Backfirin' Briggs

RedMF

Member
I've got a B&S 28N700 vertical on a riding mower, which sometimes backfires when starting and slowing down. Briggs seemed to have used two different makes of carby:Walbro and Nikki on this engine. Mine is a pump feed Walbro. Both the Nikki and some Walbro's on this engine were also gravity feed.
I've seen a replacement Nikki gravity feed advertised as having an anti-backfire device built in, which makes me think that it was a problem with this Walbro carby generally rather than mine being worn out.
Shouid I:
A) live with the backfiring
B) re-build the existing one with a kit $40
C) Replace the existing with the same again $130
D) Get the Nikki and go gravity feed $130
 
When an engine backfires that means it's running lean. I would check for a vacum leak or adjust the carb that's on it, if it's adjustable.
 
Is this an exhaust backfire? If so, and it only does it occasionally, I think for the price of a new carb that may or may not fix it, I could live with it. It's pretty much harmless.

If it is an intake backfire, and this is something new, it could probably be fixed by cleaning the carb, especially the main jet. You can probably ease it apart without needing a kit. Also check for vacuum leaks.

Have you tried a new plug?
 
I just got mower, used.
Spoke to a Briggs dealer who said the carby isn't adjustable.
He also suggested a new spark plug which I have ordered.
How do I check for a vacuum leak...would I see leaking fuel?
 
Not sure if it's an intake or exhaust backfire.....how to tell?
It only happens when slowing engine down and starting (both only sometimes) but never when at driving revs.
 
The easiest thing to do is to go to Menards or Tractor Supply and get a bottle of the fuel additive called "Mechanic-In-A-Bottle", around $5. I have seen that stuff do miracles, worked great on my sons Honda CRF450 motocross race bike. Quit backfiring in about 1 minute after adding 1/2 an oz to the gas tank. My buddy has thanked me a couple of times for telling him about it.
 
An exhaust backfire is typically a loud bang out the exhaust pipe, caused from accumulated unburned fuel in the muffler. If it pops once on a hot restart, the muffler was filled with unburned fuel as the engine was coasting down after the ignition was cut. That's what the solenoid on the carb bowl is for, to stop the fuel when the ignition is cut, doesn't always work. It can also happen when a safety switch has intermittently cut the ignition (like the seat switch), then the ignition is restored, the fuel in the muffler ignites.

If random poping out the exhaust while running, that is still unburned fuel in the muffler. If the engine is misfiring, it is a sign of intermittent ignition failure, like a bad plug, weak coil, kill wire shorting to ground. Could also be a vacuum leak, even an exhaust leak drawing air into the muffler enough to cause fuel burning in the exhaust.

Intake backfire is popping back through the carb. This can be more serious, can melt the carb, burn the air filter, etc. It is caused usually be a too lean mixture, vacuum leaks, faulty ignition, valves out of adjustment, exhaust valve not fully opening, or exhaust restriction. Continued running, if severe, can cause the engine to overheat, gall the piston.

On a simple single cylinder engine, there aren't too many places for vacuum leaks. They are usually a result of the carb mount bolts or the intake tube being loose. Some use gaskets, which can suck in, causing a leak. Others use orings which can usually just be tightened up. If a lot of hours on the machine, sometimes the throttle shaft will wear the carb housing, but that is doubtful on your model.
 
Thanks Steve.
Had a close listen and I'm fairly certain it's at the muffler. This is leaking as it looks like the muffler was damaged at some point and the adaptor flange isn't flush. This fits with your description.
Ordered a new muffler and adaptor the other day.
The spark plug was also the wrong one but changing to the right one didn't stop the pops.
Put some cleaner in the fuel too, just in case.

One other thing I found which doesn't look right, was the rear transmission to engine belt. This is so out of alignment the belt had half flipped over on itself: had been rotating half upside down.
Unless the engine should have mounts to lift it up, (there arent any and nothing in the manual) I cant see how this is a workable design, is this how they are supposed to be seated?
I can see with the top belt to the deck would have to be at an angle due to the raising and lowering of the deck (it is left quite loose) but the transmission belt doesn't look right to me.

41523.jpg
 
The belts should be aligned.

Make sure the engine pulley is fully seated on the crankshaft and the bolt is tight, washer in place. It may have come loose and dropped down, not good if it's running loose!
 
Took the engine off. Pulley doesnt look as if it has slipped and it has the right measurements.
Engine definitely never had mounts and it's the correct original engine.
Transmission is the correct model and the pulley and fan are attached properly and doesnt look as if the mounting points have ever been touched or adapted.

I just can't work out why this belt isn't aligned.

41585.jpg
 

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