Briggs and Stratton V-Twin Persistent Popping and Afterfire from Exhaust

[sgkent]

Swap the coils left right & see if the problem shifts with them

he wrote "I swapped the coils and the afterfire remained on cylinder 2."

 

[sgkent]

my general engine thoughts on this are to swap plugs and see if the problem swaps. You won't be the first person to get a bad plug to stick it in to replace a bad plug.

I have to wonder if something is going on in the valve train or intake. Doing a compression test at low RPM can't really test a moderate speed misfire. I remember having a 6-cyl Jeep head completely rebuilt at a reputable shop when I first got to Sacramento. It ran fine for about 10,000 miles then it developed a mis-fire on a couple cylinders. I finally took it to a guy with a scope who told me based on the waveform it was something with the valves. I pulled the head even though I thought he was wrong and sure enough, two cylinders had worn the new false guides so badly the valves rocked. They were randomly not seating properly. The shop gave me my money back, I found a new head for it off a totaled low mile, and life was like new again. You could have a weak valve spring, or a worn guide that isn't picked up in a low speed compression test. Likewise any air dilution can make a mixture so lean it misfires, or if the mixture is too rich it can misfire and then burn in the exhaust when air reaches it there, and the next exhaust flame hits it.

 

[slomo]

Look for leaking exhaust gasket on cyl 2. Sounds like air or fuel is passing into the cylinder/exhaust and popping. Vacuum leak on cyl 2?

Is valve lash proper?

Flywheel key okay?

You said all the above was good. Smoke test normally has nearly 0 pressure. Wondering if this was a proper check?

Now it's cam inspection time. Maybe a dead lobe in #2 cyl?

Remove valve covers. Look to see all valves move the same with engine on and off.

 

[sgkent]

maybe the cam lobe suggestion might have some value. The higher compression on that cylinder might be caused by the valve having less duration and lift.

 

[gre1252]

Swap the coils left right & see if the problem shifts with them

I tried swapping the coils and the afterfire did not move. I double checked it with the muffler off. Only cylinder 2 is popping out the exhaust.

 

[gre1252]

Look for leaking exhaust gasket on cyl 2. Sounds like air or fuel is passing into the cylinder/exhaust and popping. Vacuum leak on cyl 2?

Is valve lash proper?

Flywheel key okay?

You said all the above was good. Smoke test normally has nearly 0 pressure. Wondering if this was a proper check?

Now it's cam inspection time. Maybe a dead lobe in #2 cyl?

Remove valve covers. Look to see all valves move the same with engine on and off.

I smoked cylinder 2 from the muffler pipe and I could not see anything leaking with a bright light. I also smoked the intake from the air filter connection, while plugging the vents with my fingers on the back, and there was no visible leaks. I use a diy paint can smoker which has worked on cars in the past.

Valve lash is set to both .005 in. as recommended by the shop manual at 1/4" past TDC on cold engine. I watched all of the valve movements with both covers removed carefully while feeling the piston head movement in the cylinder. Everything looked the same on both sides. I measured the exhaust port spring compression when fully open with a caliper and both sides measure the same. I doubt there's any cam lobe issue. I thought the camshaft compression release might be at fault, but the dynamic compression test number shows numbers in the 170-180 range when shutting off the engine from high rpms. The counter weight is definitely moving on the camshaft.

Yes, the list of test above came back normal except for a damaged float bowl gasket (698781) in the clean carburetor. One of the little circle gasket on the side of the main gasket in the middle was mangled and creased. However, on this model of carb it does not seal anything. The space above and below that section of gasket is blank plastic or metal. I replaced the gasket, just in case, but the engine still pops and backfires out the exhaust.

 

[sgkent]

does the carb have two outlets, are there two carbs, or is the manifold bifurcated?

Did you swap the plugs?

 

[bertsmobile1]

When you swapped the coils over, did you try it with the kill wires removed ?
I ask this because you can get feedback ( which is what the diodes are supposed to prevent ) from one to the other
The fact that you got a faultering tach reading on one side & not the other points towards a bad magneto.

Also have a good look at the rockers
the cam grinds are identical , the followers are identical & the valves are identical
So if the cam is good both pairs of rockers should sit parallel at firing time & move the same amount.

If the head overheats then one of the guides ( usually the inlet ) can slip in the head so the inlet rocker will be out of place .

 

[gre1252]

does the carb have two outlets, are there two carbs, or is the manifold bifurcated?

Did you swap the plugs?

The carb has only one throttle plate. The part number is 597128.

Yes, I tried swapping the plugs back and forth with the coils and nothing changed.

 

[slomo]

So brother, this is puzzling. Again flywheel key just a micro tick off? You said it was checked already.

Could be excess fuel or air, SOMEHOW, getting into the muffkin, maybe dripping into it??

Carb needle and seat pressure tested to 7psi, holds for 30+ minutes?

Think your exhaust port needs pressure tested with 20-40'ish psi. These car evap system smokers are 1psi and under. Think they are 1psi MAX pressure.