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Hot!Briggs and Stratton Intek

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backporch
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2019/03/19 07:55:50 (permalink)
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Briggs and Stratton Intek

Before I tear this motor apart to verify that my camshaft compression release is broken, can anyone help me understand what I see.  When I turn manually through the compression stroke, the intake valve doesn't really close until about 1/2 way up.  Not sure if that is normal and when the compression release takes place.  I still have significant resistance at that point and the starter stops about there.  Also, what I find strange is that manually turning the motor through the power stroke is difficult.  Am I drawing a vacuum??
 
With the spark plug out, the starter will spin the motor at what I guess is a normal speed.   I have not verified that the starter is in good shape or sanded/cleaned every contact yet.
 
The motor is on a 2004 sears with an 18.5.  I purchased it non-running so don't know what is really normal

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    AVB
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    Re: Briggs and Stratton Intek 2019/03/19 08:48:03 (permalink)
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    Unless someone has been in the engine you may not be the middle of the compression stroke as the valve closes but just after BDC when the intake valve closes. The both valves stays closed until just before TDC of the compression then ACR bumps the intake valve to bleed off some of the compression. Use a dowel rod to verify the piston position.
     
    As for the resistance when hand turning this the compression that you are feeling. If compression release (ACR) was working properly there would still compression to fight but at a lesser amount. This high compression is what causes starting problems with the starter as they are not strong enough to spin through the high compression of 130+psi but when ACR is working this is as little as 70 psi.
     
    Also before tearing down adjust your valve clearances to .003-.005 IN .005-.007 EX. These clearance on OHV open up over time which leads to ACR not being able to do its job. Plus make sure the valve stem button(s) depending the setup are in place. Once the clearances are set then you watch for the ACR bump as the piston nears TDC compression stroke. If not present after valve adjustment then the ACR on the camshaft has failed.
    backporch
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    Re: Briggs and Stratton Intek 2019/03/19 11:54:47 (permalink)
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    AVB,
     
    I did have the spark plug out to determine and mark top and bottom dead center.  I adjusted the values to spec.  Found a few videos that said the adjustment should be just beyond TDC.  One video said let the piston travel down 8mm... probably to avoid the bump.   
     
    I also tried to put a compression gauge on the motor.  The starter didn't turn to TDC on compression.  Maybe 25-30 degrees before and stopped? just a guess.  gauge read 30PSI.  Could be leaky seal, dry cylinder or crappy gauge.
     
    Thanks for the info on the decompression bump. I did not know when the bump happens.  I see nothing just before TDC on the compression stroke, so my cam must be crap.
     
    Any warnings about picking up a used cam? looks like the part specified was used on lots of motors of different years and HP ratings.  Some are solid gear, others spoked design.
     
    The resistance on the power stroke has me puzzled.  I took that to mean that my decompression bump had worked, I had been passing through it too slowly and was drawing a vacuum after that point.  I guess in the event that the cam was taken out and put in incorrectly I can resolve that when the motor is open.
    AVB
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    Re: Briggs and Stratton Intek 2019/03/19 12:14:49 (permalink)
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    It will be a spoke version. You double me on this that the engine has the 793880 camshaft by looking the model and type number of your engine. The 793880 ACR do fail more than they should which why I keep 1 or 2 793880 camshaft kits in stock at my my shop. If the ACR did fail make sure you get all the pieces of the engine as it can damage things like governor/oil slinger gear.
     
    As for used just make sure it is in good shape before buying it. Most are probably from an engine that has been ran without oil that had broken the rod and put a hole in the side of the block.
    backporch
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    Re: Briggs and Stratton Intek 2019/03/19 12:53:11 (permalink)
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    owner's manual says 698492.  Looking on ebay there a lots of hits, many look like smaller motors with a solid gear. Are the cam lobes any different on those?  Thanks for the help
    AVB
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    Re: Briggs and Stratton Intek 2019/03/19 14:19:43 (permalink)
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    I haven't seen any that are solid but the number you posted is currently superseded to 793880. Since the 698492 supersedes to the 793880 the lobes should in the correct position. Apparently Briggs went from a casted camshaft to one they can assemble.
     
    kshansen
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    Re: Briggs and Stratton Intek 2019/03/20 09:08:51 (permalink)
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     "When I turn manually through the compression stroke, the intake valve doesn't really close until about 1/2 way up"
     
    If this is true then something sure sounds wrong inside.
     
    1. Intake should open close to top center and stay open as the piston moves down on the "Intake Stroke".
     
    2. Then it should remain closed on the compression stroke until piston is close to "Top Center" and then "bump open" for a short time to vent off a little compression to take load off starter.
     
    3. On the power stroke both valve should be closed till piston is at, or very close to, bottom center on "Power Stroke" 
     
    4. Then exhaust valve should open and stay open till piston is at, or very close to, top center on exhaust stroke. Just as the exhaust closes the intake should start to open. Actually both might be slightly open at the same time, this is referred to as "Valve Over Lap".
     
    Now go back to #1 and repeat.
     
    Unless I'm completely misunderstanding that sentence at the top of this page there is a cam timing problem in this engine.
    backporch
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    Re: Briggs and Stratton Intek 2019/03/20 14:58:18 (permalink)
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    The exhaust valve seems to follow an expected path.  I am probably confusing intake being open with there being a gap until closer to the top of the piston's travel.  
     
    I have definitely determined there is no bump in the intake valve.  The work I have to do on the power stroke to turn this engine by hand has me very puzzled, but my next step is to take the engine off and check the camshaft.
     
    Thanks for the help.  will follow up with what i find
    kshansen
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    Re: Briggs and Stratton Intek 2019/03/20 15:23:42 (permalink)
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    Hope you understand that many times a question or remark is made in an effort to understand what is happening. Things that one could see and know in a matter of a few seconds if there in person with hands on the engine may take several minutes of thought and dozens of words to express in text!
     
    Case in point, you seem to indicate, if I understand correctly, that the engine takes what you feel is more that the expected effort to turn when on the power stroke. What I would want to feel would be does the engine almost try to turn on it's own if you stop part way through the power stroke and begin turning it in the opposite direction.
     
    If having both valves closed and piston going down is creating a vacuum you maybe able to feel that to some extent. Can't say I ever noticed it but who knows?
    backporch
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    Re: Briggs and Stratton Intek 2019/03/21 07:25:53 (permalink)
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    Unfortunately I didn't see your last post and just got to work dismantling.
     
    I pulled the motor and opened up the case.  I found that the camshaft looks like it is intact. The bump mechanism is there and there is some very faint spring action.  Probably just the way it is supposed to be.  Oil is pretty dark and thick.  Maybe a thicker oil than I am used to SAE 30?
     
    Tomorrow I am going to visually check the cam lobes, lifters and pushrods to look for anything weird.  Any advice of what to look for would be great.
     
    I probably should have spent more time examining the wiring to the starter.  but I was sure that something was wrong because of the lack of a bump in the valves and I was pretty sure I adjusted the valves to spec... on the tight side of the range.
    backporch
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    Re: Briggs and Stratton Intek 2019/03/21 09:12:20 (permalink)
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    Here is a picture of the camshaft.  Orange flakes are a dusting of pan bolt's loctite

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    kshansen
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    Re: Briggs and Stratton Intek 2019/03/21 14:17:25 (permalink)
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    At this point I guess one has to start with the basics.
     
    If cam has not been removed make sure the timing marks on the crank gear and cam gear line up. 
    If this is one of the engines with the plastic gear and lobes inspect real close to confirm they are not loose or somehow out of position.
     
    I think it's possible to post pictures to this forum, if so how about a picture or two inside the engine showing cam, gear and lobes?
    backporch
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    Re: Briggs and Stratton Intek 2019/03/21 15:14:15 (permalink)
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    I posted a picture 6 hrs ago.  pending approval still.  If that gets approved, let me know what you think.  All metal gear.
    backporch
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    Re: Briggs and Stratton Intek 2019/03/21 17:15:09 (permalink)
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    Looks like its up now.
    backporch
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    Re: Briggs and Stratton Intek 2019/03/21 20:08:12 (permalink)
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    I took a measurement with my digital caliper.  Measured the cam diameter with and without the bump.  The readings I got in inches was 1.016 on the bump. and .9895 where the bump isn't.  thats a difference of .0265 in.  Given the gap at the intake was set to .003, seems like there should have been some movement.
     
    If I am thinking straight, the crank should rotate at half the speed of the camshaft.  At one rotation of the crank both exhaust and then intake should do there thing.  That's 1/2 rotation of the camshaft.  I was surprised to see the lobes were more than 90 degrees offset.  I think that lends some support to my finding that the exhaust was right on time and the intake started after TDC and closed after BDC.  The bump would have extended the "open time" or at least the tightness in the valve clearance till almost TDC on compression.  Its kind of what I saw.  I didn't notice the "bump" that I had seen on youtube videos was pretty pronounced.  Does any of this make sense?
    backporch
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    Re: Briggs and Stratton Intek 2019/03/21 22:18:05 (permalink)
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    Hello Mikel1,
     
    The valves were adjusted to the tight range of the clearances.  I dont have a good mower battery but I did jump start from my truck battery as well as two jump packs.  They basically all had the same result.  Stuck before TDC on the compression stroke.
     
    The wire connections to the starter were tight and looked pretty clean.  I didn't follow grounds or look to the other connections on the solenoid.  
     
    Looks like the acr on this camshaft is 90 degrees off the max cam lift for the intake. 
    Mikel1
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    Re: Briggs and Stratton Intek 2019/03/21 22:26:12 (permalink)
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    I see so can you manually move past that spot?
    backporch
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    Re: Briggs and Stratton Intek 2019/03/21 23:11:18 (permalink)
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    Sure.  I was able to rotate the motor by hand.  There was a good amount of resistance, but I was able to do it without too much effort with just two hands on the flywheel screen.  As I said before I found it weird that there was resistance on the power stroke.  I wish my compression gauge could have registered if there was any vacuum
    backporch
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    Re: Briggs and Stratton Intek 2019/03/22 08:30:41 (permalink)
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    I realize this post is getting long, so I will summarize where I am
     
    1. Couldn't crank past part way into compression stroke
    2. Set valve clearance to tight
    3. Didn't see the noticeable bump from the ACR when manually turning the motor
    4. Resistance during compression and power stroke
    5. Removed cam, ACR was intact
    6. Measured with micrometer and see difference of about .025 due to ACR
     
    issue 3 could have been me comparing my motor to what I saw on youtube
    issue 4 is odd about power stroke, but maybe a vacuum??
     
    Not sure where to go next.   I am going to reassemble when my gasket comes in.  probably also will clean up all electrical connections on starter/solenoid connections and grounds
     
    Maybe get a different starter or motor
     
    kshansen
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    Re: Briggs and Stratton Intek 2019/03/22 09:18:38 (permalink)
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    First off the cam rotates at Half Crankshaft speed. The gear on the crank has half the number of teeth as the cam!
    And the compression relief is before not after top center. The idea of compression relief is to make the crank easier to turn for the starter. If it was after top center the compression pressure would be pushing the piston down and helping the starter turn the crank!
     
    If you do in fact have 0.0265 lift at the cam from the compression relief and the valve is set to 0.003 you should see obvious movement of the valve when it comes up on the compression relief. Not sure what the rocker arm ration of an Intek engine is but should not be enough to account for the 0.0235 difference between no compression relief section of cam and compression relief point!
     
    Something is either wrong in your description of what you are seeing or something is wrong with the compression relief on the cam. I'm still trying to wrap my mind around the statement in you first post " the intake valve doesn't really close until about 1/2 way up". This makes no sense to me.
     
    Not trying to be a jerk with any of my comments but just having a problem understanding what is really going on!
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