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hey guys. I have a situation going on with 2 B&S engines. both are 18hp. Engine number on one is (31H7770297E1), Can't read the other engine number because of rust on valve cover but engine cover says the same thing as the first engine. They look like the same series of engine. The problem is that when I start engine, right off it starts surging and then like backfire. does this no matter the throttle position. The second engine just surges no matter the throttle. ok, here is what I have done to remedy with no luck. 1. new spark plugs. 2. Cleaned carbs then replaced with new carbs. Cleaned coil contacts and Flywheel Contact. Replaced Flywheel key. Sprayed carb cleaner around head gasket area while running but no change in engine performance. No oil leaks around head gasket area. Any ideas why this is happening would be of great help.

Before digging deeper into the possible causes, I would check the valve clearances on all valves. It does sound like a fuel issue, but let’s look at the easy things first.

Rivets said:

Before digging deeper into the possible causes, I would check the valve clearances on all valves. It does sound like a fuel issue, but let’s look at the easy things first.

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Thanks for answering. I forgot to list that I checked and set the valves.

Bat57 said:

Thanks for answering. I forgot to list that I checked and set the valves.

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Clean OEM carburetors again correctly. If you don’t know how to, then take them to a shop. Always want to keep OEM carburetor unless it is too far gone. Also, need to check for air intake leaks.

Tiger Small Engine said:

Clean OEM carburetors again correctly. If you don’t know how to, then take them to a shop. Always want to keep OEM carburetor unless it is too far gone. Also, need to check for air intake leaks.

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Ok.Thanks

Typically surging is lack of fuel delivery as Tiger Small Engine was talking about or a dirty carb. Check there first as he recommended.

slomo said:

Typically surging is lack of fuel delivery as Tiger Small Engine was talking about or a dirty carb. Check there first as he recommended.

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Thanks. I have taken shroud off and cleaned engine and fins already.

Bat57 said:

I have taken shroud off and cleaned engine and fins already.

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Yearly if not more often is recommended in our manuals. Good man......

As well as cleaning and adjusting the coils you might also unplug both the coil kill wires . There is a diode in each wire that allows current to go to the ignition switch to be shorted to the ground to stop the engine . If one of the diodes goes bad it will allow the pulse from one cylinder to go through the bad diode into the other cylinder coil and it will fire when it should not , the backfire could be unburnt fuel in the exhaust . If that has no effect on the problem try adding fuel to the carb with a unlit propane torch at the air cleaner, if adding more fuel stops the surge then you are back to carb work. Check the gasket between the carb and the block or plastic insulator make sure it is in the right way for holes to line up with each other .
I am close to a auction house that sells store returns so I saw a lot of Champion generators , they are brand new out of box that ran every way but right. I worked on the first one for 8 hours, I had the carb off so many times I wrecked the gasket between the carb and engine . So I made one using the carb as the pattern and it ran fine. I had already ordered a new carb and the gasket that came with it did not match the one I made , but I tried it and it ran like crap. It blocked a little hole that allowed air in. It is all ways the last place you look.

Thank you, guys, for all your help. After cleaning and adjusting everything several times. I cleaned and rebuilt 2 Nikki carbs (with OEM) parts the correct way. No change. I finally put a new Rui*ing carb on it and ran Hi-test fuel through it. It popped a couple of times then it smoothed out and ran like a sewing machine. Thanks again for your help.

Some carb info for your single cylinder Briggs engines with the Nikki carbs:
Briggs first used a Walbro LMT carb on those type engines as a more user friendly re-buildable carb.
Here is some info about rebuilding your Briggs/Nikki single barrel carbs. and parts link.
I get fast shipping from sawzilla and shipping is very reasonable.
You need to save your OEM Nikki carbs. You got lucky with the replacement. The China clone replacements are a roll of the dice.
Best to get them from Amazon for a free return refund with 30 days.
I've seen guys that sometimes have to get 3 before they get a good one. Also when replacing they usually require a static governor adjust be done.


For carb rebuilding: (also look at the Walbro LMT, they are a direct bolt on for the Nikki.
https://outdoorpowerinfo.com/repairs/

When looking for Nikki carb parts and you have a part number use Briggs as the part number instead of Nikki



Also for Briggs Nikki parts check out sawzill parts.com click on the menu link in the upper left corner and select Briggs for the Nikki parts list.

Stihl Chainsaw Parts | Husqvarna Chainsaw Parts | Sawzilla Partsct

and some links to rebuild you tube videos of the Nikki rebuild.
Nikki Carb Rebuild Single Cylinder Briggs - YouTube
and

Bat57 said:

Thank you, guys, for all your help. After cleaning and adjusting everything several times. I cleaned and rebuilt 2 Nikki carbs (with OEM) parts the correct way. No change. I finally put a new Rui*ing carb on it and ran Hi-test fuel through it. It popped a couple of times then it smoothed out and ran like a sewing machine. Thanks again for your help.

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I’m going to add just my .02 worth here and say that I run nothing but premium, ethanol free fuel in ALL my small engines. If you can’t get ethanol free at the pump you just run premium fuel and/or buy the ethanol free additive at an automotive store.

Had a generator that I could not stop surging . Turned out one of the springs that works the throttle was bad.

And it can even be bad spark plugs.

It sounds like you have a fuel issue.

Start at the fuel tank and work your way from there. Debris will collect in the fuel nipple over year of use where the line connects. Next would be the fuel lines and inline filter. Aged fuel lines get rigid and will come apart internally.

I recommend only good fuel line such as Gates brand or NAPA. Stay away from Amazon or Ebay fuel line. That stuff fails very easily with heat and Ethanol fuel blends. I purchased a bulk roll online and had to throw it away.

Next, if it is a v-twin, confirm both cylinders are producing power