What would cause this?

[OMARiverRat]

Hey everybody... you may remember me from previous posts such as "Backfire problems" and "Carburetor adjustment".... I'm back now with a problem a little more serious. 1993 Crownline 225 w/ 7.4L/Bravo (not magnum). Engine was backfiring through carburetor under load, so I replaced the carb. Runs better, but still backfiring. Also had knock coming from center of engine. Ran a compression test last weekend and everything checks out at 130psi, except for #4 & #6. #4 has 120psi, #6 has 110psi. After some advice from a marine mechanic, I pulled the manifolds, valve covers, and head off. The head appears to be fine, no broken valve springs as far as I can tell, however I'm taking it into the machine shop today to get a professional opinion. While observing the cam and lifters, I noticed that the exhaust lifter on #8 cyl is completely shot. The bottom of the lifter has been chewed up extremely bad, and the lobe on the cam appears to be damaged as well. Any idea what would cause something like this? To throw another wrench into the equation, I believe this is a counter-turn motor. When I'm standing in the boat looking towards the motor, the harmonic balancer spins clockwise. If this is a counter-turn motor, is a special counter-rotation camshaft involved in the rebuild? <br /><br />Any help or thoughts would be greatly appreciated. The irony is that the #8 cyl had excellent compression. It was #6 and #4 that were all lower than the others.<br /><br />Thanks guys, looking forward to hearing back from you.

 

[trog100]

Re: What would cause this?

cam lobes and followers are case hardened.. just a few thou of hard metal on the outside.. when this wears thru rapid wear takes place.. assuming the valve is opening and closing it dosnt have to be exact u will get good compression with such a worn lobe/follower.. as one part wears the other part wears with it..<br /><br />a badly worn lobe is just likley to produce less power from the one cylinder and perhaps be noisy in operation.. i have seen engines with half the lobes worn away on every cylinder that still ran basically okay except for a general lack of power..<br /><br />i cant be of much more help with your engine thow.. except to say the cam wants replacing and the head sounds like it could do with a general overhaul..<br /><br />the slightly low compression on some cylinders could be ring and bore wear but its more likely to be valve and head connected.. its not the cam..<br /><br />trog100

 

[TilliamWe]

Re: What would cause this?

By the way, looking at the front of the motor and it spins clockwise is the normal rotation for Chevy engines.

 

[flashback]

Re: What would cause this?

OMA, I,m no engine expert, but I can say this, lifters and cam lobes don't usually tear up like you have described.. wear like Trog mentioned is normal, but not chunks missing.. carefully inspect the push rod, rocker and valve for any damage. you also may have gotten some water in that cylinder.. in any case, find out what caused it then replace the cam and ALL the lifters.. good luck......

 

[trog100]

Re: What would cause this?

flashback.. i cant imagine anything that would cause chunks to be missing to be honest without some kinda major damage elsewhere.. i took it as slight exageration.. he he.. perhaps i shouldnt have..<br /><br />trog100

 

[Silvertip]

Re: What would cause this?

If the boat ever took a "little air" at WOT there is the possibility of valve float. If it happens at just the right time, tremendous shock can be sent through the valve train. Very carefully inspect the top of the piston to see if any contact was made. Improper hardening on the the cam and or lifter can cause lobe and lifter damage but like others said is is unusual to see pieces missing. Improper cam hardening was a problem for several manufactures back in the late 70's. Don't know if it was due to wrinkles in a new process or what. I changed a cam in my 77 Pontiac that had a couple of cam lobes that looked more like cam bearing journals. Roll the push rods across a surface plate to see if they are bent. Examine the rocker arm pivot for signs of damage. Damage on any of these parts point to valve float. Since you need to replace the cam, lifter replacement is also in order. You have a standard rotation engine.

 

[OMARiverRat]

Re: What would cause this?

Guys, thank you for the help on this. I've thoroughly inspected the tops of the pistons, rocker arms, pushrods, and everything seems to be in ship-shape. I may have exxagerating a little when I said a chunk of the lifter was missing. It appears the bottom of the lifter is worn deeply at an angle. The camshaft is still in the engine, so it is very hard to see (I'm having a special harmonic balancer puller made so I don't have to pull/lift the engine to get the timing chain cover off). I went ahead with the valve job, and I will be picking up the head tonight to re-install on the boat, as well as remove the old camshaft.<br /><br />On a related topic, I am going to purchase an engine gasket set tonight at the parts store. Does it make a difference if the gasket set is for an automotive or marine engine? <br /><br />And finally, since I have this pulled apart and I'm searching for a new camshaft/lifter set, I'm thinking about getting a little creative with the new camshaft that I use. Does anybody have any recommendations on a good camshaft which will work with this engine? What can I expect to gain out of this, without swapping the heads? Will I loose any reliability? <br /><br />Thanks guys, Take care & be safe.

 

[tommays]

Re: What would cause this?

no expert but i would stick with a marine/truck cam many high performance cams will suck water into the motor and not make power at the right rpm for marine use<br /><br />tommays

 

[trog100]

Re: What would cause this?

i would go for tommays suggestion.. stick with a "marine/truck" cam.. a boat needs a wide power band with plenty of bottom end torque.. car performance cams tend to go for top end increases but very often at the expense of bottom end torque..<br /><br />i have read that marine head gaskets are more corrosion resistant than automotive ones.. not sure how much truth there is in this or whether its just boating myth..<br /><br />trog100

 

[90skichallenger]

Re: What would cause this?

Edelbrock, comp, and crane all make a good cam for that motor. If you have money to spend and are looking at a power upgrade I would consider changing to a roller rocker valve train. All three of those companies make matched sets for your chevy motor. But definitely go with a truck cam designed to make more power and torque at lower rpms. If you call any of those people they have good tech people that will help you find the best cam for your use.<br />later,

 

[flashback]

Re: What would cause this?

OMA, If the bottom of the lifter is worn at an angle, I would think that it wasn't turning in it's bore like it is supposed to.. sorry to say, I don't know what would cause that....