OMC 4.3 stumbles when trimming up

[mike loya]

I own a 1989 Omc cobra 4.3.I have been experiencing a bad stumble or missfire only when trimming the drive up,and only at high rpm's(3000,or better).I replaced the points,plugs coil,cap, rotor,and wires,which improved over all, but did not solve the stumble when activating trim switch.I manually depressed the esa switch and it did interupt the spark,so i'm assuming its fine.I read that omc issued a service bulletin regarding this problem and it spoke of cutting the resistor wire down 22 inches.Can anyone confirm this?Also there were a couple of members who experienced this same problem,but no posts as to what finally cured this annoyance.Is trim wire chaffing in the throttle box a common problem with these omc's.the problem started out as a slight stumble, and has progressivley gotten worse.thanks for any help.

 

[Redsfilthy]

Re: OMC 4.3 stumbles when trimming up

I'm with ya, Reaper. Same engine, same problem.I'm an electrician by trade and I can tell you that the trim pump pulls a lot of amps when its working.If you notice that it doesn't do it when you are idling(at least mine doesn't).I wonder if our alternators are getting weak? When you run at high speed, your ignition demands are higher and maybe that taxes the alternator. I have the original alternator.Do you?

 

[Mufasa]

Re: OMC 4.3 stumbles when trimming up

You are correct that it is a voltage problem and there is a bulletin out on this issue. Before you chop into the engine harness try resetting the plug gap to .030". This lowers the required voltage and often will cure the trouble. I often get this complaint after a tune up has been done and now it has a problem. The RV15YC4 spark plugs are gapped wider than the RV8C plugs that are being replaced. If you put a volt meter at the coil and run down the lake you will notice that when the trim is activated it draws the voltage down far enough for the ignition miss to occur. There is nothing wrong with the alternator. If needed you can remove some length from the resister wire and apply more voltage to the coil but I suspect you will not need too after regapping your spark plugs.