Thoughts

[19cobaltcd]

The entire cooling system has been checked out. The impeller and housing have been replaced twice. The exhaust manifolds and elbows are stock new one's. The water pump on the motor is new, as is the thermostat. The hose from the outdrive to the gimbal housing was replaced. I disconnected the incomming hose from the thermostat housing and started it one time and it was pumping alot real fast. I dont know where else to look. Thanks

[DetroitJim]

Hmmm.... Now you've got me thinking. My Chris Craft did this once, there were pieces of an old water impeller up in the outlet port of the seawater pump. Also, my Sanger had a big weed up in the inlet hose. I was stumped for months on that one. What about the pickup on the drive unit or the passages inside? Maybe the inlet hose is collapsing? Or, have you checked for a blown head gasket i.e. from a cylinder into the water jacket? I agree with checking total timing, also fuel pressure at high speed. I still think it's cooling though. Late timing would be my 2nd guess.
DJ

[19cobaltcd]

I tore the motor down this winter. No evidence of any gasket leaks. I pulled every hose out straight and looked thru them, no blockages. I'm just at my wits end and dont know where else to look but at the carb... It still baffles me about the soot in the runners on one side of the carb, and why it was such a dog, just like it has too small of a carb or it was starving for air. I built this combination 2 years ago. The first summer the cam went flat on me in the first couple of hours run time. (Had a broken wire on the shift interupt). It took us all day of running tests before we found out what the problem was. So all the cranking the engine over running tests killed the new cam. Called comp cams and explained what happened and they sent a new one. Props to them. So the carb set on the shelf for over a year till I got it back together again and ran it. I have found no evidence but maybe a gasket swelled up and blocked a passage. I am going to put a kit in it but I'm still half afraid of trying it again. I was talking to a buddy and he has a 750 edlebrock on his tow truck he said we could try. Its not marine but I'm not putting the engine cover on till I figure this damn thing out. I think its worth a shot. The back barrels open about 3000 rpm and 4000 all I can get. This is the same I was getting with the motor completely stock. Go figure. Thanks for the help, I'm not giving up yet.

[Foggerjet]

Cobalt, You mentioned a water pump on the motor, do you have a heat exchanger (closed cooling) on this deal? if so, i've seen the little tiny tubes in the exchanger become blocked acting just like a stopped up radiator. Also No-one is doubting your ability, but it sounds like this thing is an ass kicker, and it will probly be some obscure, tiny detail that was missed, maybe. Check things like the intake gaskets, if they are backwards it won't get hot until under a load. I had a customer once who put the intake gaskets in with the water passages in the back instead of the front. it had the excact same symptoms you described.
hope to help
fog

[Rexone]

This sure sounds like a cooling system on the edge of it's capacity to cool for some reason. See if you have good water flow at the water out lines where they dump into the exhaust risers. Water flow in means nothing if there's a blockage beyond that point.
If theres an oil cooler in the circuit check it throughly for foriegn objects partially blocking flow or corroded up tubes.
If the carb sat for a year I'd rebuild it just on GP. gas varnishes up, gaskets crack and shrink especially the older ones. you could have a piece of crap floating around in the bowl for all you know blocking a jet or something. remote, yes... but possible.
hows the fuel filter?????? checked it? fuel pump (pressure)...checked it? both those can cause lean out under full throttle if marginal.
just stabbing in the dark with ideas.
I wouldn't change the carb until you know its the carb. Trial and error on expensive parts..., well is expensive. And many times isn't the problem.
I would think though if it was lean enough to overheat the motor "immeadiately" with a "good" cooling system, other engine damage would be occuring besides overheating. Have you taken any plug readings after running it hard and experiencing this overheating problem? if it's that lean you should be able to see it on the plugs.

[19cobaltcd]

It has the impeller in the lower unit, no seawater pump on the motor, then it has a water pump on the front of the motor that looks just like any other small block chevy pump. There is no oil cooler, just a power steering cooler on the incoming water line. I had no overheating problems until I put the cam and new carb on. I'm going to get a kit and go thru the carb. Cant afford a different one right now, (oldest kid is getting married this summer). It is not a closed cooling system. Fresh water all the way. Never got a chance to run it wide open and see what the plugs looked like. It overheated too fast. I'm going to start the summer out without the engine cover where access is simpler. The motor is in the boat, only thing left to put on is the carb. Just havent made it down to get a kit yet.

[canuck1]

The thermostat is sticking and causing the cooling system to recirc in the block
my advice is to throw the water pump on the front in the garbage, order a proper thermostat housing from rex, plumb it like a jet, or just run it without a stat for now and block the bypass

[Rexone]

As a first step in diagnosis assuming the carb rebuild does not fix it I would install a temporary water pressure gauge and a temporary fuel pressure gauge that you can watch when the problem is occuring. Might give you a clue to what's going on. The fact that you say it overheats so fast you can't even get to full throttle points to a cooling system issue of some kind.

[19cobaltcd]

Yea, these are some good ideas. I have the carb soaking in cleaner right now. Nothing obvious in tearing it down but we will see. I am going to get a pressure gages for the fuel and water. See where that takes me. The weather is almost warm enough here to start getting out and doing some testing. I hope to be doing that soon. Thanks.....

[gnarley]

Could you have a Y-pipe that has flappers in it? Those flappers come apart in some Y-pipes every now and again then get stuck in the transom adapter where it should be going through and prevanting the water from flowing out and restricting exhaust flow.
It is an idea.