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briggs 20hp intek 406777 backfire through carb/won't run

This is a discussion on briggs 20hp intek 406777 backfire through carb/won't run within the Small Engines forums, part of the Tech Support Forum category. I just put a 20hp intek back together from a recent overhaul, the mower belongs to my father in law


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Old 08-10-2009, 06:12 PM   #1
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I just put a 20hp intek back together from a recent overhaul, the mower belongs to my father in law who ran it low on oil and broke one rod and score a piston. i put a new crank,rods, and pistons as well as all new gaskets and seals in the engine. fired her up for the first time this evening and she started relatively quick everything seemed fine for a round 45 seconds then the engine died. I figured it lost its prime from were the carb was off cranked and cranked the engine it finally started again but would only run about half throttle then it died a gain after maybe a minute I think I had the throttle linkage spring on wrong so I corrected it and now it just back fires through the intake and the exhaust and won't run. but the exhaust is getting hot like the engine was running.

I am beating my head against the wall trying to figure it out I know very little about small engines but I am a diesel mechanic by trade so I understand basic principles of an I/C engine. I got $500 dollars in this motor and I need to get it running any help would be greatly appreciated.

the mower is a John Deere with a 20hp briggs intek OHV

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Old 08-10-2009, 09:10 PM   #2
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Hi smoke20

Sounds like a timing problem. After the engine was rebuilt were the valves re-done and the rockers adjusted ? I believe the adjustment is at .004, but double check the spec sheet for that engine ( just in case). Also check that the flywheel key has not sheered off, on small engines it's a common problem. If the timing is off even by a millimeter the engine will never run.

post back your findings.

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Old 08-11-2009, 09:46 AM   #3
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Are you sure your getting fuel from the fuel pump?
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Old 08-11-2009, 02:10 PM   #4
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octane thanks for the quick reply. I figured a timing problem as well but I didn't think about the key way. i thought the valve and the ignition timing is pretty much fool proof on these engines, dot to dot and key to key way. but if it slipped or was sheared that would screw it up. and that kinda makes sense as to the way the engine ran; it started out great then got out of time. and no I didn't lap or adjust the valves but I'm gonna adjust them and check the key way this evening. to be honest I didn't even have a feeler gauge that small. I watched the smalleng.com vid so I am gonna adjust them that way.

madcat. it is getting fuel and I have all the wires plugged up I took discharge line off the pump and cranked the engine and got fuel and then took the bowl of the carb and cranked the motor and fuel was pouring into the bowl so I figure I'm good on fuel. that was my first thought too
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Old 08-11-2009, 02:36 PM   #5
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Ok that didn't take long. I went out side and pulled tarp off the mower and removed the flywheel nut off and found a this

Notice the angled gap beside the key. sorry for the image quality I took the pic with my razr



As you can see the key looks to be sheared off or twisted hopefully this is the source of my problems. I'll have to bring a puller home from work tomorrow evening to get the flywheel off plus its raining this evening so I can't really work on it tonight
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Old 08-11-2009, 05:12 PM   #6
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I wish i could see your picture better. I too am working on a backfire issue. Its a little bit different than your as mine runs but runs like crap. My flywheel key is slightly tilted and the slots are just a tiny bit misaligned. I don't know if its enough to make a difference. It seems to get off everytime i tighten the bolt down. I'll try to get a picture of mine tomorrow sometime and get yalls opinion also. I used that valve adjust method in the smalleng.com video also and it worked out perfect.
Do get back with us if your key fixes your problem and if you can, try to put up a clearer picture. You can take the pic further away and i can zoom in. I would like to see how off your key is. From what i can tell, your key tilted exactly like mine. I have been trying to fix my engine for several weeks trying new things and previously dismissed this as not being to big an issue. Maybe i was wrong and need to look into it further.
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Old 08-12-2009, 09:24 AM   #7
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I will try to post a better pic this evening
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Old 08-12-2009, 03:32 PM   #8
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well it was a the flywheel key I got to get a new one tomorrow and try her a again.

hey mad cat sorry but I ripped the fly wheel off before I took another pic but if your flywheel key is twisting or slightly misaligned you might want to try a new key it might be worn or out of spec enough to twist which could have a timing effect

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Old 08-12-2009, 03:35 PM   #9
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Thanks again for all of the help guys. sometimes you can get so frustrated that you over look the obvious. I'll let you know how she runs tomorrow I might post a vid if possible
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Old 08-12-2009, 04:57 PM   #10
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Wow that's really sheared. Did you hit something? I cant help but wonder why it sheared. I wouldn't thinkl those shear just on there own for no apparent reason.Let us know how it runs
I pulled mine before and the key seemed to look normal but you may be right ,it could be out of tolerance and squished just enough to let the fly wheel move a tiny bit. I have some spar keys. I'll pull it off tomorrow and report back my findings
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Old 08-12-2009, 07:25 PM   #11
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no I didn't hit anything. the flywheel wasn't torqued to the right spec and it sheared under the load of the drive belt. after i started the mower and it ran for about 30 seconds I hopped on and hit the forward pedal, the mower moved about 12 ft and that's when she quit. looking back it all makes sense now.

but I have a another problem I got a new key and got the motor running tonight but it runs wide open when the throttle is set to idle I thought I had the governor lever right when I put the the case back together but it's running wide open on idle so something is not right. Can the case be assembled with the governor lever in the wrong position? Does anybody have any pics or diagrams of the correct linkage/spring set up? Before I had the springs on incorrect and they were pulling the throttle wide open all the time so I re arranged them and I thought they were correct but its running wide open I can force the linkage to idle by hand. it was getting dark on me so I quit tinkering for the night.
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Old 08-13-2009, 08:42 AM   #12
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Here are a few pictures .I don't know if these are your set up or not



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Old 08-13-2009, 09:24 AM   #13
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Just another note about your question. Yes the governor can be positioned wrong inside the case. When the case is put on you have to hold the governor arm so that the paddle inside the case sits on top of the governor gear assy. The gear has a point that pushes up and down according to engine speed. This in turn moves the governor arm back and forth. The governor spring (bigger one)should be loose with the engine off

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