Backfiring in carb (pulling my hair out)

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  #1  
Old 02-14-06, 02:17 PM
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Backfiring in carb (pulling my hair out)

Hi. I have a 20 hp intek that is backfiring in carb, exhaust, and pre ignition. Flywhell key is ok, diodes in ground wires are good, have swapped coils, overhauled carb 3 times, tried changing jets, replaced leaking intake gaskets, valve adjustment is at 006, cam lobes are good, 150psi compression both cyls, valve timing is good. Engine is still lean, and seems like coils are firing out of time. Runs like #[email protected]%. Any suggestion? lol
 
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  #2  
Old 02-14-06, 04:51 PM
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Is that what the manual called for on the valves? Briggs has either the owners manual on their site, use the model number, and should have the settings, and they also have a chart to go by.
 
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Old 02-15-06, 12:14 AM
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Will it run better on half choke? The diodes you checked, were they on the kill wires to the coils? Have you pressurized the cylinders to check for compression leakage?
 
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Old 02-15-06, 06:52 AM
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the valve adjustment specs are .004-.006. I have tried from .004 all the way to .012. Yes, I have ran a cylinder leakage test and the results were good. This thing does improve when spraying choke cleaner into carb, or closing choke. But It still backfires some even when extremely rich. I know it has a lean carb, but what is causing this thing to fire erratically?
Yes Cheese I was talking about the diodes on the kill wires. I checked them with an ohm meter. Current can only flow one way.
I wish I had access to briggs and stratton's service bulletins.
Almost forgot model#406777 type 0128. I just replaced the crankshaft, bored .020, replaced pistons and rods ect. Any chance the new crank could be wrong causing the timing to be off? By that I mean could the keyway be clocked differently on this crank depending on the date code? Not sure what the changes to the crankshaft were, or if it was wrong would it even fit.
 

Last edited by d_farnz; 02-15-06 at 07:16 AM.
  #5  
Old 02-16-06, 12:41 AM
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I don't think it has anything to do with the crankshaft. I'm wondering now if it hasn't got a loose valve seat. This usually causes erratic backfiring. What originally happened to this engine to rate the rebuild? Did it run hot previously? If so, even more reason to suspect the valve seats.
 
  #6  
Old 02-16-06, 05:58 AM
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I didn't hear it run before the rebuild. It came to me low on oil with two broken rods. Something caused the cylinders to be worn enough to burn oil (most likely the lean carb caused extreme cylinder temps.) I just dont know if it was backfiring or not. The valve seats seemed to be ok, and the valves weren't worn too bad. But it's a though. Might be moving some after running for a few seconds. But wouldn't that show up on a leakage test?
Also, if I kill one cylinder or theother it quits backfiring. Dont matter which one. As soon as both cylinders are firing again it starts again.
 
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Old 02-16-06, 07:53 AM
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Since this engine has two seperate ignition coils, it's unlikely that you have a coil related problem. What I would do is the following: 1- Eliminate the possibility of chassis related safety interlock switch trouble by locating the junction block on the engine and removing every wire except the wire(s) going to the coils (kill wires). NOTE: you will have no way to shut down the engine and will have to use an insulated handled pliers to pull off each spark plug lead from each spark plug. 2- Go back and set the valve clearances at .003" and be sure to set them while the respective piston (adjusting cylinder #1 first then cylinder #2 last) is at the proper position - 1/4" past top dead center on the power stroke and adjusting the intake first and then the exhaust last. This is per Briggs Service Bulletin #729. The only other Service Bulletin that relates to your engine is # 731 which addresses a rich condition at idle speed on engines with a plastic intake manifold whereas you need to install a swirl plate, part number 791677, between the carburetor and manifold to combat this issue. By the way, the correct crankshaft is part number 699701.
 
  #8  
Old 03-18-06, 12:25 PM
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JohnDeere 9hp OHV won't start, backfiring thru carb

I have a 9HP John Deere rider that I inherited from a buddy that moved to Canada. It sat for about 3 years in a storage building before I got it. It wouldn't start at first, so I did a carb tear-down (I've done a number of Quadra-jets and motorcycle carbs, so this was a cake) and slapped it back on. Started right up and ran all last summer and fall. Since we live in the South, it's been run a few times over the winter (kids hauling leaves and branches). Now the thing won't start. I pulled the carby and it looks fine. Plug I cleaned up (not that it was that bad). Tried starter fluid, no luck. It cranks and cranks all day, but mostly it just backfires through the carb. I was going to tear the dumb thing apart and check the valves, but that's quite a job, so I figured I'd ask for some advice. What's the path of least resistance? Are there some simple things to check first? I have no idea how to check timing on a lawn mower (I hooked up the gun and spent an hour looking for somewhere to point it -- just kidding).

Thanks for your consideration,

Bob in SC (where the grass grows fast)
 
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