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Seems to be a common problem, but not sure what else to do. Working on a Craftsman 21" self propelled / walk behind model number 917.377823. Briggs engine 7.0 hp OHV model 128602 0304 E1 for a neighbor. Mower cranks with 2 pulls when cold (first start) but after shut down to empty bag it will not start without some serious pulling 20-30 times and sometimes backfires thru carburetor slightly (pops). A quick google search returned a hit to make sure engine isn't low on oil as they Briggs have a safety mechanism if engine is too hot, oil level is correct, also same post told me to watch the butterfly valve on the carb. I pulled the intake off to see it and marked the plastic lever above it connecting to the auto choke/throttle cable. Post said B.F. valve will be open at first start but should be closed on hot start. This mower has valve open anytime the engine is off, valve open on cold start and open on hot start, but nearly closed and fluctuating with engine running. From what I can tell that seems normal as you don't have centrifical force to actuate the lever near the flywheel so the spring on the auto choke/throttle pulls the B.F vale in the open position when engine shuts down. I manually closed the B.F. valve after engine was hot and it seemed to start better with about 5 -8 pulls. Not sure if my info is bad or not. Should the butterfly valve be open at cold start and closed or partially closed at hot start? Next task is to pull the carbureator off and start cleaning it and the fuel bowl internals, but I am waiting to see if anyone else has feedback. Thanks for any info and suggestions.

the choke butterfly should be CLOSED when cold, and OPEN when hot.
Might have an issue with the thermostat on the muffler.

Not sure if this the correct one, having trouble finding out if it uses centrifugal force to open the choke, or the thermostat type on the muffler...

The butterfly at the front of the carb ( nearest you ) is the choke and should be closed when the engine is cold then open up fully and stay open till the engine gets cold again.
If it flutters then the spring needs to be replaced.

My auto choke system looks nothing like the video, maybe i am not using correct name for it, the butterfly control cable goes to a lever at the back of the engine that moves when the mower is running and you can see the butterfly valve nearly closed and fluctuating when running, shut off mower and the spring pulls the butterfly opens. Here are some pics of what I have. The black line on the plastic piece that the cable hooks too is for me to know what position butterfly was in when running and in stopped / hot condition. The last couple pics I am manually holding the lever back/closing the butterfly. I don't see any kind of thermostat in the muffler area either.

If butterfly is supposed to be closed on cold start and this one I am working on is exact opposite i.e; wide open at cold start, i can see right down the intake like in above pic 0536. how is this mower starting after two pulls? It seems this machine is opposite of what its supposed to be. I am confused.

Oh, you don't even have a choke..... that's the throttle which controls engine speed. a choke would be right behind the air filter. that metal link going into it is the governor link, and the spring that goes to the control panel on the front is the governor spring, when tension is increased on that spring, it runs faster. it may need more tension or a new spring.
A choke would be where i circled.

I will try to post a video of it running tomorrow, but the only thing visible when running is the plastic lever on top of the butterfly, that is why I marked it with a line in the direction of the valve. Strangest thing.

Your carb primer bulb is probably not working right. The prime from this primer bulb acts as the choke sending xtra fuel in the carb!

Ok, my bad. I was thinking this is choke system. this is throttle system, sorry. Ok now what. LOL.

you tube mechanic said:

I will try to post a video of it running tomorrow, but the only thing visible when running is the plastic lever on top of the butterfly, that is why I marked it with a line in the direction of the valve. Strangest thing.

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I'm sorry........I CAN'T RESIST!

mower king said:

Your carb primer bulb is probably not working right. The prime from this primer bulb acts as the choke sending xtra fuel in the carb!

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primer bulb?? How does that come into play when engine is hot. 3 prime pumps at cold start and it fires right up. I have never used the primer after engine hot. Are you supposed to?

mower king said:

I'm sorry........I CAN'T RESIST!

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LOL. Yuk, yuk , yuk. still better than paying $30 hr to a shop.

you tube mechanic said:

primer bulb?? How does that come into play when engine is hot. 3 prime pumps at cold start and it fires right up. I have never used the primer after engine hot. Are you supposed to?

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You might have to prime it, even hot. As a engine gets wear on it, it doesn't react the same as when new....same with the carb. Try it, to find out.
Nothing fires an engine faster than fresh, raw fuel!.......well, there is a couple things but I don't print it, don't really want anyone getting hurt!

Ok, so now I am straight on this is the THROTTLE. Does the throttle butterfly have anything to do with original problem. Pulling on the rope forever to get it to fire up again after dumping grass catcher. It seems to start up pretty well if you let it cool down. So maybe back to this thermostat thing? I am not seeing one around my muffler area. Here is a pic of muffler.

mower king said:

You might have to prime it, even hot. As a engine gets wear on it, it doesn't react the same as when new....same with the carb. Try it, to find out.

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Ok I will try to prime it tomorrow on a hot engine to see if it helps it start. This is a 15yr old mower by the way so it won't surprise me if the primer is malfunctioning. There again as I stated above this is why I am trying to fix it for the neighbor instead of a small engine shop at $30 an hr.

Spark plug looks fine by the way, first thing i checked along with the oil level.

Scrubcadet10 said:

Oh, you don't even have a choke..... that's the throttle which controls engine speed. a choke would be right behind the air filter. that metal link going into it is the governor link, and the spring that goes to the control panel on the front is the governor spring, when tension is increased on that spring, it runs faster. it may need more tension or a new spring.
A choke would be where i circled.

View attachment 53990

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Ok, no choke, not sure I understand the concept of no choke system, but I got it. I sure miss the mowers I grew up with. Mounted on the handle was a throttle/choke control with a turtle and rabbit icon. Worked great until all these kill levers and such came along.

you tube mechanic said:

Ok, so now I am straight on this is the THROTTLE. Does the throttle butterfly have anything to do with original problem. Pulling on the rope forever to get it to fire up again after dumping grass catcher. It seems to start up pretty well if you let it cool down. So maybe back to this thermostat thing? I am not seeing one around my muffler area. Here is a pic of muffler.

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If there is no choke then there is no need for a thermostat to control it
You may have a worn out carb or an air leak around the carb.

you tube mechanic said:

Ok, no choke, not sure I understand the concept of no choke system, but I got it. I sure miss the mowers I grew up with. Mounted on the handle was a throttle/choke control with a turtle and rabbit icon. Worked great until all these kill levers and such came along.

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as bert said, since you don't have a choke, you won't have the thermostat to control it.
if you don't have a choke it uses a primer bulb... usually a red rubber button you push.

Understood. Thermostat is for choke system engines. I have the primer bulb system. I am going to prime it with it hot and not starting to see if that fixes anything tomorrow. If priming helps, then I will change just the primer bulb. Still a little confused about how this is causing hard to start condition on a hot engine. I thought the primers sole purpose in life is to prime a cold engine on first start and that is it. How is it affecting a hot engine? Rubber leaking maybe allowing too much air in the carb???

The traditional problem in a Briggs and Stratton engine that won't start when hot is that the valves are out of adjustment. As the engine wears the valves sit lower in their seats and they'll leak slightly when the engine is hot because the valve stems expand with heat. An engine so encumbered will run but not start, at least not willingly. Your car doesn't have this difficulty because its valves adjust automatically, but a small four-stroke engine needs an occasional valve adjustment. When I last did this I fashioned a feeler gauge out of a plastic bubble pack and gently applied a file to the valve stems, for the older B&S engines don't have adjustable valves. Yours may be improved in that regard.

Mark Kinsler

Because of idiot emission laws mower engines are jetted way too lean.
The volume of fuel sucked into the engine is a function of how fast the air is rushing past the venturi in the carb & the height of the fuel in the float bowl .
When running at idle the engine is doing something like 1500 rpm
When pulling the starter cord you would be lucky to be doing 500 rpm
So the slightest constriction to the idle circuit will mean there is not enough fuel to start the engine
From cold you have a primer which will squirt some fuel directly into the carb throat or raise the height of the fuel in the float bowl so it is easier for the air rushing past the venturii to suck fuel up into the carb throat.
The joke of this is every push mower in the entire USA running 24/7 for an entire year would not put out as much pollution as a single jumbo doing the NY to LAX route an there are near 50 of them a day

bertsmobile1 said:

Because of idiot emission laws mower engines are jetted way too lean.

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It's not just carbs. The bloody stupid gas cans that take a three armed contortionist to pour are equally as bad. I love the guys selling water can nozzels that just happen t fit gas cans.

Test of primer bulb results: cold start- 3 x push on primer bulb + 4 pulls on the rope = starts right up. Run for 10 minutes, shut off and let 5 minutes pass to simulate time to dump grass catcher. Hot start- 35 pulls on rope and at pulls 12,15 and 21 back fired through carb and popped pretty loud, after 35 pulls sputtered a bit and died. 3 x push on primer bulb + 4 pulls on the rope, started right up. Repeated 10 minute engine run and shut off, waited 5 minutes. Pulled 5 time on rope, no start. 3 x push on primer bulb + 3 pulls on rope, started right up. Looks like the bulb is the issue and I am going out to get one. Removed bulb and I don't see any obvious defects but will post again if a new bulb fixes the issue. Thanks for everyone's help, even the ones who busted me out for being a YouTube Master Mechanic, LOL.
Say what you like, but Youtube and Forums have saved me a lot of money as usually there is someone out there that has seen or done it all. Its all good.

you tube mechanic said:

Test of primer bulb results: cold start- 3 x push on primer bulb + 4 pulls on the rope = starts right up. Run for 10 minutes, shut off and let 5 minutes pass to simulate time to dump grass catcher. Hot start- 35 pulls on rope and at pulls 12,15 and 21 back fired through carb and popped pretty loud, after 35 pulls sputtered a bit and died. 3 x push on primer bulb + 4 pulls on the rope, started right up. Repeated 10 minute engine run and shut off, waited 5 minutes. Pulled 5 time on rope, no start. 3 x push on primer bulb + 3 pulls on rope, started right up. Looks like the bulb is the issue and I am going out to get one. Removed bulb and I don't see any obvious defects but will post again if a new bulb fixes the issue. Thanks for everyone's help, even the ones who busted me out for being a YouTube Master Mechanic, LOL.
Say what you like, but Youtube and Forums have saved me a lot of money as usually there is someone out there that has seen or done it all. Its all good.

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Alrighty then....that means the primer bulb is working and there is nothing wrong with the bulb, sounds like it primes each time you want it too......so NO NEED to change it!
Some engines need primed every time to start up....especially with no choke assembly!
Even 98% of NEW MOWERS, even HIGH $$$ Mowers (if carbureted), have to be choked to start every time, even after being ran and are hot!

wrldtvlr said:

I love the guys selling water can nozzels that just happen t fit gas cans.

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Yeah..............i put those on my water cans. Honest.

ok, just saw Mower Kings post. but I already spent a whopping $4 for the bulb and installed to find out that didn't really fix the underlying issue, but at least I know how to get it started again in hot condition. I couldn't figure how changing a rubber bulb that was not cracked was going to fix the issue but for the cost I was willing to rule that out. So still having a hard to start problem when engine is hot, but if you give it 3 primes then it starts after about 4 pulls. If you don't prime then your pulling for about 30 times and it backfires at least 3-4 times in during the 30 pulls.

Pull the spark plug out and take note of the color of it around the bottom of it where the electrode is....white? Brown?Black?

mower king said:

Alrighty then....that means the primer bulb is working and there is nothing wrong with the bulb, sounds like it primes each time you want it too......so NO NEED to change it!
Some engines need primed every time to start up....especially with no choke assembly!
Even 98% of NEW MOWERS, even HIGH $$$ Mowers (if carbureted), have to be choked to start every time, even after being ran and are hot!

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I do not have any choke on this mower. Carburetor has a priming bulb on the intake, next thing in the carb is the automatic throttle control as see in pictures I posted on 8/29.There is no manual choke auto choke or thermostat to control an automatic choke.

Scrubcadet10 said:

Pull the spark plug out and take note of the color of it around the bottom of it where the electrode is....white? Brown?Black?

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Spark plug looked ok, just pulled it out of a hot engine, man that sucker was hot. here is a pic. I have a brand new autolite 3924 that says it crosses to a RC12YC i can install if needed. Electrode looks like a new one.

you tube mechanic said:

I do not have any choke on this mower. Carburetor has a priming bulb on the intake, next thing in the carb is the automatic throttle control as see in pictures I posted on 8/29.There is no manual choke auto choke or thermostat to control an automatic choke.

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yes, I know that....prime it to start it = no more problem!

My next step I think is going to be pulling the carb off and removing fuel bowl. Give it a good cleaning and see what happens. It will have to wait til next weekend though. At least I know how to get the hot engine to restart with the primer bulb even if it isn't that healthy on the engine. Not sure if priming everytime is hurting anything. Again, it is a 15 yr old mower and it's tired.

you tube mechanic said:

My next step I think is going to be pulling the carb off and removing fuel bowl. Give it a good cleaning and see what happens. It will have to wait til next weekend though. At least I know how to get the hot engine to restart with the primer bulb even if it isn't that healthy on the engine. Not sure if priming everytime is hurting anything.

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no comment needed

that's about the 500th time ive seen that

Scrubcadet10 said:

that's about the 500th time ive seen that

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Soon to be 501......and counting lol

It was funny the first time (You Tube Master Mechanic). I can take it, but I repeat, "say what you want about You tube, but there is usually someone else out there that has gone through this" So I use You tube and the Forums a lot to help in repairs. No budget to take it to a repair shop every time it acts up. If my posts on this forums help the next guy then I am happy to help.

Scrubcadet10 said:

that's about the 500th time ive seen that

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If you read total bull dust often enough you will believe it is true .