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Briggs Platinum 120000 Engine Cold Start ok, Hot Start struggle

#1

J

jchau

I am a little stumped on what the issue or fix but I have a Briggs Platinum (120000 series) engine on a Craftsman lawnmower (2013 model). This engine has the auto choke and primer. It has been acting up last year and this year too. It used to start with one pull and now on cold start most of the time it will work within 3 pulls. On a hot start I struggle with getting it going until the engine cools.

Engine Detail:
  • Model: 128M02
  • Type: 0962
  • Code: F1

Things I have tried:
  • Auto choke Thermostat replace: It was a little sticky at the end of last year so I thought that was the main culprit for the hot engine issue but problem still exist
  • Spark plug has been replaced with Champion RJ19HX. I am getting spark with the spark plug checker.
  • Air filter has been replaced
  • Carburetor bowl and main jet (bolt holding the bowl) has been cleaned (without removing the whole carb) and spray as much around the choke
  • Resistive check the ignition coil, measurement seems normal. Also re-gapped the coil to the flywheel.
What works:
  • If I get a rod or something to manually close the choke when the engine is hot, I am able to start the engine and remove the rod immediately right after the engine seems to start up.

I am a little confused what is causing the problem and what needs fixing.


#2

7394

7394

Welcome Aboard !


#3

R

Rivets

If it came to me with these symptoms I would be checking the float level. Seat may have swollen causing a lean running condition, which you rectify by choking the engine on hot start up.


#4

J

jchau

Rivets said:
If it came to me with these symptoms I would be checking the float level. Seat may have swollen causing a lean running condition, which you rectify by choking the engine on hot start up.
Click to expand...
Question about the seat, even though it is a small part to replace but will require taking apart the whole carburetor, it is worth replacing the seat only or should I replace the whole carb? Also when I search up the carb there are a lot of aftermarket ones, are they trustworthy or not worth the effort?

I have tried tipping the mower to the side to start like what this video is showing
it seems to work with a hot start. Signs of a carb needs a full rebuild or replace?


#5

sgkent

sgkent

there are 3 things that any engine needs to work. Compression, spark at the correct time, and air to fuel mixture. Fuel ages so something sitting all winter doesn't behave like fresh fuel. I personally have found the best way to start any pull engine is to prime it when not fully warm, pull the rope slowly until the engine resists a bit then pull hard, or if the rope is too far out, let it in and pull hard. Doing that my Toro or Edger etc will start almost the first pull each time. I used to rebuild the carbs on anything I owned with great success, but finding good quality parts have been harder so I try to find a new old stock carb when i can now, replace, and rebuild the old one as a spare at my convenience. Older mowers and small engines tend to loosen the head gaskets over time and that can cause issues. So can tight valves on high use engines. Ethanol in the fuel can cause issues. Many years ago I had a boat whose engine did not behave properly and I spent hours on it. I discounted that the fuel was bad because it was only about 60 days old. In the end the problem was the fuel was stale. The point being, don't discount the small things. Figure what of the three items is missing when something won't start before throwing parts at it.


#6

J

jchau

sgkent said:
there are 3 things that any engine needs to work. Compression, spark at the correct time, and air to fuel mixture. Fuel ages so something sitting all winter doesn't behave like fresh fuel. I personally have found the best way to start any pull engine is to prime it when not fully warm, pull the rope slowly until the engine resists a bit then pull hard, or if the rope is too far out, let it in and pull hard. Doing that my Toro or Edger etc will start almost the first pull each time. I used to rebuild the carbs on anything I owned with great success, but finding good quality parts have been harder so I try to find a new old stock carb when i can now, replace, and rebuild the old one as a spare at my convenience. Older mowers and small engines tend to loosen the head gaskets over time and that can cause issues. So can tight valves on high use engines. Ethanol in the fuel can cause issues. Many years ago I had a boat whose engine did not behave properly and I spent hours on it. I discounted that the fuel was bad because it was only about 60 days old. In the end the problem was the fuel was stale. The point being, don't discount the small things. Figure what of the three items is missing when something won't start before throwing parts at it.
Click to expand...
I have put fresh fuel with 0% Ethanol and was having issues.

If I need to close the choke on an auto choke system seems like I have air fuel mixture problem? I cannot prime the engine as this is also auto prime with auto choke.


#7

R

Rivets

On Briggs carbs I always replace the float needle and seat before replacing the carb. But with that said, I’ve done more than I can count and have a couple of tweaks to get the float set slightly rich. Briggs part number #398188. Not everyone on this forum agrees with me, so it is up to you.


#8

J

jchau

Rivets said:
On Briggs carbs I always replace the float needle and seat before replacing the carb. But with that said, I’ve done more than I can count and have a couple of tweaks to get the float set slightly rich. Briggs part number #398188. Not everyone on this forum agrees with me, so it is up to you.
Click to expand...
Yeah I am just debating if I should just swap for an Amazon special carb since the float and needle is half the price but more work to do and do the needle and seat swap on the old one later in time.


#9

R

Rivets

I can’t answer that question.


#10

J

jchau

well I don't want to take it all apart and open a can of worms to find out I need to replace more things like gaskets, pins, etc. :LOL:


#11

R

Rivets

I don’t mean to sound rude, but from your last post I suggest you stop right now and find a mechanic. You definitely don’t know what you are doing. I suggested you had a choice of replacing the float needle and seat or replacing the carb. You don’t seem to understand in both cases you need to only remove the air filter assembly and the carb. In either case you also should replace the gasket. No one has suggested anything different. By continuing yourself, you could easily get get in over your head. Just have to be honest, sorry.


#12

Scrubcadet10

Scrubcadet10

those carburetors are very very simple.


#13

S

slomo

sgkent said:
there are 3 things that any engine needs to work. Compression, spark at the correct time, and air to fuel mixture.
Click to expand...
4


#14

S

slomo

Rivets said:
On Briggs carbs I always replace the float needle and seat before replacing the carb. But with that said, I’ve done more than I can count and have a couple of tweaks to get the float set slightly rich. Briggs part number #398188. Not everyone on this forum agrees with me, so it is up to you.
Click to expand...
I set mine to JUST below level on the float setting, slightly low. All of my mowers tend to run on the rich side with clean air filters and no chokes on. Plugs are tinted black with carbon.


#15

S

slomo

Scrubcadet10 said:
those carburetors are very very simple.
Click to expand...
For some people. Not everyone is a super tech like you, Bert, Star and so on...... Huge group of very helpful people on here.


#16

sgkent

sgkent

people aren't being mean when they suggest taking it to a shop. They are trying to save the owner money, time and grief when they sense someone might be in over their head. Lots of people here have thousands of carb rebuilds under their belts, and sometimes even they hit one that doesn't want to play by the rules. Some parts aren't meant to be rebuilt these days too.


#17

Hammermechanicman

Hammermechanicman

slomo said:
I set mine to JUST below level on the float setting, slightly low. All of my mowers tend to run on the rich side with clean air filters and no chokes on. Plugs are tinted black with carbon.
Click to expand...
Is it running rich or is the spark plug not operating in the self cleaning operating range. Many people don't run equipmebt hard enough to get the plug temp into the self cleaning range. Unlike a car engine with water cooling and a thermostat. Most air cooled small engines have no temperature regulation so they are designed to operate in high ambient temperatures at max power. Nearly all the flathead push mower engines don't get hot enought to get the plug into the self cleaning range. Carbon deposits from running rich and carbon deposits from being too cold look very similar. OHV OPE engines have higher compression so they run hotter than a flat head engine so they tend to get the plug(s) into the self cleaning range much better than flat head engines. A flat head briggs uses a champion plug with a 19 heat range (RJ19LM) whereas a OHV uses a 12 heat range plug (RC12YC) which is a colder plug. Every time an engine is started the spark plug(s) are developing carbon deposits, regardless of the mixture, until the plug(s) reach the self cleaning range and burn off the carbon.


#18

S

slomo

Hammermechanicman said:
Is it running rich or is the spark plug not operating in the self cleaning operating range. Many people don't run equipmebt hard enough to get the plug temp into the self cleaning range. Unlike a car engine with water cooling and a thermostat. Most air cooled small engines have no temperature regulation so they are designed to operate in high ambient temperatures at max power. Nearly all the flathead push mower engines don't get hot enought to get the plug into the self cleaning range. Carbon deposits from running rich and carbon deposits from being too cold look very similar. OHV OPE engines have higher compression so they run hotter than a flat head engine so they tend to get the plug(s) into the self cleaning range much better than flat head engines. A flat head briggs uses a champion plug with a 19 heat range (RJ19LM) whereas a OHV uses a 12 heat range plug (RC12YC) which is a colder plug. Every time an engine is started the spark plug(s) are developing carbon deposits, regardless of the mixture, until the plug(s) reach the self cleaning range and burn off the carbon.
Click to expand...
Run at a min of 3400 to 3600 revs ALL the time. Even on models with adjustable throttle levers. Max revs all the way.

These slow burn combustion chambers and no efi create a rich looking plug.


#19

J

jchau

Anyway issue has been fixed by swapping out with an Amazon special carb. All by myself in 15min or so ?

Thanks for the suggestions


#20

E

efred

slomo said:
4
Click to expand...
Speaking of compression, have you checked the compression, both cold AND hot? There are tolerances in the valve train to accommodate the expansion of those parts, and the wear on the engine changes those tolerances. It's possible that you're not getting enough compression. It's an easy check.


#21

S

slomo

air
fuel
spark
compression

Adding 1. Good yank on the rope or a strong battery.


#22

L

lbrac

If you decide to rebuild the carb, get a rebuild kit with all the gaskets, o-rings, float needle/seat, etc. That should resolve not having a part that needs to be replaced once you get it apart.


#23

S

slomo

Issue was resolved last page.


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