Briggs 16.5 twin cylinder starving for gas / backfires

11 years ago

I have a White/MTD w/ a Briggs 16.5 HP 42A707 (420000) OHV twin cylinder low emission that slowly began backfiring on shutdown, and now pulses unless I pull the choke partially. Plugs are burning very lean. I changed the fuel, the fuel filter, and a cracked fuel line. Tried pouring Gumout directly into carb. Oil has always been changed - no oil filter on this model. Air filter is clean. Really gets rough at lower speed, almost won't run, but does sputter at full throttle. Never used to do this, shutdown backfiring seemed to start last summer, now getting worse. Does this sound more like a plugged carb jet / main jet? or a valve clearance issue? Not sure if this model has a fuel pump - the carb inlet is higher than the bottom of the fuel tank. There is a small rectangular box on the face of the carb - pump?? I maintain the thing well. Dave

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Comments (9)

agletdave
Original Author
11 years ago
Someone mentioned in another blog that a plastic manifold causes problems w/warpage. My manifold looks like cast iron, but there is a black square between the manifold and the carb, but when I tap it it feels like steel not plastic. Like I mentioned, engine now sputters at full throttle, so I dont think it is an idle jet.
agletdave
Original Author
11 years ago
below is a link to photobucket with some pics of my carburetor.

Here is a link that might be useful: photobucket pics
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As a matter of fact, I have seen something almost exactly like you are describing. It was on my own B&S powered (422707) Huskee Garden Tractor just last week. Started it up this spring and it did not want to stay running unless you had the choke pulled nearly to the full closed position. I pulled the top of the carb off and way down in the main jet well I saw some black junk sort of sloshing around. I went ahead and removed the carb from the intake and carried it to my workbench. I poured the gas out of the bowl straining it through a paper towel lined filter funnel to catch the material for a look-see what it could be. I was unable to say for certain but I saw a worn area on the bowl cover gasket that appeared to have a notch worn in it (by the float pivot tab) and the bits of material may have been "shavings" from the gasket. Anyhow, the main jet had a little of this stuff in it and that was why the choke had to be fully applied for the engine to stay running. The clogged main jet could not deliver enough fuel for the engine to run on with the choke open, the air/fuel ration was simply too lean. By closing the choke, the amount of air allowed into the carb was drastically reduced and that amount of air was nearly the correct volume to match the paltry amount of gas coming through the partly clogged main jet. I gave the carb a total cleaning soak in a bucket type carb cleaner and now the engine runs strong again. Something could be lurking in the bottom of your bowl too. If your carb has an after-fire fuel solenoid, a sticky or corroded plunger valve on the solenoid might be causing the extreme lean mix you obviously have now. Carb cleaning time.
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Before we start guessing, you need to do the tests suggested by Walt. It is not unheard of for a magneto (coil) to become weak and not be able to fire off a spark when conditions inside the cylinder change. So, a check for presence of spark should be done when the engine dies. This kind of test is more meaningful if the genuine Briggs spark tester is used because that tool allows the spark plug to remain in the head and thus the effects of Combustion pressure and cylinder temperature can still influence the "sparkability" of the coil. Simply checking for spark by laying a plug out on the engine does not allow for those influences. But you still need to check for spark any way you can when it dies. Temperature changes and the effect of increased compression pressures can literally suppress a spark from ocurring in a warm engine. Applying some choke will have a direct effect on the compression pressures in an engine (make them lower) and that could be why the engine is "smothering down". Another thing that could cause the engine to die out is starving for fuel. If it is dying due to lack of fuel, you can determine that by spraying a short burst of spray carb cleaner into the carburetor throat as soon as the engine dies. If the engine tries to run strong for a second or 2 and dies again, there is a fuel delivery problem. If it is from too little fuel, pulling the choke will keep it running because the choke is reducing the volume of air that can mix with the insufficient fuel volume. The engine might not die with the choke pulled, but it will not make much power either. Another thing that could be killing this engine is an exhaust valve that is too tight (too little clearance). if this were the case, when the valve heated up, it would not seat completely and compression would be lost out the exhaust, resulting in the engine losing RPM and power. This is why you need to check compression when it dies.
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I can't see how adjusting the valves would be related to blowing a head gasket. IF the gasket was "slightly blown" AND the intake valve lash was so loose that the cylinder wasn't getting a full "charge" of air on the intake stroke, than possibly allowing the engine to develop more power than it had been, might have been enough to put the gasket "over the edge". That's getting to be quite a stretch though and still wouldn't be the root cause. Besides, you'd probably have trouble starting the engine for multiple reasons. (compression release not functioning amongst them) How would putting the piston @ BDC be useful? IF you are going to change the head gasket, you want the piston in a position where both push rods are loose. The fact you asked that question makes me wonder about the procedure you used to adjust the valves.
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I just read this and it sounds to me like it may well be the woodruff key in the fly wheel that sits on the crankshaft has moved a little or even snapped......Some times as you mow and it say hits a bit of wood or comes under heavy load that key can move a little or as i say snap. Then when you try to start it you may get a kick from the starter pulley like a horse kicking you in the arm...I would take the top pulley off and the NUT then look down and see if the KEY is in line with the pulley SLOT. If its slightly off line even 3 or 4 mill of line i would take the flywheel off and renew the key or check it and reseat it....It may be some thing else but if you have tried every thing and it still kicks check that..Its quite common...It just put the timing out a little........................................I have a 6 year old Sears Craftsman lawn tractor with a 24 HP OHV Briggs & Stratton engine. When I was moving the lawn the other day I heard a "pop", then the engine died. Now when I try to start it the engine kicks and backfires, but never comes close to starting. We checked the fuel and the spark and both are present. I can't help but think if it died suddenly, and now is backfiring when I try to start it that it must be something related to the timing. Unfortunately, I don't know anything about how the timing works on that engine. Is it possible the timing chain/belt (if it has either) could have jumped a tooth? Any other suggestions for what this could be, and how we would go about diagnosing?
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agletdave
Original Author
11 years ago
Heres a link to partstree.com for my engine.42A707-2238-E1 You can zoom in or go back to complete parts list on their page

Here is a link that might be useful: partstree
agletdave
Original Author
11 years ago
Now that I see it does have a fuel pump on partstree, and that is part of a "carb rebuild kit", I'm really suspecting the fuel pump. Any comments anyone?
walt2002
11 years ago
"Any comments anyone?"

Yes, assuming your model number is correct, why did you put that it was an OHV engine?

I would remove the hex head plug at the lower right side of the carb when looking at the Intake Manifold. Be aware that gas will run out and maybe water/crap. Blow compressed air in there and or probe with a tag wire. See if that helped.

Check for leak in the fuel pump vacuum line running from the crankcase to the bottom of the fuel pump. Is the line from the fuel pump to the gas tank clear?

Walt Conner
agletdave
Original Author
11 years ago
Walt - the engine owners manual states it is a "twin cylinder overhead valve (OHV) air cooled engine." It does not show any discrepancies between the 2 models covered in this manual (420000 and 4600000 series) for that statement. There is a discrepancy with the mower sales sheet however, that sheet simply calls it an "Industrial plus opposed twin engine", whereas some other models specify "OHV". Hard to believe the engine manual is wrong, but I guess it could be.
agletdave
Original Author
11 years ago
the vacuum line is black and looks good. I'm gonna try a pump diaphram rebuild kit first. I'm guessing that rubber diaphram has been weakened over time. I imagine it gets worse at low speed because of slower pump cycling / weak diaphram.
bluemower
11 years ago
I checked my owner's manual and indeed - it states this is an OHV low emission engine. I assure you it is not ohv.

the engine performance improved when you partially closed the choke. This is an indication of a lean fuel air mixture. The fuel pumps are usually very reliable, but there could be a leaking diaphram. My suggestion is to renew the entire carburetor and fuel pump. The pump pulse hose should be clamped. Hoses develop cracks and become hard - especially around the ends. Air leaks are common. I know you replaced a section of hose. The fuel hoses around the engine frequently become hard, then cracks form. The fuel pump draws in air. Also the ends of the fuel hose also allow air leaks.

The current ethanol fuel sometimes leaves residue on valve stems. The valves may not be closing fully due to sticky valve stems. There could be excessive carbon on the valve seats and in the combustion chamber.

The opposed twin was a great engine and is well worth maintaining provided the equipment is in reasonable condition.
agletdave
Original Author
11 years ago
Well, it ended up being something in the carb, not sure what. Like an idiot, first I bought a fuel pump rebuild kit, not knowing that you can't buy a full carb rebuild kit that doesn't already come with the pump kit. So when the fuel pump kit didn't help, I had to buy the full carb kit, which worked for who knows why (new needle,main jet, pilot jet, gaskets. My carb looked really clean inside. Now I have an extra fuel pump kit.

Briggs 16.5 twin cylinder starving for gas / backfires

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Briggs 16.5 twin cylinder starving for gas / backfires

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