!8.5 hp Briggs backfires when throttle is open

Bruce.BuchananSeptember 15, 2013

This is my first post and I hope this is the right place, so if not please suggest where. I am a diesel mechanic but this small engine stuff is different and I am stuck. Thanks in advance for any help.

My 18.5 hp Inteck Briggs 31P777 0293E1 03 112ZD
is backfiring through the carburetor if I open the throttle any at all. The engine is in good solid shape and has no wear to it. It will start up ok and idles smooth. If you open the throttle the engine does not speed up it instead will start a loud backfiring out of the carburetor. You would think that the intake valve is sticking but after disassembling the head and going through it, it�s not the head. The tappets are not sticking and it is not that the valve clearance is wrong. It is not the ACR opening the valve during the compression stroke. The flywheel key is not sheered and the flywheel is correct with the crank. In short everything it should be causing this is not what is causing this but the engine is real sick. I have worked quite hard at checking the above mentioned things but am open to second guessing if anyone has any ideas.

Details: Engine was running fine and instantly developed this condition while at full power, there is fire and burned fuel in what is backfiring from the carburetor so there has to be spark, fuel and an open intake valve to explain this. I am concluding that somehow the plug is firing during the intake stroke rather than the valve is open at the wrong time.
Does any one have any ideas on this? Has any one ever heard of a coil causing spark at the wrong time?

Thanks , Bruce Buchanan

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Bruce, being a diesel mechanic, you may have a dial indicator that you can set up to measure the lift of the cam.
These engines have something of a reputation/history for wiping out cam lobes. It would be a stretch to think a cam lobe would wipe out "instantaneously" so this step may be just to assure the cam is in good shape and eliminate that as a possibility.
The symptoms of exhaust and compression coming back through the intake valve would point toward the exhaust cam lobe being the culprit. It's not that the engine would be firing at the wrong time, it's that if the exhaust lobe is gone, the combustion chamber is not purging on the exhaust stroke. That leaves a significant volume of "compression and exhaust" remaining in the CC which will rush out through the intake valve as it opens for the next intake stroke.

Measure at each rocker arm/push rod position and see how much lift the cam is providing and compare the intake lift to the exhaust. The values should be nearly identical.
Briggs does not give a specification as to actual lift, instead they give a physical dimension size of the lobes to be determined using a micrometer after the camshaft is removed. That is all fine and good for checking during rebuild but does not help when diagnosing on an intact engine. Best you can do without tear down is the measure the lift of each and compare the two values.

    Bookmark   September 15, 2013 at 11:03AM
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Hay! Thanks,
I will do that, your idea makes reason as to the symptoms I describe. I am not thinking that this real good running engine became real sick by a part wearing out suddenly to this extent. I am curious though as to the limited opening of the exahust vlve? I think I have a part failure and it is driving me nuts so thanks for a new aproach.

    Bookmark   September 15, 2013 at 11:27AM
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Yuarha Genius!

Well I have been goofing around with this for two week-ends. All kinds of test, checks, and even checks recommended by Briggs certified Mechs but not this check I feel stupid.... I got ...420 on the intake and THEN I got a wopin big .140 on the exhaust. I don't know what it should be but, "that's not a match"

I am dumbfounded how this came on so quick but in reflection it has worsened terribly as I have continued to run the engine probably another hour since it started. At first it ran ok at 1/2 throttle and now only at idle.

So thanks I did not want to change a cam shaft but I have probably worked harder already at this to have already done so.

Well this is pretty good form my first post.

    Bookmark   September 15, 2013 at 12:21PM
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***"Yuarha Genius!"***
Not at all! I are a mechanic, that's all.....and a danged ugly one to boot.

    Bookmark   September 15, 2013 at 5:25PM
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Well I got all my parts today and have the case back together with new camshaft in it. When I turn over the engine by hand in the upright position (shaft horizontal) I hear a soft clank about once a revolution. When I place the engine in its shaft vertical position I do not hear the clank and all seems smooth and normal. I have concluded that this sound is coming from the governor slipping up and down while lying on its side. I have reopened the case and can find nothing out of order to cause a sound like this except the governor. Can any one explain this I donâÂÂt feel comfortable proceeding with assembly until I can figure this sound out.

My other question is, I want to remove and replace the intake valve seal. Can some one explain the procedure? It appears that it just pulls off and new one is pressed back on. I canâÂÂt find instructions and this bugger dose not look like any automotive seals I have ever done.

Thanks in advance for any help,

    Bookmark   September 27, 2013 at 8:56PM
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I can send you a Service Manual for your engine IF you like, address below, put in proper format and remind me, engine model number and what you want.

Walt Conner
wconner5 at frontier dot com

    Bookmark   September 27, 2013 at 9:49PM
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