350 Magnum problems at WOT and starting

[Sammy805]

I have a 1990 Stinray with a 350 Magnum and it is bogging down at WOT. It will reach peak RPM's, but after 15-20 seconds it starts to lose RPM's without missing. If the engine is run at other throttle positions it runs good, up to WOT. It is also having problems starting, the starter engages, but the motor does not turn over. Once it does turn over it takes a lot of cranking to get it started. Also, at low RPM's it has a poor idle. I have also noticed that the breathers have steam coming out of them, I saw this when I removed the flame arrestor and saw water on the secondaries, but there is no noticable water in the oil.<br /><br />I have done several things to fix the problems and they are listed below:<br />replaced starter<br />replaced fuel pump<br />replaced both fuel filters<br />checked fuel pressure at WOT, 7-9 psi<br />blew out fuel lines and tank lines<br />rebuilt carb<br />new wires, cap, rotor, and plugs<br />removed check valve, still ran the same<br />replaced the check valve<br /><br />I have read several posts, I have some of the symptoms that are posted, but I can not find any with the same as my boat.<br /><br />The manifolds and risers have never been replaced and I know they are in the time frame of replacement.<br /><br />I am planning on removing the manifold and inspecting, and also doing a compression check on the enigne.<br /><br />I know there are several good mechanincs on this forum and any help or ideas would be appreciated.<br /><br />Thanks

 

[Bondo]

Re: 350 Magnum problems at WOT and starting

The manifolds and risers have never been replaced and I know they are in the time frame of replacement.

That's where I'd Start Looking.........

 

[kdmiller8251]

Re: 350 Magnum problems at WOT and starting

Is the oil milky???<br /><br />I am with bondo, check the manifolds and risers quick.... could hydro lock it if water is getting in the cylinders...<br /><br />T

 

[johnkc]

Re: 350 Magnum problems at WOT and starting

yep, manifolds.

 

[Sammy805]

Re: 350 Magnum problems at WOT and starting

Well today I pulled the exhaust off of the even side and did a compression check. I have 0 in 4 and low on 8, so now I have decided to rebuild the 350 magnum.<br />If you have ever rebuilt this motor, I would appreciate your feedback.<br /><br />Thanks

 

[K Hultgre]

Re: 350 Magnum problems at WOT and starting

Slow down on the rebuild unless you have that many hours on it and a main or rod bearing is knocking. If you have cracked manifolds a rebuild on the engine will not fix the manifold - have them pressure tested.<br /> "0" on cylinder 4 and "low" on cylinder 8 does not necessarily point to the pistons and rings, how much compression on "2" and "6". Have you tried to squirt some oil into the cylinders and take the readings again? <br />While unlikely the head gasket could be blown between cylinders, I have seen them blown between cylinder and water passage, it is also very possible you could have a cracked head or valve problems (search using member 33513 or "got the lifter out" under the I/O forum lots of good info that may pertain to your problem). Could you retake your compression readings with and without squirting oil on all 8 cylinders. I would think this would tell you if a complete rebuild is necessary.<br />Also, as tallman asked "is the oil milky"?

 

[Sammy805]

Re: 350 Magnum problems at WOT and starting

My reply was short, here are the complete test results dry and with oil.<br /><br /> dry oil<br />1 160 160<br />3 140 165<br />5 165 165<br />7 150 155<br />2 160 160<br />4 0 0<br />6 160 160<br />8 125 140<br /><br />The 0 reading could be numerous things from head problems to real bad rings.<br /><br />The engine has 500 hours on it and with the readings on 3, 7, and 8, it shows that the rings are worn.<br /><br />The exhaust manifolds did not show signs of water leaking inside them and the exhaust valves were clean, but I will replace them when the engine is worked on.<br /><br />Does anyone manufacture a complete rebuild kit for marine engines?<br /><br />Thanks

 

[CCrew]

Re: 350 Magnum problems at WOT and starting

You've got issues with 3, 4, and 8. May be a head on the 3/4 side, just wear on the other bank. 500 hours isn't worn out, and should be an excellent rebuild candidate.<br /><br />Block preparation, bearings, pistons, rings are no different than a car engine. Use a marine gasket set, and if you need a cam, there are plenty of aftermarket marine grinds. Crane makes several. Use Brass freeze plugs and transfer items like electrical/distributor/carburation/waterpump. Short of the gasket set, the only thing in the long block that's different is really just the freeze plugs and cam. Everything else is just re-used. <br /><br />A small block chevy is one of the easiest to rebuild, cheapest to get parts for, and most all machine shops can deal with them. Personally I'd tear it down and go for it, but I've built a ton of motors much wilder than that, so a sbc I could do in my sleep. <br /><br />If you're not comfortable with it, any decent machine shop should be able to do it without it costing an arm and a leg. <br /><br />Just my .02..<br /><br />-Roger

 

[Sammy805]

Re: 350 Magnum problems at WOT and starting

Well, here is the update.<br />I have pulled the heads and taken them to the machine shop. Machine shop said that I had weak springs and 5 intake valves were tuliped. He said that the valves probably tuliped due to the weak springs. There was also a burn mark between 4 and 6, possible head gasket leak.<br />He replaced all of the springs, intake valves, and milled the heads.<br />I hope the weak springs were the cause of my problems.

 

[Walt T]

Re: 350 Magnum problems at WOT and starting

They werent. How does the #4 cylinder look? Can you see where a head gasket may have been leaking?