Briggs 25 hp V Twin Backfiring through carb

12 years ago

Yep, it's me again...

I just replaced the sump gasket on my Briggs 25 hp V Twin model number 445777-0168-E1. While in the process, I replaced the starter, the carb, cleaned the heads and valves, relapped the valves and adjusted them.

I replaced the filters, oil, plugs and voltage regulator. Got it all back together yesterday and it ran great and mowed great EXCEPT, it started leaking oil from the bottom of one of the heads. Took it off, and sure enough the head gasket had blown. So I got a new head gasket and put it on, readjusted the valves on that side and off we go.

Except now I have a new problem. It backfires through the carb under load. Normally I would think to check the valves and make sure I adjusted them right (and that's the first thing I will do), but I didn't replace the other head gasket and I bet it may be blown too. Unfortunately they only had one in the local shop and had to order the other.

The other thing that has me wondering, is that one of the intake bolts, on the side I DID NOT replace the head gasket on, is stripped out. I mean it is not putting hardly any pressure on the intake manifold. So could be it getting an excessive amount of air in on that side causing the backfire?

Remember, the only thing that changed from yesterday (when it wasn't backfiring) is that I replaced a head gasket and readjusted valves on one side. Which did involve removing the intake manifold, of course.

So what do you guys think? Fix that bolt, replace the other head gasket and readjust the valves and go from there?

Backfiring through the carb is normally caused by a bad fuel/air ratio isn't it? Which could be caused by a bad head gasket or a leak at the intake manifold, right? Or there could be crap in the carb, I guess, but this is a new carb and it didn't do it yesterday.

I forgot to mention, it runs a little rough at low idle today too. Didn't do that yesterday.

I have another concern I would like to ask about as well. I had to have the bolt in the bottom of the crank machined out, and when they drilled it and put it a helicoil, it is just a hair off-center. This really worries me. Worst case scenerio, what can that tear up? The clutch? Belt wear? I don't feel or see any abnormal vibration in the clutch and the bolt is spinning like crazy. You can't even tell it is off-center while looking at it when it is running.

Thanks for the help.

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Comments (5)

mownie
12 years ago
While the engine is at idle RPM, spray a short, light burst of carb cleaner on the gasket seam of the intake manifold where it fastens to the head. A change of RPM that coincides with your spraying indicates a vacuum leak at that spot.
Any vacuum leakage there will alter the air/fuel ratio to the cylinder.
Due to the nature of "harmonic vibration", a poorly secured gasket location might in fact "open wider" at specific RPM points, and will even erode the gasket if not repaired.

The appearance that the crank thread repair is slightly "off center" may only be an illusion.
It WILL NOT matter if the threaded hole is merely a thread or so off center anyhow...............the mounting bolt ONLY serves to secure the clutch and pulleys to the crankshaft.......it plays no role at all in centering the pulleys to the crankshaft.
Centering of the pulleys to the crank is achieved by the bore of the pulleys fitting the outside diameter of the crankshaft PTO surface.
cedarghost
Original Author
12 years ago
Nice tip once again Mownie. I tried it and it does leak there, sure enough. I went ahead and took the other head off to check the gasket and while it's all there, it's also pretty ragged (meaning that it is separating into layers). I'll get a helicoil kit for the bolt and my head gasket should be in tomorrow.
I'll let you guys know how it goes. As far as running while not under load, it low idles at 1723, high idles at 3575, starts real quick and sounds smooth going up and down with the throttle. Couldn't have done it without your help.
I was hoping as much about the bolt being off-center. I noticed it was just a bearing under there anyway and that the washer looks like it should let the bolt spin using that bearing.
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***" The camshaft and crankshaft timing marks are off by about 6 gear teeth."*** SAY WHAT?? !! ?? I don't think we are getting a complete story on the recent history of this engine. There are only a couple of ways the camshaft gear and the crankshaft gear can "get out of time". One way is for them to have been installed "out of time", or two, the engine was treated carelessly while the sump cover was removed for some kind of repair. It is possible for a camshaft to slip out of position if the engine is tipped the wrong way while the sump cover is off of the crankcase. It is also possible for the cam gear and the crank gear to "jump one another" if the engine is rotated while the sump cover is off of the crankcase. But both scenarios REQUIRE that the sump cover be removed from the crankcase. The camshaft and crankshaft gears DID NOT become "untimed" while the engine was operating. And, how is it that you are able to report that you have learned the camshaft and crankshaft are off by about 6 teeth, without some sort of preface to the news such as " I took the sump cover off and I see that the camshaft and crankshaft are not timed correctly."?? You would not keep secrets from us, would you? :^) To get the camshaft into the correct index, it is best if you remove the push rods (so there is no downward pressure on the camshaft) and then slip the camshaft out of its bearing bore in the crankcase just enough to turn the cam gear and crankshaft to where the timing marks on each align correctly. Do not put the push rods back in place until the sump cover has been installed and properly torqued.
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cedarghost
Original Author
12 years ago
Got my helicoil kit in and the other head gasket on. She runs and mows great. Only backfired a little when I was in some really deep grass (about 12 inches) and it was still wet this morning. I know, I shouldn't mow when it's too wet, and I didn't mow too much, just wanted to test her out. :)
mownie
12 years ago
OK, good.
12" of wet grass will impose quite a load...............so if you are only getting sporadic cough back through the carb I would attribute that to some minor "hunting" of the governor.
The real evaluation of fuel ratio and cylinder balance should be done after you have mowed with it a couple of times under "normal conditions" by reading the coloration of the spark plugs.
cedarghost
Original Author
12 years ago
Just an update. The mower runs and cuts great. I have an occasional backfire under really heavy load, but other than that she is good to go. I mowed 4 1/2 acres with it yesterday and some of it had gotten pretty thick while the mower has been down.
My backfiring, was either the bad head gasket on one side, or the stripped intake manifold bolt on that same side. In any case I replaced them both and no more problems.
Thanks a bunch.

Briggs 25 hp V Twin Backfiring through carb

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Briggs 25 hp V Twin Backfiring through carb

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