Only 2 spark plugs firing on 3.0 mercruiser

[Provi1983]

Re: Only 2 spark plugs firing on 3.0 mercruiser

How are you confirming the cylinders are not firing?

I haven't confirmed that the cylinders aren't firing. I confirmed that the plugs to the #3 & #4 cylinder aren't firing.

 

[stonyloam]

Re: Only 2 spark plugs firing on 3.0 mercruiser

Change the points, and set the gap, and bet it will run OK. Best guess is that for some reason the points are not opening (or not properly closing) on the #3 & #4 cam lobes, either a worn cam, cam follower or the point contacts.

 

[Scott Danforth]

Re: Only 2 spark plugs firing on 3.0 mercruiser

I haven't confirmed that the cylinders aren't firing. I confirmed that the plugs to the #3 & #4 cylinder aren't firing.

Specifically how are you confirming this?

are you simply letting the wires hang loose, are you using a spark tester? are you trying to hold the plug against the block when your having the motor cranked?

 

[Provi1983]

Re: Only 2 spark plugs firing on 3.0 mercruiser

Didn't have a chance to work on it today, so hopefully I'll get to work on it tomorrow. I'm not a mechanic and my knowledge is limited when it comes to working on boats. However, I have done a lot of reading trying to understand all of the components of an ignition and how they function. The replies have been very helpful in narrowing down the possible causes and I appreciate everyone taking the time to try help me find the problem. That said, the problem has to be the distributor. I have verified that 2 of the 4 terminals on the distributor are consistently getting voltage and the other 2 are not getting voltage(at least not enough to create a spark on the plugs). No, I did not have a spark tester when checking the plugs, but I know how to check without one. Even though I never saw a spark on the 2 plugs (removed from #3 & #4 cylinder) during the countless number of times of checking with different wires and plugs, I went ahead and picked up a spark tester today and I'll check them again.

 

[harleyman1975]

Re: Only 2 spark plugs firing on 3.0 mercruiser

Good man. Once you have definately determined that you have no spark via a method that is "industry recognized" You will know that the distributor is the problem. Now whether it is the lobes on the cam, the actual point set, the cap, the rotor, the condenser or the plug wires that will be the next level of diagnostics. Make sure the plug wires are 100% by checking with an ohms tester, make sure the "towers" in the cap don't have anything stuck down iside of them (like the old end of an old plug wire), make sure the points are opening and closing the same on each lobe (.017 on feeler gauge is a good place to start) check that the cap is fitting the distributor base tightly and the alignment pegs are in the right spot, check cap and rotor for cracks. points and condenser are chep to replace if it is determined that the lobes are not worn...make sure to lightly grease lobes and oil felt wick. Good luck!

 

[Walt T]

Re: Only 2 spark plugs firing on 3.0 mercruiser

maybe a bent distributor shaft?

 

[stonyloam]

Re: Only 2 spark plugs firing on 3.0 mercruiser

You need to replace the points!

 

[Scott Danforth]

Re: Only 2 spark plugs firing on 3.0 mercruiser

using an ohm meter you should be able to see just the points opening and closing. if that works, you know the coil is firing for every lobe of the distributor.

from there chase the secondary wires out with the ohm meter. more than 8000 ohms per foot and you have a problem. I suspect something simple like molding flash in the plug towers.

if the distributor shaft is bent enough not to move the points on 2 cylinders, then its bend enough for the rotor to make contact with the cap.

 

[Provi1983]

Re: Only 2 spark plugs firing on 3.0 mercruiser

I was able to work on it some today and determined that the points need to be replaced. First, I used a spark tester on the two wires from #3 & #4 cylinders. It flashed one time on #3 wire and nothing on #4 wire. I went ahead pulled the points out of the distributor and noticed the arm was bent such that only half of the surface covered the stationary point, which caused it to wear unevenly. I decided to go ahead and clean the points using a file and set the gap just to see if it would work. I hooked everything back up and put the spark tester on back on the #3 wire and light was flashing. Moved it over to the #4 wire and it was flashing on that wire as well. I hooked up the muffs and ran the engine for a while and it ran great. Now I just need to order some new points and I'll be back on the water again. I really appreciate everyone's help, it saved me a lot of frustration and probably saved me a lot of money.

 

[bonzoscott]

Re: Only 2 spark plugs firing on 3.0 mercruiser

I couldn't figure how it was consistently firing on 1 & 2 but not 3 & 4. And I don't get the picture of how you mean the arm was bent enough that only half the surface was mating on the points. It should have done the same on 1 & 2. Glad you got 'er going. But if the same problem comes up later, take a look at the disty cam. Points were easy enough to just change out.

 

[stonyloam]

Re: Only 2 spark plugs firing on 3.0 mercruiser

Told Ya so . You will want to get your hands on a dwell meter to properly set the points. Much more accurate than just setting the gap, will also give you an accurate tach. reading to set the idle. The dwell is the time (in degrees) that the points are closed allowing current to flow through the coil, which allows a maximum voltage to be generated when the points open.

 

[harleyman1975]

Re: Only 2 spark plugs firing on 3.0 mercruiser

Glad you got it Now look how much you learned! Nextime you can be the guy saying try this and try that!