My frustration has driven me to join the forum...I have a 1989 Mastercraft Maristar 240 competition ski boat running a 454 big block. Last season I rebuilt the Holley carb and did valve adjustment- it ran awesome!! A 24 foot, 3500 lb boat with 6 adults aboard running 63 mph was a blast. As I came back down to idle speed for one last "prop check" I noticed a loud noise like a rod knock. No hydro lock or over-rev. happened~ so i was puzzled. Anyway- I tore into it- pulled valve covers and put rod end clips on to run it.. I found that #8 was the culprit. I ran full compression check, all were at 145-150 but #8 was at 120. I backed off the rocker then readjusted as it ran until the noise stopped. Then took another compression check and found zero- valve fully compressed. I readjusted until i got 120 back to the cylinder and droped some light oil in spark plug hole and rechecked- still 120 and holding - no boost or loss so piston and rings are good. I thought it was hydraulic lifter collapse so i fished it out and replaced the lifter- put it all back and started... noise is still there... I looked at the rocker it is moving operating same as the rest on visual inspection. The old lifter looked fine- no real indication of over wear- i am being told by friends that the cam lobe has developed a groove. I just started thinking as im writing this that the rod may be a little bent- still rotates and pushes but is like 30 psi loss and a slap.. could this be a $49 set of rods or should i dig the entire cam out? Sounds like a stupid question now...thinking about the differences in the tear down to get to the cam... but seeking some outside advice..
What does the bottom of the lifter you pulled out look like?If it's worn a bit it's normal.main thing is it should be shiny.That will be a good indicator of if it's a cam lobe.Putting a dial indicator on it at the rockers and checking the lift is the best way to know for sure.A bent pushrod will still run fine ,you just need more lash to make it stop clacking.None of these will sound like a rod knocking though.Sounds more to me like a valve is maybe bent , and it won't close all the way.Hence the 30Lb drop in compression.Would also explain why when you adjusted the lash you lost compression.The lash adjustment may have pushed a valve that's was already not closing,just a little farther open.And I imagine this could sound like a rod knocking if the valve is contacting the piston at TDC.Testing idle vacuum could help find out.Bent intake valves tend to make vacuum gauges go crazy.Check your oil in bright sunlight.If your engine / cam is grinding itself up,your oil will have glitter in it.
The older I get,the faster I was.
Thanks for the reply.. I was afraid of having to pull the head and dropping a new intake valve- only because I haven't done that before- but I have seen some good tutorials on u-tube.. I will go the $25 valve rout before rods or cam.. Thanks for helping me make my mind up..
let some more replys show up before tearing into this motor..GM Marine spec 454s are built with slight variations in parts..including almost unavailable main bearings... for some reason.. when gm built the marine 454/7.4 motors.. they line bored the mains to get them really straight.. this created main bore diameters that were OVERSIZE.. this is on the OUTSIDE of the main bearings.. not the crankshaft side.. the main bearings might be available thru the marine parts division.. i don't know if all the marine spec motors had oversize main bearing bores. but i have run across a few.. another thing.. i want to verify that the cylinder effected is the last on on the RIGHT Side of the motor.. with the distributer at the back of the block.. not the last on on the left side of the motor.. spark plug wires too close together on the last two cylinders on the left side.. 5 & 7. can cause piston damage.. as they are only 90 degrees apart and when the wires are too close.. the #5 wire induces a spark in the #7 wire.. causing some of the air fuel mixture to be burned away.. leaving that cylinder TOO LEAN and piston damage will happen.. this requires one of the tiny tipped scope cameras to look thru the spark plug hole with the piston down to see into the cylinder.. since you have a compression tester.. do you have access to an air compressor also..???if you *** can you find the proper coupler to hook your compression tester hose directly to your air compressor hose... so you can bring the crank around till the effected cylinder is at TDC on the end of the compression stroke.... then.. making sure that every thing is clear of rotating parts.. the check valve removed and safely stored from the end of the compression tester.. hook up the air line to fill the cylinder with compressed air.. where is the air leaking out of now...the intake port.. the exhaust portthe crankcase breather ??? or the radiator.. if it has a cracked head or blown head gasket.. you can also do this test with the rocker arms off... and the piston at the bottom.. i normally do this with just one spark plug out at a time... so if the head gasket is blown between cylinders the air does not just leak out the next spark plug hole... this is an easy way to find exactly where the leak is....there should be an engine ID number or tag somewhere on the engine to identify the exact build info... you will need this to get replacement parts that are marine spec.. hang out for more answers...
I have seen weak and/or broken valve springs cause low or dead cylinders. Luckily for the customer the keepers and retainer stayed in place. The valve spring was just slighty shorter in appearance while still on the motor, it barely caught my attention but was causing run problems.just some input, food for thought