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18 HP Briggs spits gas, makes popping noise


msidan's Avatar
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08-08-06, 01:43 PM   #1  
18 HP Briggs spits gas, makes popping noise

I have a riding lawn mower with a !8 hp briggs engine that ran great up till about a week ago. I mowed a big, thick lawn , running it for about an hour and a half. When I turned it off, it made this pop pop pop pop pop noise as it shut down that I never heard before. It still starts right up and runs good a side from a few of those "pops" every so often, but every time i turn it off it makes those poping noises.

I replaced both spark plugs, fuel line, fuel filter, and vacuum hose. I also change the oil. There is no visible smoke from the muffler.

Today I mowed another lawn and after about 15 min., when I was in some thick grass, it died out. It started right back up, but when I engaged the blades, it died the same way in no grass. I took off the air filter and started it without it. As it started, some gas was blown out of the carburetor, but it still started and sounded normal once it was running. After waiting about 10 min., I started it and it ran with the blades on.
I have been told that it could be the coil heating up from some rust on the flywheel or coil.

Any ideas?

 
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08-08-06, 07:13 PM   #2  
18 twin? single cylinder? doesn't sound like the coil (and they won't heat up from rust) kinda sounds like the carb at the moment..... does it have a in-line fuel filter?

 
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08-08-06, 08:44 PM   #3  
yes i argree it is probably the carb, the poping noises is unburnt fuel going into the hot muffler possibly from too rich of a fuel mixture, there should be a screw on the bottom of the float bowl try turning it in 1/4 turn then shut off your lawnmower, try giving the carb a cleaning and blowing out all jets with compressed air, and yes, check for clogged fuel filter

 
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08-09-06, 10:11 AM   #4  
Yes it is a twin cylinder (ever hear of an 18 HP single cylinder?)
I adjusted the jet on the carb, but it still makes that popping noise wherever it is adjusted. I have replaced the fuel filter.

These pops don't sound like fuel explosions, more like there is something wrong with a one of the cylinders. When I turn the key off and it starts shutting down, then I hear the pops. Just before the flywheel stops spinning, it jerks in the opposite direction about 1/4 turn before stopping. It did not used to do that.
Could it be a broken timing key or compression ring?
It sounds perfectly normal when running.

As for cleaning the jet(s), do I just unscrew it and blow compressed air through it?

 
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08-09-06, 10:06 PM   #5  
Sounds like a carb cleaning would be the next step. You can remove the top of the carb and clean out the bowl and blow it out. The main jet is located at the bottom of the bowl, accessible through the drain plug.

Yes, there are 18 hp single cylinder engines.


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08-10-06, 02:57 PM   #6  
Because the engine is a belt drive, it would be highly unlikely for the flywheel (timing) key to have sheared but it shouldn't be overlooked. Since you have fuel spit-back out the carburetor, it could be one of two things: 1- Intake valve issue, with a lack of valve clearance being the most likely issue, or 2- Ignition timing advanced by way of a sheared flywheel key whereas the ignition is timed before the intake valve has time to close. My first step would be to remove the flywheel and inspect the key for shearing and then, if this is OK, do a leak-down test of each cylinder. Bear in mind that this is based on your description and not any first hand diagnosis on my part. An explanation for a sheared key were if you recently had the flywheel off and didn't properly torque the nut to spec (145 ft. lbs.).

 
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08-10-06, 03:00 PM   #7  
I have never had the flywheel off, but I bought it used a month ago and do not know what the previous owners have done with it.

I am wondering how to know when I have reached 145 ft. lbs when I put the nut back on. I do not have an impact wrench.

 
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08-10-06, 08:04 PM   #8  
A torque wrench. Generally, an impact wrench would tighten it tight enough, or you can tighten the stew out of it with a long handled ratchet and a strong arm. Tighten it as far as you can, then give it a little more of a turn . (Unless you're a bodybuilder with 22" biceps). A few foot pounds over won't hurt anything. Make sure to hold the flywheel still without damaging the fan or ring gear.


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10-06-06, 09:14 AM   #9  
An update...

I never did get the flywheel off and since it ran all summer, I just let it go.
It still sounds rough starting and shutting down but now I here this clicking noise when I throttle down; almost like the sound of loose change in a dryer. That can't be good. It only sound like that in low throttle. Occasionaly it will "gunshot" backfire when I start it.

 
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10-06-06, 02:38 PM   #10  
It sounds like an ignition problem and you will want to test the strength of such by way of an inline spark tester on each cylinder. If this tests OK then you need to have a look at the flywheel key for being sheared. The spark test will tell you if you have a good ignition coil or not, provided you don't have a kill circuit issue. If after you perform the inline test and suspect a coil problem, you first must eliminate the kill circuit in order to isolate the coil as the actual problem by simply removing the kill wire AT the coil and then testing again.

 
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10-27-06, 08:20 PM   #11  
I don't have an inline spark tester, but I get a spark from both plugs.

I noticed an oil leak under one of the heads (not a big drip, just a little oil mess) and took off that head. The head gasket looks fine, but there was a TON of carbon/grime buildup on the head, piston, and valves, which I removed with a wire brush.
I'm beginning to suspect rings on one of the pistons.

I did get the nut off the flywheel, but could not pull the flywheel off. It says "Do not pry". I'v heard of something called a wheel puller to get the flywheel off, but I don't know where to find one. Is there another way to get the flywheel off?

The engine is also doing something new; it starts but, at 3/4 throttle, it starts itself in low idle and slowly works itself to full throttle, all the while spitting black smoke. Once it's running, there is no visible smoke.

 
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10-28-06, 01:32 AM   #12  
Does the oil smell like gas?


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10-28-06, 07:52 AM   #13  
It's hard to tell; I can't tell if I'm smelling gas, or just used oil. The oil level is right on the "full" mark and it seems to be as thick as the oil should be. (doesn't look watery)

If it means anything, when I took one of the heads off the engine, there was oil coming from the bottom of that piston. Not a lot, but thats what made me suspect the rings.

 
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10-28-06, 12:33 PM   #14  
I would suspect the intake valve if it's blowing back through the carb.. I had a B&S 12 hp do that, and it turned out to be a crack near the intake valve that was allowing pressure blowback through the carb..

 
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10-28-06, 03:28 PM   #15  
Now that you mention the intake valve, (the valve from the carb?) when I took the head off, there was this ring that had fallen loose around that valve. I pushed it back in and didn't think anything of it. Could that be stopping the intake valve from closing when the engine runs?

I haven't noticed any more fuel blowing from the top of the carb.


Last edited by msidan; 10-28-06 at 03:44 PM.
 
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10-28-06, 05:51 PM   #16  
You Bet! Sounds like the valve seat came loose from the block. Sometimes you can peen them back in place, but it does not alway work.

 
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10-28-06, 08:31 PM   #17  
What do you meen "peen" the valve seat? If it does not stay in the block, is the whole engine junk?

 
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10-29-06, 06:26 AM   #18  
Well first you reinstall the seat back into the block. Then take a small center punch, you peen the area around the valve seat. Start at 12:00 then 6:00, 9:00 and 3:00 o'clock positions just slightly outside the valve seat so as to force the aluminum from the surounding area to pinch the seat in place. Once you have it secured, follow up at 1,2,4,5,7,8,9 and 10 o'clock with the punch. You may have to reface the seat, reset the lash and reseat the valve so that it seats properly, as sometimes the seat is no longer in the original position in the block.

If you can't get the seat to stay in the block, then yes you would have to replace the block in order to get it to work right.

 
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10-29-06, 03:13 PM   #19  
Correct. That ring is the source of your problems. Glad you mentioned it being loose. Peening the seat usually works well. If you haven't done this sort of thing, it might be best to let someone who knows how to do it work on it. It doesn't take much time, and how well it's done can mean the difference between having a good engine or a piece of junk.


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12-04-06, 02:49 PM   #20  
Well, all the experts in my area either never heard of such a procedure or were too far away, so I did it myself. I started the engine in 20 degrees (F.) and the rough sound is gone, but there are still some small backfires from the muffler. The oil really smells like gas, so I am going to change it. Now that it's colder out, I was going to put a multi-grade oil in. Should I use 5w-30 or 10w- 30?

 
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12-04-06, 11:01 PM   #21  
Straight 30 is generally fine unless you plan to actually be using it in very cold weather (below freezing). Then, you might switch to 10-30.


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12-04-06, 11:35 PM   #22  
I never will use 30 weight below 40 degrees......
The one time I did with a old engine.... it was just above freezing..., was a drain on the starter to start it...., not to mention for some reason the engine started using oil pretty good after that...


When its going to be under 50 degrees outside...... I use either 10w-30, or 10w-40 conventional in the old ones....
Or 5w-30 synthetic in the new ones all year round... I've used 5w-30 conventional in one.... and it did burn through it even though it was cold outside.... the switch to 10w-30 stopped it...
(5w-30 has more viscosity improvers and pour point depressants..... these shear very easy, making the oil lighter and easier to burn, 5w-30 though flows better then 10w-30 and both are 30 weights at operating temp)
Its just good to have oil slinging around faster.... and get in all the tight places.... then have straight 30 weight thats like molasses in the cold.....

 
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12-05-06, 08:19 PM   #23  
True, if in doubt, use the thinner oil. Oil weight is not an issue in my neck of the woods. The daytime temps are rarely below the 40's even in the coldest months of the year. We use straight 30 year round.


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12-07-06, 11:23 PM   #24  
Excellent Thread !!!

This will be a great thread to refer some of the newer guy's to. Great series of Q&A all the way to the root problem and beyond! I had forgotten about that ring myself and I wasn't thinking of it any longer. We had this same issue about two years ago. May have been more but I remember it being way back! SO I relearned one today. The process of "peening" it back in place and how critical the process is was explained extremely well !

 
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