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Omc cobra, mercruiser 5.7 problems

Kylenorth

New member
I have been chasing a running issue with this 5.7 "vortec" since I bought the boat. It has turned into more issues. I will try to provide everything done since day one. Looking for others thoughts, opinions, insight.
Bought boat and "gone through" engine, out of the boat, last year. Set valves. Checked compression.
Installed mercruiser 350 vortec. No spark.
Installed new Pertronix ignitor and flamethrower coil. Added electric fuel pump.
Hard to start( fires for a few seconds and dies), but once warm will idle and start easily. Has quadrajet carb.
Took to lake couples times. Engine would reach 4000+ rpm, then back itself down to around 2500-3000.
Checked valves while engine running.Looked at head castings and found, they are not vortec heads but rather a 64cc(if I remember right) chamber, older heads with center bolt valve covers. Used boat in this condition for a few more outings until stored for winter.
This spring, installed higher GPM carter fuel pump. Took to lake and reached 4000+ rpm without ever backing itself down.

Found only 4-9 volts at positive terminal on coil with ignition on. Was able to pull 1-3 plugs wires off cap without a difference to engine rpm.(specific 3, not any 3). Also found adding jumper wire from Batt pos. Terminal to pos. Terminal on coil smoothed out rough idle somewhat.
Replaced spark plugs with autolites for vehicle(they didn't have the marine ones). Took to water and with 4 hours of run time, engine lost power at 3500rpm. Not quite a backfire, but was making noise through carb and possibly exhaust. Found one spark plug had no center electrode left. Did not look like electrode burned off.
Put old plug in when returned to home. Tested with water muffs and engine gave over 4000 rpm and sounded healthy.(no load) Still rough idle and starting.
Added relay so coil could receive full battery voltage. Took to water. Boat reached wot a couple times. Later in the day, would not go over 2600rpm without engine sounding bad. Backfiring, noise in carb. Brought home.
Yesterday checked compression. 5 cylinders were 165. Other 3 were 130, 115, and 0.
So after reading my novel, thoughts? Valves? Gaskets? Timing? Issues with spark? Maybe more than one problem? Thanks
 
Found one spark plug had no center electrode left.
Yesterday checked compression. 5 cylinders were 165. Other 3 were 130, 115, and 0.

Ayuh,.... My guess is, it's been runnin' lean the whole time, 'n you've detonated it to dead,....
 
Thanks for the response. I have brought engine to higher rpm and checked plugs that appeared to be black/brown, but still very possible. I do not like quadrajet carbs. The engine does seem to run rich at idle. when its died, you can smell the gas fumes. Either way, 0 psi is not a good sign, checked valves by sight to see if there was something obvious. The electrode of spark plug that disappeared happened to of been in cylinder with 0 psi. Engines ready to pull and will do in the near future, expecting the worst. Does anyone think i have more than one problem occurring? i should say, the boat got on the trailer under its own power this last time. so it does run, just continued to idle rough and couldnt give it any high rpm. maybe theres been a hole in a piston for some time now
 
maybe theres been a hole in a piston for some time now

Ayuh,.... Ya won't know til ya pull the head,...

Were ya gettin' Massive crank case pressure, lotsa fumes outa the vent tubes,..??

If not, the zero compression is probably a badly burnt valve,....

Carbs, all of 'em have several circuits or actions,...
idle, mid-range, wot, as well as choke, 'n accelerator pump,...

The color of the spark plugs only reflects the last few minutes of operation,....
Wot spark plug checks, ya run Wot for a few minutes, 'n chop the throttle as quickly as feasible, 'n shut it off,....
 
No excessive pressure from case. Yes ive checked plugs that way. It does make sense pistons could be heating up at high throttle. expanding and increasing friction, causing a rpm reduction. I dont know if that idea could last very long but engine has plenty of power, and always starts and runs the same since purchased, after everything thats happened. Other than loss of high rpm. Black plugs could have been from trailing fuel entering chamber after spark was shut off, i dont know. last year i believed fuel pump was starving engine at high rpm which most likely aided in the issues occurring currently. This is a 28' cruiser boat set up for salmon fishing. Only reason to go wot has been for test purposes, 10 seconds at a time. appreciate your help bondo, anxious to tear into it. What causes an electrode for disappear from a plug? have never seen that before. it looks almost if it broke out, but may have melted. I ruled on a faulty plug since it only took 4 hours and the old plug has lasted after incident in same chamber. But i may be wrong
 
Well so far the side with 0psi, the head has two similar intake valves, and two different. 3 similar exhaust and one different. The one with 0 psi, is cupped in, to where it's not seating correctly. Giving 0 psi. Another cylinder is leaking minimally into the other at gasket explaining 115psi. Cylinder walls look perfect and pistons good. Can see ring gap and looks within spec. How the intake valve cupped inwards? No idea
 
And I found flat tappet lifters...so there's that. Unless they make single roller lifters without needing the cross brace to the other.
 
this 5.7 "vortec"

Ayuh,.... So are the heads Vortec,..?? are they salvageable,..??

Sounds like the rest of the motor is earlier,...

Still sounds like massive detonation from a lean condition,...

Ya might consider a long block crate motor replacement,....

'n, ya, they make retro-fit roller lifters,... check the better hot rod parts distributors,...
 
Based on head #s- 14096217. 87+ 65cc 1.94 & 1.5 valves, centerbolt. the engine block number does show it to be vortec.

I think im understanding what the guy did. this 88-89 boat, which i dont think vortecs were around then, had a frozen block. He said he bought this Vortec mercruiser from a guy and went through it. I think he just bought the block or block and lower assembly, and put the original heads on it. this might explain the one intake valve being dished in so bad. Water may have been in the cylinder during freeze?? And either stopped seating when ive had it or i never did compression test that cylinder when purchased. Im wondering if he put the original cam/ lifters in this vortec block. im not convinced it has a hydraulic roller cam. I didnt pull a lifter out, but they sure look like flat tappets ive installed on older engines.
Its hard to explain to someone without them seeing for themselves.

Basically i have a red headed stepchild of an engine. Vortec block, older but fairly decent heads, and possibly older cam and crank assembly.

As far as are the heads salvagable, not without some time. ive found a couple cracks around the spark plug hole and another on the other head. all on the outside. They may never spread, or they may one day, one month, one year from now. Assuming they have been like this since I got it. And engine hasn't seen over 180 185 deg.

Looking at getting other heads, or buying valve and trying to see what can be done with cracks. And also have looked into 350 long blocks and 350 vortec short blocks.

After all of this, i agree something else is going on. could be lean on top end, could be an erratic pertronix igntor. What has never changed is a rough idle, hard cold starting, smooth running above idle, and plenty of power.
 
This is an older post, but my issues have not changed greatly. Now looking for advice on future actions to repair and create a trustworthy engine. I'd like to stay with my heads/intake and exhaust manifolds.
I replaced the one cupped intake valve, lapped, and achieved 165 psi. Ran on water muffs before bringing to water. Engine could not go over 2200 rpm without moving dist, and then only reached 3000. I did not want to continue messing with it as I didn't want to destroy engine completely.
Standing in front of engine and looking towards transom:
130. 170 (rear two)
110. 75
135. 165
135. 165psi (front two)
75psi is the same cylinder which previously had the tuliped valve.
Will post pictures of the piston face, casting #, etc
 
Wow..... you've got quite a few things going on here.

First, when we change cylinder heads, we must be aware of the piston dish volume contrasted to the cylinder head's combustion chamber volume.
In other words, we would NOT install a cylinder head with a larger chamber than what was originally selected for that particular piston profile. If so, the Static Compression Ratio will be very low.
If we use a cylinder head with a smaller chamber than what was originally selected for a particular piston profile, the opposite occurs!

Secondly, some of your cylinder pressures are far too low, and the spread between the low and high is out of the acceptable range.
Are you cycling each cylinder through at least 3 compression cycles during your readings?


Just an FYI.....
A flat tappet camshaft must be accompanied with flat tappet cam followers (aka lifters).
A roller tappet camshaft must be accompanied with the correct roller style cam followers.

If you are wanting roller cam followers, you must also use the camshaft that is designed for them.
And yes, as a retro-fit for earlier blocks, there are roller followers that "link" (link bar) themselves above the follower bores.

shopping




Ignition advance is given the last word regarding the placement of LPCP (location of peak cylinder pressure).
Too much spark lead and too early, will cause very destructive Detonation.
Pre-ignition and Detonation are not the same phenomenon.

Make certain that when you adjust the ignition advance, you not only look at BASE advance, but that you look further at what the progressive and total advance are doing!
Too much advance can cause damage in many areas.





.
 
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Thanks for the reply. My first mistake was trusting the seller about the engine. He said he replaced the old clacked block motor with a vortec and installed the original cam. I'm assuming he installed the whole top end. I also noticed a larger harmonic balancer installed. The boat came with a smaller diameter one as a random part. The irregularities have lost my trust in this engine.

I seen the piston profile doesn't mirror the head, which has brought concern about the compression quench.
I did cycle 3 compression strokes when testing, and the lowest cylinder, 4 times. I'm guessing the variance is due to poorly seating valves. The ring gap is within tolerance and the gaskets were new before this last outing and show no signs of leaking.

I can't find any specs on the camshaft but I do plan on replacing with a hydraulic roller. The block accepts a spider as well. The block is a gen 1 zz4 roller cam.

The timing advance is a whole different story. Timing to recommended degree, engine was know where near starting. Had to advance way more than I should have to run. I was wondering if the pointer on the plastic timing cover is in a different location than the original ( large balancer installed).
Pertronix ignitor with centrifugal advance dist. I plan to switch to hei with vacuum advance if it's the safer bet.

Boat is an 89, heads are 64cc chambers. Main journal and rod clearanced .0015. tight?
I appreciate the help.
 
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the standard pointer should have been changed is the balancer used was 'larger'... you need to check the balancer and the tab at TDC - best to use a piston stop.

If you go with a roller cam, make sure the distributor's gear matches it....

you need to use a marine rated distributor and make sure the advance curve is correct - NO vacuum advance used on a marine distributor....
 
Thank you. I'll most likely put to correct balancer on it .
Should I ignore the piston style, and just do a valve job to the heads and see where that takes me?
Also with water reversion, that doesn't happen at rpms such as 2200 right?
Thanks
 
..............................
Thank you. I'll most likely put to correct balancer on it .
I would still do the PPS procedure.
That way you will know for certain that you are working with an accurate #1 TDC mark.
For testing/checking only, this can be done without the use of a degree wheel.


Should I ignore the piston style,
As said earlier, the cylinder head's combustion chamber volume must be a match for the piston profile.... more specifically, the piston's dish volume.
Visa-versa as well.
The combined piston dish volume and cylinder head chamber volume (along with bore/stroke) determine the Static Compression Ratio.

and just do a valve job to the heads and see where that takes me?
Before removing the cylinder heads, do a cylinder pressure leak-down test.
This test will show you where the pressure is escaping.
Once the cylinder heads have been removed, this test cannot occur.

Also with water reversion, that doesn't happen at rpm such as 2200 right?
Water reversion is more so of an issue at low RPM and is a result of the camshaft's over-lap duration.

By the way, 2,200 RPM is a risky RPM to cruise at for any duration.
At 2,200 RPM and under load, detonation potential is greater..... especially with the GM full dished pistons.


Thanks
 
I found a marine engine builder that sells the same setup as this one. 880 block, 217 heads, flat tappet. I was believing this engine was unique but it doesn't look that way
 
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