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Old 06-13-2006, 05:55 PM
MichaelW MichaelW is offline
Join Date: Jun 2006
Rep: 10 Posts: 4
Location: Pennsylvania
OMC 4.3 misses, sluggish - just bought it.

So my wife talks me into buying a very nice 1987 SeaRay 19 Bowrider. It is in fantastic shape, well cared for, no signs of any abuse at all. The only problem is that it has no take off nor can I get it above 38 MPH at 3800 RPM. It also has a vibration like a dead miss when accelerating.

From what the seller told me (and I am inclined to believe him) is that it ran very well. Now I have it and it doesn't run well. The following is what I know to date.

I took the boat to the river and on the way I filled it up with premium gas and noticed that it was very low on fuel.

I took it out and learned of the running issue.

I then ask a few people at the river and decided that new spark plugs were the first step. Installed them and nothing changed significantly.

Another river man told me to change the fuel filter with a new one. I did that and nothing changed significantly.

So now my next step is to put in some drygas in case there is still some more water somewhere in the system. I am somewhat inclined to agree based on earlier fact of the nearly empty tank over the winter.

However, my concern is that I will spend the entire summer chasing this issue instead of fixing it. Then, if the carb needs to be overhauled, what about updating it from the two barrell to the four barrell. I know the manifold will have to be changed but will this change yield the other 30 hp or do I need a camshaft to get the full 30 hp? Is there anything else I can do to improve the power in the 4.3?

Additionally, the shift seems a little stiff and where can I lubricate the linkage?

Finally, since the ignition must remain with points, is there a better coil to use over the stock unit.

I apologize for the rambling on but I like the boat and despite being very mechanically inclined, I am an import guy and new to the domestic motors such as the 4.3 V6.

I appreciate all that any of you can help me with.


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Old 06-13-2006, 07:10 PM
Frosty Frosty is offline
Previous Member
Diagnosis is very difficult isnt it? Its almost dam near immposible from the end of a telephone line. Your next step would be a compression check. Gonzu usually has a go at these enquiries, but to diagnose on "some kind of vibration" is very very difficult. I suggest if you dont want to spend the summer looking at it then to go and have it looked at professionaly. Are you sure the prop is not damaged,-- that could cause your vibration and poor performance. Good luck
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Old 06-13-2006, 10:26 PM
gonzo's Avatar
gonzo gonzo is offline
Senior Member
Join Date: Aug 2002
Rep: 2031 Posts: 11,854
Location: Milwaukee, WI
A 1987 4.3 doesn't have the counterbalance shaft and will have a small vibration. Changing parts is an expensive way to troubleshoot. If it was stored for a long time, rebuilding the carburator seems like a good idea. Did you read the old sparkplugs? It will tell you if the carburetor was running rich, proper or lean. Did you do a compression test?
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Old 06-15-2006, 09:30 AM
MichaelW MichaelW is offline
Join Date: Jun 2006
Rep: 10 Posts: 4
Location: Pennsylvania
OMC misses & etc.

First off, I thank you for your answers to my questions and will attempt to inform some more.

Plugs: The plugs were replaced and I noticed that the plug closest to driver (as you face towards the stern it is immediately to the left closest to you) was wet and the other plugs were sooty and black in color but dry. All were the same.

Prop: The prop has damage albeit small around the external edges and I want to replace it but I want to wait until I get the motor running better so I can pick the right stainless steel prop. I guess I will have to buy a newer aluminum prop (cheaper) untill after I fix the other problem.

Local Shop: I asked two of the local shops but they don't seem interested in helping because they are suggesting I do some of the work before they get involved. I have the boat on a local river and will have to take it away to a more land locked shop that wants the work. The local shops are not very high end and I am also somewhat doubtful of their ability.

Compression: Looks like I'll have to dig out the old compression gauge and check that also. What should the readings be on a marine V6 other than relatively even among the cylinders?

Carb: If I have to rebuild the carb I am then leaning towards getting the larger carb and manifold which then leads into the question of the camshaft difference between the 4.3 2V versus the 4.3 4V; Is there a difference? Is there a performance camshaft available and suitable for marine application?

I thank you all in advance for your help and will keep you posted of results.


Michael W
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Old 06-15-2006, 10:11 AM
Frosty Frosty is offline
Previous Member
Continue with the compression check, yes they should be similar. Clean or replace the plugs again and take care on the one that you thought wet. Get the right gaps, and make sure the choke is comming off. I would not think about performance parts untill you get the engine running properly or you might be throwing good money at a bad engine. 38 MPH at 3800 sounds like you have 1 cylinder off to me.Check all the plug leads at both ends and especially to the suspect wet cylinder. Put a spare plug in the lead and crank it to see if the lead is live. Is it wet with fuel or water? Run the engine with the lead on and then off, if it doesnt make a difference then thats the one. You could do that with all cylinders. Please be carefull of rotating belts etc, what I have advised you to do is on a static engine. I wouldnt be too concerened at the carburettor yet if its running 5 it can run 6.
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Old 06-15-2006, 11:04 AM
MichaelW MichaelW is offline
Join Date: Jun 2006
Rep: 10 Posts: 4
Location: Pennsylvania

Thanks for the idea. I will do so soon and advise as the the outcome.

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Old 06-20-2006, 09:37 AM
MichaelW MichaelW is offline
Join Date: Jun 2006
Rep: 10 Posts: 4
Location: Pennsylvania

Here is the latest update. I put in the fuel system antifreeze (called Heet) and ran the boat for about 45 minutes at nearly three quarters throttle. It improved performance by about 50%. The rough idle is improved as is the vibration. However, there is still a long way to go. With the improved running of the motor I am still limited to about 38 MPH at 3800 RPM. It just won't go the rest of the way to what I read should be about 42-46 MPH at about 4400 rpm. Now, do I have the carb rebuilt or do I go for the larger carb with the better manifold. Do I have have to replace the exhaust manifolds to get the full benefit of the larger carb? Should I even bother with the camshaft?

Additonal observation. I noticed that when running the motor at moderate speed and using the trim adjust the rpms drop off significantly and then return when I stop adjusting the trim. I think I read somewhere that the trim is draining the ignition system. Did I read correctly? If so, what do I do to fix this?



Last edited by MichaelW : 06-20-2006 at 09:40 AM. Reason: Additional observation
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Old 07-03-2006, 02:39 AM
julius750 julius750 is offline
Junior Member
Join Date: Feb 2006
Rep: 10 Posts: 17
Location: Melbourne Australia
If you are looking to improve performance I can supply one piece stainless watercooled headers which will make a significant improvement without altering the carb setup (includes all brackets for a direct swap) . No matter what you do at the inlet end, you have to be able to get the exhaust gasses out efficiently.
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Old 07-16-2006, 11:41 PM
cirrus142 cirrus142 is offline
Join Date: Jul 2006
Rep: 10 Posts: 3
Location: Virginia
i had these problems and more with the exact same driveline. Last year I rebuilt the carb; this solved the random stalling, delayed starts, and slow response/speed.

another useful tip (which i'm currently trying to work on) is to adjust your shifter bracket and cables the proper way. there is a ton of info at the following links.

main page

shifter adjustment
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Old 07-17-2006, 01:51 AM
Redsky Redsky is offline
Senior Member
Join Date: Jul 2006
Rep: 9 Posts: 107
Location: pacific Nw
omc 4.3 v6 = gm v6 that era hmmmm hmmmm ?
90% of what i know about gass motors is gm made
1 vibration check the harmonic balencer bolted to the crank snout,, iv seen these go bad in cars the rubber connecting the 2 parts wears out...and or check the water pump pully for play
2 dont know what the fuel tank looks like and or how hard the lines are to get at...however you might try taking off the fuel cap and taking the line off at the fuel pump and blowing back into the tank with compressed air same with taking off the fuel pump-carb line....and perhaps the fuel pump should be checked for proper pressure delivery at 77 blazer stranded me one night 60 miles from home in wva with a dead fuel pump one day...ended up putting that one on in the dark by feel.......does the thing have a eletric choke on it? iv seen old eleteric chokes act up when they havent been used in a long time....another thing iv seen gm made engines do is fracture the distribuiter cap and top end high energy preformance goes to crap. wet cylinder what is wet with..fuel/ oil / water? if oil or water and its a aluminium motor/aluminium heads the answer 80% of the time is somone over heated it and warped the head or block when that happens you get classic blown head gasket. if its a warped head/block my first choice would be to look for a smashed vw tdi and yank the 90 hp tdi and eletronics for the desil and replace the v6 with that
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