Engine Shaking

[ed hobbs]

My powerplant is a 1987 5.7l OMC w/ cobra outdrive. My challenges have been many, since this was a project from the start. I have had the boat out and ran it at WOT since I purchased it. Recently, I took it out and could not get it above 3000 RPM, and it would surge. I replaced fuel filters and cleaned up the fuel delivery components. When I put it back in to test it, I could get it to about 2800 RPM then the engine would shake ever so slightly. The harder I pushed it, the more it would shake, with no increase in RPM. I have ruled out the prop, since it does the same thing with both my props . The engine has been completely tuned up. It idles and runs perfectly to 2800 RPM, when it starts to shake. I suspected the u-joint, and have pulled the outdrive off. Nothing seems loose in the joint, but I'm not sure how to test it. The gimble bearing seems smooth and has no play in it either. How can I test the u-joint? Is there another area I should be looking at?<br /><br />Thanks in advance!

 

[Trent]

Re: Engine Shaking

Do a compression check both wet and dry to begin with.

 

[Texan_Ron]

Re: Engine Shaking

have y ou checked to make sure the fuel line is good ( kincked, pinched loose fittings ) also what size of fuel line you have installed? also if this engine has a carb have the float and needle valve checked also

 

[ed hobbs]

Re: Engine Shaking

Last compression check in May was 140-165. <br /><br />My fuel line is at least a 1/4" ID rubber hose. I cleaned out the antisiphon valve and blew air backwards into the tank through the hose.

 

[Texan_Ron]

Re: Engine Shaking

Maroon 1/4" is not big enough for a small block v-8 muchless a lawnmower your engine could be striving for fuel make sure atleast 3/8" or larger might think about the fuel pump also

 

[ed hobbs]

Re: Engine Shaking

I was going to say 3/8", but I was going on memory. I tried to err on the conservative side. I'm sure it is 3/8", but I'll measure to be sure.

 

[arks]

Re: Engine Shaking

This might sound obvious, but have you tried running it in neutral above 3000 RPM? If it still won't run smoothly, then it's not a problem with the drive. It almost sounds like it's missing. Are you sure the plug wires are routed properly?

 

[ed hobbs]

Re: Engine Shaking

I checked the plug wire routing twice. I originally felt it was missing as well, but it runs so smoothly up to 2800. Wouldn't it show up sooner in the RPM spectrum? I put the timing light on each wire to make sure it was getting power. Some of them looked as if they were trying to spark in between every once in a while. I had intended to replace the plug wires, but the mechanics I talked to said that plug wires don't go bad. I thought, "why do they sell them then?" I did accidently start the engine with the drive up the day I took it out to test it. That is why I thought I might have strained the U-joints.<br /><br />Thanks for the ideas.

 

[GodsBeast]

Re: Engine Shaking

Having any trouble starting it at all, just a while back I had moisture getting under the Dist. cap in the rotor. Mine came on sudden, ridding and all was fine. Stoped and refueld. When we took off, it ran like crap, would not get over 3000. Finally took Dist. Cap off, and roto cap, wiped dry, sprayed with WD-40, wiped out, put back on and it ran good. I also had a service bulletin on my Mercruiser 350 Mag. Took back in and they changed the whole Dist. <br /><br />Mine acted just like your describing though, would not speed up, and surged. It finally would not start once we got back to the dock. All over a little moisture under the cap! Once cleaned it ran like a champ. Hope this helps.

 

[Avalanchediver]

Re: Engine Shaking

Marooned,<br /><br />Sounds like your ignition (spark) is breaking down under load. As the cylinder pressures rise with increased load, the resistance at the spark plug gap increases. If you have any weak points in your ignition system, this is how they may show their first signs of trouble. I have seen many condensors fail on boats with point systems unter the distributor cap.<br /><br />If I were you, I would replace cap wires, plugs, rotor, and if you have them, points and condensor.<br />I don't recall if an 98 5.7L cobra has electronic ignition or not. This is relatively inexpensive and will rule out a ot of trouble if it is not the primary problem.<br /><br />Good Luck,<br /><br />Eric Pfeifer

 

[ed hobbs]

Re: Engine Shaking

I recently replaced the cap, rotor, points, condenser, plugs and coil. One mechanic suggested I try the old condenser after I was having this trouble. I did, but there was no improvement. It does rev up past 3000 in neutral, without shaking. I think I'll try new plug wires next. Any other ideas?

 

[eoinodonnell2000]

Re: Engine Shaking

as i see you said you were a omc champion my topic is directly under yours please read to see if you can help me

 

[navigator336]

Re: Engine Shaking

It seems you've worked the ignition angle pretty hard. You might try pulling the plug wires off one at a time and seeing how each one effects engine smoothness. If one or more has no effect on rpm's when pulled off, that will isolate the problem (if you get "lite-up" grabbing the plug boot, that's a sign of poor insulation. Any arcing on the coil to the low voltage leads? Arks, suggested seeing if it will run over 3000 rpm's in neutral. Did you try this? <br /><br />It sounds like it's a fuel delivery problem to me.

 

[K Hultgre]

Re: Engine Shaking

Have you pulled the dist plate (where points and condenser mount) and checked your spark advance weights and springs. It may be all gummed up not allowing the spark to advance. Or if you have vacuum advance the vacuum diaphram may be ruptured or hose has a hole in it not allowing vaccuum to reach diaphram. <br />Have you checked to see if your dist. is advancing the spark with a timing light under load? If you search the site for spark advance you could probably find the total deg your dist should be advancing and at what RPM. <br />I remember something about 28 deg total advance but cannot remember what RPM or type of engine. Hopefully someone in the know will post with the exact specs.

 

[ed hobbs]

Re: Engine Shaking

I did check the mechanical advance, both by hand and by timing light. It checked out okay. Also, it will rev in neutral over 3000 rpm. The problem is under load only.

 

[navigator336]

Re: Engine Shaking

When you hit 2800 rpms and it starts shaking, will it continue to go faster if you give it more throttle, or does it stumble and rpm's become unstable no matter how much gas you give it?

 

[K Hultgre]

Re: Engine Shaking

Do a search under the I/O board using this search string "I cant get more than 2400 rpm's " and "'87 OMC 5.7 stalls over 3000 rpm ". Also could be the exaust flappers restricting the exhaust if they broke loose and jammed somewhere. Good Luck, keep posting.

 

[ed hobbs]

Re: Engine Shaking

Thanks for the responses! <br /><br />navigator336:<br />At 2800, it starts shaking. If more throttle is applied, it shakes harder, but does not increase in rpm.<br /><br />kevin h:<br />I will check out "I cant get more than 2400 rpm's". The other reference, "'87 OMC 5.7 stalls over 3000 rpm " is one I started regarding my original problem, which I thought was fuel delivery. There was no shaking then.

 

[timothyl]

Re: Engine Shaking

Marooned, The problem sounds like a u-joint. With the outdrive in nuetral, check for any play in the u-joint. Any play at all will cause vibration.

 

[ed hobbs]

Re: Engine Shaking

tlands:<br />I did check for play when I had the outdrive off. I couldn't detect any, but I wasn't sure if I was strong enough to move it. In my original post, I asked if there is a test (beyond just wiggling with my hands) that would reveal play in the u-joint. Any ideas?