![]() ![]() During the portion of the 4 stroke cycle where air is being drawn into the combustion chamber (intake stroke), the intake valve is open because that tappet is riding on the cam lobe (large eccentric area). The camshaft base circle is the portion of the cam where the brief "high point" (that opens the valve) is situated. TDC is the same point in the piston travel, whether you reach it by turning the crankshaft CW or CCW. The difference (in the compression pressure) is that when turning backward, the compression continues to build up without any partial release of pressure just prior to the piston's arrival at TDC. At the points in the cycle where the compression tests are being taken, whether turning clockwise or counterclockwise.both valves are closed at the time the piston reaches the TDC mark. So whichever method you use, your timing ends up in the same place!Ĥ stroke cycle engine. When I set the marks up aligning the cam gear mark with the #2 crankpin, and then slowly turned the engine over, (with my "loose" ball bearing slid forward so I could peek behind,) I saw the crank gear marking come into perfect alignment with the cam gear marking, just like the plain bearing instructions demand. I found that while the plain bearing instructions are to align the cam gear marking with the crank gear marking, and the ball bearing engine instructions are to align the cam gear marking with the marking on the #2 crankpin, these timing procedures result in the SAME timing. ![]() Enough to see the crank gear behind the bearing. My PTO end ball bearing, while supposedly a press fit, moves a bit forward on the crankshaft. I took the sump cover off and checked the situation out. I thought perhaps the machine shop did not follow those instructions. plain bearing set ups.Įarly on, I thought I had found why my compression was low after the rebuild when I saw the manual describe the two methods. ![]() ![]() One issue I can address mentioned above is the dual timing instructions for ball bearing vs. It confirms what I sort of stumbled onto myself. The engine manual has a couple of pages devoted to compression testing and performance issues. Consider performing a blow down compression test to determine if there is excessive leakage at valves, head gaskets, or piston rings. Setting the clearance on the higher side will increase compression.ģ. Make sure the pistons are positioned 1/4" past top dead center. Make sure the battery if fully charged.Ģ. I have tested many of these and normally see about 95 psi and I am at 7,250 feet.ġ. Compression testing is recommended to compare readings between cylinders. This description is from an older Briggs repair manual.īriggs does not publish any compression figures. After the engine starts, events occur so quickly, the engine does not experience loss of power. This prevents excessive compression during engine starting. The intake lobe on the camshaft is designed to hold the intake valve slightly open (1/100 of an inch) during a portion of the compression stroke. The Briggs opposed twin has "easy spin" compression release. Does this confirm an "intact compression chamber", and that I should put the engine back in the tractor and cut grass?Īny briggs bulletins or explanations of this system? any list of which engines do or do not carry this compression feature? In normal operating direction, checking compression with engine starter, I only got around 55lbs. I put a bolt in the PTO end of crankshaft and spun the engine backwards a few turns with a variable speed electric drill with a socket on the end. How does this not effect compression/power at operating speeds? This seems to tell me the compression release cam lobe "system" is at work. I checked the valve action with head off and find that the intake valve stays open a few thousanths until very near TDC. I checked compression and got numbers in the low to mid 50lb and thought the rebuild was done poorly.Īfter much investigation and testing: does this engine have an intake valve compression release built into the cam lobe? I have a briggs manual for this engine.not a word. I have had this engine re-bored and piston. ![]()
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