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  #1  
Old 09-14-2007, 01:26 PM
TommyC TommyC is offline
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Puzzler w/ B&S Intek 7HP mower engine

confused:
Attention Please:

Firstly, I'm new to this nice forum, and have searched on all B&S threads. Some clues but nothing on the head. Also, I'm very mechanically inclined and have rebuilt many 4-cycle engines.


Engine: Model 128602; Type 0212 E1
Problem: Engine will not start. Acquired the mower slightly used and not running. Cleaned up the carb and it ran fine but always required a prime for each start (even when warm) and almost always started w/ a backfire. After several weeks, it ultimately - on its "last minutes" - ran poorly (no throttle response). Attempts to start yielded at best only a backfire out thru the carb.

I removed the carb and thoroughly cleaned w/ solvent & compressed air. W/ carb attached to filtered fuel line, I blew compressed air thru its throat to create a venturi effect. Atomized fuel mixture emerged from the carb outlet, which tells me it probably ain't the carb.

Other conditions:
Intake gap: 0.010" (obviously a little high)
Exhaust gap: 0.007" (nominal)
Armature air gap: 0.010" - 0.011" (nominal)
Plug Gap: 0.020"

Compression status is reasonable (I think rings are fine).

With valve cover removed, I noted TDC and observed the valve train for 720 degrees. During the compression stroke, at between "7:30 & 11:00", I notice that the exhaust valve opens about 0.055"! This puzzles me no end and I must say I have never seen this behavior (pattern). Has ANYONE see this or can ANYONE confirm that this is "normal" on this B&S engine???

In searching thru the B&S threads, I've seen comments by folks like RestroRob suggesting the coil (i.e., that the B&S coil eventually "wears out"). I've also heard that they can fail from "overheating". The strange valve anomaly notwithstanding, I wonder if my symptoms point to this? Is there any simple, objective way to test the coil (I don't have a shop manual)? BTW, I tried a new plug and no difference ...

Any help or insight any of you B&S heads can offer is very welcome!
I'm especially curious about the valve pattern I described.

Thanks,
TC
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  #2  
Old 09-14-2007, 01:47 PM
Bill Kapaun's Avatar
Bill Kapaun Bill Kapaun is offline
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Are you setting the valves with the piston 1/4" PAST TDC?
With the compression release "bump" on the camshaft, TDC is NOT the way to set the valves on these engines.
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Old 09-14-2007, 02:28 PM
TommyC TommyC is offline
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Bill - I haven't yet adjusted the valves - just reported their clearances. I suspect the valve clearance has nothing to do w/ the problem, or at best is a very minor factor, Your 1/4" past TDC is duly noted. Not sure what you mean by the "compression release bump" on the cam shaft. Can you pls elaborate?

I'm still waiting to hear if anyone one shares my puzzlement about the exhaust valve OPENING on the compression stroke!

TC
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  #4  
Old 09-14-2007, 03:00 PM
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Bill Kapaun Bill Kapaun is offline
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There is a slight "bump" on the cam to open the valve SLIGHTLY and relieve compression when starting the engine.
Until you verify the valves are set WITH THE PROPER PROCEDURE, your observations are pretty moot regarding ANY valve opening/closing function.
You have to establish that you have a correct baseline before you proceed. Basics first!
Until you do that, we're just "spitting in the wind".
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Old 09-14-2007, 04:58 PM
TommyC TommyC is offline
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Bill-
I just searched on "compression release bump on cam shaft - B&S" and got this hit:
http://www.freepatentsonline.com/5301643.html

I also found this (even better):
http://www.patentstorm.us/patents/53...scription.html

The more formal name for the concept is:

Low oil sensor using compression release to affect engine operation

The engine protection device affects the engine operation when a low oil level is sensed by releasing substantially all of the compression pressure in the combustion chamber to prevent engine starting, to stop a running engine, or to reduce engine speed. The engine protection device includes a paddle or float whose movement is responsive to the crankcase oil level, an arm connected to the paddle or float, and a pivotable, yoke-type mechanical compression release mechanism that pivots in response to the movement of the arm. The compression release yoke has a first cam member that engages a valve tappet surface. When a low oil level is sensed, the paddle or float pivots the arm and the first compression release member, causing the first cam member to engage the tappet surface to substantially open the intake or exhaust valve.

Note that Briggs & Stratton is listed as the "assignee" for this patent, which was issued in 1994,. I thought this might shed light on the pattern I reported, which I assumed to be a clear SHOW STOPPER for an engine to run.

I then discovered THIS on B&S website (which appears to be the complete answer to my own question, and that to which you refer):


Compression Release System

Some small engines incorporate a compression release system to decrease operator effort when pulling a rewind starter. A compression release system is a system that relieves excess pressure during the compression event by allowing a small amount of compressed gas to be released through the muffler or carburetor.

The compression release system lifts either the exhaust or the intake valve slightly off its seat during the compression event (this is why the piston is positioned past top dead center to about 1/4" down from the top of the cylinder when adjusting valve clearances). This releases pressure and reduces the force required to pull the starter rope or load on the starter motor. A compression release is used on most Briggs & Stratton engines and does not affect engine performance above engine starting rpm.


In all my life I have never heard of relieving the compression so some wimp can start his mower. And I have to disagree with B&S' statement "A compression release ... does not affect engine performance above engine starting rpm." Mechanically speaking, anything that undermines compression affects engine performance.

Having said that, the best I can tell w/o a dial indicator is that when the piston is ~1/4" past TDC, the valves are "on the cam". The only place where the valves clearly come off the cam is at the top of the compression stroke (as strange as that may sound). And back to the ignition coil, I think I've just discovered that the primary lead has faulty connection at the coil, so THIS seems to be the best bet as to the show stopper. I also plan to remove the flywheel and make sure that the timing (key) hasn't drifted.


Sorry this was so long.
TC
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Old 09-14-2007, 06:15 PM
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Bill Kapaun Bill Kapaun is offline
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Any thing you wrote about the "low oil sensor" and compression release has NOTHING to do with this engine.

The ACR is strictly to ease starting effort when cranking these engines. This was very useful on the larger engines, and has "trickled down" to the smaller sizes. I agree that many smaller engines don't need it for 95% of us. It's a selling point however with terms like "EZ Start" or similar.
You also have to remember that there is a segment of the population that is getting older and/or handicapped that appreciate this "feature".

"In all my life I have never heard of relieving the compression so some wimp can start his mower. And I have to disagree with B&S' statement "A compression release ... does not affect engine performance above engine starting rpm." Mechanically speaking, anything that undermines compression affects engine performance."
Well, you still have a lot to learn
Just "how long" is "in all your life"? Just because you haven't seen it doesn't mean it doesn't exist!
Some releases are activated by a spring loaded lever on the cam. Once the engine reaches a certain (low) RPM, inertia pulls the lever out of the way and it has no further effect on valve action.

IF you are actually serious about checking to make sure the valves are set right, you can remove the spark plug and use something like a wooden pencil for a depth gauge to measure the piston travel.
Put Piston to TDC, insert pencil and mark it. Make another mark 1/4" away from the first. Rotate engine until 2nd mark is is now where the first one WAS when pencil is stuck back in. Check valves and adjust if necessary.
If you don't want to do this, refer to previous post about spit and wind.
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Old 09-14-2007, 08:14 PM
TommyC TommyC is offline
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Bill-

1) I thought I made it obvious that the patented compression release I discovered had nothing to do w/ the problem I'm reporting. In describing B&S' ACR, my exact words were: "which appears to be the complete answer to my own question, and that to which you refer." I mentioned the former (for low oil) because you hadn't explained the piece I later discovered on B&S website (I asked you to please elaborate). You didn't read my post very carefully.

2) As an engineer, I find your statement "Well, you still have a lot to learn" a little disingenuous and condescending. I would like YOU to explain how releasing compression - as these engines are designed to do - does NOT rob HP! In the net, what you end up with is probably a 4-5HP engine (vs. 7HP). The same effect (for the wimpy mower) could be accomplished by using a mower w/ a 3.75HP engine whose cam does NOT release compression. And when you wrote:
Some releases are activated by a spring loaded lever on the cam. Once the engine reaches a certain (low) RPM, inertia pulls the lever out of the way and it has no further effect on valve action.

I think what meant to write:
Once the engine reaches a certain (minimum) RPM, inertia pulls the lever out of the way and it has no further effect on valve action

That might make more sense. Unfortunately, the claim that "A compression release ... does not affect engine performance above engine starting rpm." is ambiguous. No details are provided on B&S website (I posted the full description of the topic) - nor anywhere else I can find - to describe a mechanism that cancels compression release above a certain RPM.

3) Then you wrote:
IF you are actually serious about checking to make sure the valves are set right, you can remove the spark plug and use something like a wooden pencil for a depth gauge to measure the piston travel.
Put Piston to TDC, insert pencil and mark it. Make another mark 1/4" away from the first. Rotate engine until 2nd mark is is now where the first one WAS when pencil is stuck back in. Check valves and adjust if necessary.


Remove the spark plug and use something like a wooden pencil for a depth gauge? What makes you assume that this isn't exactly what I did? Do you assume all LawnSite participants are morons? Didn't I start out by saying that I've rebuilt many 4-cycle engines? I didn't appreciate your condescending attitude, Bill. I was hoping for some intelligent engagement on this.
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  #8  
Old 09-14-2007, 07:44 PM
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Restrorob Restrorob is offline
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Quote:
Originally Posted by TommyC View Post
I've seen comments by folks like RestroRob suggesting the coil (i.e., that the B&S coil eventually "wears out"). I've also heard that they can fail from "overheating".
Tommy,

I would never use the term "wears out" referring to any coil.

Because, They don't !

The second part is very much possible if the engine is overheated thus causing the coil to fail after the fact.

In most cases when any brand coil fails on these small engines it's as the engine heats to running temperature.
As the coil heats up more resistance is obtained which reduces spark output to the plug, This is why in most cases a engine will start and run fine cold but drop spark. Then after a cool down period start sparking again.

Now,
Sorry to inform you but your engine has neither a mechanical nor electronically controlled low oil shut-down system listed.

Mr Bill has went out of his way to give explanations to your questions, He has also sent you in the right direction you need to be to repair this non-running engine.

If I were you I think I would be saying "Thanks Bill" instead of trying to second guess him.....

Good Luck, With your repair.
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  #9  
Old 09-14-2007, 10:12 PM
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Bill Kapaun Bill Kapaun is offline
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I give up! Totally IGNORE EVERYTHING I said.
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  #10  
Old 09-14-2007, 10:50 PM
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Restrorob Restrorob is offline
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Quote:
Originally Posted by TommyC View Post
Unfortunately, the claim that "A compression release ... does not affect engine performance above engine starting rpm." is ambiguous. No details are provided on B&S website (I posted the full description of the topic) - nor anywhere else I can find - to describe a mechanism that cancels compression release above a certain RPM.

Tommy, Maybe this will help.

As they say "A picture is worth a thousand words"

The first I threw together, The second (Out of a service manual) list the function and different styles.







Maybe your not looking in the right places ?

And something that baffles me a little;

Quote:
Originally Posted by TommyC View Post
Didn't I start out by saying that I've rebuilt many 4-cycle engines?
None of these "many" engines had some type compression release for you to already have learned the operation ?
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Originally Posted by piston slapper View Post
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