serious misfire on replaced engine

[sholm22]

wellcraft 175 4.3l mercruiser engine , 2001 . just replaced engine with new g.m long block and new modified two piece manifolds , one manifold was running excessively hot but think that was due to a hose restriction . Now we have a serious misfire when cold , stops after about half an hour of running , engineer wants to do carb service but we are wondering if this is a waste of time ,could it be due to an electrical fault ? if so what are the likely causes ? or has anybody got any other ideas ?

 

[Sagabu]

Re: serious misfire on replaced engine

Run for a few minutes and romove the plugs one at a time and inspect for unburnt fuel (foul plug). It will be wet. <br /> If you do not see this, use a propain tank w/2' hose and run it along all intake gaskets and carb base as well as the vacume conections. If the engine races the problem is found. <br /> Is it a studder (off and on) or consistant?<br /> Did the exauhst develope a small leak at the gasket?

 

[KaGee]

Re: serious misfire on replaced engine

Very difficult to second guess a mechanic. Rebuilding the carb should have been part of the original game plan anyway IMO.

 

[rbezdon]

Re: serious misfire on replaced engine

The answer to your question sholm is yes it can be electrical, namely an ignition problem. It also could be fuel as your mechanic is thinking or mechanical as the other postings suggest by looking for leaks in vacuum, exhaust etc. since we all can not see/hear the engine and IF you trust your mechanic, then go with what he says. Yes it is a bit of a guess on his part but it should be an educated guess as he has access to the engine.

 

[Jilly - 5]

Re: serious misfire on replaced engine

One half hour is a long time for a misfire to go away, well beyond engine reaching operating temperature and any sluggish valves loosening up. Does misfire really go away, or is it "smoothing over"? Check firing order and whether two wires might be switched. When cold, misfire would be pronounced. After running, spark might be cylinder-jumping and it just seems that the problem has gone away.

 

[magster65]

Re: serious misfire on replaced engine

Your mechanic has likely checked the firing order and timing already so his diagnosis of carb. troubles is next on the list. When it's cold it needs to be rich, if it's not it'll backfire or 'cough' until it's completely warmed up. It doesn't sound like an electrical fault to me either.