1989 4.3 V6 RPM's falling off

[jasoutside]

Hi fellas, I reall need some help getting this Merc to run properly. Let's start with some background so you can see what I have done so far:

This engine is a donor so these are the first trips I have see it in action. First couple of trips the engine wanted to stall right out advancing to 2K RPMs. I was suspicious of carb and dist/cap/rotor/plug wires. So, all of that was replaced. New dist/cap/rotor/plug wires and a new carb was installed.

Took her out again and she ran great through about 3700 RPMs. My WOT only got me to 3800/4000 range and that was it. It felt like missing or bogging down a little, I could just detect it. Here it was suggested I was running too rich and the carb needed some tuning. I also needed to go from the 23P prop to something smaller.

So, I spent some time tuning the idle jets. I thought I had it correctly tuned. I also swapped out the 23P for a 19P prop.

Then I took it for a run, check out the video....

It seems that my WOT RPMs are now correct at about 4800 range but at 1:35 in the video that engine is really unhappy. It starts missing/sputtering/bogging down from 4800 to 4000-4200 RPMs. I don't actually ease up on the throttle until 1:52 so you can hear it struggling between 1:35 and 1:52 in the video.

That was from a few days ago. I tuned the carb a bit more, thought I had things dialed in.

I took the boat with us up here to the big lake. Man she ran great! For about 10 minutes and then the same stuff started happening again. So I backed it down to about 3800 and she ran great for about 10 -15 minutes. Then fell off again, stalled out actually. I got it started up and idled back to port.

Well, beyond my knowledge base. So I bit the bullet and took it in to the local Mercruiser dealer up here. They have had it for two days and they can't seem to pinpoint the issue either.

They have checked over the carb, the fuel, timing...all seem to check out so far.

They are going to get to looking at it again tomorrow morning.

Suggestions? Any thoughts on next steps?

The mechanics are thinking about taking apart that carb to check it out.

Thanks in advance guys

 

[Watermann]

Just the way she ran so good and then started acting like it was starving for fuel makes me ask about vapor lock and if your tank vent is clear. If the carb was bad it would be bad all the time so my suggestion is it would be an upstream interruption of flow. Especially since it ran again fine for 10 min and started acting up again.

 

[Patfromny]

Fuel or have them check the coil. Coil could be over heating. Sounds like fuel though. Check the filter when this is happening. If it's not full it could be the pump, a clog or vapor lock. Did you change all your fuel lines to ethanol resistant? Could be getting pieces of fuel line into he carb. A new coil is cheap too. It could be cracked or just dropping voltage when it warms up.

 

[Teamster]

Just the way she ran so good and then started acting like it was starving for fuel makes me ask about vapor lock and if your tank vent is clear. If the carb was bad it would be bad all the time so my suggestion is it would be an upstream interruption of flow. Especially since it ran again fine for 10 min and started acting up again.

Same thing I was gonna guess,..........

 

[Rick Stephens]

Nice test is hook up a tank for a fuel source and bypass the tank and everything before the fuel pump.

One thing to know. setting idle mixture only effects idle mixture. Soon as you open the throttle, even the littlest bit, the mixture is controlled by other ports, jets and needle valve. If you have problems at higher RPMs it may be useful to have the carb done by someone who knows what they are up to.

 

[achris]

What size boat do you have that engine in? 23" prop sounds too big for anything more than about a light 18 footer.

Chris.....

 

[jasoutside]

Just the way she ran so good and then started acting like it was starving for fuel makes me ask about vapor lock and if your tank vent is clear. If the carb was bad it would be bad all the time so my suggestion is it would be an upstream interruption of flow. Especially since it ran again fine for 10 min and started acting up again.

I know the vent lines on both tanks are clear so that part should be good to go.

 

[jasoutside]

Fuel or have them check the coil. Coil could be over heating. Sounds like fuel though. Check the filter when this is happening. If it's not full it could be the pump, a clog or vapor lock. Did you change all your fuel lines to ethanol resistant? Could be getting pieces of fuel line into he carb. A new coil is cheap too. It could be cracked or just dropping voltage when it warms up.

Fuel lines are all brand new, haven't even got a tank of gas through them yet.

Spin on fuel filter is full too.

Coil? I'll ask about that....

Thanks!

 

[jasoutside]

Nice test is hook up a tank for a fuel source and bypass the tank and everything before the fuel pump.

One thing to know. setting idle mixture only effects idle mixture. Soon as you open the throttle, even the littlest bit, the mixture is controlled by other ports, jets and needle valve. If you have problems at higher RPMs it may be useful to have the carb done by someone who knows what they are up to.

I bypassed everything with a portable tank and was getting the same results, but, that was on the original carb. I don't think the tanks and fuel lines are the issue but I should probably run that test again with this new carb on just to be certain.

 

[jasoutside]

What size boat do you have that engine in? 23" prop sounds too big for anything more than about a light 18 footer.

Chris.....

Hey Chris,

That 23 prop originally came with this driveline out of the donor boat so I just thought I would start with it. It's been switched out for a 19 and that's what I'm running in the video there. It was spinning at 4800 and about 39 mph when it was running properly.

 

[Bondo]

Ayuh,.... Sounds like it's runnin' outa gas,....

Fuel tank,..?? Fuel pump,..??

 

[jasoutside]

Ayuh,.... Sounds like it's runnin' outa gas,....

Fuel tank,..?? Fuel pump,..??

I feel like I can rule out the fuel tanks and lines and filter. All that is new or clean stuff plus I ran off of a portable tank right before the fuel pump with similar results. But, that was with the original carb, so, I need to run that test again just to be sure.

Fuel pump? Maybe? That was my initial thought. But if it was a bad fuel pump wouldn't it show through all along rather than intermittent?

The marine mechanics said they were "dumb founded" yesterday.

 

[Patfromny]

It sounds like loss of fuel but acts electrical. If the fuel pump was bad it would probably sputter as soon as the bowl emptied. You had that boat going a whole long time before it started to fail. That is why I would suggest ignition investigation. A coil or bad ground or whatever can be intermittent. But stranger things have happened. The fuel pump could be intermittently failing and not filling the bowl. When you bring the rpm down and it runs smoothly that is where everyone is thinking fuel. The fuel pump is catching up to the engines demand and refilling the bowl so the engine smooths out again. Weird to say the least but it does present as a fuel issue. The mechanics can inspect and test the coil but unfortunately not a mechanical pump. Especially one that, if bad, doesn't act that way all the time. Might be worth it to change the pump. Especially if you are going to be using her on big water. Worth the piece of mind? Sorry to be spending your money. It's easier than spending mine.

 

[zdeyonker]

Have you checked the timing advance, I would do a resistance check of the coil when warm as well. If it is an old oil filled, they are notorious for working well when cold and failing when warm. Could also check the sparkplugs, a hairline crack in the porcelain is pretty common when you overheat one in one of these engines and it will produce a misfire like condition.

 

[jasoutside]

It sounds like loss of fuel but acts electrical. If the fuel pump was bad it would probably sputter as soon as the bowl emptied. You had that boat going a whole long time before it started to fail. That is why I would suggest ignition investigation. A coil or bad ground or whatever can be intermittent. But stranger things have happened. The fuel pump could be intermittently failing and not filling the bowl. When you bring the rpm down and it runs smoothly that is where everyone is thinking fuel. The fuel pump is catching up to the engines demand and refilling the bowl so the engine smooths out again. Weird to say the least but it does present as a fuel issue. The mechanics can inspect and test the coil but unfortunately not a mechanical pump. Especially one that, if bad, doesn't act that way all the time. Might be worth it to change the pump. Especially if you are going to be using her on big water. Worth the piece of mind? Sorry to be spending your money. It's easier than spending mine.

I hear ya Pat, I sure don't wish to be stuck out on that huge inland ocean!

Yah, the fella at the marina did mention he felt as though it might be electrical and it was losing a cylinder along the way. He was also thinking that maybe something might be blocking up the float bowl. That would stink since it's brand new.

The last time I ran it wanted to stall right out at the lower RPM's too (after running 3800 fine for about 10 -15 minutes).

I don't mind buying an installing a new fuel pump jus to be sure. Although it'll cost me about 5x now that it's at the marina vs. me just doing it. Rats.

Maybe a coil? Maybe carb issue? Hmmmm...

 

[jasoutside]

Have you checked the timing advance, I would do a resistance check of the coil when warm as well. If it is an old oil filled, they are notorious for working well when cold and failing when warm. Could also check the sparkplugs, a hairline crack in the porcelain is pretty common when you overheat one in one of these engines and it will produce a misfire like condition.

Yah, the guys did check timing and it was on.

 

[Watermann]

Did you check on that fluttering choke plate issue with that old stove pipe choke? Once the motor is hot make sure the plate isn't being sucked closed.

 

[jasoutside]

Did you check on that fluttering choke plate issue with that old stove pipe choke? Once the motor is hot make sure the plate isn't being sucked closed.

Well, I didn't mess with the old carb any longer. The new carb however was not choking correctly so we adjusted that for much easier cold starts. When it's warm it stays full open like it should.

 

[Watermann]

Ok, I didn't know if the new carb came with a choke or you had to reuse the old one.

 

[jasoutside]

Whelp, they just called me, they said come pick it up. About a half dozen guys from two different marinas and they can't figure out what's wrong with it.

Fuel is good
Fuel system is good
Fuel pump is good
Carb is good
Coil is good

???