omc cobra 5.0

[hijinks1951]

so boat was given a tune up and then run once or twice before being winterized and stored for 18 months. Started on second try to move to ramp for trailer. Relocated it and took it out, ran ok except it would not start easily after planing for 20 mins and WOT was only 3600 RPM (never got more than that when I bought the boat and thought is was prop pitch). Noticed water coming from the valve cover breathers and oil inside valve covers was milky. Crankcase oil looks ok. Disconected breather tubes from flame arrestor and boat started better after being run probably because water was not going into carb. Replaced exhaust mainifolds and risers but still the same, replaced intake manifold gasket and reduced water flow from valve breathers considerably but not completely but now WOT is only 3000 RPM and boat won't plane. Set dwell per specs and used timing light to adjust timing, BUT it seems way advanced in order to get the engine to start/run smoothly.

 

[Coors]

Re: omc cobra 5.0

Search the threads for pressure test in here.

 

[hijinks1951]

Re: omc cobra 5.0

compression test next weekend, then I will think about trailering it for service. I still don't understand the WOT issue as it revs fine in neutral

 

[Coors]

Re: omc cobra 5.0

Not compression test; a pressure test.
could be crap gas.

 

[hijinks1951]

Re: omc cobra 5.0

Figured I would do a compression test anyway, as I plan to pull the heads off if I see any bad cylinders, The gas tank was a 1/4 and I just dropped in 30 gallons of fresh gas. I need to run a couple of oil changes to clen out any residual water but something has been stopping it getting to the WOT of 4200 rpm for a while

 

[Gary H NC]

Re: omc cobra 5.0

An engine with bad head gaskets or other problems will rev up with no load on it,you can't really go by that...

 

[Coors]

Re: omc cobra 5.0

An engine with bad head gaskets or other problems will rev up with no load on it,you can't really go by that...

start with the tests; it will make it simple.

 

[hijinks1951]

Re: omc cobra 5.0

Will run the tests this weekend, what I don't understand is why I can only get 3000 rpm out of the engine now, all I did was replace the intake manifold gasket. I tried adjusting the timing when the boat was underway under full power. At least before I was getting 3600 rpm and planing. You think new points/condensor might help? Appreciate your input as I am far from being a mechanic

 

[Haut Medoc]

Re: omc cobra 5.0

I would do a complete tune up....
Points, cap, rotor , plugs, condenser,.....At least.....
Change all fuel filters.....
I would drain every drop of gas out of that tank & start new, I'll bet you have alot of water in it from sitting that long.....
You may have a vacuum leak now that you have changed the gaskets....
Check your timing at idle, not under load......
I would check to see if the advance weights are not stuck by seeing if the timing advances with increased rpms........
In order.....
Compression test.....
Vacuum test.....
Fuel pressure test......
My bet is tuneup & replacing the filters & draining the bad or contaminated gas will work wonders.....
A litttle Sea foam additive would not hurt either........​

 

[hijinks1951]

Re: omc cobra 5.0

Will run the tests this weekend, what I don't understand is why I can only get 3000 rpm out of the engine now, all I did was replace the intake manifold gasket. I tried adjusting the timing when the boat was underway under full power. At least before I was getting 3600 rpm and planing. You think new points/condensor might help? Appreciate your input as I am far from being a mechanic

 

[Haut Medoc]

Re: omc cobra 5.0

Don't adjust the timing with the boat underway....
Do the full tune up & the compression test......
Post results & we will proced from there......
Without performing any tests, it is all conjecture.....​

 

[hijinks1951]

Re: omc cobra 5.0

the original owner is a friend of mine and had all the maintenance records! The fuel filter has not been changed since 1994!!! I suspect/hope that there is probably some correlation between the low WOT and this. More to follow after all the tests AND new fuel filter and carb filter

 

[bruceb58]

Re: omc cobra 5.0

Not sure how you were timing your engine...hopefully with a timing light. If not, you need to to correctly retime your engine.

Before you adjust the timing, set your dwell with a dwell meter.

 

[hijinks1951]

Re: omc cobra 5.0

dwell meter and then timing light, had a problem getting the dwell set correctly so am going to replace points and condenser. We would gap it and expect the dwell to increase but it decreased (lower gap greater dwell right?) When we set the timing it was way up from the timing marks to get a smooth idle (500 rpm).

Optimistically, I don't have cracked block, heads or manifold but have bad gas, bad points and bad carb,

 

[bruceb58]

Re: omc cobra 5.0

Next time you adjust your dwell try this method.

1) Run the engine and measure your dwell.

2) Take distributor cap off and measure your dwell while cranking. The dwell amount will be a little lower or higher.

3) Set the dwell to the proper setting while cranking taking into account the difference between the values measured in steps 1 and 2.

4) Put the cap back on and verify the dwell is at the proper setting.

Smaller gap increases the dwell.

I also suggest using an advance timing light or getting the stick on timing tapes so that you can measure the total advance at higher RPMs. This will verify that the advance weights are working properly.

 

[hijinks1951]

Re: omc cobra 5.0

Brother in law didn't make it with the tools etc so all I got done this weekend was a new points and reset the dwell and timing. Changed the oil and filters twice, first time with a quart of transmission fluid and pumped oil out through the dipstick, second time I drained it through the drain. First time it miked up in 15 mins, second time it stayed as clear as day.

Then I ran it for two hours water skiing this am with still some condensation from the valve breather hoses but absolutely no sign of any water in the oil, even the rocker arm cover caps stayed completely clean and no signs of milkiness anywhere. I suspect the amount of condensation is due to old gas.

But 3600 rpm is it for this motor! I suspect it is because it only has a two barrel rochester, even though according to the manual they never put a two barrel on a 5.0 GM engine. Any think putting an Edelrock intake manifold and a Holley 4 barrel carb on it it would enhance performance. Right now I am getting GPS measured 26 knots at 3500 rpm. Top speed is not the real issue as I want to pull out of the hole faster.

 

[Coors]

Re: omc cobra 5.0

Brother in law didn't make it with the tools etc so all I got done this weekend was a new points and reset the dwell and timing. Changed the oil and filters twice, first time with a quart of transmission fluid and pumped oil out through the dipstick, second time I drained it through the drain. First time it miked up in 15 mins, second time it stayed as clear as day.

Then I ran it for two hours water skiing this am with still some condensation from the valve breather hoses but absolutely no sign of any water in the oil, even the rocker arm cover caps stayed completely clean and no signs of milkiness anywhere. I suspect the amount of condensation is due to old gas.

But 3600 rpm is it for this motor! I suspect it is because it only has a two barrel rochester, even though according to the manual they never put a two barrel on a 5.0 GM engine. Any think putting an Edelrock intake manifold and a Holley 4 barrel carb on it it would enhance performance. Right now I am getting GPS measured 26 knots at 3500 rpm. Top speed is not the real issue as I want to pull out of the hole faster.

Dirty hull, or propped wrong, but you need any tool you will ever need, and spare parts on-board at all times.

 

[hijinks1951]

Re: omc cobra 5.0

did a compression test and all are in the 120's. boat seems to run well except for total power, slight "pulsing" at idle. Lifted me out on skis easily but took too long to plane.

Still only 3600 rpm and 26knts. but i have 3200 lbs of boat and 200 hp!

Good news is no water in oil or valve covers.