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I have a 407777 0128E1 000815YG which in the last weeks started acting up.

After the last cold spell my mower would not start which I chased down to carb problems. Also, the fuel cut off valve on the Nikki carb has been dripping for a couple of years. That thing drips fuel onto the muffler guard so a fire is definitely a risk. It is leaking through the valve body and not on the sealing crush washer. After pricing the valve I decided to try an Amazon replacement carb.

I installed the carb and reset the valves. Now it starts and runs well off idle but will not idle at a fixed RPM. It doesn't surge or hunt like when the little spring is missing on the throttle lever on the smaller engines, instead the idle RPM will be 1300 then creep to 1800 then drop back to 1400 or so; the cycle takes several seconds. When the RPM drops to the lower range it then coughs like it is coughing out of the air intake tube.

What would cause this erratic idle?

As an experiment, I reinstalled the factory carb and it would not start and the carb dumped lots of fuel. I don't know if the float needle is not seated, or if either the bowl o'ring or the float itself is bad. I was in the carb before buying the Amazon carb (when the mower would not start) so maybe I didn't get the needle in the float. Either way, if I open it again it will be to rebuild it.

I have looked at the Briggs parts and can't find a complete carb rebuild kit. My carb has the aluminum alloy float bowl. Are complete rebuild kits available or must all the parts be ordered separately - I see no mention of rebuild kits on the Carb, fuel supply parts diagram?

The fuel cut-off switch is listed as $145, yet a complete carb 791230 (part 125) is $187. My carb is 791093 266 7723-741 I have no idea how to read the carb markings.
https://shop.briggsandstratton.com/pages/parts-lookup#/Briggs_&_Stratton/407777-0128-E1/Carburetor,_Fuel_Supply/4077770128E1/29870005A/y

I would appreciate help as follows:
1. what would cause the inconsistent idle RPM on the Amazon carb?
2. are Briggs complete carb rebuild kits available?

First the engine idling at a too low of rpm. Its governed idle speed should be set at 1750. And with the throttle held against the idle stop @1200 rpm. By idling too low it will act the way you are describing due to the governed idle setup.

What would cause it to vary the idle speed by several hundred RPM?

The idle speed screw is functioning and it is very sensitive. A quarter turn results in significant speed change. As long as the screw setting results in less than 2000 RPM the varying speed starts.

I agree when the idle drops too low around 1400 or less it coughs - so I presumed that cough is caused by low idle RPM.

It sounds like it has some trash or old gas in the carb to me. Plus the new replacement carbs are just OK in my opinion. The older original carb is much better. Was your old nikki carb a 3 or 4 bolt?
I was having some of the same problems with my opposing twin Briggs.

I gave up on finding a rebuild kit for the Nikki and bought a new 791230 Nikki on Amazon for $145 + tax.

kirkkw said:

What would cause it to vary the idle speed by several hundred RPM?

The idle speed screw is functioning and it is very sensitive. A quarter turn results in significant speed change. As long as the screw setting results in less than 2000 RPM the varying speed starts.

I agree when the idle drops too low around 1400 or less it coughs - so I presumed that cough is caused by low idle RPM.

Click to expand...

You seems to not understand this particular engine setup. It has a tendency to vary the throttle vane at idle to maintain a steady speed. Just setting the idle speed by the screw at a higher speed doesn't really fixes this inherit problem. This is why this engine is setup with a governed idle speed spring and the idle speed screw is use the bare minimum idle speed and the governed idle speed spring sets it at 1750. The governed idle speed is set by bending a tang on the control assembly. Note if the governed idle spring is missing then it needs replacing or the engine will try to idle at bare minimum idle speed.

When this setting is not I even seen run at near full throttle or die.




As for the carburetor repair kit I figure someone would have chimed in with its PN of 797890. An after market version could be gotten of Fleecebay for $10. The last price update I got from Briggs the kit listed for $90.

I certainly don't understand the idle setting. After I get the new carb installed I will try to follow those instructions. It is difficult to identify exactly what is being depicted when just looking at the drawings - hopefully when I am looking at the motor it will make sense.

Thank you very much for the help.

The governed idle spring is 691019 (for my reference).

Goverened engines can be confusing
The Idle adjuster sets the MINIMUM idle speed when YOU have throttled down the engine, not the normal idle speed when the engine is being used .
The running idle is set by bending the tangs unless you carb has an adjustable idle jet ( and it should not )
The book uses the terms "high idle" = running idle with throttle WFO and "low idle" which is the minimum speed with the throttle shut down fully .

I got the new Nikki carb installed and it now starts and runs like it should. I used it all day yesterday and haven't even bothered to check the carb setting. I know it is idling around 1700rpm which is below the spec of 1750 so I may try to adjust it as described above. I may use the junk Amazon carb to rebuilt the original Nikki just to have as a back up.

What is it about the cheap carbs that causes them not to work properly? On my tractor (I don't have a mowing deck) the RPM was all over the place and moved 500 RPM or more up and down (not surging). I presume the fuel passages are not correct.

My tractor gets used year round at a minimum of 6 miles per week picking up trash along the road.

The factories that make the carbs test them
The good ones go in a box to be sold as brand name spares or fitted to a new mower.
In a US 9 or Aust ) factory, the bad ones would be broken down for scrap and tossed back in the furnace.
In China, the actual profit margins the factories work on are very small, usually less than 10% so they can not afford to do this.
SO the quality control rejects get sold off as QC rejects .
Because China is still a 3rd world country there are people desperate to make money any way they can so the 2nd tier factory will buy those rejects and try to fix them.
What they fix will get sold into the aftermarket parts stream & what they can not fix gets on sold to 3rd tier factories who will attempt to fix them as well.
Again what they fix get sold as good & what they can't get sold as scrap to 4th tier factories, and this process continues at infinitum.
Eventually you end up with carbs ( or other parts ) that have all of the defective bits in them or problems that can not be fixed that are nothing more than scrap .
These end up being bought by less than honest vendors who either sell them direct via the platforms specifically designed for the sale of stolen or defective goods like ebay or amazon or dishonestly resell them to surplus parts vendors who again use the same platforms to hide their real identity and protect them from any legal repocussions because no one actually knows who they are .
All that Amazon knows is their vendors have a verified bank account ( easy to do ) and a real email address .

Add to that some vendors who believe all Western people are greedy fools so it is their patriotic duty to scam them for every cent and those will deliberatley buy known defective goods and sell them with true but misleading descriptions like "OEM" or "Fits" but nowhere do they say "good" or "works" .
And again Bezos does not care. All he cares about is his cut of the sale price + if it is warehoused by him, his warehousing & distribution fees .

This is why I keep advising people to use Alibaba or better still Ali Express for single items and Made in China for bulk orders .
Unlike the USA based platforms both of these guarantee the quality of the goods the vendors sell.
The vendors do not get paid for what you buy till either you respond that the goods were received & were of god quality or 90 to 120 days pass since the goods were marked as received .
Thus no junk on any of these platforms but they are a bit harder to use and because they are Chinese converted into English, searching can be a bit klunky till you get used to it .
I just had 2 carbs arrive broken.
Sent the photos back to AliExpress & got a refund from AliExpress within a day plus 2 replacements are on their way from the vendor with appologies for any inconvienance caused by the bad shipping
Try that with either AMazon or ebay and you will be chasing up retrobution for months .

So that is why there are so many bad parts out there.
Ebay & Amazon could fix it in an instant but they want the extra money.
You end up buying 5 parts till you finally get a good one & they get 5 lots of fees .
Feel like your fellow countrymen are playing you for a sucker yet ?

kirkkw said:

I have a 407777 0128E1 000815YG which in the last weeks started acting up.

After the last cold spell my mower would not start which I chased down to carb problems. Also, the fuel cut off valve on the Nikki carb has been dripping for a couple of years. That thing drips fuel onto the muffler guard so a fire is definitely a risk. It is leaking through the valve body and not on the sealing crush washer. After pricing the valve I decided to try an Amazon replacement carb.

I installed the carb and reset the valves. Now it starts and runs well off idle but will not idle at a fixed RPM. It doesn't surge or hunt like when the little spring is missing on the throttle lever on the smaller engines, instead the idle RPM will be 1300 then creep to 1800 then drop back to 1400 or so; the cycle takes several seconds. When the RPM drops to the lower range it then coughs like it is coughing out of the air intake tube.

What would cause this erratic idle?

As an experiment, I reinstalled the factory carb and it would not start and the carb dumped lots of fuel. I don't know if the float needle is not seated, or if either the bowl o'ring or the float itself is bad. I was in the carb before buying the Amazon carb (when the mower would not start) so maybe I didn't get the needle in the float. Either way, if I open it again it will be to rebuild it.

I have looked at the Briggs parts and can't find a complete carb rebuild kit. My carb has the aluminum alloy float bowl. Are complete rebuild kits available or must all the parts be ordered separately - I see no mention of rebuild kits on the Carb, fuel supply parts diagram?

The fuel cut-off switch is listed as $145, yet a complete carb 791230 (part 125) is $187. My carb is 791093 266 7723-741 I have no idea how to read the carb markings.
https://shop.briggsandstratton.com/pages/parts-lookup#/Briggs_&_Stratton/407777-0128-E1/Carburetor,_Fuel_Supply/4077770128E1/29870005A/y

I would appreciate help as follows:
1. what would cause the inconsistent idle RPM on the Amazon carb?
2. are Briggs complete carb rebuild kits available?

Click to expand...

You ought to take off the valve covers again, then rotate the flywheel by hand while watching the rocker arms move. One of them may move only slightly. Brigg's engines from around that time had problems with a cam lobe wiping out, and this test will tell you if that is the case. Take a look, it costs nothing to check.

With the new Nikki carb the engine runs virtually like new. I adjusted the valves and my method is not based on TDC. Instead, I just turn the motor and adjust the closed valve based on when the other valve opens.

All the valves were opening and closing normally. BTW the engine has 570 hours on it.