28N707 15.5 HP Compression Release

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  #1  
Old 03-13-11, 11:19 PM
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28N707 15.5 HP Compression Release

I threw a rod on the Briggs 28N707 1026E1 980615ZE 15.5 HP engine on my riding mower. The smaller screw simply came unscrewed. There was no thread locking compound on it. It should be noted that this was a replacement engine the previous owner had installed new and, while several years old, it has very little wear... low hours.

After I replaced the rod and re-installed the engine, it started but knocked so badly I thought something was broken or breaking. After removing the spark plug and hand-turning it through a couple of cycles, I tried it again.

The engine runs fine at mid-RPMs and (very briefly) near top speed, but knocks like crazy at idle speeds. I tried adjusting the valve clearances to many different settings all the way from .003" on both to .007" intake & .011" exhaust, as well as .013" TDC & .017 TDC - Intake & Exhaust respectively. It behaves pretty much the same way at any settings.

I pulled the engine again, tore it down and inspected the centrifugal compression release mechanism on the camshaft... it looks fine ! No wear, moves freely, spring is returning it to cam shaft, etc. A previous thread here (02-13-08 #9) shows a picture of the camshaft and says:
[You can rotate the engine slowly by hand and watch the intake rocker and valve, The valve will start closing then once closed it will re-open slightly then close again. This is the release working to lower the compression. If you don't see this happening you will have to take the sump/pan back off and remove the cam to properly check, I left this pic large to show greater detail. The lobe to weight reject dimensions are .010"].

My Intake Valve does make that little after-bump as described. That post further refers to a Table No. 4, Page 18 (which I don't have) for Lobe Reject Dimensions, and I would like to know what it says for mine and how to measure, BUT I don't think there's anything wrong with mine.

After pulling the engine again, tearing it down and inspecting the mechanism, I am at a loss for any ideas to solve the problem. When I re-assembled the engine, I found it so hard to hand-turn through the compression stroke, that I'm pretty sure it's going to behave the same way when I get it back on the mower.

I have one other Clue/Question... When I ordered the Rod from B&S, I also ordered a new Crankcase Gasket; they list Three different thicknesses .015", .009" and .005". I asked the order-taker at B&S why three and which one I should have... she put me on hold for quite a while and eventually came back and said I need .015" but she didn't know why. Does anyone here know?
Any input, suggestions, education, ideas, comments, etc. from you kind folks would be greatly appreciated.
Kind Regards to All -
Romotor
 
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  #2  
Old 03-13-11, 11:37 PM
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Was the rod journal smooth and polished looking? Did you check the clearance at the rod? Did you check the weights for excessive play?
 
  #3  
Old 03-14-11, 08:40 AM
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Yes, the journal is as clean as a whistle, no galling, and everything is tight, including the balancer. Of course, with the crankcase pan off, there's a bit of end-play in the crankshaft. The new spec rod (not the oversized one) fits nicely. As I indicated, this engine has low hours on it. Incidentally, B&S specifies 160 and 260 Inch/Lbs on the two rod screws. Good luck finding a torque wrench that will fit inside the crankcase and accurately reach those settings! I used Locktite and common sense tight for aluminum. Their screw came out and caused all of this problem in the first place... no evidence of thread-locking compound. Do you have any information on the Compression Release, the sump gasket thicknesses and the Lobe Reject Dimensions?
Thanks !
Romotor
 
  #4  
Old 03-14-11, 08:56 AM
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Correction re: "The new spec rod (not the oversized one) fits nicely"

The new rod is the standard one not the UNDERSIZED one.
 
  #5  
Old 03-14-11, 09:12 AM
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I have both a 1/4 and 3/8 drive torque wrench that cover that range. I can get both in the 28 series. You need to position the crank just right (different) for each bolt. Re the centrifigal compression release. I didn't think they used one. I thought the release was based on the grind of the cam lobe. Also , the .015 gasket is standard. If you need to increase the crank end play then you have to add one of the other gaskets. You do not use just a thinner gasket instead of of the .015 to reduce end play but must add a thrust washer inside on the crank.
 
  #6  
Old 03-14-11, 10:24 AM
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Thanks, Racing ~
re: gaskets... of course... end-play... I shoulda' thought of that. Didn't know about the thrust washers though.
My torque wrench has a 1/2" drive, a large head and it's calibrated in FT/lbs not INCH/lbs. I guess you could convert, but 160 Inch/Lbs is only 13.33 Ft/Lbs and that felt unreliable to me when I tried in on a bench vise. Maybe that's another reason the original screw came out and threw the rod. I searched all over Memphis for a small inch/lb wrench... like looking for a needle in haystack. Not likely to ever need again (i hope) anyway.
Thanks for clearing up the gasket question. Oh... yes it's a centrifugal release... go here and scroll down to #9 for a picture: http://forum.doityourself.com/outdoo...n707-15hp.html
Regards,

Romotor
 
  #7  
Old 03-15-11, 09:55 AM
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Well, I was wrong about this* part...
>After pulling the engine again, tearing it down and inspecting the mechanism, I am at a loss for any ideas >to solve the problem. When I re-assembled the engine, I found it so hard to hand-turn through the >compression stroke, that *I'm pretty sure it's going to behave the same way when I get it back on the >mower.
When I re-installed the engine on the mower again, it ran much better... far less knocking and banging than
before... I have no idea why, though. Also, I can't get it to idle slowly like it did before the disaster (thrown rod). Same carburator settings, same throttle cable adjustments, etc. Otherwise it runs fine at moderate speeds and higher. Any suggestions from the Members as to how to get the slow idle back would be appreciated.
Kind Regards,

Romotor
 
  #8  
Old 03-16-11, 04:23 PM
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I sure would appreciate some help with this.
Anyone?
Thanks !
Romotor
 
  #9  
Old 03-16-11, 06:42 PM
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I almost wonder if your engine mounting bolts are tight enough, or maybe the key made into the stack pulley is worn and letting the pulley slap. Since it got better after removing the engine and remounting it, while you made no other changes, it points to that as a possibility. The engine will make a lot of knocking noise if the mounting bolts are not tight.
 
  #10  
Old 03-17-11, 06:59 AM
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BINGO !!!
Thanks, Cheese -
That's exactly what was causing the knocking (motor mounts). The first time I only tightened one bolt, just to see if it was even going to run... the second time, I tightened two diagonal bolts that were easy to reach. Just to confirm, I loosened all four just a bit, and it was a very ugly racket. Thanks for your diagnosis... greatly appreciated. Now, maybe messing with the valve clearances and carburator settings a bit will get the idle back down to normal... that's still somewhat a mystery.
Cheers !
Romotor
 
  #11  
Old 03-17-11, 10:07 AM
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Glad to help.

Can you manually push the throttle closed to make it idle?
 
  #12  
Old 03-17-11, 11:47 AM
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No Sir, Mr. Cheese ... When it's all the way down, the engine still running pretty fast (medium). I have tried adjusting the "fixed" end of the throttle cable (outside sheath) to different positions, and I have run out of travel on the throttle mechanism itself... i.e. when the white plastic slider that the throttle wire sticks into is pushed all the way forward, it's still running medium speed. The idle adjustment screw (phillips head at top of carburator) does nothing when backed further out... tightening it increases RPMs, though. The govenor is working OK when I chop leaves, but do you suppose it's out of adjustment, causing the fast idle ?
 
  #13  
Old 03-17-11, 11:31 PM
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What I mean by manually pushing the throttle shut is at the top of the carb where the phillips screw is. If you manually make the throttle shaft stop close against the screw, will it slow down then?
 
  #14  
Old 03-18-11, 09:46 PM
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Thanks, Mr. Cheese for helping me with these problems.
Yes, if I push up there I can hold it by hand in a nice idle OR even shut the engine down if I turn the butterfly all the way closed. The idle screw (phillips head) isn't doing anything. If I push the white plastic slide - the thing that the throttle cable wire attaches to - all the way forward, it's still running too fast... with or without the throttle cable wire. Obviously something needs adjusting, but I just don't know what or how. I suspect the governor but I'm reluctant to mess with that, because when I'm operating it (mulching leaves), it's working perfectly... powering up as it should under heavier load. What's really frustrating is that I didn't mess with any of those things when I replaced the connecting rod... at least not intentionally... LOL ! The good news is that idle or no idle, I'm back in business.
 
  #15  
Old 03-18-11, 11:48 PM
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The spring that connects to the governor arm is what puts the tension on it to set the speed. Does this spring get loose when you set the throttle to idle with the engine running?
 
  #16  
Old 12-30-12, 08:27 AM
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UPDATE: I used the mower for a season, but I could tell the starter was really struggling to turn the engine over... I often had to key-click (bump) past high the compression point. It was so bad I was afraid that I would burn out the starter motor. I re-adjusted the valve setting SLIGHTLY and (amazing) it started cranking normally... no strain on the starter! Valve Setting is critical!
Best Wishes to ALL -
Romotor
 
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