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Hot!Briggs Engine 7.75hp - popping, stalling - Snow Blower

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TurboMatt
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2019/11/20 11:07:27 (permalink)
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Briggs Engine 7.75hp - popping, stalling - Snow Blower

I'm currently working on a snow blower I purchased second hand. The snow blower is a Craftsman 26" unit with a Briggs Stratton Intek 7.75 horsepower single cylinder engine. The previous owner said it wasn't running right and assumed it needed carb work from sitting. The way he describes how it ran is exactly how it runs now, with a bunch of new parts.
 
Before I tried to use the unit, I knew it would need some maintenance. So, I changed the oil with SAE 30, new Champion spark plug, new OEM carb. As well, I removed the gas tank to be cleaned. I also installed new fuel lines.
 
The engine will start but will only idle for about 20 seconds. If I touch the throttle or the governor arm, the exhaust will pop and the engine will stall. The fuel is fresh 87 octane. 
 
Anyone know of anything common that could cause this since it's basic single cylinder engine? Maybe the coil? Not sure what else can act up on these engines.
 
Thanks!

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    Roy
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    Re: Briggs Engine 7.75hp - popping, stalling - Snow Blower 2019/11/20 11:34:37 (permalink)
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    Can you give us the model and type number?
    TurboMatt
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    Re: Briggs Engine 7.75hp - popping, stalling - Snow Blower 2019/11/20 11:38:52 (permalink)
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    These numbers are on the PTO side of the engine that the small engine parts store locally asked form.
     
    030815 FE
    12C314 0131 E1
     
    The Craftsman Model is 536.887752
    AVB
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    Re: Briggs Engine 7.75hp - popping, stalling - Snow Blower 2019/11/20 13:29:04 (permalink)
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    Start by checking the valve clearances. It is an OHV which tends to open the valves clearances.
     
    Intake Valve.004-.006 in (.10-.15 mm)
    Exhaust Valve.009-.011 in (.23-.28 mm
    TurboMatt
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    Re: Briggs Engine 7.75hp - popping, stalling - Snow Blower 2019/11/20 13:53:56 (permalink)
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    I'll measure the clearances this evening and report back.
    TurboMatt
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    Re: Briggs Engine 7.75hp - popping, stalling - Snow Blower 2019/11/20 21:10:06 (permalink)
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    The valves both measure exactly in spec so we're good there.
     
    After I checked the clearance, I re-installed the valve cover and tried to run it again with a fresh tank of 93 octane.
     
    Here is what I found.
     
    It does take some time to start using the electric starter. When it does start, it will only run with the choke 1-2 clicks from fully closed and with the throttle set to WOT. I can see the governor arm moving to open the throttle blade at the carb while cranking. When it does start, the governor arm backs off slightly.
     
    When it's running, the muffler seems to get much hotter and faster than it should. With that, I noticed that the side of the plastic gas tank was slightly melted. Not sure if this could mean that there is an ignition timing issue. The previous owner only said that it started to pop while running and he gave up on it.
     
    When it does start, if I try to lower the throttle or open to choke, it will pop then shut off.
     
    I also hooked up an in-line spark checker while running and the spark never dies while the engine is running.
     
    Here is a quick video I took to show what I mean about the choke. The throttle is set to full which is supposed to be 3700 rpm. It doesn't sound like 3700 rpm to me?
     
    EDIT: Doesn't look like I can share links. Go to Youtube and paste /watch?v=D0fvf8BqgWI
    post edited by TurboMatt - 2019/11/20 21:28:25
    AVB
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    Re: Briggs Engine 7.75hp - popping, stalling - Snow Blower 2019/11/21 07:29:44 (permalink)
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    It sounds like you are running extremely lean. Are you sure you ordered the correct OEM carburetor? With engine partially choked it will not make full rpms.
     
    IPL says 794587 which has been superseded to 798917 and will cost $63.45 or more if OEM (MSRP is $70.50); otherwise, you are getting a cloned carburetor. The $63.45 is my dealer cost and there are few dealer with volume discounts that would sell at this price online.
    TurboMatt
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    Re: Briggs Engine 7.75hp - popping, stalling - Snow Blower 2019/11/21 08:55:09 (permalink)
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    It does seem like it is running extremely lean. What worries me more is how hot the muffler gets and so fast. 
     
    As far as the carb, that is the part number I received. I just checked the box and it does list 798917 as the part number. The gas tank seems to have a nice steady flow leading down to the carb as well. One thing to note is the engine ran the same with the old carb and the new OEM one that's on the engine now.
    post edited by TurboMatt - 2019/11/21 09:08:39
    TurboMatt
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    Re: Briggs Engine 7.75hp - popping, stalling - Snow Blower 2019/11/21 09:08:09 (permalink)
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    I just spent some timing looking at other aspects that could be out. From what I can tell, ignition timing is fixed? Is that true for mostly all B&S small engines?
     
    I'm trying to think of what else I could check.
     
    TurboMatt
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    Re: Briggs Engine 7.75hp - popping, stalling - Snow Blower 2019/11/21 22:05:46 (permalink)
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    Does anyone know if anything can cause ignition timing to change or is it fixed?
     
    As far as an update, I did a compression checking. Using the electric starter, compression is 120 psi.
    AVB
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    Re: Briggs Engine 7.75hp - popping, stalling - Snow Blower 2019/11/22 07:59:24 (permalink)
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    With the camshaft having an ACR (automatic compression release) you should be only seeing 70-90 psi. I don't think  the starter going to be spinning the engine fast enough to achieve idle speed which disengages the ACR. This would indicate that the ACR has failed. And plus with the camshaft having plastic lobes and gear they may have loosen and be shifting around but this rare.
     
    As ignition timing it about 20 degree BTDC but the coil's electronics can fail shifting this point. As far as I know Briggs isn't using the coil's electronics to shift this timing variably at the current time. Valve timing is fixed. Now if the flywheel key should happen to partially shear then the ignition can shift enough to cause problems.
    TurboMatt
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    Re: Briggs Engine 7.75hp - popping, stalling - Snow Blower 2019/11/22 08:55:40 (permalink)
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    Do know the easiest or best way to check the ACR or cam function? While I had the valve cover over, I did spin the engine over by hand and with the starter. The valve train appears to function smooth for what it's worth. Should I plan on tearing the engine down at this point?
     
    I've been looking for a factory repair manual but cannot locate one. Anyone know where to find one?
    AVB
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    Re: Briggs Engine 7.75hp - popping, stalling - Snow Blower 2019/11/22 09:43:19 (permalink)
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    This should be the service manual.
    https://drive.google.com/open?id=0B-g4TpKUJnh7WXBmMnJ2akI4S0E
     
    As for checking the ACR operation is fairly simple. With the valve cover as turn the engine through as you approach TDC compression stroke the exhaust valve should open slightly and close again. This is called the ACR bump.
    TurboMatt
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    Re: Briggs Engine 7.75hp - popping, stalling - Snow Blower 2019/11/22 09:53:05 (permalink)
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    Thanks a bunch for the manual link. I browsed through it and it looks like it's probably the one I need.
     
    I help my fingers on the push rod side of the rocker arms while pulling the starting cord and don't recall feeling that. I'll pull the valve cover this evening and check again. If that is in fact the issue, will it be best to remove the engine from the snow blower frame? Not sure how much needs to come apart.
     
    As well, if the ACR is screwed up, could that cause the running issues I'm describing?
    AVB
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    Re: Briggs Engine 7.75hp - popping, stalling - Snow Blower 2019/11/22 10:29:18 (permalink)
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    IF it has a bad ACR then engine would need removing as you will pull the crankcase cover/oil pan to get to the camshaft.
     
    As the running issues it could but I haven't experience this issue so far but engine in this class are seldom in my shop mostly larger engines on riding mowers.
    TurboMatt
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    Re: Briggs Engine 7.75hp - popping, stalling - Snow Blower 2019/11/22 10:32:18 (permalink)
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    Ok, sounds good. I'll pop the valve cover off again and check for the bump you describe.
     
    It's definitely an odd issue. Most of the small engine stuff I've run into is runs fine or doesn't run. This is somewhere in the middle. I'll post results when I have more information.
    AVB
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    Re: Briggs Engine 7.75hp - popping, stalling - Snow Blower 2019/11/22 10:43:14 (permalink)
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    I would appreciate the updates as I hate intermediate problems. They are a lot harder to fix on anything I repair. The source of the problem can be something totally unexpected at times.
     
    Any info can be use to help others when something like happens again.
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    Re: Briggs Engine 7.75hp - popping, stalling - Snow Blower 2019/11/22 10:49:41 (permalink)
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    I absolutely agree. I will take a video of it running tonight to clearly illustrate the issue. That way we have a good foundation from initial issue to resolution.
    TurboMatt
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    Re: Briggs Engine 7.75hp - popping, stalling - Snow Blower 2019/12/18 19:21:12 (permalink)
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    Ok, first video is with the engine running. This is at full throttle and full choke. If I leave the engine alone, it will run for about 15 seconds like this then stall. I can wait a few seconds and it will start again then run shorter and shorter until it won't start again. This is with fresh fuel, brand new Briggs carb, new plug and the valve adjustment checked.
     
    https://www.youtube.com/watch?v=D0fvf8BqgWI
     
    Second link is with the engine all torn apart. From what I can tell, the mechanical aspects are ok. The cam was in time with the mark on the crank gear. As well, it looks like the piston and valves are in great shape. Please take a look and let me know what you guys think. The only thing I can think to try at this point is something with the alternator? Thinking it make have a weak output? Open to ideas!
     
    https://www.youtube.com/watch?v=sqdi9LsSEDM
     
    Thanks!
    Roy
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    Re: Briggs Engine 7.75hp - popping, stalling - Snow Blower 2019/12/18 21:12:35 (permalink)
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    The alternator has nothing to do with the way the engine runs, it only powers the light and the heated grips if so equipped. A failed ACR won't affect running but will cause hard starting, the engine will kick back when the rope is pulled or stall the electric starter. Everything you say points to fuel starvation.
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