FindMe

Jayl13,
Well these guys have given you a lot of good suggestions and food for thought, but you need to approach this in a methodical manner if you want to fix it without shooting parts at it from a bazooka First, the temp problem and the motor that is popping need to be isolated (is it the same eng)? If you don't know for sure which motor is doing it, hook up a vacuum gage to 1 motor at a time at a source BELOW the throttle plates to ensure you are seeing manifold vacuum. On a Mercruiser Qjet a good place to hook up is the little hose that goes to the choke pulloff, as they all work on manifold vacuum. You do not want to see a increase in vacuum by snapping the throttle, but a decrease... that is the difference between ported and manifold vacuum. 1st reading I want you to record is the amount of vacuum you see at idle in both motors at the same idle speed, and is the needle steady, jumping or? Next, record how much you have @ 1500 or close to where the popping begins (with steady throttle) in both motors, then @ 3100 - 3500 rpm on plane, cruising with steady throttle. Now, to confirm which motor is popping, make it happen and watch the vacuum gage as it will spike violently when the pop occurs (test both motors). Now you will have some useable info, and know if the motor that is popping is the one with the oil press problem. For a qjet to operate correctly, you must have good steady vacuum to operate the primary power piston, which raises the metering rods out of the jets for enrichment. Although im not convinced you have a carb problem, I have seen about 50 or more qjet off idle problems caused by the transfer tubes that are pressed into the airhorn falling out and laying in the fuel bowl. What this does, is takes away the vacuum signal that operates the "transition" circuit, and pulls the fuel from the bowl into the transfer chamber where the fuel emulsifies (turns from liquid to vapor), as raw fuel does not, and will not burn. You can remove the top of the carb, and there should be 2 large tubes in the middle, with 2 smaller ones to the outside of the larger ones. I have won a lot of money betting on these tubes laying in the bottom of the bowls on several different applications that have several hours on them, or subjected to severe duty, and if there is a problem in the off idle transition to the main circuit THAT is the reason 99.9% of the time, but lets not go there just yet OK? Now for the popping... are you SURE the ign system does not have a problem i.e. bad or cracked plug, bad or open wire, weak coil output? The only way to confirm these are to scope check BOTH motors, as they need to run, and be set up as a matched pair in EVERY respect including initial advance, advance curve, and total advance! Another question, who adjusted the valves and using what procedure? Are they hyd. with 3/4 to a turn of preload ? Are you sure this was done correctly? Sticking valves cause popping, and the fact that you have ate pushrods scares the crap out of me. 1st of all, you NEVER put more than 1 shim under a valve spring PERIOD... so that concerns me as far as the builder goes. Next do a static compression test to ensure all cyls are equal in compression... and do both motors for obvious reasons. If you have one or more different readings on the motor that is losing oil pressure, and you have confirmed that motor is the one popping with the vacuum gage, then I want you to do a running compression test. Believe it or not, this WILL NOT blow your gage off the hose, and a very accurate way to identify valve train problems. Cylinders operate differently under a load than they do cranking, so if you have confirmed with a scope you have no ign problems then do this. Remove 1 plug at a time and install the compression gage, start the motor and let it idle. The compression will not go higher than cylinder pressure is able to deliver, and the readings should be fairly close to your cranking test results. A bad valve spring, sticky guide, valve hanging open will all show up clear as day by the reading you see i.e. needle jumping and not steady, irregular needle reading etc, and the highest attained reading should remain on the gage ( if it goes to 175 lbs, it should stay there and not bleed off) while running. I would also do that in both motors, as valve timing, timing chain and the sealing of the motor will affect the readings, and remember BOTH MOTORS MUST BE THE SAME! I could write all day, but do these steps first and post your results, and then we can go off of that info for the next logical step.
So in review...
#1. Vacuum readings first as stated above.
#2. Verify ign system, plugs and wires are ok by hooking up to a scope.
#3. Static and running compression tests.

I agree that eng temp, loss of oil pressure id #1 problem, but we very well may see a reason for that based on these tests, and should not assume or proceed with any other remedies until these answers are recorded. Remember, to diagnose something like this, you must follow a direct guideline, not miss or assume any results by guessing, and remain methodical to properly eliminate possibilities in order of likely occurrence. I also realize that you have been given a lot of good advise here, but you need to start at point A, and go from there if you intend on fixing this thing. I do this for a living, certified to the tits, and very successful in finding problems without undue expenditure, but you have to understand that I am a professional in the top of my field, and need correct, honest reliable info on the things suggested above to help, and if you don't complete every step in order and completely, or are not honest with the results, then I stand a snowballs chance in hell of helping you. I hope this makes sense to you, and I will be looking to see if you need to "FindMe"..... Good luck my friend!

PS... I have a vacuum gage reading chart, and as soon as I can find it, I will post it to help you understand what you are seeing, and when this is all solved, if need be I can guide you to making the qjets work like a charm fairly easy and cheap. They are great carbs, but ya gotta know how to fondle them... lol