Poulan V-Twin Mower backfires when load is applied from the blade

[bricha2]

Another pictures of the valves

 

[Spanner]

Thanks Guys for all your comments.... Here's what I find from the Spark Plugs... I switched them from L-R. The Left side (non-backfiring side) spark plug is a Champion RC12YC. This SP appeared to be dry and good. The Ride Side (backfiring side) SP is a Champion RC14YC. It appeared a little wet from oil. When I switched them, the backfiring was not as constants as before, but still backfired a little. I don't think the mismatch plugs are the problem, though....

I then went to the 2nd option of removing the valve cover. Now, I don't know much about valve, but willing to learn. The rockers on the Left side (Backfiring side) appear to be level but the top rocker was a little loose. The (non backfiring side) the bottom rocker was a little loose as well but not backfiring. The top rocker was not even or level like the left side. See my picks of the valves. Another note, the bottom side underneath the Valve, muffle connection was wet with oil. I have also provided a picture of that as well. View attachment 517988View attachment 517989View attachment 517990

Always some reason anybody would fit a single odd spark-plug so there must have been an issue there for a while?

The oil on the exhaust is from a leaking rocker cover gasket by the look.

In Para. 1 you say: "The Left side (non-backfiring side) " & " The Ride Side (backfiring side)..." {Ride = Right?}

In Par 2. you then say: "...the Left side (Backfiring side)..."

So I'm a little 'confuzzed'.

I'm also wondering how you determine a particular side is backfiring?

Not much can be determined from the pic of the rockers to me anyway.
they look 'normal'. One valve slightly open, one closed.

I would still fit two new NGK Spark Plugs (of the same rating) to start with.

 

[CincyFlyer]

Another pictures of the valvesView attachment 517991View attachment 517992View attachment 517993

It's more likely that a valve is "tight", not closing completely, than a valve is sticking or the guide has moved. Note that you should check the rocker arms with each cylinder at top dead center on the compression stroke. This would mean that both valves will be closed, and you can check for clearance (easy to set, BTW). On some engines you can look in the plug hole and watch the piston come up. When it's near the top, and neither valve is depressed, you're good. This will be in a different spot for each cylinder. A "tight" valve is a bad thing, and if not adjusted soon, may cause damage that requires disassembly. I've adjusted the valves on my Briggs 21-hp V-twin twice in 160 hours, takes a trivial amount of time.

 

[frugalangler]

One other thing that can cause "backfire" or the symptom of such (in this case it's not backfire - that is through the carb, but firing late enough that the ignition is not complete when the exhaust valves open and the flame front is exhausted out the exhaust port.), is out of time - I've seen the key on the flywheel shear or partly so, enough that the spark is retarded enough to cause it to fire way late in the 'power stroke' where it should have already been fired, and it's actually firing into the exhaust stroke. This can happen if the mower hit a solid object, the shock can cause the weight of the flywheel to shear the key. Not a real common problem, but in my many (too many to admit how many) years of servicing small engines, I've seen this a few times. Only way to know is either a timing light or simply pop the top housing, remove the flywheel nut, and observe the positional relationship of the shaft key-way to the flywheel key-way.

 

[Texasmark]

The Engine is a Briggs and Stratton Model 40777, Type 0167 E1, - Poulan PPR20H42STA - Yes, The choke is all the way pushed in and the air filter is clean and new.. Again, while running in idle the mower runs perfect, no surging or sputting, just runs fine.. But as soon, as I apply the blade, it start to backfire thru the right-side exhaust... I have cleaned the Carb several times, replaced any gaskets or O-rings that look worn....

Go to the Yesterday's Tractors www. Start in the Garden Forum section and look for a post titled "Jeffcat" authored by me, dated around the last week of July. Read all of it. Then go back to the index and go to the Tool Talk section. Go down to the first days of August to the same "Jeffcat" post name, authored by me and read that.

When finished, do as I did. Problem solved. As he said therein, the carburetor is the culprit and (using his words) it is putting .5 the required gas in one side which won't pull a load and 1.5 times the required gas in the other side, making for an overrich mixture and your problem.

You may get by with your plastic intake manifold, but if you read up on them they are a problem as was the OEM carburetor and new designs were required to get the problem solved. If you go to Partstree.com and insert your model number you will find the new part numbers listed in the BS parts lists showing full well that the problems were discovered, solutions were designed and manufactured and are ready for the consumer.

I got my Carb off ebay and the manifold from PT as I recall....total outlay about $150. For the record my engine is the 40H77 type O123, B1. Problem affects most all of the V twin, vertical crankshaft around 22-25 hp which use the Niki twin barrel carbs. 18 and 20 hp use a different carburetor (Walbro single barrel).

 

[bricha2]

Thanks Guys, for all your help. These are the things that i have done, One post mentioned, to have the spark Plugs replaced. I have replaced the Spark Plugs and the problem is still there. Another post mentioned a flywheel problem. Now, I do have several teeth missing from the flywheel. It's such a problem that if the flywheel is not in the right position, the mower will not startup. The Starter will just spin with nothing to engage. I have to turn the flywheel 90 degrees to find a good tooth. Once found it will power on fine. However, once running again it will run fine in idle, again once the blade is engaged it start backfiring again. Texasmark, I will start working on your resolution next... Thanks