1993 Omc 4.3 cobra idling problems

[welch181]

I have a 93 OMC 4.3 Cobra and my problem is it will not idle at less than around 1100 RPMs. Anything above that the motor runs fine, just stalls at anything less than 1100. First I changed plugs, they were in pretty bad shape, then checked wires and distributor for spark, all checked out fine as far as I can tell. Took fuel/water separater off and drained in a glass, appears no water at all in fuel. With engine running I can pull the plug wires on both center cylinders and get absolutely no change in engine speed, but do with all other cylinders. Compression is good on all 6 cylinders. No excess fuel in carb bowl from what I can see looking down through. I could only find one vacuum hose on the carb, and it appears to be ok and sealed properly. When i pulled the plugs again today, they appear to be quite black already, boat has not even been in the water since changing them, just running on muffs while trying to figure out this idling problem.

What other causes should I be looking at next, I have checked about all I know to check at this point.

Any suggestions would be greatly appreciated!!!

 

[Don S]

Re: 1993 Omc 4.3 cobra idling problems

When was the last full tuneup? Points, condenser, cap, rotor, plugs, the normal tuneup stuff?
Did you set the dwell, then the timing, then adjust the carb? if not, all are suspect.
You also can't look down the throat of a carb and see the fuel level.
Is the firing order correct?

Did your problem slowly develop over time, or something that just started after some other work being done?

 

[welch181]

Re: 1993 Omc 4.3 cobra idling problems

Do not know when last tune up was but it has been switched to an electronic ignition system as I found when pulled cap. Does that system still have dwell? I dont think the electronic system has points, isnt that the purpose? I really dont know. Have not made any timing or carb adjustments just started with the plugs and went from there. I had no problems at all until 3 weekends ago, then when returning from being out all day, slowed up for the no wake zone at the ramp and it idled down and stalled. First thought being so sudden was plugs. So I replaced them first and worked back to wires when the new plugs made no change to the idling problem. The firing order went back on as they came off individually one by one, but I cant be certain it is correct, but it never had any problems prior with that firing order.

Thank you for your input

 

[Badzook]

Re: 1993 Omc 4.3 cobra idling problems

Funny,

I'm dealing with a very similar problem right now.

Mine is an 95 OMC Cobra Vortec 4.3L but same ignition. I was running around the lake just fine, was idling towards the dock for pullout ( Very Lucky for me ) and the engine just quit. Would not re-start. Plenty of fuel ( Could smell it... ). Mine actually died and won't re-start though...

Pull your Dist cap and look at the terminals for corrosion.

My problem was a direct result of the dist cap having badly corroded terminals inside. The spark jumps around under the cap and fries the module or strains the coil until it fails.... Very common problem on GM Trucks.

I am a Oldie Auto Dealer tech so finding this was easy for me today...

( Shame on me for not tuning my boat..LOL )

Quickly found my ignition coil to be very weak.... I thought the boat was running a bit sluggish too.... However, a bad Ignition Module can cause what you describe as well.

Go get an ignition testing spark plug with the clamp on it. ( about 10 Bucks ) Do not use a spark plug. Using a standard spark plug will fool you into believing you're ok. A weak spark will jump that gap. Your ignition is capable of 40k volts and should jump a .70 gap with ease. Pull the coil wire from the cap and check it there. A bad coil or Module will show up as a no spark with this tool....

Check the trigger signal coming into the coil with a test light. If it is erratic suspect the Module or pick-up coil. If there is no switching suspect Module or pick-up coil. If it blinks sharply and brightly suspect Coil.

I know you mentioned no water in the gas..... But...if your spark tests ok.... Pump some gas from your tank and check to see if it is bad gas?? Does it smell like gas or turpentine?? Bad gas will kill your idle too.

The fuel filter on thet boat is pretty good at filtering the fuel and I doubt there is dirt in the carb. But, it could be sludged up from gas turning bad in storage.... Try running some injector cleaner through your boat ( Yes, it works on Carbs too ) ... I use a strong mix like double the instructions and it works great for me in all my vehicles...

Keep us posted..

Badzook

 

[welch181]

Re: 1993 Omc 4.3 cobra idling problems

Ok to update thus far... Have not had a chance to pull the carb yet, did however notice that the idle mixture screws on either side will effect the idle. Does that mean that my problem is definately in the idle circuit on the carb? Is there anyway to clean that without a full rebuild? With the mixture screws I have it down to just under 800 rpms on muffs, have not had a chance to see what that correlates to in the water yet. If i change mixture to run any lower rpm, there is a huge hesitation and almost stall when the throttle increases.

Am I correct to say that everything is still pointing to the carb idle circuit? Could the 10% ethanol mix this year be what has caused this problem to develop in the carb?

Thanks for all the help and advice so far.

 

[mthieme]

Re: 1993 Omc 4.3 cobra idling problems

Sounds like the carb to me.
I have a 4.3 cobra too.
I rebuild it about every 4-5 years due to (severe) hesitation when accelerating after sitting all winter. Sometimes it works out after a little running (like this spring).
I actually have a spare now so I can just swap them out when needed. 2 bbl Rochesters are very common.
Rebuilding the carb is not a great big deal. The kit runs around $20 or so and you'll need a couple cans of carb cleaner and compressed air. Pick a work area that is clean and put all the little parts on a rag so they won't move and get lost.