B&S 4hp engine - starts then dies

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  #1  
Old 04-05-14, 08:09 PM
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B&S 4hp engine - starts then dies

Model/code 10A902 2189B3

Prime it, starts (with difficulty) but will run for 5-10 sec then gradually slow and die. New clean gas (emptied tank and refilled with fresh stuff).

Pulled carb, cleaned thoroughly. Twice. Carb cleaner flowed freely through all passages, no obvious cracks. Changed spark plug. No difference.

Replaced diaphragm, replaced o-ring on intake tube. No difference.

Replaced carb (with brand new one). No difference.

Pulled and examined intake tube, no cracks. Took off muffler in case that was clogged or something hardly believable (no difference). Pulled off valve cover and spun motor, observed stems moving in and out w/o any obvious sign of problems although I have no idea how to check adjustment.

I would give up and buy a new mower but since I have $55 invested in new parts I'll humor additional suggestions to try.
 
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Old 04-05-14, 10:38 PM
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Does it have sufficient compression?
 
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Old 04-06-14, 07:12 PM
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Ah, you kindled long repressed memories and made me recall I do have a compression tester in a box in my garage. Long story short, there is some amount of compression but it is too low to read on the gauge. "Some amount" because I do hear a "phfft" when I press the pressure release button after yanking the starter cord for 5 minutes, but it is not enough to clear the big broad "0" white mark. I can't find compression rating in the operation manual but I presume it is non-zero?
 
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Old 04-06-14, 07:14 PM
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You might want to test the compression tester on another engine to make sure it works.
 
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Old 04-06-14, 11:51 PM
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Yeah, for it to start at all, you have some compression, but if you are below 80 or so, it may not keep running. A compression gauge isn't the ideal way to check compression on that engine, but it can help give you an idea. The test used by the manufacturer is to put the plug back in and remove the plug wire and give the engine a good spin backwards by hand and see if it will bounce forwards when it hits the compression stroke.
 
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Old 04-08-14, 07:37 AM
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From my recollection, I could not even spin the engine backwards-(clockwise, looking up at the blade - basically the opposite of the cutting direction) (by grabbing the blade and trying to turn in that direction) but it spins pretty freely in the cutting direction.

It will be a couple days before I can get to this project again, but will investigate the bounce-back concept more. Assuming there is a bona fide loss of compression issue, is this engine worth repairing by me the garage mechanic that knows just enough to be dangerous? I guess that would depend a lot on the damage but let's say it is rings, how much do they cost and how long would it take for me to disassemble-reassemble (take your experts-time and double it)?

As a side question, is it normal for the exhaust valve to open ever so slightly and rotate a small amount after each time the intake opens?
 
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Old 04-08-14, 05:56 PM
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Replacing the ring(s) is pretty easy on small engines.

The only time the exhaust valve should open is on the exhaust stroke of the engine, not when the intake is open.

I suggest that if you have the time that you should do the ring replacement. However, you should first determine for sure that the compression is low. Have you tested the compression in another small engine with your gauge. Also, make sure you tighten the compression gauge firmly so that its o-ring is compressed.
 
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Old 04-08-14, 11:05 PM
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The exhaust valve thing is normal on that engine. It has a compression release effect, which is why a compression gauge isn't the preferred method of checking compression. Rotating the engine backwards makes the compression peak the most, since it keeps the compression release out of the equation.

Replacing rings is very easy for a mildly mechanical minded person, and still very doable for a novice. It shouldn't take more than a few hours if you really take your time. These engines are rarely worth putting rings in though. For the time and money, and the fact that the rest of the engine likely has a lot of wear and the cylinder may be scored and worn heavily, re-ring jobs on low hp engines is just about a thing of the past.
 
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Old 04-09-14, 08:56 PM
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I was able to look at it this evening again. It does spin backward (and forward) easily, no bounce back effect whatsoever. Makes a slight honking noise actually, seemed like coming out of the carb. Didn't have the valve cover off so not sure it was intake or compression. I wish I had more time, I would like to tear it apart just for curiosity's sake and rebuild it just for the fun of it, but alas I'm stuck with more money than time and grass that hasn't been mowed for 2 weeks now. Thanks for your input.
 
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Old 04-10-14, 02:24 AM
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I was able to look at it this evening again. It does spin backward (and forward) easily, no bounce back effect whatsoever. Makes a slight honking noise actually, seemed like coming out of the carb. Didn't have the valve cover off so not sure it was intake or compression. I wish I had more time, I would like to tear it apart just for curiosity's sake and rebuild it just for the fun of it, but alas I'm stuck with more money than time and grass that hasn't been mowed for 2 weeks now. Thanks for your input.

If you hear a noise coming from the carb when you spin the engine, I would check your intake valve clearance. Not sure exactly what engine this is and I do not have my reference links any longer
However if this is OHV engine valve adjustment should be easy enough.

Noise from carb on compression =intake valve
muffler = exhaust valve
oil fill = rings

If the rings are bad you may not hear any thing from the oil fill without doing a leak back.
 
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