1987 OMC 7.5L engine pops and lack of power

[Chappy509]

Hey guys, new to this forum... I have a 1987 Century 4500LX with a OMC 7.5L. All of last year and this season up to a few weeks ago the boat ran great. Engine always could run 4500-4800 rpms and up to 50mph. Past few times out has been running worse and worse. Runs fine in N, but under a load the engine pops and has no power, can only get about 4000 rpms and about 40 mph... And thats pushing it and pops pretty bad with real lack of power. Did some needed maint., plugs,wires, cap, rotor, points and fuel filter. The Holley carb has also had a rebuild kit. None of that helped at all.... Thinking it must be a fuul delivery or ignition issue. I know i need to check the fuel pressure still..

Was looking to see if anyone else has had this issue or for any helpful input.

Thank you

 

[LAC_STS]

Re: 1987 OMC 7.5L engine pops and lack of power

Hey,


My engine did this exact same thing popping and lack of power. Most everyone who looked at it said it should be fuel related. But after going through everything fuel related ten times I found the problem to be in the IGN.

The problem was the resistor wire that goes from the alt to the + side of the coil. It looks just like a normal wire but really is a resistor wire. It is suppposed to lower the V when the engine is at idle and then it increases the V when you give it gas.

I am assuming you have a points IGN correct? If so it is very easy to test if its the problem.

Take the boat out and run it. When it startes running like crap and popping take a piece of wire and wire it from the + side of your battery to the + side of the coil. If the resistor wire is truely the problem the engine should take off and clear right up when you hook that wire up.

Don't leave the wire on there for along time though. It will burn the points up and make the coil hot as h***. I wouldnt leave it on for longer than you need to see if its the problem. 5-10 mins is cool.

If that is the problem then you can either buy a replacement resistance wire or you can buy a ballast resistor and wire it up in place of the resistor wire. You just have to make sure to get the correct ohms one.

 

[Chappy509]

Re: 1987 OMC 7.5L engine pops and lack of power

Thanks for that info. And yes it does have points ignition.... I will hopefully get the boat back in the water tomorrow night and try that.

 

[LAC_STS]

Re: 1987 OMC 7.5L engine pops and lack of power

How long has it been since you replaced the points and condensor? If yo dont know or its been a while maybe just install new ones, set the timing and dwell right and eliminate that as a source of your problems. Points and cond are not that much $.

And check the fuel system really good. Take a look inside of the fuel tank. Look for water or crud. Make sure you have no restrictions and all that good stuff.

There is a thread in the FAQ that desribe in detail how to test both the IGN system and the fuel system.

 

[Chappy509]

Re: 1987 OMC 7.5L engine pops and lack of power

Points and condensor were replaced 2 weeks ago when i did the tune up after it started acting up. Tonight double checked over basics.... Found a broken spring in the distributor Tried jumping the coil resistor as recommended, also no help. Fuel spray and pattern in carb looks fine. I know that still doesnt elimanate a fuel issue, however having someone else with me tonight to help and running boat with sundeck open noticed it was a ticking noise from bank 1 (cylinder 2 or 3 area) also sounds like it has a skip to it. So it could also have a internal issue?

Hopefully tomorrow night i will have time to check inside fuel tank. Also going to pull plugs out and check compression, and pull valve cover to find what engine tick is... Might have a loose rocker or broken valve spring? Need to keep digging a little deeper i guess...

 

[Chappy509]

Re: 1987 OMC 7.5L engine pops and lack of power

I will also check out that FAQ thread.... Thanks

 

[southkogs]

Re: 1987 OMC 7.5L engine pops and lack of power

Do a compression check on your cylinders. I was getting a popping noise on mine and had zero compression on two outta' six ... looks like a bad head gasket for me.

 

[HT32BSX115]

Re: 1987 OMC 7.5L engine pops and lack of power

Howdy, I had one of those......

Found a broken spring in the distributor

That could cause your problem. I have to assume that you have double checked your spark plug wiring and timing? Also carefully check the PCV valve and associated hoses and fitting for vacuum leaks.

How many hours on the engine?

 

[Chappy509]

Re: 1987 OMC 7.5L engine pops and lack of power

Not really sure how many hours on engine. I dont have an hour meter(i know i should). Had a brand new replacement engine when my father in law owned it, but that was 6 years ago. He ran it one summer after new engine, then it sat in shop for 4 years till i bought it, 2nd season for me..

Anyways, im getting on the right track. Did compression check..ok, 130-150psi on all cylinders. Pulled valve cover on bank 1 where the noise was coming from.... Cyl. 4 intake valve rocker as loose as could be. valve not even tring to open so much slop... So, either i have a colapsed lifter or a bad camshaft.
Going to take oil filter off in am and cut open to inspect. Look for metal debris from cam(hopefully not). Also need to take intake off to check lifters but might not get time till weekend to do that.

 

[HT32BSX115]

Re: 1987 OMC 7.5L engine pops and lack of power

Not really sure how many hours on engine. I dont have an hour meter(i know i should). Had a brand new replacement engine when my father in law owned it, but that was 6 years ago. He ran it one summer after new engine, then it sat in shop for 4 years till i bought it, 2nd season for me..

Anyways, im getting on the right track. Did compression check..ok, 130-150psi on all cylinders. Pulled valve cover on bank 1 where the noise was coming from.... Cyl. 4 intake valve rocker as loose as could be. valve not even tring to open so much slop... So, either i have a colapsed lifter or a bad camshaft.
Going to take oil filter off in am and cut open to inspect. Look for metal debris from cam(hopefully not). Also need to take intake off to check lifters but might not get time till weekend to do that.

When I tore mine down I couldn't understand why I couldn't get more than about 4000 RPM at WOT.

I found that it had several flat lobes. I probably had 4 or 5 valves that were only opening just a little but the compression check was fine.

I fixed it a different way......see my signature.

 

[Chappy509]

Re: 1987 OMC 7.5L engine pops and lack of power

When I tore mine down I couldn't understand why I couldn't get more than about 4000 RPM at WOT.

I found that it had several flat lobes. I probably had 4 or 5 valves that were only opening just a little but the compression check was fine.

I fixed it a different way......see my signature.

Haha.. Ya i noticed that the other day

 

[HT32BSX115]

Re: 1987 OMC 7.5L engine pops and lack of power

Yeah. Sorry about that, believe me I tried to keep it going but with the ex-manifold and coupler scarcity I just couldn't make it happen.

Good thing is, parting it all out a little at a time over about 3 years, I got $3500 for everything!

 

[Chappy509]

Re: 1987 OMC 7.5L engine pops and lack of power

Got the intake off tonight. Found cylinder 4 intake valve lifter collapsed.. Checked cam with a boreascope, cam looks fine. Swapped a lifter to confirm rocker arm was tight. Order up some lifters and gaskets tomorrow.

 

[HT32BSX115]

Re: 1987 OMC 7.5L engine pops and lack of power

Got the intake off tonight. Found cylinder 4 intake valve lifter collapsed.. Checked cam with a boreascope, cam looks fine. Swapped a lifter to confirm rocker arm was tight. Order up some lifters and gaskets tomorrow.

Good for you! Strange that lifter didn't pump right back up...........You might try turning the oil pump with a drill to make sure you have good oil flow to each lifter.........

Also, make sure you check/replace your riser gaskets. Mine were leaking when I took my 460 apart. It didn't affect the running but the slight leaking made it hydrolock briefly if I tried to start it within 30 min or so of shutdown.....and of course, that little bit of water would sit there on top of the piston after shutdown.....