Mercruiser Engine Misses Under Load

[numbered01]

I am still having a problem with my 1987 Mercuiser 4.3 liter V6. Engine number B602338. It has the 4 barrel Rochester carb. When under load the engine misses constantly and sputters. It works good in idle speed. I changed the water/fuel seperator and changed the carb filter, also added 10 gallons of fresh gas, put in a bottle of Heet, added a bottle of Sea Foam and finally added the ethanol additive. Starts great except for when I take of it doesnt want to go up to full speed and starts to bog out.

Is there anything else to check? I bought the boat in the spring and dont know when or if the carb has been rebuilt, or when she had a full tune-up. I just dont want to throw parts into it and find out that it's not the problem.

I know after the first season boat stands for (Bust Out Another Thousand). HA HA HA.

 

[Don S]

Re: Mercruiser Engine Misses Under Load

also added 10 gallons of fresh gas, put in a bottle of Heet, added a bottle of Sea Foam and finally added the ethanol additive.

Have you tried running the engine on gas ???????

Seriously though, did you check the old fuel filters for water? If you have water in the tank, your new filters could now be full of water or the carb plugged up from previous water and corrosion.

When was the last full tuneup and compression check?

 

[Bondo]

Re: Mercruiser Engine Misses Under Load

I am still having a problem with my 1987 Mercuiser 4.3 liter V6. Engine number B602338. It has the 4 barrel Rochester carb. When under load the engine misses constantly and sputters. It works good in idle speed. I changed the water/fuel seperator and changed the carb filter, also added 10 gallons of fresh gas, put in a bottle of Heet, added a bottle of Sea Foam and finally added the ethanol additive. Starts great except for when I take of it doesnt want to go up to full speed and starts to bog out.

Is there anything else to check? I bought the boat in the spring and dont know when or if the carb has been rebuilt, or when she had a full tune-up. I just dont want to throw parts into it and find out that it's not the problem.

I know after the first season boat stands for (Bust Out Another Thousand). HA HA HA.

Ayuh,... It sounds like you've got a Carb problem...

 

[numbered01]

Re: Mercruiser Engine Misses Under Load

How do you check the filter for water? I still have the old one.

I will try to run on gas only, then if it runs right I will drain the tank and add new gas.

Also could it be the fuel pump or anti-siphon valve?

I already ordered the carb rebuild kit, just waiting for it.

If I rebuild the carb I should drain the gas first, put fresh fuel in it, add new filters and then install the rebuilt carb.

 

[Don S]

Re: Mercruiser Engine Misses Under Load

How do you check the filter for water? I still have the old one.

Pour the contents into a clear container. Gas will float on top of water.

What about the tuneup and compression? You seem to be fixated on the fuel system only.
If you have a cylinder with low compression, or a plug not firing, or a corroded cap cross firing or not firing, you will also have a miss under load.

 

[jimmy wise]

Re: Mercruiser Engine Misses Under Load

Check cap,rotor, points, and look over the plug wires. Really look in the cap

 

[sgwill122]

Re: Mercruiser Engine Misses Under Load

Jimmy is right, first step is to check cap & rotor and replace if you find any corrosion. Unforunately your description could be lots of things. Next thing to do is pull your plugs and look at them, do a compression check while you have them out. The plug condition should point you in the right direction, just google spark plug troubleshooting and you will get lots of pictures explaining the different fouling conditions. If you suspect fuel delivery problems pull off the flame arrestor and spray fuel into the carb while accelerating(be ready to jump overboard when your weed sprayer turns into a flame thrower:facepal m I just did it and it is not as bad as it sounds. If adding fuel helps then time to troubleshoot fuel pressure or fouled carb. Fortunately all these steps wont cost you much and should get you a direction to pursue.

 

[888]

Re: Mercruiser Engine Misses Under Load

Jimmy is right, first step is to check cap & rotor and replace if you find any corrosion. Unforunately your description could be lots of things. Next thing to do is pull your plugs and look at them, do a compression check while you have them out. The plug condition should point you in the right direction, just google spark plug troubleshooting and you will get lots of pictures explaining the different fouling conditions. If you suspect fuel delivery problems pull off the flame arrestor and spray fuel into the carb while accelerating(be ready to jump overboard when your weed sprayer turns into a flame thrower:facepal m I just did it and it is not as bad as it sounds. If adding fuel helps then time to troubleshoot fuel pressure or fouled carb. Fortunately all these steps wont cost you much and should get you a direction to pursue.

Do as sgwill122 says in the order its posted and you'll be boating in no time without replacing good parts.

 

[numbered01]

Re: Mercruiser Engine Misses Under Load

So I finally fixed my problem with my engine. The ignition sensor assembly was failing. When It was pulled out it had rust all over the contacts. The new one from Mercury was enclosed in a pod of some kind so water couldnt get to those contacts. Put it in, started right up and took her out and ran great, no misses.

Thanks for the replies back in helping me correct this problem.

Now its time to bottom wash, winterize and storage until next year.

Have a great off-season!!!!