Compression Question


Old 05-23-03, 12:25 PM
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Compression Question

Briggs model 12v802...what should the compression be?? I can't find the spec anywhere in my service manuals. This motor has great spark, runs when I pour fuel down the carb, and runs perfect...been through the carb several times replacing and cleaning. Valves appear to both be functioning properly....possibly not seating?? I did a compression test and got 50psi on 3 consecutive tests....
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Old 05-23-03, 01:42 PM
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You're going to laugh, but I remember reading in a Briggs manual that the compression spec was like this:

"Rotate the flywheel back a bit. If the flywheel moves forward on its own, there is sufficient compression.". Really scientific. LOL

What is the actual problem with the engine?
Old 05-23-03, 01:45 PM
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Problem won't suck fuel. That typically sounds like Briggs manuals...the one I have says something to the effect that "we don't publish compression figures because of the special tools needed"

go figure...
Old 05-23-03, 02:21 PM
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Factering in the compression release loss 50 would probably be more like 60 or more once started. Really like to see 80 at a minimum. I don't have the specs in front of me but I think somewhere in the 90's is where it should be. Check the intake tube to make sure its tight. It runs from the carb over to the fitting that holds it to the engine.....Mike
Old 05-23-03, 05:30 PM
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If you put gas in the carb. and it ran good the compression should be OK. You could have a dirty intake screen in the fuel tank or carb. or have a dirty jet in the carb. or a leaking gasket or "o"ring some where between the carb. and the combustion chamber. Did you replace the carb. or just parts in the carb.? Sometimes it is hard to clean every small passage unless you take it off and submerge it in carb. cleaner.
Old 05-23-03, 10:40 PM
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Hello DennicG!!!!!! I had a similar problem with a briggs engine not sucking gas. Cheese had pointed out the problem. Turns out the intake valve's seat wore so much that it didn't have sufficient clearance, so I had to grind the valve stem down a little. You can sometimes tell by looking at the valve seat, if the seat isn't shiny and it has a black deposit on the seat, then you know the valves were allowing compression to blow past them. Hope you find the problem. I know it can be aggrivating to try forever and have the engine not start. Good luck!!!!
Old 05-24-03, 11:26 AM
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mike- thanks for those figures...already been over the intake issue...triple checked

Roger-It has been my experience that even an engine with little compression will run when force fed takes compression to suck the fuel though...the intake screen is clean and replaced, as is everything else in the carb...I soaked it in carb cleaner and cleaned it as thorough as I have every other carb I have rebuilt...but thanks for the input...gotta explore every avenue.

Mower- valves were my guess, I do understand what they should look like, and how they function...which is why I did the compression test...but didn't know the figure.

I think I have located the source of the problem, now just see if the customer wants to pay me to go into the engine...its a pretty beat up mower...I was kind of surprised there was even oil in
Old 05-26-03, 11:14 PM
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What did you find?

My guess, assuming the carb and intake are in good working order, would be that either the valve clearances are too tight, or the exhaust is restricted.
Old 05-27-03, 12:05 AM
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checked the exhaust restriction as well....still waiting to see if the customer wants me to go into the engine...been there done that too many times and ate the work I did...

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