1988 4.3 OMC Cobra Misfiring

[lasko1]

The problem: I'm getting a misfire at ~3800 rpm or 33 mph. I don't notice a misfire below or above the 3800 rpm.

Background: I had my 4 barrel Rochester carb rebuilt due to: 1- Hard starting when warm AND 2- Would bog down when going from 3/4 to full throttle.

Also had points, condenser, and rotor replaced by local shop. Not sure what they gapped the points at, but he said the dwell was set at 39 degrees and timing is at 8 degrees BTDC.



The engine idles fine and gets out of the hole fine. When I go full throttle (4600 rpm and 40 mph), I don't notice any misfire. When I slow down to the certain rpm and speed (3800 rpm and 31 mph), I get the misfire.

Been told by one person that its the carb and another says its electrical. I tend to think its on the electrical side due to the "sharp" misfire/cutting out and no bog down.

Any advice is greatly appreciated.

Dave

 

[indybleck]

Re: 1988 4.3 OMC Cobra Misfiring

Plugs and wires in good shape? A friend of mine had a similar problem with a 4.3 and found out he had a bad coil wire.

 

[lasko1]

Re: 1988 4.3 OMC Cobra Misfiring

New plugs this spring. Gapped at .035 Same wires as before the carb rebuild and the points, rotor, and condensor rebuild. Appear to be in good shape.

They didn't replace the distributor cap during the other maintenance due to it "looking" clean. I'm thinking of replacing it. Plug wires would be next.

Also thinking of using a timing light on the coil wire to see if i can see the misfire happening here. Hoping this will tell me if its electrical.

 

[indybleck]

Re: 1988 4.3 OMC Cobra Misfiring

Did you have this problem before the rebuild on the carb?

 

[Bondo]

Re: 1988 4.3 OMC Cobra Misfiring

Did you have this problem before the rebuild on the carb?

Ayuh,... Pull the fuel Filters, 'n look for crud...

 

[lasko1]

Re: 1988 4.3 OMC Cobra Misfiring

I did NOT have this misfire issue prior to the carb rebuild. The two issues I was having prior to any maintenance was: 1- Hard starting engine while warm and 2- Engine would bog down when going from 3/4 to full throttle.

Small inline filter in carb has been replaced along with the fuel/water seperator.

Pulled spark plugs a few minutes ago. All plugs looked the same EXCEPT #2 cylinder which was more of a tan color than the other cylinders which were all gray'ish.

I did have to idle in the river for about 1/2 hr just prior to docking it the last time out due to lots of boat traffic.

 

[indybleck]

Re: 1988 4.3 OMC Cobra Misfiring

Check the #2 wire and look at the filter again like Bond-o suggested. I got rid of the paper filter in the carb and changed to a Quicksilver filter that is basically a screen that fits into the carb housing. I got it from West Marine.

 

[lasko1]

Re: 1988 4.3 OMC Cobra Misfiring

Checked all the plug wires and they all seemed to be fully attached to the plugs and cap. I plan on changing the distributor cap and the plug wires. These haven't been changed since I've owned the boat which is about 5 yrs.

Does anyone know how much resistance my meter should read when I do an ohms reading on each of the plug wires? Even though I plan on changing all the wires, it'd be nice to know if it is the #2 wire or any of the others for that matter.

 

[Bondo]

Re: 1988 4.3 OMC Cobra Misfiring

Checked all the plug wires and they all seemed to be fully attached to the plugs and cap. I plan on changing the distributor cap and the plug wires. These haven't been changed since I've owned the boat which is about 5 yrs.

Does anyone know how much resistance my meter should read when I do an ohms reading on each of the plug wires? Even though I plan on changing all the wires, it'd be nice to know if it is the #2 wire or any of the others for that matter.

Ayuh,.... It's like a compression test,... it ain't so much the number, but the consistency...

 

[lasko1]

Re: 1988 4.3 OMC Cobra Misfiring

Sorry for the delay, but I'm back.

Ok, I pulled the distributor cap off and installed the new one. I noticed that one of the spark plug holes had quite a bit of green corrosion in it. I though "AHa" here's my problem. I got my new wires and re-installed them. Took the boat out the other day and it STILL MISSES at the 3800 rmp only. Also, it starts great when cold but a little harder once warmed up. I have to put the throttle forward a little and then it'll start up.

Maintenance so far:
- Carb Rebuilt
- New points, condensor, rotor, and dist. cap
- New plug wires

What to check next???

 

[Wismeijer]

Re: 1988 4.3 OMC Cobra Misfiring

Sorry for the delay, but I'm back.

Ok, I pulled the distributor cap off and installed the new one. I noticed that one of the spark plug holes had quite a bit of green corrosion in it. I though "AHa" here's my problem. I got my new wires and re-installed them. Took the boat out the other day and it STILL MISSES at the 3800 rmp only. Also, it starts great when cold but a little harder once warmed up. I have to put the throttle forward a little and then it'll start up.

Maintenance so far:
- Carb Rebuilt
- New points, condensor, rotor, and dist. cap
- New plug wires

What to check next???

Hello Dave,

Did you already find a solution?

I got a same problem. When my RPM is above the 3500 the engine goes down to 1000 rpm or something. You will get a stop shock. I hope you already have any idea.

Glenn

 

[lasko1]

Re: 1988 4.3 OMC Cobra Misfiring

Hello Dave,

Did you already find a solution?

I got a same problem. When my RPM is above the 3500 the engine goes down to 1000 rpm or something. You will get a stop shock. I hope you already have any idea.


I have not found the solution, but the issue has moved up to the 4100 rpm range now. For now, I'm just not running the boat in the "bad" rpm range. The boat runs great above or below the 4100rpm, so I just avoid that range. Not the proper solution, but it does help save on fuel by running slower.
Also, when the "miss" does occur, I don't see any drop in voltage or rpm. It's just quick intermittent misses.

If I find anything out, I'll be sure to post it on here so others can troubleshoot it a lot quicker.

Thanks, Dave