Help! 4.3 Lean or hot on #5

[Grouchybear35]

Help please! My OMC cobra 4.3 has been running very hot or very lean on cylinder # 5. The others look fine. With the heads off the combustion chamber of # 5 looks much lighter in color and the electrode on the plug has burned away. I have honed the cylinders and will replace the rings (although they where probably okay) and the heads are at the machine shop being reworked. I just want to be sure that when I put it back together I have resolved this problem and will be ready for a long summer on the water.

 

[Walt T]

Re: Help! 4.3 Lean or hot on #5

year? efi or carb?

 

[WillyBWright]

Re: Help! 4.3 Lean or hot on #5

Sounds like a burned exhaust valve. The head job should solve that. If it's a multiport EFI it could be that injector. Has VP ventured into that arena on 4.3s?

 

[Grouchybear35]

Re: Help! 4.3 Lean or hot on #5

Sorry guys, the engine I belive to be a "94" and it is a 4 barel carb.

 

[Bondo]

Re: Help! 4.3 Lean or hot on #5

What did the Intake Gasket look like in that area ???????

 

[Walt T]

Re: Help! 4.3 Lean or hot on #5

I agree, look for a possible vacuum leak that affects that cylinder, or excessive carbon build up.

 

[Grouchybear35]

Re: Help! 4.3 Lean or hot on #5

Guys,<br /> I did not notice anything unusual looking on any of the gaskets but I did not pay that much attention to the intake gasket. If the intake was leaking would it still idle smooth at 600 rpms?

 

[Bondo]

Re: Help! 4.3 Lean or hot on #5

Grouchy,<br />Just How Smooth did it Run on 5 Cylinders Anyway ??????<br /><br />About the Only thing that'll cause the Damage you state is,<br />Lean mix or<br />Really Advanced Timing.....<br />If the carb was plugged, you Should have at least 2 more lean cylinders.... Even though #5 Is the Farthest from the carb..... <br /><br />600rpms Sounds a little low.....<br />My Merc. 4.3LX idles about 700/750rpms....<br />On All 6 Cylinders....

 

[Grouchybear35]

Re: Help! 4.3 Lean or hot on #5

Bondo,<br /> The day that this happened or became noticable started out running good on six or so it seemed. I had experienced dificulty shifting and someone said to try to get the idle down to 600 or at least close to it. Once in the water and warm I turned the idle set screw down while watching the tach, to right at 600 rpms. later that day I noticed a strange noise coming from #5. I think this was the lack of a plug firing due to the electrode being burnt off. Once I noticed this we pulled out of the water and did not run agaiin until tear down.

 

[Bondo]

Re: Help! 4.3 Lean or hot on #5

Do you still have the Intake Gaskets ???<br />Has the Intake been Cleaned Yet ???<br />You Might Still have the answer out in the Garage.....<br /> Close Inspection , Might reveal a Leak......<br /><br />Have you ever had any luck " Reading " gaskets ??????

 

[Grouchybear35]

Re: Help! 4.3 Lean or hot on #5

Bondo,<br /> The intake should be just as it was when I removed it. I can't see it from here, I am in Montreal and it is in Tennessee. I will check it out this next week end.

 

[Grouchybear35]

Re: Help! 4.3 Lean or hot on #5

Another question,<br /> Are there any differnces between marine gaskets and automotive gaskets for oil pan, heads, intake and exhaust ?

 

[Bondo]

Re: Help! 4.3 Lean or hot on #5

With the Quality of gaskets like Fel-Pro, Even the automotive gaskets are Just as Good as Any Marine gasket sets.......<br />With the old steel shim gaskets there were a few issues.... Now, Most All head sets come with the better Composite Gaskets.....<br />Check with your MachineShop, They Should have already bought a Complete Head Set... They need the valve seals out of the set to do their work for you.... All the rest of the gaskets needed Should be in there........<br /><br />I guess you'll also need a new base gasket... Get 1 of the new rubber 1 piece gaskets... It's much easier to get right, than the old 4 piece 1s....

 

[Grouchybear35]

Re: Help! 4.3 Lean or hot on #5

Thanks again Bondo!