removed it was pushing air out of the intake. I did start it with a hand drill spinning it fast enough for it to catch and run. Power output should be 20 amps - I get 11 amps max. It starts hard on a good day. And it's impossible to start when it's cold out. I'd quess my exhaust valve isn't closing, just like you showed. I love little engines and I haven't had reason to mess with one for a long time. I'm looking forward to this! And I thank you again for the information.
 
Soloboss said:
Rockcrawler, I've been looking for information like your post for a long time. Not only did you have excellent information, the pictures really tell the story and I THANK YOU A TON! I have a 1996 Generac 2400 generator with a Briggs 5 hp just like the one you show. Of course mine has a fixed rpm ( really hate to start an engine at full throttle) and no adjustments.
I tried to start it yesterday and with the air filter removed it was pushing air out of the intake. I did start it with a hand drill spinning it fast enough for it to catch and run. Power output should be 20 amps - I get 11 amps max. It starts hard on a good day. And it's impossible to start when it's cold out. I'd quess my exhaust valve isn't closing, just like you showed. I love little engines and I haven't had reason to mess with one for a long time. I'm looking forward to this! And I thank you again for the information.
Click to expand...

Glad this was of use to you. Let us know how you go :)
 
Valve Job

Rockcrawler, It was a good first day of the year! I got enough confidence from your post that I pulled the head and valves from my little 5 horse engine. I initially checked valve closing using an LED in the intake and exhaust ports with the valves closed. In a total dark shop, I saw light 360° around the intake (YIKES!!) - no clearance at all! It's no wonder the carb was spitting gas back at me when I pulled the rope. The exhaust had .003 after I removed the carbon and the valve would actually close. I lapped both valves, set the clearances and repeated the light check. Absolutely perfect.
Tomorrow I'll get gaskets and reassemble the engine. I'll post the results. No need for photos - mine looks just like yours.
Mark
 
Generator Success!!

GENERATOR ACTUAL OUTPUTS PRE VALVE JOB:
Gen - No load = 130v
6 amp resistive load @ 120v = 720w
10 amp resistive load @ 110v = 1100w
13 amp resistive load @ 109v direct = 1400w
additional load just labored the engine so the voltage dropped dramatically as the load increased.

GENERATOR ACTUAL OUTPUTS POST VALVE JOB:
8.5a resistive load @ 129v = 1100w
14.5a resistive load @ 123v = 1783w
21 amps resistive load @ 115v = 2400w

It's back to original specs! It still starts hard, gotta be a vacuum leak in the sucker pipe as you showed.
One last thank you and I'm outa here. Your post made my day!
 
rockcrawler - I just made the last improvement to the little generator. I did in fact have suction leaks at the vacuum pipe that works the fuel pump. Since it's cold in the shed (19°F) I didn't seal the rubber boots but I did put little wire ties around the boots so they gripped the aluminum tube. And then tilted the little generator toward the carby. And it started on the first pull. And to think that a week ago it wouldn't start even when I used the drill to spin the motor!
 
Really glad you sorted out your generator Mark. B&S motors are amazing, they seem to go forever with a bit of a service every few years. It's like having a brand new unit now eh?

I had to do a similar thing to mine, put a small hose clamp on the right-angle boot that goes on that pipe.

Alex
 
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