Briggs Backfiring! New Carb, and Cam done. Now what?

[suparoood]

Hey all. New here.
So I work on plenty of small engines, but this one has me stumped. I am working on my brother's John Deere 190C lawn tractor with a Briggs and Stratton V Twin 25hp Engine. It has multiple problems, but the biggest issue is it currently backfires through the carb, and fouls plugs up. It doesn't do it when first started, but has to run for a minute or so before it starts, then it starts doing it badly. It had poor maintenance most of it's life. But I'll go through what I've done, and my bro.
-fuel filter
-new spark plugs 12yc champions
-oil change
-valve adjust to .004-.006"
-clean carb, drill out swollen gaskets. glue back in loose jets (I know, I know, I was stuck otherwise)
-check for constant spark with inline spark tester - good
-compression test - good, I think over 80psi but was months ago. Runs smooth when not back firing, and not hard to start
-leak down test - damn near perfect - I did not suspect this to be true, but verified tool operation on known bad just to be sure!
-replaced cam to eliminate any chance that the tiny amount of wear on lobes was enough to cause problem - had 1.221 at the worst, where 1.228 is spec, and below 1.225 seems to be the reject spec.
-replaced carb after knowing about the glued in jets, and swollen gaskets anyways, and to try to make sure the issue was solved.

The full story - It started popping like crazy. He changed plugs, fuel filter, then called me. I did valve adjust and carb clean. No better. Saw that spark plugs and air filters were problematic online for some people. Put non-Canadian Tire plugs in, directly from John Deere - this is the champion 12yc. Ran better for a bit, then fouled again and ran like poop again. Also found after adjusting valves, engine would struggle to turn over sometimes stopping on compression stroke - suspected faulty decompression valve as per many videos, as well as some on worn cam lobes causing backfires. Thought for sure this was my problem. Took it all apart. Engine not equipped with decompression, darn, nor are the cam lobes crazy worn, though still just a tad below spec, so replaced anyways. Put new carb on to make sure glued in jets or other issues in carb weren't the cause, or would be a future failure.

The only thing I have seen online that others have found is trying a 14yc plug that runs hotter, and some others have had problems with the air filter not crossing directly and causing an issue. Every part we have put on was OEM from JD dealership. The engine backfires with or without the filter on. I am tempted to get some plugs for a 3RD!! time and try that. Otherwise the only thing I can see is a set of lazy coils that just isn't showing up during the inline spark test (the kind with a little light that lights up, and you can watch while it's running normally). I'm not interested at throwing more money at this thing. It's making me look stupid when it's what I do for a living! (though not this type of engine specifically). I plan to recheck the spark on Monday. Who has some other ideas?

 

[Rivets]

Have you looked at the valves themselves. Sounds to me like you may have an intake valve which is either slightly bent or a valve guide which has carbon buildup. Either case would cause the valve to stick as soon as they get warm.

 

[suparoood]

I only looked through the intake and exhaust ports when I had the engine out. Both are very clean (surprisingly) and move very nicely through their guides. I'm actually surprised how clean everything is in the head despite the general lack of maintenance. No visible carbon, and I would think it is eliminated having tested for leak down and having none at all. It's possible there is a minor bend that maybe causes the valve to stick when warmed up though, or a tiny amount of carbon in the guide like you say, that does the same thing. Unlikely, but not impossible. And that's kind of the stage I'm at with this thing! I don't really want to pull the head though, if I don't have to. Thanks for the response

One other note with this engine. Very odd. During disassembly to get to cam, I found a piston pin locking ring laying in the pan. No damage anywhere on the sidewall of the cylinders. Not even a scuff, or scratch. I'm guessing it got hot once and popped out. But due to no movement in that direction, I think it shouldn't be a source of the problem (though may cause catastrophic failure eventually - which we are both just hoping for at this point! haha)

 

[bertsmobile1]

Take the valves out and have a good look at the valve seat as well .
A loose seat will also present similar problems when hot untill it gets chronic

 

[tkos115]

Sounds like it could be a bad coil acting up, and when it warms up it gets worse. Usually coils will get worse when they warm up when they are starting to fail. It happened to a few engines I've worked on. See how the spark looks when it's cold, then check it when it gets hot and starts acting up. To me that would explain the fouling and why it gets worse after it runs for a few min. I would physically look at the spark by removing the plug and grounding it to something metal to see the spark, or get a spark tester that allows you to see the spark jump a set gap.

 

[mechanic mark]

Please post B&S model xxxxxx, type xxxx, trim xx, & code xxxxxx from engine, thanks, Mark

 

[suparoood]

Sorry, I thought I did. Model 445777 Type 0264 E1, Code 051020yc

 

[mechanic mark]

Results

www.briggsandstratton.com

Read through operators manual to see if you may have missed something. Also check parts manual & see if you installed correct carburetor.
example: https://shop.briggsandstratton.com/products/briggs-and-stratton-791230-carburetor, this is your carburetor not a cheap knockoff like the ones sold on Amazon.

Briggs and Stratton 797890 Carburetor Overhaul Kit - Jacks Small Engines

Overhaul kit from Jacks.

Amazon.com : Briggs & Stratton 491055S - 2 Pack : Lawn And Garden Tool Replacement Parts : Patio, Lawn & Garden

Amazon.com : Briggs & Stratton 491055S - 2 Pack : Lawn And Garden Tool Replacement Parts : Patio, Lawn & Garden

www.amazon.com

Try these B&S spark plugs if you care to. Questions: What brand & weight oil is in engine? What octane gas is in tank & are you using fuel treatment & brand?
Thanks suparoood, Mark

 

[suparoood]

I got the parts directly from John Deere, so I sure hope they are correct! Being that the new carb is acting exactly like the old one though, I wouldn't think the new and old have the same issues by fluke. But I'll try to check the part # tomorrow. I checked the spark with inline tester again today. I think the tool itself isn't the best. Cheapo from princess auto. It seems to "miss" the odd time on both sides - but not how I've seen misses before in automotive where it clearly doesn't spark, and you can hear the miss at the same time. This would backfire and pop randomly with no sign of missing spark, then "potentially" lose spark (I don't trust tool) with no sign of backfire or running rough. Basically the visual cues not lining up at all with the audio cues of backfire. I did get the fan and shroud off, and verified that the key is not sheared off - DARN. Would have been an easy fix. My bro is currently installing RC14YC plugs, as we read somewhere that the 12s may not run hot enough. We figured what's the harm? Worst case is he has a spare set of plugs for in a few years.

I looked it up on the JD website. It has their part numbers, rather than briggs. But a search brought up this website that lists the JD part number and says it is the OEM replacement for 791230 like you say. So I suspect it is right. https://www.greenpartstore.com/John-Deere-Carburetor-Assembly-MIA10632.html

I'm kind of at a stalemate. I don't want to throw more money at it, even though it's not mine. So if I take the heads off - costs new head gaskets at minimum. And unless there is a glaring problem, I may be stuck going that route and not finding anything. Or, I can suspect the coils are getting weak and replace them, probably for $$$. Which to me, "shouldn't" cause backfire, only misfire. - though I never rule anything out.

This thing is eating my lunch. Ironically I keep finding things wrong, but the issue never gets solved. (crazy loose terminal at solenoid, though wow, that should fix hard start after clean and tighten. NOPE)(Backfiring, and hard start? 90% chance worn camshaft and failed decompression valve - NOPE not equipped, and wear too minor)(also needs an ignition switch because sometimes you turn it and nothing at all until you wiggle or tap it, with clean connections)

I will be calling my friend at JD to see if he knows anything to look for. I will also test my spark tester on a known good engine to see how accurate tool is, since it's relatively new to me still. Otherwise I'm crossing my fingers on these spark plugs. Otherwise I may spend another few hours staring blankly at it trying to convince myself its anything but the valves/heads

 

[Scrubcadet10]

might want to read through this thread, https://www.lawnmowerforum.com/threads/briggs-v-twin-backfiring.50066/