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Hello!
Can anyone help with a diagnostic procedure to reliably rule out a broken compression release mechanism/pin versus a valve clearance that needs to be tweaked, or other possible causes for a B&S not turning over easily? I have a B&S 28W707 16.5HP which is showing symptoms resembling compression release issues when starting cold. When attempting key start, it turns a little then locks. The battery does not seem to be the issue, as jumping it does not change anything and also it does not get weaker with the number or length of turn attempts. When I turn the engine by hand, it reaches a compression point past which it is very hard to turn and I can only do so using both hands. It does start eventually but the only way I am able to do it is by turning it by hand a couple of turns then by key for about 6-7 seconds which, needless to say, floods the engine to the point where I have gas sprayed out the exhaust... Once started/hot, I can easily turn it off and on again without issues, which I find confusing. It started doing this after I recently adjusted the valve clearances, which I did find to be out of spec. The reason I did the clearance check in the first place was because it would after-fire in the exhaust when I shut the engine off. (The fuel valve does seem to work, as I can hear/feel it click with the switch.) After clearance adjustment, I am no longer getting the after-fire, except rarely, but now I am getting the hard starts. I can of course open the valve cover and check the clearances again, but I doubt I got them wrong the 1st time, hence this question. Is there any way I can rule out a broken compression release mechanism without opening up the engine? I am also confused as to why it starts easily after it has run for a few minutes: would this be an indicatation that tweaking the valve tolerances could solve the problem, since the running temperature seems to be a factor? Or am I missing something else?

For specs: engine manual lists the following clearances: 0.08-0.13 mm intake and 0.13-0.18 mm exhaust. I set them at 0.10 and 0.15, respectively.

Thank you in advance for any help; I am comfortable getting my nose into things but I am not always good at diagnosing them

Your valve clearance is way off. Only adjust valves when cold.

Pull the spark plug so you can turn the engine by hand on the top screen. Remove the valve cover and rotate until 1 of the valves is fully open. Adjust the rocker lash to .005 inch with a slight drag on the feeler. Once that one is done, rotate the engine so that the other valve now opens and the first is fully closed and do the same adjustment of the other valve.

I have done it this way for many years, and .005 inch is the "sweet spot" for valve adjustments on these engines.

If the compression relief does ever break on these, the engine will fight the starter no matter hot or cold.

Auto Doc's said:

Your valve clearance is way off. Only adjust valves when cold.

Pull the spark plug so you can turn the engine by hand on the top screen. Remove the valve cover and rotate until 1 of the valves is fully open. Adjust the rocker lash to .005 inch with a slight drag on the feeler. Once that one is done, rotate the engine so that the other valve now opens and the first is fully closed and do the same adjustment of the other valve.

I have done it this way for many years, and .005 inch is the "sweet spot" for valve adjustments on these engines.

If the compression relief does ever break on these, the engine will fight the starter no matter hot or cold.

Click to expand...

Don't confuse metric clearance with imperial clearance. He set the valves at .10 and .15mm which is roughly .004 to .005 inch.

Auto Doc's said:

Your valve clearance is way off. Only adjust valves when cold.

Pull the spark plug so you can turn the engine by hand on the top screen. Remove the valve cover and rotate until 1 of the valves is fully open. Adjust the rocker lash to .005 inch with a slight drag on the feeler. Once that one is done, rotate the engine so that the other valve now opens and the first is fully closed and do the same adjustment of the other valve.

I have done it this way for many years, and .005 inch is the "sweet spot" for valve adjustments on these engines.

Click to expand...

Thanks, Auto Doc.

I did all that and more. I actually adjusted the clearance by the book, with the piston 1/4" (6mm) past the top dead center. While it is possible I may have been off by 1/16' or 2mm on the downstroke, I doubt that should result in what I am experiencing. I strongly doubt everyone who has succesfully adjusted the valve clearance is that surgically precise...

As ILENGINE pointed out, the clearances I gave in my initial post were in mm but the screenshot I attached shows them both in metric and imperial.

Since my original post, I have re-opened the valve cover and checked the clearances again. They were still in the spec range, I did not get them wrong the first time and the nuts had not loosened. Both times the engine was cold.

So I still do not have an answer to the problem. If anyone has any ideas, please share. It is still hard to start and tends to lock. Yesterday it took even longer than usual, about 10 seconds, and I do not know if the longer start was random or an indicator that it is getting worse. By the time it did start, there was fuel sprayed and dripping out the exhaust... I might be tempted to blame a spark plug that acts weird when cold - I will replace it, although it is new - but that still would not explain the engine lock, hence my original post.

That pretty much leaves a failed compression release mechanism on the camshaft. I'm not sure if Briggs & Stratton had caught on their pitiful slow production for the camshafts, but an aftermarket one is a waste of time and money unless you want practice opening this engine up again and again.

Parts Tree shows to have an updated cam, but I do not know if it is B&S Make sure to enter all of your engine model and type numbers, this link is just a close reference:

Bolts said:

Thanks, Auto Doc.

I did all that and more. I actually adjusted the clearance by the book, with the piston 1/4" (6mm) past the top dead center. While it is possible I may have been off by 1/16' or 2mm on the downstroke, I doubt that should result in what I am experiencing. I strongly doubt everyone who has succesfully adjusted the valve clearance is that surgically precise...

As ILENGINE pointed out, the clearances I gave in my initial post were in mm but the screenshot I attached shows them both in metric and imperial.

Since my original post, I have re-opened the valve cover and checked the clearances again. They were still in the spec range, I did not get them wrong the first time and the nuts had not loosened. Both times the engine was cold.

So I still do not have an answer to the problem. If anyone has any ideas, please share. It is still hard to start and tends to lock. Yesterday it took even longer than usual, about 10 seconds, and I do not know if the longer start was random or an indicator that it is getting worse. By the time it did start, there was fuel sprayed and dripping out the exhaust... I might be tempted to blame a spark plug that acts weird when cold - I will replace it, although it is new - but that still would not explain the engine lock, hence my original post.

Click to expand...

You have to look for a slight bump in either exhaust or intake in order to diagnose for sure, that the compression release on the camshaft is broken. If you do not know how to do this, watch YouTube videos or do more research. This is the only sure fire method of properly diagnosing a broken compression release.

Maybe this will help:

Try rotating the engine crankshaft backward by hand (by the screen above the flywheel) until it just gets to the compression. Then use your electric starter normally. It might give the starter enough umph to over come the problem. Let us know. Good luck.