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6.75 Briggs Auto choke

#1

Sawfxr

Sawfxr

I'm helping a co-worker with a newer Toro that has the air vane operated auto choke. It also has the bi-metal arm that's supposed to hold the choke off while starting the engine when it's already hot. This mower had a huge mouse nest under the flywheel housing and the owner removed it himself. Since then the engine surges bad as the air vane arm keeps moving the choke lever on and off.
I can't figure out how or if the bi-metal arm connects to the air vane. I'm thinking there was a second spring that got unknowingly removed with the mouse nest debris. Is there supposed to be another spring connecting the two parts? Do the two arms directly hook together? Or does the bi-metal arm just act as a limiter to the movement of the air vane arm?
Thanks in advance for all and any replies.
E.


#2

R

Rivets

There is NO second spring. If the bimetallic choke arm is set properly the cause of the problem is probably dirt in the carb main jet. These carbs are very sensitive to dirt.


#3

pugaltitude

pugaltitude

There should be a spring connected from the choke lever on top of carb to a metal cover with a little
hook cut out where the carb fits to engine.
The thermostat (bi metal arm) is common for seizing and sticking which can keep choke on causing rough running and bad starting.
Model,type and code should help to identify spring.


#4

Sawfxr

Sawfxr

Hey, Thanks for the replies.
The bi-metal arm appears to be doing it's thing as it does move back and forth when heating up and cooling down. I just was not sure if there was a second spring used to connect it or not . I did figure out that the hook at the end of the air vane did belong around the bi-metal arm and not in front of it as I found it. Now that I'm sure it's together correctly I hoped the hard starting when hot would be gone. It is still a problem. As far as the surging, I have soaked the carb twice now for a total of about 2 hours and it still surges. If I hold the choke partially closed with my finger it smooths right out so I'm sure its a carb issue still. My next thought is for $35 I can replace the carb with a primer type carb and change the air filter base to the primer type for an additional $18. I have another similar mower with the primer setup (no choke plate) and it starts every time when hot with one pull. A new primer carb should kill both problems ;>)
Thanks.
E.


#5

R

Rivets

Just remember something after I saw your post. I think that there is a replacement carb to be used on engines with your problem. Ask your B&S dealer if they have service bulletin #760. Why I can remember that number I don't know. If they do ask them to see if the recommended carb has been superseded to a different number. I will try to remember to look for this SB when I go in to work Monday, but my short term memory has gone the way of the two cycle engines, so it may take some time to find it.


#6

pugaltitude

pugaltitude

Briggs ready start has had cold and hot start problems over the years.
The thermostat has to be set up correctly. on the plastic air vane near the thermostat there are 4 adjustment lugs. Bend the top of the thermostat so it aligns in between the lugs with forward to back and side to side adjustment.
Also depending on the manufacture date depends on the spark plug gap. Need model type and code to give you that info.
Also does it have a metal spark plug cover. They prone to causing no spark when hot.
Governor spring can also be updated to stronger spring from the older quantums.
If all else fails try a carb part no 794304 which now has an idle circuit built in to help start.
Good luck.


#7

Sawfxr

Sawfxr

Thanks again for the replies.
pugaltitude,
Do you have a page from the service manual detailing the adjustment of the thermostat?
The numbers on this engine are 126T05 / 0123-B1. No metal spark plug cover.
I had a mower in the garage with the primer system on the B&S engine. Since that mowers carb was already off, just for kicks I switched the carbs and this problem mower starts when hot with no hesitation. We were about to pull the trigger on ordering the primer carb and air filter base later today.
Thanks
E.


#8

R

Rivets

This auto choke is so new that it is not in the service manual yet. I forgot to grab the instruction sheet at work today. I will try to remember tomorrow.


#9

Sawfxr

Sawfxr

OK. We did the carb swap to the primer style and she starts like new hot or cold. One pull.
But... it still surges up and down. :mad:
I sprayed carb cleaner around everything looking for an air leak but didn't find one. It will stop surging if I hold the throttle shaft steady with my finger, then let go and it will stay smooth. Do a little bit of cutting heavy grass and once the governor moves the throttle then it's back to constant surging until I touch the throttle shaft again.
I'm wondering if it could be valve related??
Thanks.
E.


#10

R

Rivets

Main jet needs cleaning.


#11

Sawfxr

Sawfxr

Rivets,
It's a brand new carb right out of the box. It was only on the motor for less than a minute before it started surging. I didn't bother pulling the bowl because of the fact it was a new B&S carb and not a used one. I experimented a bit this morning by putting more tension on the governor spring and raising the running RPM's a little. The surging seemed less and was not as bad with about 150-200 more RPM's on the handheld tach.
E.


#12

R

Rivets

Ok I guess I can't help. Good luck.


#13

pugaltitude

pugaltitude

:biggrin:
Sawfxr said:
Rivets,
It's a brand new carb right out of the box. It was only on the motor for less than a minute before it started surging. I didn't bother pulling the bowl because of the fact it was a new B&S carb and not a used one. I experimented a bit this morning by putting more tension on the governor spring and raising the running RPM's a little. The surging seemed less and was not as bad with about 150-200 more RPM's on the handheld tach.
E.
Click to expand...

Sounds as though its sucking air somewhere. Check the oring and intake manifold.
Also the spring for the ready start carb is different to the primer.
The primer spring is stronger.
Did you fit the primer as well or just the carb?


#14

J

JosephP.

Am a newbie to site and am having the exact same issues with warm starts on my 675series ready start mower.I have had this issue since 2010 when I purchased it.When I change bags I have to wait. I keep my gas can,gas on mower,carb impeccable.Run my fuel through a 15 micron sock,drain my mower of gas,take float bowl down,etc.etc.etc. Point is I have avoided all the issues of starting my mower that I could think of thinking it was something I may have been doing wrong or missing. Then I started looking up online and finding that I am not alone.I have had my thermostat arm exposed 1/2 a dozen times evaluating and tweaking the adjusting piece on the choke itself.Replaced it,re built the carb and still,,,,starts perfect 1st time cold,then once hot (ZIPPO) nothing. Is there a air filter box and carb that I can go back to,"old school"with. I consider myself to be pretty hands on being as how I run my chainsaws,back pack blowers and have a pretty good basic concept of this stuff but this auto choke just ain't working. Is there a up to date diagnostic check for these systems or is a new carb my best bet? Even a way to manually work the arm once the engine is hot ?????


#15

T

tspikewell

JosephP. said:
Am a newbie to site and am having the exact same issues with warm starts on my 675series ready start mower.I have had this issue since 2010 when I purchased it.When I change bags I have to wait. I keep my gas can,gas on mower,carb impeccable.Run my fuel through a 15 micron sock,drain my mower of gas,take float bowl down,etc.etc.etc. Point is I have avoided all the issues of starting my mower that I could think of thinking it was something I may have been doing wrong or missing. Then I started looking up online and finding that I am not alone.I have had my thermostat arm exposed 1/2 a dozen times evaluating and tweaking the adjusting piece on the choke itself.Replaced it,re built the carb and still,,,,starts perfect 1st time cold,then once hot (ZIPPO) nothing. Is there a air filter box and carb that I can go back to,"old school"with. I consider myself to be pretty hands on being as how I run my chainsaws,back pack blowers and have a pretty good basic concept of this stuff but this auto choke just ain't working. Is there a up to date diagnostic check for these systems or is a new carb my best bet? Even a way to manually work the arm once the engine is hot ?????
Click to expand...

I have had luck taking off the air filter and using a screwdriver or other implement to hold the automatic choke in the open position and then starting the mower.

If the choke is in the shut position it will not receive enough air to run.

Then put air filter housing back on...and continue mowing...


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