Thread: Boxxer 421 back firing and running rough

I've had a lot of problems with my Briggs & Stratton engine. When the engine is hot the carburetor will back fire and not want to throttle up. Some times it will open up and begin to run fine. The problem has been persistent. I even had the carburetor rebuilt last December, but it did not solve the problem. I finally broke down and had a brand new carburetor put on today thinking surely that would fix it, but it did not even help so that's about 450.00 down the drain counting the rebuild and the replacement. I also had the exhaust heat exchanger taken out and cleaned a couple months ago at Jondon St. Louis and a compression check done which improved some after the heat exchanger was cleaned. If I remember correctly the compression was only about 40 psi after the heat exchanger was cleaned and was much less before. Btw, this problem occurs even with the diverter off. This is driving me nuts and nobody seems to know how to fix it. I'm having a compression and leak down test run tomorrow and see if the local small engine guy has any idea what's wrong, but this guy works on lawn mowers and weed eaters not truck mounts. Does anyone have any idea what could be causing this. Next step is a new engine since over haul is out of the question.

does it eventually run and your using it eveyday or is this a unit that you are trying to get running?
it may be an electrical problem. when i put my new briggs 31hp engine in my tm it ran perfect while testing. then i wired some gauges and stuff (with the battery connected, oops) and shorted a wire on the frame. didn't think much of it. then went to run it after wiring everything. it would start and idle a little, pop and backfire, then when i tried to throttle up it got worse like the timing was way off. pulled the front cover, adjusted coils, swapped coils with old engine, nothing changed. it was driving me BONKERS. i couldn't believe my new engine was jacked already. i was going to pull the flywheel off to see if it spun on the crankshaft. then my buddy pulls the kill wire off the coil and says try it again i gave him a scarcatsic "yeah right" and started the engine, it ran perfect. there was a little diode on the kill wire to the coils that i apparently fried when i shorted a wire. swapped it with an old engine and it runs fine now. you can check it by pulling the front cover off and removing the kill wires to the coils. it should still die when you kill the power by shutting the fuel valve if equipped, (takes a few seconds though). that is if it may be electrical, if your sure it is a fuel or compression problem then i just did a lot of typing for nothin. but i feel your frustration.

youmight need this lil bugger. heres the link http://www.jondon.com/catalog/produc...c043d0ee1c8142

Kevin has a good point. I had a geo metro had the same issue. My god I was ready to give it a molatove cocktail good-bye. and as it turned out a diode called fusible link. Not a fusible link but that is what they called it.
another thing to check also the cooling screens that cover the flywheel. remove the shrouds everything. Sometimes junk builds up in the cooling fins, and then more or less overheats everything. resulting in poor performance lunging, backfiring etc.

Check wires going to the backside of the ignition intermittent powerloss to the cut-out solenoid
check voltage going to the fuel cut-out solenoid on the carb. should be energized with the key in run, no-voltage with key off.
Man if I was there I would be more than happy to diagnose it for ya. But it would cost you a Starbucks lol

let us know wht it was when you get er fixed

Thanks for the replies. I'm still not sure what the problem is so I just took it into a small engine shop for some testing. I'm leaning towards an electrical problem, but from what I've read and been told there at least a few different things that could cause this. Yesterday I disconnected the left spark plug while the engine was idling and it immediately died. I did the same thing on the other side leaving the left one connected so it was only running on the left cylinder and it continued to run. That told me something is wrong on the right cylinder or right coil. (I think) I am having a compression check, leak down test, and the coils checked. I'm also having the pressure from the fuel pump checked to make sure it's the required 2 or 3 psi at the recommendation of a Jondon tech.

Just in case I went ahead and put another spark plug on the right side and it seemed to help, but it will still back fire when you throttle up if the engine is hot. It did not die when I removed the left spark plug today with that new spark plug on the right. The plug that was on it was fairly new and in good shape appearance wise. I hope to know more this afternoon. I just wish I had done all these test before I shelled out nearly 300.00 for a new carburetor!!!!

A word to wise. Testing is cheap. Parts can get pretty friggin high! I wish I had been wise about this and tested first.

Pat I would love to have you diagnose it for me, but you would have to settle for folgers or Maxwell house. No Starbucks here anymore. The closed years ago.

Could be something simple like a valve adjustment. If your intake valves are not closing then combustion can flow back through the intake causing backfire.

Ara is right it could be a valve (comp. check should verify that).
however i would put my money on the right side coil. i have had this happen a few times and the worst is when they don't go out completely, they go intermittent and that can frustrate the CR@P out of even the most sane person. hard to figure out the problem because you can check your spark and still have good spark (or at least you think). they can also be fine when cold and short out when they get hot. good news is i believe those coils are around a $100 to replace (about $200 less than your spare carb. lol.)
if the coil tests good have them check the kill wire diode that goes to the coils also, if its bad it can jump spark from the other coil which will cause backfiring. to test it just remove the wire and see how it runs or use an ohm meter. the diode is about $2 to replace.
good luck and let us know what happens.

I also think a bad coil on the right side fits. Engine bogging down on throttle up... It definately fits. The plug wire removal confirms it. It could be a PITA to remove if there is an obstuction like a panel in front of the shrowd.

If JonDon did a running compression test that is called a dynamic compression test. You say it read about 60psi while running. Then you say your small engine guy says it should be around 115psi. In my mind he is probably talking about a static test at 115psi which would be double what it should be. Static test or what is called a cranking test is going to be higher than a running test. A running compression test is not done in high rpm but rather in low. I dont know why they just didnt do a cranking compression test but no big deal. And a running test is done with a scope analyzer. I think the numbers sound about right. 60x2=120 . I think your compression is acceptable enough not to cause backfire.
Do not go over 3 psi or you could flood the carb. Get the pump from JonDon because 1 psi is not sufficiant. Also the coil should have been checked for resistance. My suggestion would be to replace the easiest thing first. Which would be the fuel pump. Then go from there. That is how it should be done anyway. Always fix the easiest stuff first. If the coil has a semiconductor and it was fried it would ground out the coil. If I remember correctly only one coil has a kill.

No Randy you are not underatanding what I am saying. A running compression test will show about half of what a cranking test shows. If you do a running test at 60psi then a cranking test should be near 120psi. EAch side has its own value. Your number at 60psi is good. The range is normally 50-75 psi on a running test. I think someone is crossing these numbers on you. Running number and cranking numbers are different in spec.

Hang on, the small emgine guy said the combined total is 115 psi? each cylendar should be unique with its own psi rating. I am thinking Ara was right about the valve adjustment. It could be that the lock nut has backed out allowing too much slack. if the valves clearance is too great it will allow gas to flow out everywhere exhast and intake during the combustion cycle causing backfires and odd sounding exhaust tone hence the bogging sound.

If your fuel pressure was 1 psi why didnt jondon put a new fuel pump on for you. This is starting to sound like a wild goose chase for you. The new fuel pump should be your first order of business. Don't forget to install a new filter with that fuel pump.

Originally Posted by Jaeman

What was the result????

Ara, of course, was right about the tests. I was going back and forth between Jondon and the small engine shop and didn't understand the difference in the test. As far as fixing the problem, I had what i think was a valve adjustment. I think it was the one on the left side, if I remember correctly, they said was way off. Since then the machine has been working fine. I haven't had to replace the engine yet. Replacing the carburetor did seem to help some over all so that wasn't a complete loss. Also, having the exhaust heat exchanger cleaned at Jondon made a very noticeable difference both in how the engine ran and it got my solution temps back to like new again. I believe the exhaust heat exchanger being somewhat clogged up and the valves being out of adjustment caused the problems I was experiencing.

What Ara said here explains it pretty well I think, "No Randy you are not underatanding what I am saying. A running compression test will show about half of what a cranking test shows. If you do a running test at 60psi then a cranking test should be near 120psi. EAch side has its own value. Your number at 60psi is good."

Best thing I ever did to my boxxer was; let someone remove every bit of hydra master from it and start over; haven't heard a peep out of it since other than oil changes!!!!!

adjust your valves wand wizard...low compression...intermittantly runs just fine....back firing through the carb....as it warms up...classic symptoms of valves being too tight...