http://videos.streetfire.net/video/wm/F556ECEB-3062-4525-976B-CBDFFDB8FE6A.htm

They have six attempts at killing it with a 60HP NOS shot....
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https://www.halfdonethings.com/

That looks suspiciously fake to me.

Where did the enrichment fuel come from ? Does anyone think a briggs big-end would handle the sort of pressures nitrous would create ? Where did the flames keep appearing from ? Why do they look like pyro effects ? why are there no closeups of the injection ? If you even doubled the power output on a unloaded free-revving engine, what do you think the RPM would likely do ? (unless the valve train floated). With no cooling system, I would expect a briggs would seize in seconds.

There might be answers to all those questions, but I've actually had a nitrous system on my car, so I know a little bit about it, and there seems to be a growing trend to make fake "cool" videos these days. I wonder if there are a bunch of "viral video" marketing dudes behind this stuff sometimes.
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After watching the video about 6 times here are my conclusions .They were injecting the nos into the front of the carburetor choke body with out the air filter .It look s like they are projecting a gas stream across in front of the carby . The first few runs look like they were running the motor at high rpm and it basicly stalled it out .The fire would probaly be from a mixture of air fuel and nos rejected by the engine and posibly ignited by it backfireing .All the shots are taken from the air intake side of the engine . The rest of the vids with it running with nos streaming out of the carby were with the motor running at signifantly lower revs .Most briggs and straton motors have the valves timed so that the inlet valve stayes slightly open after intake until about aproximatly 30degres before TDC this results in some air and fuel mixture being pushed back out of the carby on each stroke .so what we are posibly seeing is those gases mixed with nos pushing most of the nos out of the carby intake .Most of the time when they did have it running with out stalling it would have been running with hardly and throttle opening so would have been basicly running with high intake vacume so wouldnt have been makeing much more power than it would operating normaly on full throttle . Take the cam out grind the cam so the inlet valve shuts just after bdc and stays shut and has full compresion then try it i doubt it would last very long before explodeing also the cooling system looked to be intact and they might have been venting some of the gas into it as there seemed to be a few diferent streams of vapour comeing out .Im gueseing nos would act like any other gas and provide a rapid cooling effect .

Well, Andrew's the expert when it comes to lawnmower engines, so if he thinks it looks genuine, I'll settle for that.

They must have had a fuel mix in with the Nos though, since the gas by itself would just lean the engine out, so you would need some sort of fuel pump to provide the fuel pressure.

I read a book on tweaking up the "''tata digger" (as they called the briggs for some reason) for go-kart use. It seemed they were very well "value engineered" motors where no part was really any stronger than it needed to be - I guess they would figure out the optimum design after making it for as long as they have.

One thing that stuck in my mind was that they found it necessary to make a plug for the exhaust that you put in immediately after stopping the engine after some hard laps. Apparently the cool air easily coming back up the low-restriction racing exhaust pipes would hit the extremely hot exhaust valves and warp them !

a friend and I were working on an idea to try super-charging one using a smog-pump once, but never got around to it. guess I was being too critical.. Thanks for the evaluation there andrew.
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There was fire, therefore it was cool

The first bang that happens and the fire erupts on one side, I believe the exhaust blew apart... I could be wrong though...
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https://www.halfdonethings.com/

Actualy looking at the vid again , the first two tries when they had it running at full throttle was taken from exaust side of the engine the flames are comeing out the other side through the carby a good back fire wil actualy spray raw petrol out of these carbys as back presure with the butterfly partial closed will go into the carby bowl - tank and force raw fuel out the air vent which is on the front of the carby .
They had the carby on providing the air fuel mix. By putting the nos in through the carby choke only a small amount of it would have actualy been induced into the cylinder most of it ,as you can see by the stream of vapour is just geting pushed back out and away from the carby by the open intake valve back wash . The valves also stick at high revs on worn B/s motors so the flame would have been from it back fireing through the inlet valve and igniting the fuel geting pushed out through the carby ..There probaly would have been a bit more fire action and rev increase if they had removed the muffler .which is on the other side of the mtor .