140 mercruiser missing after it gets warm

[splatter]

Boat runs great for about 30 minutes . Then if starts missing bad . Sounds like one or more of the plugs isn't firing . Gets worse the longer it runs . If I cut it off and let it cool off it runs good again . Also I smell gas when this is happening . I figure that is the gas not getting burned because its missing so bad .

I replaced the points ,plugs,wires,rotor button and distributor cap at the end of last season .

Mt best guess is a faulty coil breaking down after it gets hot.

what do you guys think ?

 

[smassey22180]

Re: 140 mercruiser missing after it gets warm

Could you hook a timing light up to see if you were loosing spark?

 

[MikDee]

Re: 140 mercruiser missing after it gets warm

Sounds like a bad coil?

 

[harleyman1975]

Re: 140 mercruiser missing after it gets warm

I would lean to the condenser being bad.

 

[splatter]

Re: 140 mercruiser missing after it gets warm

I would lean to the condenser being bad.

Condenser was also replaced at end of last season . Of coarse that doesn't mean its good .

 

[splatter]

Re: 140 mercruiser missing after it gets warm

went ahead and ordered another coil so I should know next weekend .
Thanks

 

[boozer1966]

Re: 140 mercruiser missing after it gets warm

I have been chasing a very similar problem since last year. It turned out to be one of the spark plugs that I had installed which was the wrong heat range. Yes, that's right. 3 of the 4 spark plugs were NGK, the fourth was an autolite of a totally different heat range. I am not sure how it ended up in there, but I feel like a total idiot. It would run great for a while, then start missing. Once it cooled, same story. In any event, once it starts missing, stop and pull all of the plugs and see what they look like. That way you can tell if it is specific to one cylinder, or if its a more general problem, like a coil. Alternatively, I found that cleaning up all of my ground connections made a positive impact as well.

 

[splatter]

Re: 140 mercruiser missing after it gets warm

Ok it wasn't the coil . I replaced it and I'm still getting the same thing . I'm gonna pull the plugs and look at them when I get home tomorrow but I don't hold out much hope of that being the problem seems to me if that were the trouble it would have showed up last year.

Also I noticed that once it starts sputtering I can make it worse by trimming the motor .If I hit the trim switch it sputters on Q . But only after the trouble starts .

 

[splatter]

Re: 140 mercruiser missing after it gets warm

Oh one other thing . The coil on my boat is suppose to have an external resistor . I can't find one not even a pic of one in the manual . I know what a resistor looks like on a PC board but I'm not sure just what this one would look like

 

[dubs283]

Re: 140 mercruiser missing after it gets warm

The coil on my boat is suppose to have an external resistor . I can't find one not even a pic of one in the manual

the resistor is built into the primary ignition lead wire in the engine harness, you can test it by verifying you have approx 9 volts at the (+) side of the coil with the key switch on

check the quality of the ignition spark when the engine acts up, should be nice strong blue, if weak and orange you definitely have an ignition issue

 

[wrench 3]

Re: 140 mercruiser missing after it gets warm

Your trim pump should go directly to the battery. To effect the running the hole system voltage would have to be dropping.

 

[splatter]

Re: 140 mercruiser missing after it gets warm

Your trim pump should go directly to the battery. To effect the running the hole system voltage would have to be dropping.

Thats what I'm thinking . Just can't figure out why

 

[dubs283]

Re: 140 mercruiser missing after it gets warm

it makes sense the problem gets worse with running the trim, it puts a good load on the battery and with the engine not running well it only makes things worse

you need to inspect the ignition system fully

 

[splatter]

Re: 140 mercruiser missing after it gets warm

Replaced the condenser and no change . I also set the gap on the points again,checked the plugs and the plug gaps before I took it out . Also checked all the connections and every thing looks good . I did notice the points have a hole in the center of the stationary side . Is this normal ? I looks like it was made that way . I didn't notice it last season when I put them on but I don't remember points being like that in the past .

Any way I'm thinking about putting a pertronix electronic ignition on it. At least that would eliminate 2 componets in the chain .Then I though I would change the plugs ,wires , distributor cap and rotor button again . I hate to do it since they are all new but something some where is breaking down and if I do all that I will basically have a new ignition system.

Also I should add . I just changed the outdrive before I put the boat in the water this year . I can't imagine how that could affect anything but I though I should throw that out there .

 

[splatter]

Re: 140 mercruiser missing after it gets warm

a friend suggested that it could be vapor lock .

I'm thinking if it was vapor lock it would die completely or run rough even when i back off the throttle .They way its running now is that when it starts missing if I drop the RPMs to about 1k it runs ok and only misses when I give it more throttle . Also if it was vapor lock wouldn't it be hard to start after I cut it off ? As it is now if I cut it off it will start right back up .

 

[Grub54891]

Re: 140 mercruiser missing after it gets warm

If you have the old cap and rotor laying around,put them on and try it. The new ones could be bad also. Seen it happen!

 

[Fastatv]

Re: 140 mercruiser missing after it gets warm

I had a similar problem....did the tune up thing, no change. My "missing" engine issues went away after I cleaned all electrical connections, I mean all of them and after cleaning, I used dielectric grease on everything. Now runs great!

 

[MikDee]

Re: 140 mercruiser missing after it gets warm

I had a similar problem....did the tune up thing, no change. My "missing" engine issues went away after I cleaned all electrical connections, I mean all of them and after cleaning, I used dielectric grease on everything. Now runs great!

YES! That is my thoughts exactly,,, A sudden strong electrical draw with a poor connection can draw power from the motor! I've had it happen to me, Clean it all up!

 

[splatter]

Re: 140 mercruiser missing after it gets warm

If you have the old cap and rotor laying around,put them on and try it. The new ones could be bad also. Seen it happen!

Unfortunately I threw it away

 

[splatter]

Re: 140 mercruiser missing after it gets warm

I took off all the connections and cleaned and reattached everything ,used dielectric grease on all connections . Now it won't start at all :facepalm:.
I did the connections one at a time so I know its all hooked up correctly . However when I pulled one of the connections off the solonoid the bolt it was attached to was loose and wobbled around . I tightened the bolt and reconnected the wire . It seemed ok but who knows ? If the solonoid was defective the starter wouldn't turn over would it ?
also I found a ground wire coming out of the wiring harness that was disconnected it connects to the trim pump . So i reconnected it . I'm wondering if this is somehow killiing the spark on the coil and thats why it was unhooked . I ran out of time and didn't think to disconnect it till later so i will disconnect it and see what happens today when I get home .

When it wouldn't start I did check the spark at the top of the coil and at the plugs .I had some spark at the coil iit was blue but seemed weak to me . The spark at the plug was so weak I could barely see it .

Also the resistor wire on the coil is it connected to the positive side ? If I open it up how much resistance should it have on it ? And can someone tell me how much voltage I should have on the positive side of the coil before starting the motor with the ignition on and after after starting the motor ?
Sorry for all the questions and thanks for all the help .