Gents,<br /><br />I recently bought a 1990 Wellcraft St. Tropez 3300 with twin 7.4L Mercruiser inboards, supposedly recently rebuilt. The starboard engine runs great but the port engine makes a constant popping sound through the carburetor and sometimes through the exhaust while cruising above 3500 rpm. It does not do it as often at lower rpm’s. <br /><br />I had the carburetor rebuilt and it improved a little bit, especially at idle and low rpm. Ignition system looks OK. I can not tell much about the sparkplug wires. Would a simple continuity check be enough to tell if they are good?. What resistance should I expect? <br /><br />One more thing, apparently when the engine was rebuilt, they found that one of the heads cracked. So they were replaced with a newer generation heads. I’m not sure if they changed the valves and valve springs. On of the possibilities is that they did not and the “weak” springs do not close the valves entirely thus causing the backfire. <br /><br />I need guidance as to what to do next.
. my fiancee lives in caracas. I hope to vist the puerto la cruz area this winter. I am looking to relocate from the states and all I knoww are engines and machine shop work. well 6 years of electronics in the navy. I am a retied Firecontrolman.<br /> anyway your problem can be caused by many things. a sticking valve is most likely. a bad spring will show up with a simple vacume test. a leakdown test is much more accurate than a compression test and is able to pinpoint where in the cyl its not sealing. the cyl in a perfect motor will have 0% leakage past the rings,head gasket and valves. 10 to 15% is the max allowable leakage before bad things happen. the advance on that engine should be electronic. I think it has the merv thunderbolt type IV. maybe a typeV. post back if you know what ign system it has. I just replaced the port engine on a 86 33 st tropez. beautiful boats. some techs that are not used to the older 454/7.4 cannot seem to get lifter preload right when adjusting valves. a leakdown can tell ya if the valves are leaking. my only beef with the 33 st tropez is the dang V drives . I hates V drives. makes me work to hard to line up the shaft coupler.<br /> anyway email me at