Ignition or Fuel?

[84Monaco]

Any advice you guys have would be great!

My boat runs great at all speeds/RPMs for the first 35-40 minutes, but once my engine is warmed up well it will not run between 1200 and 2500 rpm. If I can get it above 2500 it still runs like a charm. At idle it runs fine too.

It does this both when throttling up or down. If it's at 3000 and I slowly lower RPM it will begin cutting out until the throttle position is nearly to idle. Then it smoothes out again. It does this whether or not it is under load (in gear on the water or on a hose in neutral). Also of interest, the tach goes crazy and the voltage drops by 2-3 volts in this "bad" RPM range.

One strange additional clue that I discovered by accident is that when I'm throttling up if I tap the trim in the "up" direction for a fraction of a second, it will grab and rev up.

Here are the vital stats:

'84 Mercruiser 350ci 260hp
Thunderbolt electronic ignition system
New plugs, wires, cap, rotor
Fuel separator checked, no water in fuel, only a few small specs of junk
Rochester 4-barrel carb

So here is my thinking:

The fact that it starts acting up once hot, and since adjusting the trim (also an electrical system) affects it, makes me think it's electrical. Does this point to the coil or is there something else I should be looking at first?

I tend to think it isn't fuel related since it runs great at higher RPM, and it seems like if it were an accelerator pump problem it would not happen on deceleration. Could the choke have anything to do with this? That's the only thing I know of that changes after about 30-40 minutes related to the carb.

Seems like these symptoms/factors should point fairly clearly to something, but I don't know enough to know what. I've looked through the adults only section and a number or related posts, but couldn't find anything that seemed to match my situation.

What do you guys think? Thanks in advance!

 

[6meter]

Re: Ignition or Fuel?

Certianly sounds electrical. Need to check wires at ignition switch and neutral switch for being loose or stripped insulation. Almost thinking throttle position shorting out neutral switch.

 

[NHGuy]

Re: Ignition or Fuel?

I read about one guy who found a bad ignition controller/amplifier doing something similar. He says he iced it down and the problem went away. So when he put in the amplifier he was all set, same engine & ignition as mine. Thunderbolt IV.

 

[84Monaco]

Re: Ignition or Fuel?

Hi guys! Thanks for the posts. I'll check both of those this evening on the hose. So he just iced the amplifier module and it temporarily fixed it? Anybody know of a source/price if this turns out to be the problem?

Also, I'm not sure I understand "throttle position shorting out neutral switch." Can you explain this a little further?

Thanks again. I'll let you know what I find!

 

[84Monaco]

Re: Ignition or Fuel?

Ok, so I tested it again tonight. As if on cue after running for about 35 minutes without trouble it started going nuts in the 1200 - 2500 RPM range, even in neutral - no load.

I iced down the amplifier for about 10 minutes, but the results were inconclusive. It certainly didn't cure the problem, but may have improved it slightly, very hard to tell. Maybe just my imagination. Certainly didn't erase problem.

So I switched to the coil and chilled it for a while with no effect, although it was hard to tell if I really cooled the coil much.

Finally I verified that an "up" tap to the trim would make it spring to life while in the "dead" zone. This makes me think it's a problem in the primary ignition wiring/switch, but I don't know why the difference in temperature would affect this. The volt meter going crazy kind of supports this idea too.

So is there any way to safely hotwire the ignition to bypass any potentially faulty wiring?

 

[84Monaco]

Re: Ignition or Fuel?

So I thought I had it fixed by cleaning the connections in the cannon connector for the main engine wiring harness at the engine end.

So I took it to the lake and found that I was still having problems, but did get more data...

If I run the boat with the doghouse removed on the lake the problem goes away after a couple of minutes, which tells me that unquestionably temperature is the trigger. With the cover off and 35mph air flowing over the engine the problem goes completely away. i can stop and take off again with perfect, smooth operation after running that way for a few minutes. I put the doghouse back on and in less then 10 minutes it was acting up again.

So...

The problem is in the engine compartment and must be one of the following to the best of my understanding:

1. Ignition module/amplifier
2. Coil
3. Wiring harnesses/connections

What do you guys think?

I think another ice test is in order with bigger ice packs/on for longer.

Any other suggestions?

 

[84Monaco]

Re: Ignition or Fuel?

Cooling the coil fixed it!

Ran it tonight and iced amplifier first and then coil with better icing system. Amplifier had no effect, cooling the coil cleared the problem up.

Will let you know if this fixes it!

 

[84Monaco]

Re: Ignition or Fuel?

Well, replaced coil and it did not make any difference. Now I'm wondering where I screwed up!

Any ideas?

 

[84Monaco]

Re: Ignition or Fuel?

Ok, so I hotwired the positive terminal of the coil to the battery. The only effect this had was keeping the engine running when I turned off the key - so I think that rules out all of the 12V supply wiring/switches.

Now I've pulled the amplifier off of the riser and have it sitting on top of the doghouse between ice packs. This doesn't seem to have made any difference.

Could it be the electronic pickup in the distributor?

I'd love some feedback if anyone has any bright ideas?

 

[84Monaco]

Re: Ignition or Fuel?

So i picked up an ignition sensor and installed it. Now I have no spark at all. I've checked and quadruple checked the connections. Everything was worked, albeit with a problem once warmed up, but now it will turn over, but not even try to start. I pulled the coil wire from the distributor and put in a grounded spark plug. No fire...

If anyone is holding out on something I should be doing now this would be a great time to chime in, because I just don't know what to do next...

 

[84Monaco]

Re: Ignition or Fuel?

Well, it was a stupid problem. The rotor wasn't seated quite all the way and so the sensor wasn't getting a pulse from the tone ring. Put everything back together again and problem solved, including the performance issues.

 

[Jaysad]

Re: Ignition or Fuel?

Hello,
Please help? I have a OMC ford 5.0 HO in a 1990 Chris Craft. I have installed the pentronix ignition, Flame thrower coil (now MSD Blaster 2), New ESA. When first installed everthing seemed to be working fine. Now at WOT (4000)RPM's it starts cutting out, or missing. Also when I trim up or down it will miss out.
On the + side of the coil I have red wire from pentronix ,wire going to silon., and a wire going to the Alt. On the - side I have black wire to the pentronix, one going to the ESA, one going to tach. With key on 9v, start 11.5v, once it's running I get 9-10v. ???

Is the wire going to the Alt. the resister wire? It's not a copper wire, it's like chrome or something. If I remove this wire and replace it with a copper one what would happen???

 

[whs109]

Re: Ignition or Fuel?

Not so stupid of a problem...

I installed a Pertronix "ignitor" electronic ignition module and the engine ran rough right off the bat. I e-mailed Pertronix, and they suggested the rotor was not seated all the way and suggested placing a socket over the rotor and lightly hammering it down into place. I did so and realized I had not seated the rotor all the way. I did not think to take a hammer to the rotor to fix the problem!