Leak Down Test

[Capt Frank]

Bought a 2003 19.8 Carolina Skiff with Yamaha 90 four Stroke . Took it to the dealer for compression check and to have the carbs cleaned because the motor was not running right. Mechanic tells me the compression was fine except for one cylinder which they thought might be carbon build up. Get a call and they are telling me that the head needs to be pulled because after the carbs were cleaned the motor is backfiring thru the carb no 1 and they checked the timing which was all right.. They then did leak down test and said one of the cylinders is losing compression. I don?t understand the ?leak down test? Could backfiring be caused by loss of compression or could it still be a carburetor problem. Any help would be appreciated

 

[rodbolt]

Re: Leak Down Test

backfire on that engine can be caused by a bad or stuck intake valve, usually its a clogged jet.
a leak down test is more accurate than a compression test.
a leak down injects a known air pressure and measures how much makeup air needs to be added to that sealed cylinder to maintain the known amount being injected.
15% is the limit but 20 will work ok.
however with a leakdown test if you suspect a stuck valve it will come out of the carb throat, the exaust or the crankcase breather tube letting the tester know EXACTLY what failed.
most likly still a carb problem.
compression test on that motor is not only useless but a leakdown test is faster.

 

[Capt Frank]

Re: Leak Down Test

Thanks Rodbolt for the help! This is an amizing site for help.