Pressure testing the fuel pump on a 2007 Mercruiser 3.0 TKS Mercarb

[flyman219]

We have set the carb's idle mixture screw and also the idle speed set. I ordered a shop built rebuilt carb and put on the boat last month with a new filter under the fuel pump, the new carburetor did not have the "cap" over the mixture special idle mixture screw that required the special tool bit to adjust it. The rebuilt carb has a simple screwdriver slot like you would expect to find.

Mike

 

[flyman219]

Well I'm back and have to admit that I didn't listen to the sage advice given early in my dilemma. Compression test ? Hmm, the motor turned over pretty even without a uneven speed (dead hole jump in rpm each cycle) , idled pretty well all things considered, and not much of a miss until after 10 minutes of 3400 RPM running. I am surprised the motor would idle with only three holes firing!

Bottom line is I did a compression test yesterday,

#1 120

#2 ZERO!

#3 100

#4 120

As bad of news as this is, I'm rather happy to get to the bottom of the loss of power issue. At least now I know what I'm dealing with, the head is coming off later today. As this episode evolves I'll start another thread if I need more advice, I promise I'll listen to the sage advice that is so freely given by those with more experience in the future. My apologies to Chief, Alldodge and Funtimes and anyone else who suggested the compression test.

Mike

 

[Scott Danforth]

Zero is pretty bad, however 100 and 120 are bad as well.

perform a leak down test to determine are you dealing with a valve sealing issue or a ring issue prior to pulling the head.

 

[flyman219]

Sorry Scott, too late for the leak down test now as the head is off,

I just rechecked the specs from Mercury and my repair manual and both of them call for 100 as a MINIMUM. The #1 and #4 at 120PSI is what I think the motor can produce in all cylinders. The #3 at 100PSI was due to a slight burn through in the head gasket to the #2 Zero hole that had the tulipped valve.

I'm expecting to see 120 on all four after the head gets out of the machine shop and on the motor again with the new head gasket.

Mike