Briggs & Stratton 450 148cc popping exhaust and rough idle

[LochlanR]

My B&G 450 engine is popping out of the exhaust and almost stalling itself but then goes back to normal engine speed.

I have replaced air filter, cleaned fuel tank, cleaned carb, and gapped the coil but no luck, I plan to sell the mower so can't have it causing issues. The governor springs have been replaced but they were cheap so not sure if they are the issue but seem alright.

 

[slomo]

engine is popping out of the exhaust

Look at the flywheel key.

 

[LochlanR]

Look at the flywheel key.

Flywheel key is all good

 

[slomo]

Pull the flywheel and look again with magnifiers on. Or maybe you have good eyes? Several on here have stated the same thing. 9 pages later the flywheel gets pulled and discover a bad key.

If the key is truely good then it might be camshaft time. Or possibly water in the fuel maybe? But you said it clears up later.

Does the mower sit outside? Maybe gets rain on it? Maybe water in the tank that you flushed?

Post up a video so we can help you more. Like the starting procedure to full revs.

 

[LochlanR]

Just took flywheel off, the key is perfect, the mower has been serviced with new fluid and the tank has been cleaned and filled with fresh fuel, it starts on first pull, runs, then occasionally pops on the exhaust, then sometimes drops it's revs and picks itself back up again. I would record it but it's just started raining as I took the flywheel off

 

[sgkent]

run some upper cylinder lubricant / cleaner in the fuel for a tank. Maybe a sticking valve. Or pull the head off, pull the valves out, clean them, lap them, set their lash, and replace the head gasket.

 

[Rivets]

Post the engine numbers so we have an idea which of the hundreds of Briggs engines you have? Sounds like a rich running condition.

 

[LochlanR]

run some upper cylinder lubricant / cleaner in the fuel for a tank. Maybe a sticking valve. Or pull the head off, pull the valves out, clean them, lap them, set their lash, and replace the head gasket.

I have some REDEX I use for the car occasionally would something like that work?

Post the engine numbers so we have an idea which of the hundreds of Briggs engines you have? Sounds like a rich running condition.

9T502 0173 H1 1209055

 

[Rivets]

Thank you for those engine numbers, because they make a big difference in how to proceed. Your engine was built in 2012 and has a pulsa-jet carb. When you removed the carb to clean it, did you replace the pump diaphragm and gasket #750283, plus tighten it down properly? Problem with these carbs is that the tank will warp, allowing an air leak and the diaphragm will not function properly. Have also seen many DIY guys install the diaphragm and gasket in the wrong order. Both of these conditions will cause your problem. It’s not uncommon for these diaphragms to dry out. I’ve attached a service manual for your engine. Please review section #3 on carburetors and check your system. http://www.tuks.nl/WFCProject/img/E...210-Briggs-Stratton-Service-Manual-L-Head.pdf

 

[sgkent]

I have one of those carbs and tanks that I had to use my skills as a prior machinist to file the tank flat because it had warped over time. Replacing the carb did not solve the issue but filing the tank did. If you have it apart again, lay a good straight edge across the tank and the screw holes etc to see if it is warped. If it is not leave it alone but it if is than a very light filing with a flat wide mill file in a series of a stroke on one way then a stroke from a different direction etc. One time forward, then one towards you then one left then one right then X etc., repeat. Never allow the file to tip or it will ruin the tank, and keep it 100% flat all the time until there is an evenness to the pattern. I used a sharpie on the high and low spots so I could tell where it was cutting etc.. Once that was done it ran fine again. Some places sell the tank and carb as a unit.

The valves can be sticking too. Especially if old ethanol fuel was left in it. A friend has run into that many times on the engines he has worked on for others.

I am not familiar with the REDEX brand. Some people like Seafoam. Those carbs have a lot of plastic in them so one has to be careful with what additives and the amounts that are used. Maybe someone else here can suggest a brand. I know that PB Blaster now has a small engine spray designed to free up rings and loosen sticky valves, but I have not tried any. https://blasterproducts.com/product/small-engine-tune-up/