# Briggs & Stratton 450 series compression



## Francophile69 (Nov 13, 2015)

Problem starting a 148cc Briggs and Stratton

Would anybody know what adequate compression should be.

I'm guessing around 100psi and that the 75psi I am achieving is too low/.

Thanks


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## AVB (Nov 28, 2014)

Actually 75 psi is fairly good. It actually when the readings are below 65-70 psi you need to worry. These engines has a decompression system that makes taking compression readings fairly useless as gauge of engine condition. Briggs recommends doing a leak down test to gauge the cylinder condition. A leak down also tell you if you have a leaky valve, bad head gasket, or cylinder/piston problems. 

If haven't already check the valve clearances needs to check especially if fairly heavy used engine. Depending if it is a L-head or OHV. The clearances can close up on L-head making the decompression work more lowering the overall compression. On ohv these clearances tend to open up making the decompression less effective thus making it harder to pull through at startup.


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## Francophile69 (Nov 13, 2015)

Hi
Thank you for your reply.
Wife is in England at the moment and popped into a B&S dealer. They cannot say what the compression should be as B&S will no longer release such information! How strange, I wonder what the big secret is?

Could you elaborate on the decompression system at all? I am intrigued by that. I will also try and find out about the leak down test. 

I have not, as yet, checked the valve clearances, although they certainly look ok I am running out of things to check so guess that will be next. Everything appears spot on and yet it wont start, very frustrating!


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## AVB (Nov 28, 2014)

The decompression system sometimes called compression release can be one of several types depending on the engine's design. All of the system provides a monetarily opening of one the valves to lower starting compression. All these are design to ineffective at running rpm speeds. 

The ramp and bump systems uses a small area specially ground on either the intake or exhaust lobes of the camshaft just before TDC of the compression stroke. Less prone to failures but does fail due normal wear of the camshaft lobes.

Then there is cam type system that consists of small d shape pin or sometimes half moon shaped disc attached to a weighted swing arm and a spring. This system also creates the bump of the valves at just before the TDC stroke of the compression stroke. This system is primary used on OHV engines but can be found in some L-head designs. It also more prone to failure since it involves moving parts.

There a few other systems but are not normally used on Briggs engines so I didn't go into there descriptions.

With this decompression systems as the valve train wears they can become over active or non active as previously noted. I have had L-head engines come into the shop where the valve clearance have reduced completely to zero clearance and would not even start. With OHV when the valve clearances open up due to wear they become very hard to either pull through a start cycle or the electric starters can not even turn the through the cycles. Also with weighted cam type system when they fail they can literally pull the rope out your hand as they back up due to the high compression. This really hurts and can cause some serious injures.

These decompression systems requires the engine to be 1/4" pass TDC of the compression stroke (piston down from the top the TDC stroke) when adjusting the valves to minimize their effects.

You noted the clearance look good. To me it is hard to tell the difference between the few thousandths of inch that can cause problems without feeler gauges. Most specs usually usually only allow a plus or minus .001" difference when adjusting the clearances.

Hopefully this explains the decompression systems. I wish I still had a copy of the article I had that went into details about them.

Here is a Wiki article on the leak down test. It explains why a compression test is also important; although, it is not as important as the static test. 

https://en.wikipedia.org/wiki/Leak-down_tester


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## Francophile69 (Nov 13, 2015)

Hi. Thanks very much for your reply. Very interesting.
I'll check the valve clearances next. Looks like the clearance is more important than I realised! Thanks again.


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