Popping and backfiring and gas smell

chuck55

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Hoping someone can identify my problem with popping/backfiring and producing a gas smell

So here is the symptoms and data
1. Engine is Briggs 44R677 0021G1. The carburetor is a nikki-6 model 796227
2. Backfiring is through the muffler
3. Valve clearance is good at .004 to .006 per the mfr specifications
4. Pushrods are not bent
5. Cam is not a problem - the rocker arms seem move equal distance when rotating engine by hand
6. The engine is only 4-5 years old and has been maintained
7. Fuel is reaching the carburetor, the lines are clear and the fuel pump works
8. Diodes are not the problem. I've removed the kill wires and it backfires with them off
9. Flywheel key is good. I don't see much more than minor surface scratching on it
10. Does not backfire when running on one cylinder. With left kill wire attached and right removed and ignition key in the "off" position, the engine runs on right cylinder (with unattached kill wire) and no backfiring. I reverse this so the left kill wire is removed and right kill wire is attached and the engine runs on the left cylinder while ignition is in the off position. Again no backfiring. It appears the carburetor solenoid does not shut gas off with one kill wire attached, so it kept running with ignition set to "off"
11 (2) years ago I took it to a shop and they said it was the carburetor. I replaced the carb myself and still had the problem. I replaced the carb again and same problem. This will be the third carburetor I am on.
12. I hooked a spark checker up and both plugs are sparking fine as far as I can tell.
13. I've removed the gas and replaced with high octane gas and popping continues

Right now I think the carburetor is probably the problem. The replacement carburetors cost like $30 to $50 and an OEM carburetor is $170 to $250. It might be possible to get 2 bad carbs in a row.....

Let me know your thoughts. I could really use the help
 

Rivets

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If you have been installing the cheaper aftermarket carbs off uglybay or the cheap clones, it is definitely possible to have two bad carbs. I’ve found that over 50% of the time I’m asked to repair a unit that the customer has try to get by cheaply, I’ve found the wrong or bad carb installed. As a service tech for many years I wouldn’t even think about trying to save a customer money by using these cheaper clones. Not worth risking the customers time, my time, my money or reputation, experimenting to find a less costly solution.
 

bertsmobile1

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Right now I think the carburetor is probably the problem. The replacement carburetors cost like $30 to $50 and an OEM carburetor is $170 to $250. It might be possible to get 2 bad carbs in a row.....

Let me know your thoughts. I could really use the help
The last warehouse I was involved in running rented their spare pallet spaces to ebay/amazon vendors ,
Nothing odd to despatch the same items with a dozen different letterhead invoices from the cheapest with plain B & W A5 invoice to the most expensive with full colour A4 invoice with lots of photos of the supposedly factory on it.
Items were all the same , some times we stuck different brand name decals on them, particularly with the stuff that came in plain white boxes.

So yes very possible to be sold 2 or even 10 defective carbs from the same batch from what to you appears to be 12 different vendors who are all the same sleezebag ,

And there is a big difference between certified OEM parts with a warranty and a defective part that the OEM factory sold originally as scrap metal.
Both can be legally sold as OEM
Also a big difference between a defective part that "FITS" a particular machine and a good part that will WORK on the same machine .

You seem to have done a good job of diagnostic testing.
As it runs fine on on either cylinder alone but backfires through the carb with both sides firing that eliminates the magneto .
So try adding a bit of choke very slowly
If the backfiring initially gets better then gets worse it would indicate a blocked carb fuel passage.
Next pinch off the fuel line with something like a clamp so you can do it slowly
If the backfiring goes away or at least gets better then that would indicate flooding or too high a float level .

To get a backfire in the muffler you have to be passing unburned fuel into the muffler that either ignites on some thing in the muffler that is red hot or is ignited by the following exhaust pulse
You can also try opening up the exhaust valve lash to around 0.010" which will delay the following exhaust pulse just a tiny amount .
You might also have a restriction in the muffler
 

ILENGINE

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This engine wouldn't have the plastic intake manifold does it. They are known to warp at the carb to intake junction. Sometimes due to overtightening the carb mounting studs.
 

chuck55

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Nov 29, 2021
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Thank you all for your feedback.

Yes it does have plastic intake manifolds, and I noticed they didn't form a good seal when I installed the latest carburetor. I used black silicone gasket material around the outside where it didn't quite seal. I know gasoline will eventually eat away at this but I wanted to see if/how the new carb worked. I think I did a pretty go seal job, but I can't see all the way around the thing either. I heaped lots of material in those areas. I think I sealed it.

Still backfires though
 
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