1988 4.3L OMC Cobra Power Loss

[bshepos]

I'm having problems with my 4.3L OMC Cobra Engine. Problem started last year and has progresively gotten worse. Last season my boat lost power on the high end around 3400rpm and this year can not get it up on plane. Since then I have been trying to fix the problem but not sure what it is. Things I have done within the last year; Rebuilt carb, changed oil, plugs, wires, fuel filter, replaced ignigtion coil and distributor. Problem with loss over power goes away if I adjust the timing forward, but engine knocks. Any suggestions or other suggestions I should try.

 

[Lou C]

Re: 1988 4.3L OMC Cobra Power Loss

Anti siphon valve on the gas tank? That happened to mine last year, I had it replaced and all has been well.

 

[edfresh]

Re: 1988 4.3L OMC Cobra Power Loss

Check the compression. How old are the exhaust manifolds? I had a similar pblm on my '87 OMC 4.3L last year which was caused by bad compression in the center 2 cylinders. The exhaust mans had rusted letting moisture back into the cyls causing the valves to run too hot causing them to burn and recess into the heads causing low/no compression. New mans and heads later, it is a strong motor. ($1500 dollars in parts!) I hope this is not your case.

 

[bshepos]

Re: 1988 4.3L OMC Cobra Power Loss

Lou C, this might sound like a stupid question but, where is the antisyphon valve located.. I change the filter element on the fuel system going into the carb and even replaced all old fuel lines from the gas tank. Is that the same thing? <br /><br />Ed Fresh, I don't think it is the manifolds, they have been replaced 3yrs ago. As far as the compression goes, I will check that last. But I don't think it is the compression because last year the engine would gain/loss power at the high rpm's, very unpartictable when it we lose and gain.<br /><br />Thanks for the advice.

 

[Bondo]

Re: 1988 4.3L OMC Cobra Power Loss

Just a Crazy Thought,..... And,.. I don't even Know if it applies to an old OMC..............<br /><br />Check your Exhaust Flappers............ If 1 has fallen off,.. It Can Seriously Restrict the exhaust,+ cause a Loss of Power.....<br />Possibly Intermittently......

 

[Lou C]

Re: 1988 4.3L OMC Cobra Power Loss

It will be on the top of the gas tank, follow the line from the fuel filter back to the tank. It is a right angle fitting with a spring loaded ball inside, what it does is keep fuel from siphoning into the bilge if the fuel line ruptures. When they rust inside they can restrict fuel flow. Normally the suction produced by the fuel pump overcomes the spring tension and fuel flows. I think they are required by Coast Guard regs. <br />The Cobras do have exhaust flappers, you would have to remove the ex manifolds and rubber ex hose to be able to check them.

 

[waterone1@aol.com]

Re: 1988 4.3L OMC Cobra Power Loss

It might be worth it to put an advance timing light on the engine and check the advance curve. Make shure you are getting full advance at the propper rpm.

 

[bshepos]

Re: 1988 4.3L OMC Cobra Power Loss

Would the exhaust flappers be a problem if it does it while sitting in the driveway?

 

[Haut Medoc]

Re: 1988 4.3L OMC Cobra Power Loss

It could.

 

[trog100]

Re: 1988 4.3L OMC Cobra Power Loss

"Problem with loss over power goes away if I adjust the timing forward, but engine knocks."<br /><br /><br />that one is the key.. think on it..<br /><br />trog100

 

[Lou C]

Re: 1988 4.3L OMC Cobra Power Loss

I did that check when I was having trouble with mine, I found the ignition was advancing as per the OMC spec so that was not the problem. It is usually best to rule out ignition problems first.

 

[trog100]

Re: 1988 4.3L OMC Cobra Power Loss

i have trouble envisaging a situation where advancing the ignition timing causes a sigificant power gain as seems the case here.. unless the ignition is well retarded in the first place..<br /><br />if the ignition timing is about right.. over advancing it will cause a pre-ignition knock.. fair enough.. but it should not cause a sigificant power gain.. in fact it should not cause any power gain at all..<br /><br />let me throw one in.. the only one i can think of.. the knock isnt a pre-ignition knock at all its a worn bearing knock.. with the timing about right the engine knocks and the power is there.. retard the timinng to hide the bearing knock and the power drops off..<br /><br />if anybody else can imagine a different situation where advanceing the timing both produces a signiifcant power increase and an engine knock.. clue me in.. cos i cant..<br /><br />trog100

 

[Norman Buus]

Re: 1988 4.3L OMC Cobra Power Loss

I had an engine on the dyno once. It continued to produce more power as we advanced the timing until it pinged so bad we were afraid to go any further.<br /><br />Have you tried reading your plugs? Run it under power for a few minutes and pull the plugs. Most popular repair manuals have a blurb on what the different deposits on the plugs mean. First thought - do they all look about the same or do one or more of them have a heavy carbon build up that would indicate it is missing?<br /><br />"very unpredictable when it would loose or gain" That can be a fuel supply problem. While it is idling, it pumps the carburetor bowl full. When you open the throttle, it goes great until the bowl drains down below the jet. Then it looses power, slows down until the system can deliver more fuel than it is using. Once the bowl is filled past the jet again, it goes great. You have changed the fuel filter and are checking the anti-siphon valve. That will leave the fuel pump plus, I think, a screen just where the fuel line enters the carburetor. Hope this helps. Good luck.

 

[bshepos]

Re: 1988 4.3L OMC Cobra Power Loss

trog haven't tried your suggestion yet. <br /><br />However for the rest of you, I have tried several suggestions. I went back and checked the anti-syphon valve all is well there. Double check the gas, replaced the fuel filter and the filter before it goes into the carb.<br /><br />Tonight I plan on checking the exhaust flappers and how easy is it to check for warn bearing?<br /><br />Thanks for all the help.

 

[av8or09]

Re: 1988 4.3L OMC Cobra Power Loss

I have a 89 SUNBIRD 4.3 OMC/Cobra and last year I had an exhaust flapper get stuck and it did act just the way you described. Loss of upper end power and would not get on plane. Sounding like it was working real hard at full throttle too. My problem was the flapper had fouled up and stuck closed. I pulled both flappers and cleaned all the components,( I RTV'ed the rubber bushings to the exhaust pipe and greased all components, then replaced and everything went back to normal. I did not replace with new parts due to the great condition of the old. That was 30-40 hrs ago and no problems. Hope this helps!!