Yamaha Non starter.

Mercury · Yesterday, 19:50

Hi, I am having trouble with my Yamaha 4 hp 2 stroke. I have cleaned fuel line and carb, reed valve look good new gasket on reed valve & carb, and I have spark. Tried a compression test but it seems low to me around 80 psi. but not sure what it needs to be, does any one know ?

beamishken · Yesterday, 21:24

80 psi does sound a bit low but its notoriously difficult to get an accurate compression test result on a pull start engine even if it is as low as 80psi I'd still expect it to start. If the plug is okay & you definitely have spark I'd try some neat petrol down the plug hole & see if it fires on that, just dont go too mad & that it cant turn over. If it fires you know its fuel related if not then you need to look elsewhere

jeffstevens763@g · Yesterday, 22:41

120/150 psi normal

Fenlander · Today, 00:18

Like Ken I'd be straight to proving if it's fuel delivery or not. Either his method into the plughole or mine where I put 2-stroke mix into a trigger spray and spray this into the intake while pulling over with the throttle part open, choke off or part open.

If this doesn't make it fire and you revisit the spark aspect I wonder if you've fitted a new plug? Assuming you tested the plug by observing it spark out of the cylinder I have known a plug that will show OK like that but not fire correctly once tightened in.

Re compressions yes you'd hope for higher but again as Ken hints there are variations in testing, including the accuracy of a particular gauge, that make comparisons with a target reading difficult.

Has this motor been fine up until a first start up this year?