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  1. #1

    Default Briggs 18hp twin runs backwards!

    I've got 18hp twin briggs that runs backwards. It ran backwards when I first started it up. I adjusted the ignition gap and it ran normal for a little while. Then it switched to backwards again and has been backwards ever since.

    Things I did before starting it up: (1) new stator, (2) full carb clean (3) new flywheel key, (4) new spark plugs, (5) new oil.

    I am having trouble torquing down the flywheel nut. I've heard it needs 145 ft-lbs. (is this right?) I can't lock the engine to get the torque on it. I already bent a belt pulley when I was using a belt and vice-grips to stop the engine so I could tighten it up. Any advice about how to lock this engine so I can tighten that flywheel nut?

    I've been playing with the ignition gap but I can't get it to run normal again. Does anyone know why this motor runs backwards and how I can fix this?

    Thanks,
    Backman

  2. #2
    Veteran Member
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    John Deere 4520,

    Default Re: Briggs 18hp twin runs backwards!

    Does this engine run backward when shuting off. cannot see it running at any other time.
    If so look closely at the valve botton of carb. it is supposed to stop gas from entering the cyl. when ignition is shut off. be careful when removing dealiers like about $75.00 to sell new one.
    Electronic ignition shouldn't change good chance wouldn't give spark if it did.
    if all is good looking maybe a air leak in gaskets.of carb.
    ken

  3. #3

    Default Re: Briggs 18hp twin runs backwards!

    Hi Ken, thanks for quick response. Can you please explain exactly where this valve is? Is it in the carb? Intake manifold? That carb is clean as a whistle I promise, with all new gaskets. It has one adjuster screw and I expect it is for idle. I've got it at 1.5 turn open now.

    Here is a more detailed description of what is going on: The engine starts right up great but unless I keep it choked it takes off accelerating and accelerating without leveling off. I get nervous and kill it before it flies into outer space. I could probably ride this mower down the highway (but it would be going backwards!).

    I know this is hard to believe but the thing just runs backwards from starting it up to shutting it down. Sometimes the flywheel kicks back a quarter turn when shutting down. When I put it in reverse gear I go forward and when I put it in 1st I go backward.

    Just one time I was able to get it running right and it didn't accelerate out of control and I went forward in 1st gear. Haven't been able to get it back there though. All I did was flip the flywhere 1/2 turn and reset ignition gap.

    I'll look into possible air leaks. Thanks.

    -Backman

  4. #4
    Veteran Member
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    Kubota B6100

    Default Re: Briggs 18hp twin runs backwards!

    I don't understand how this thing could run backwards unless the starter is spinning the wrong way. Being a 4 stroke you wouldn't think it would be possible to run backwards.
    Mike
    B6100 w/ power steering - link below
    http://www.tractorbynet.com/forums/k...has-power.html

  5. #5
    Veteran Member K7LN's Avatar
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    Default Re: Briggs 18hp twin runs backwards!

    Quote Originally Posted by Backman View Post
    The engine starts right up great but unless I keep it choked it takes off accelerating and accelerating without leveling off....

    All I did was flip the flywhere 1/2 turn and reset ignition gap.
    The acceleration is probably caused by the governor not being able to function. If it's an air vane type of governor, the air flow will not be correct if the engine runs backwards, thus no control.

    How were you able to flip the flywheel 1/2 turn? The flywheel key should prevent this from happening. I'm surprised that it runs at all. I suspect that the flywheel provides a signal for the ignition to fire. If it's off a half turn, then it's out of time with the cam and valves. Restore it back to normal and I would expect it should run normally.
    JD 790 w/70 FEL & 7 BH on turf tires
    JD 455 w/60" MMM & 54" front blade

  6. #6

    Default Re: Briggs 18hp twin runs backwards!

    Hi K7LN, thanks for the explanation of the governor. That makes perfect sense!

    I'm sorry but I misspoke about turning the flywheel. I turned the whole engine around 1/2 turn before resetting ignition gap because I was worried that magnet pulling on ignition coil was causing problems during gap setting. Flywheel key holds flywheel to crank and I did not turn flywheel separately from crank.

    KubotainNH, the starter may very well be turning the wrong way (and I have no idea why) but like I said I got the thing to run normal once and the only thing I did was reset ignition gap after turning engine (not just flywheel) 1/2 turn.

    Thanks,
    Backman

  7. #7

    Default Re: Briggs 18hp twin runs backwards!

    Quote Originally Posted by KubotainNH View Post
    Being a 4 stroke you wouldn't think it would be possible to run backwards.
    I have seen much skepticism about 4-stroke running backwards in this forum. In fact, the first person I called about this said "4-strokes can't run backwards" and then hung up. But I promise you this engine is running backwards. It is running quite nicely actually, just backwards (and too fast). I may have to post video of it if this is in fact "impossible." I can't explain it anyway, so please don't ask me to explain it. It just is. I just want it to run normal so I can mow tomorrow!

    -Backman

  8. #8
    Veteran Member K7LN's Avatar
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    Default Re: Briggs 18hp twin runs backwards!

    I gather that you've had this engine for a while and it used to run normally. My only guess at this point is that while installing the stator, you got something out of time. The other thought is that while trying to tighten up the flywheel nut, something got stripped or somehow knocked out of time by jumping a tooth or something similar. I'm not at all familiar with the engine, so I can't speculate any further. Ignition gap would have very little effect on the problem as long as you're in the ballpark with the gap size.
    JD 790 w/70 FEL & 7 BH on turf tires
    JD 455 w/60" MMM & 54" front blade

  9. #9
    Super Member Scooby074's Avatar
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    Default Re: Briggs 18hp twin runs backwards!

    To torque the flywheel you can wedge a prybar carefully in the fins and go at it. Or you can just run the nut on with a airgun

    You need to check the gap at the coil. Use some brass feeler gauges. .008-.012 is the spec for most twins, what model engine is it?

    I bet you sheered the key . new key and " torque" the flywheel and you should be ok

  10. #10
    Super Member crazyal's Avatar
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    Default Re: Briggs 18hp twin runs backwards!

    Just curious but do you have the plug wires on correctly? Also you may have a air leak somewhere in the intake system. It sounds like the timing is very screwed up (hense the plug wire question) and you're igniting the gas too soon or you're running very lean and you're getting pre ignition (air leak).
    Kubota L4240,Case 580K backhoe, Case 450 Dozer

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