B&S just backfires

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  #1  
Old 10-19-05, 02:48 PM
bigjohn
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Angry B&S just backfires

I’m dealing with a Briggs and Stratton I/C Gold 28N707 15.5 HP on a Craftsman riding mower. It will not fire at the right time to run. It just wants to pop back into the carburetor. The spark is excellent. The intake valve had no lash … 0.000” both valves are now adjusted slightly loose according to specs (in .005”-007” and ex .009-.011). I’ve tried it exactly on the specs, no difference Compression is an easy 125 #. I went back to look at the flywheel key … it looks perfect. Not once has it made an attempt to run. The magneto coil looks like it has never been moved... going by rust and dust.

Squirt gas or starting ether into the spark plug opening or into the carburetor … all it wants to do is backfire into the carburetor, a good strong one. For that matter crank it with the choke on and all it will do is backfire regularly and repeatedly on the fourth stroke.

History: this mower set up idle outside for at least two years.
Was given to me with the comment “nothing wrong except it will not start. “
It first needed a flywheel ring gear and key … replaced both.
The carburetor was stopped up and I cleaned it now gas can be smelled on cranking with choke fully on. And for what it’s worth... a new battery with a full charge and also a booster if needed. It spins around more than fast enough.

1. It has blue flame spark,
2. 125# compression,
3. fuel supplied testing three different ways
4. And it spins fast enough. .

I am not a profession mechanic, not even close. I’m just a retired old geezer that likes to learn. Any and all help will be appreciated.
 
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  #2  
Old 10-19-05, 07:34 PM
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strong smell of gas? have you checked the fuel float for heaviness or sloshing? may be sinking. letting too much gas by, and of course gas fouling the plug, unless thats not the case, try a little gas down the carb, then see if it'll start. how are you testing spark?
 
  #3  
Old 10-20-05, 02:04 AM
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There's a few things I'd look at...first being the spark plug. Even though it has a strong spark, it could be bad.

Second, have a look at the wires coming from under the engine shroud behind the starter. Make sure one of them is not pinched behind the starter.

Third...if the above don't prove successful, I think I'd replace the coil.
 
  #4  
Old 10-20-05, 04:22 PM
bigjohn
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Still Backfires only

V8driver .. Thanks for your response.
As for the smell of gas, it only happens when I crank the engine with the choke fully on for a long period (10 or 15 seconds), all the while it’s popping back thru the carb. Crank it from a cold start and no choke…. no popping and the plug is dry.
As mentioned in my post, I have squirted gas and sprayed starting ether into the spark plug hole …. It’s always the same story. Also putting both into the carb throat same story.
As for the sparkplug... I failed to mention I bought two new spark plugs plus I have the one than came in the motor. All have been separately installed with identical results.
I have checked the spark by pulling the plug and grounding it and watching the spark... a good blue one in this case. Also I have a tune-up meter that has a meter reading for spark as well as a light that illuminates if it has a good spark. Both indicate a satisfactory spark. If you have another idea please post.
Again, Thanks
 
  #5  
Old 10-20-05, 04:39 PM
bigjohn
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B&S still backfire's

Cheese … Thank you for your response.
I have tried three different plugs with the same results. I checked all of the wiring in the mower and all are clear and in good shape. No pinches, no binding and no scrapes (broken insulation). I have the shroud sitting off to the side so I have a good view.
One thing I’m not too happy about is the counter man gave me two plugs with the right number but not resistor plugs as the B&S manual list as OEM. The plug that came with the motor is resistor type. It appears unworn and clean, but that the first thing that I changed. Could that resistor plug retard the spark just long enough to “retard the spark”? I don’t think so but I could be wrong.
You suggest replacing the coil. I’ll go for that too but I live a way out of town and it will be several days before I get to the parts house, however I will.
Thanks again, all help is appreciated
 
  #6  
Old 10-20-05, 04:51 PM
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Did anyone try to "fix" the engine before you got it? I'm wondering if someone opened the engine up and did not get the cam and crank aligned properly...
 
  #7  
Old 10-20-05, 05:20 PM
bigjohn
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Originally Posted by Pilot Dane
Did anyone try to "fix" the engine before you got it? I'm wondering if someone opened the engine up and did not get the cam and crank aligned properly...
Yes.. I could see that the starter had been recently replaced but not the flywheel plastic ring gear.. that was missing several inches of teeth, which was the reason he could not get it to turn over. It does not however have any signs of it being opened up. Thanks
 
  #8  
Old 10-20-05, 09:08 PM
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is the plug getting wet?....well anyway, those light testers really do good. though the timing.....cam, crank idea does sound like a maybe. see if the shop if you have one around can test the coil right. that or try it on another engine.
 
  #9  
Old 10-21-05, 02:55 AM
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Sounds like a faulty ignition coil. Briggs part number 492341.
 
  #10  
Old 10-28-05, 08:12 AM
bigjohn
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B&S still backfire's

As recommended above, I got a new ignition coil and installed it. With an experienced friend looking over my shoulder, I went back over all the settings for the engine as published in the B&S manual.
Well, I think I have tried all the suggestions above, and I do appreciate all the responses and did my best … but it still backfires the same.

As a last effort, I am considering leaving the flywheel key out, put the flywheel back on without a key… and rotated a few degrees to retard the spark … any comments? Otherwise I’ll give it up.

Again, thanks.
 
  #11  
Old 10-28-05, 12:38 PM
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no you have to use the flywheel key..... that timing is precise, if its off by a little it will be off timing, then no go. plus if it even did start, i'd imagine the flywheel may spin in place if not down really tight when its shut off. was the key still in good shape? flat and smooth on alll sides?
 
  #12  
Old 10-28-05, 02:53 PM
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Take the kill wire off the coil - AT THE COIL - and run it. You will have to pull the spark plug wire off (with an insulated pliers) to shut it down but this will eliminate any safety switches/modules as a problem.
 
  #13  
Old 10-28-05, 05:04 PM
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If I Am Recalling Correctly There Was A Problem With The Cam Shafts On Some Of These Engines As Well As Head Gaskets Blowing From Piston Towards The Valves....
 
  #14  
Old 10-29-05, 12:15 AM
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I agree with Puey61...remove the kill wire from the coil and try again. It still sounds like an ignition problem. Removing the flywheel key won't fix the problem.

I am going to mention one more time to be sure....look at the wires behind the starter. The black one in particular. You said it looked like the starter had recently been replaced. This is a common area for a problem, and the symptoms of the problem are exactly what you're describing. You may want to remove the starter and make double sure that black wire isn't being pinched behind it or near the top where it goes under the engine shroud.
 
  #15  
Old 11-02-05, 11:40 AM
bigjohn
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B&S 15.5 still backfires

I have followed the instructions in each any every one above, and I thank you for taking your time to help me try to solve this vexing problem. I take your suggestions seriously enough that I printed them all out and took them to the shop and checked them off as I completed each task ….I did not miss a comment .. And it still backfires the same as always.

Here is a today’s new observation….. I completely removed the intake valve adjustments from the head (the valve assembly on the bottom closest to the spark plug) shot fuel into the spark plug hole replaced the plug… cranked it and it still backfires the same. Put it back together, adjusted the valves and I still get 125# compression.

Maybe a burned or stuck intake valve? While the adjustment assembly was removed, the intake valve moved relatively free when I pressed it straight on the valve stem. That valve has got to be as shut as it gets with the adjustors removed. If it is burned or what, how can I have 125# ?? Can not be..

Well, when the mower was given to me, I said I needed a challenge. Looks like I found one. As I see it now it looks like I need to pull the head and inspect. As usual, all suggested are appreciated.
 
  #16  
Old 11-02-05, 10:29 PM
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Any chance someone has been into this engine? Maybe the valve timing is off, although the compression reading is too good for that I'd think.

Another thing...You mentioned a flywheel key replacement. If the flywheel nut was not tightened VERY well, it may have just sheared again. You didn't put oil or grease on the crankshaft before reinstalling the key, right? If so, it is almost certainly sheared again.
 
  #17  
Old 11-03-05, 05:23 PM
bigjohn
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B&S 15.5 still backfires

Flywheel was torqued by the numbers and has been checked several times. It is still good. I again talked to the guy who gave the mower to me… he says it definitely has not been opened up .. and it shows no sign of being opened etc.. it still has a fair amount of dirt on the motor case. Thanks again Cheese.
 
  #18  
Old 11-06-05, 10:58 PM
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Try removing the fuel supply line, drain the carb, and a fresh plug (in case the others are gassed now). Make sure there is no gas in the crankcase, and give it a shot of starter fluid down the intake and see if it starts. I'm wondering if it's not flooding, and gassing the plugs.
 
  #19  
Old 11-10-05, 10:02 AM
bigjohn
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More of the same

Thanks Cheese...
I followed your suggestions exactly… this turkey still pops back through the carburetion. Crank it with the choke off and it will not fire... back or forward. Pull the choke on and it immediately starts to backfire every time the ignition fires... I put a strobe light on it to watch if it popped on every firing... It does. You mentioned gas in the crankcase… I sniffed the oil dipstick and no smell of gas. The spark plug has never been wet except when I squirted too much into the spark plug hole. Still working at it....
 
  #20  
Old 11-10-05, 10:07 AM
bigjohn
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More of the same

Originally Posted by bambiblaster
If I Am Recalling Correctly There Was A Problem With The Cam Shafts On Some Of These Engines As Well As Head Gaskets Blowing From Piston Towards The Valves....
bambiblaster...
This message still haunts me because I have no idea how to check it out. If indeed the cam problem is the case, maybe it explains the problem Thanks, John
 
  #21  
Old 11-10-05, 07:28 PM
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The head gasket blowing wouldn't cause this problem. The cam could if it was in wrong. You can watch the valve movements to tell if the cam is timed right. Rotate the engine until it is on top dead center between the exhaust and intake stroke. Insert a small screwdriver through the plug hole to feel the top of the piston. Rotate the engine slightly back and forth while feeling the piston move up and then back down over tdc. When the piston is exactly at tdc, both valves should be closed. As the engine rotates slightly one way or the other, the intake and exhaust valves should begin to open and close correspondingly, with both closed when the piston is at the end of it's stroke. If not, the cam timing is off. There's not any way for this to have happened without the engine being disassembled or the gear stripped (which I've never seen on that particular engine). I'm starting to think you may have a valve seat that has come loose or something to that effect.
 
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