Mercruiser 3.0 bad hesitation at WOT please help

[richa311]

My boat with a mercruiser 3.0 i/o runs great at all throttle positions up until WOT where it will run fine for about 10 seconds before hesitating badly. If I pull back about 400 rpm it seems to be fine.

Here is some history on the boat and specs.
1st off I just bought the boat and it had set for a couple years.

It's a 1989 Glasstream bowrider with mercruiser 3.0
Prop is a 14.5 x 19 pitch
Compression is at 145 to 155 on all cylinders

When I bought the boat it wouldn't even run. The fuel pump was seized badly and carb needed rebuild.

Carb was rebuilt and sourced out a used fuel pump from marina.
Installed fuel pump and boat started up and after slight adjustments seemed to run good on the hose.
I installed new plugs, changed the oil and changed out the outdrive oil.

Next took it to the lake and it would idle fine but ran like crap with no power under throttle.

Brought it home and discover the fuel tank had tons of water in it. Drained fuel tank added fresh fuel.

Brought it back out to the lake next day and the boat ran great and at all throttle positions and would run at about 37 mph at just under 5000 rpm. Then when going back to the launch the boat started acting up and wouldn't take any gas. If you tried getting on plane it would act like it was getting chocked out but would run at idle fine.
I brought it home figuring maybe there was some water left in the tank and it got stirred up from going over wake. After going through everything I could think of I noticed the vent line from fuel pump and carb had fluid in it. After a quick search here I learned the used fuel pump was shot and sending the fuel into the carb which explained why it wouldn't take throttle. I took the pump apart and sure enough the large diaphragm was shot.

So I ordered a sierra mechanical fuel pump 18-7278 and while waiting for that I also installed a fuel/water seperator, new fuel lines, checked the anti syphon valve (missing will replace when boat runs correctly) added new 3/8 fuel lines and routed the vent line for fuel tank correctly. (It was looped down so water could run down into line and the vent on aft of boat located at swim deck was rotated so vent holes were pointed up allowing water in)

Once fuel pump arrived I installed the new pump and ran the boat on the hose and it seemed fine. Only thing noticeably different this time was a noticeable knocking sound. Only thing I can imagine it being is the new pump being louder then the old.
I brought the boat to the lake and ran it at low speed till it was warmed up so far everything was good.
I then throttled up to about 3000 rpm and ran it on plane at that speed just fine. I then tried to do a wot run and it was fine for about 10 seconds then started hesitating pretty badly. It's not backfiring or popping really but more like someone is pinching a fuel line or something. It kind of goes in short bursts like it gets fuel and looses it???
I checked the plugs and they definitely look lean. The plugs used to have white center electrode, light brown/grey ground strap, and the bottom 1 or 2 threads were light brown.
Now the center electrode is real white, ground strap has gone to a grey/white color, and the ring just under last thread had turned grey/white also.

I went through everything I could think of again. Only things that changed since running it last time were fuel pump, water seperator, new fuel lines.
So I ran it with a line right from the boats fuel tank to the fuel pump bypassing water seperator and it ran the exact same. Then I ran with a line to a plastic fuel tank with known good fuel to fuel pump and it ran the same. Then I thought maybe I messed the vent line up somehow so I unscrewed the boats fuel cap out almost all the way allowing the tank more vent and that made no difference.
I also played with timing even though I have no idea how that would have changed anyway and that also made no effect other then making it run worse so I put it back as it was.

I'm at my wits end here. I have no idea what else could it be.
My only guesses are I either received a faulty fuel pump (no fluid in the vent line this time) or somehow something is clogging a high speed jet. It was just rebuilt though and would the high speed jet only affect the last 400-500 rpm of wot.

Sorry for the long post but I wanted to be sure to give as much info as possible so it could be accurately diagnosed.

Thanks in advance for any help or suggestions

 

[Bondo]

Ayuh,.... Welcome Aboard,..... Go back to the Carb, ya pumped more crap into it after ya rebuilt it, so it's probably fulla crap again by the sounds of it,....

 

[richa311]

Thanks Bondo looks like that's my next step as I'm out of other ideas. I installed a new fuel pump today same kind I suspected could be faulty.
Issue is still there so I'm hoping your right about the carb. I'd love to have this in the water this weekend.
I'll post back results as I go.

Thanks again

 

[richa311]

Ok I'm at a loss... I just pulled the carb off and did a thorough cleaning with compressed air and carb clean. There was a decent amount of grit or sand material in the bowl.
Anyway cleaned and made sure accelerator pump was good etc.
Put it all back together and went back to the lake. It ran great at all throttle positions once across the lake but then started hesitating over 4000 rpm again. It's fine until you start going over 4000 then it starts hesitating / fluttering.
Could I really have sucked up more crap even with the water fuel seperator and brand new fuel filter in bowl of new fuel pump?

Any other ideas?

 

[alldodge]

Ok I'm at a loss... I just pulled the carb off and did a thorough cleaning with compressed air and carb clean. There was a decent amount of grit or sand material in the bowl.
Anyway cleaned and made sure accelerator pump was good etc.
Put it all back together and went back to the lake. It ran great at all throttle positions once across the lake but then started hesitating over 4000 rpm again. It's fine until you start going over 4000 then it starts hesitating / fluttering.
Could I really have sucked up more crap even with the water fuel seperator and brand new fuel filter in bowl of new fuel pump?

Any other ideas?

:welcome: to iboats

Your second sentence tells you what happened, the carb had grit and sand in it. Do you have a fuel filter installed? If you do, you need to replace it again, suggest using a spin-on water separator type.

 

[richa311]

I do have a fuel filter in the bottom of the fuel pump. The filter and bowl were clean. I also have a water fuel seperator filter installed.
This is driving me nuts so I went back out and pulled the carb off again. The bowl was totally clean this time no debris at all.

 

[jimbo_jwc]

Check the filter in carb where the fuel line goes into it , my 3.0 was collapsed and acted similar .

 

[richa311]

I had already removed that filter. It was toast and I didn't see the need to replace it with the other 2 filters in line before the carb.

 

[richa311]

I think my next step is I'm going to install an electric fuel pump with 5-7 pounds of pressure in place of mechanical pump to make sure it's not a fuel pressure issue. I can't imagine I got 2 bad pumps but after review on this sierra pump there were people that had to order more then one pump before getting one that worked correctly.
It's just so odd that it runs fine until that last 500-700 rpm.

 

[richa311]

Ok took carb off the boat again today.
Took some time inspecting the float needle and seat.
I found that with the bowl bottom half of carb off and I held the float up with my hands to simulate a full carb bowl then tried blowing in fuel inlet with my mouth I wasn't able to get any air in. So it's stopping fuel as its supposed to.
What I did notice and may be an issue? When I let the float drop simulating an empty bowl and tried blowing in the fuel inlet I really had to blow hard to get the float needle to drop out of the seat to allow air to flow. I tries that multiple times and it always did the same thing.
Could this be the issue? A sticking float needle or would the 6 pounds of fuel pressure easily over come that?

Thanks again for the help and suggestions

 

[Scott Danforth]

did you replace the needle and seat when you cleaned the carb? did you replace the fuel filter in the carb inlet?

 

[richa311]

When I bought the boat I was told the carb had just been rebuilt and cleaned. It wasn't until yesterday after Bondo suggested the carb be cleaned that I took the carb and noticed the only new components in the carb were the gaskets.
I did clean the carb really well yesterday though and needle and seat looked fine. It wasn't till today that I ran the tests to make sure needle and seat were performing correctly that I noticed needle sticks.

I'm on my way to marina now to source a new one.

My question is could this cause my fuel starvation problems and only at wot.

As far as the filter in carb inlet I removed it. I have 2 other filters before the carb is the one in the carb really needed?

 

[richa311]

Ok new needle and seat installed which it definitely needed.
Problem is still runs the same. Fine right up to between 4000 and 4200 rpm then it starts cutting out or surging would be a better discription.
I have no idea what else to check.

I really am suspecting it to be these sierra fuel pumps.
It ran fine before old pump lost the diaphragm.
I should also note the arm on the old pump was straight and angled down. The arm on this pump has a curve to it. From what I can tell this new pump is a replacement for the old style?

 

[alldodge]

A cough or such at higher rpm is the engine running out of fuel. Put a fuel pressure gauge using a Tee to check the pressure at the carb. You should have 4 to 7 psi at idle. Also check your anti-siphon valve at the tank. You can also try to run the boat from a gas can (safely) to take your anti-siphon valve and tank pickup tube out of the equation

 

[richa311]

I'm going to buy an in line pressure Guage today to rule out the fuel pump.

I did run the boat off another gas can to make sure it wasn't something at the tank hindering fuel flow. It made no difference.

A friend seems to think it's an electrical problem specifically timing. I marked the engine and distributor as to be able to put it back in its current spot and we went out on the water and he tried to make slight adjustments while at speed but it did not help the issue only made it run worse. If it were electrical wouldn't the plugs be rich instead of lean due to not being able to burn the fuel?

A couple other things have come to mind though.
-1st I just figured out what the large jet in bottom of the bowl is (power valve) Anyway I've never seen one in a carb before. Well that happens to be the one thing I couldn't get out of the carb. It's seized in there pretty good and I didn't want to damage it. I did spray a liberal amount of carb clean and compressed air though. With the Sandy grit in the bottom of the carb bowl and now that I know that feeds extra fuel at high throttle I'm wondering if it could be clogged? Again would that cause the surging at the last 400-500 rpm of WOT though?
I should also note that when you go to wot it's fine for 10-15 seconds before starting to surge. Again leads me to believe it's running out of fuel.

2nd issue I don't think could be related but I'm grasping at straws here.
The last time boat was in the water when it was running great the day the original fuel pump failed I was flustered and in a hurry to get the boat out of the water due to a busy boat ramp and forgot to hit the trailer button to lift the outdrive all the way up.
Well anyway it dragged the prop on the cement ramp and gouged the prop good. It put some grooves in it so I smoothed them out.
I'm going to get a new prop but not until I figure out this issue.
My question could the prop somehow cause a hesitation or surging that you can actually hear happening and watch on the rpm guage?

Thanks again guys. I have to be getting close to figuring this out.

 

[Scott Danforth]

you have to re-time the motor every time you pull the distributor.

however your surging at high RPM is the motor running out of fuel. you still have dirt lodged in one of the passages. I usually have to run a fine wire or nylon line thru all the passages to make sure that they are clear.

 

[alldodge]

I'm going to buy an in line pressure Guage today to rule out the fuel pump.

I did run the boat off another gas can to make sure it wasn't something at the tank hindering fuel flow. It made no difference.

A friend seems to think it's an electrical problem specifically timing. I marked the engine and distributor as to be able to put it back in its current spot and we went out on the water and he tried to make slight adjustments while at speed but it did not help the issue only made it run worse. If it were electrical wouldn't the plugs be rich instead of lean due to not being able to burn the fuel?

Your only running at WOT for a small amount of time as compared to low and mid rpm's. If your running a bit rich your plugs would be black, they wouldn't get black just from a short time of missing at WOT.

A couple other things have come to mind though.
-1st I just figured out what the large jet in bottom of the bowl is (power valve) Anyway I've never seen one in a carb before. Well that happens to be the one thing I couldn't get out of the carb. It's seized in there pretty good and I didn't want to damage it. I did spray a liberal amount of carb clean and compressed air though. With the Sandy grit in the bottom of the carb bowl and now that I know that feeds extra fuel at high throttle I'm wondering if it could be clogged? Again would that cause the surging at the last 400-500 rpm of WOT though?
I should also note that when you go to wot it's fine for 10-15 seconds before starting to surge. Again leads me to believe it's running out of fuel.

The power valve is used during transition between mid to higher rpm. The accelerator pump provides the initial dump of gas upon acceleration. The power valve is held closed by vacuum. When the engine rpm increases above mid rpm the main jets start taking over. A symptom of a bad power valve is running rich if leaking and a mid throttle stumble if it won't open.

If you have points you might be getting into points bouncing. What ignition system do you have?

2nd issue I don't think could be related but I'm grasping at straws here.
The last time boat was in the water when it was running great the day the original fuel pump failed I was flustered and in a hurry to get the boat out of the water due to a busy boat ramp and forgot to hit the trailer button to lift the outdrive all the way up.
Well anyway it dragged the prop on the cement ramp and gouged the prop good. It put some grooves in it so I smoothed them out.
I'm going to get a new prop but not until I figure out this issue.
My question could the prop somehow cause a hesitation or surging that you can actually hear happening and watch on the rpm guage?

Thanks again guys. I have to be getting close to figuring this out.

A dinged prop will only cause some vibration and/or loss of top end speed. No direct relation between engine rpm

 

[richa311]

you have to re-time the motor every time you pull the distributor.

however your surging at high RPM is the motor running out of fuel. you still have dirt lodged in one of the passages. I usually have to run a fine wire or nylon line thru all the passages to make sure that they are clear.

I didn't actually pull the distributor only tried advancing and retarding the timing slightly to see if it would clear up. I'm going to check the fuel pressure at the carb today and if it checks out ok then I'll pull the carb again and try running a thin piece of wire through all the passages.

I have electronic ignition so no points or cap and rotor to mess with.
I was misinformed on the operation of the power valve I guess. I was told the plunger in the center of carb is held up from vacuum in the carb but when you get to wot the carb looses the vacuum allowing that plunger to drop. When it drops it pushes the pin in the center of the power valve down allowing fuel to enter. I was told this is to compensate the main jets as they can't put enough fuel into the engine at wot. I was thinking maybe that would be a reason for it to be running lean at WOT. Guess not though. Boat doesn't hesitate until just before full throttle

 

[richa311]

Yesterday when I got home I installed a pressure guage just before the carb and it held steady a 5 psi of pressure. I'll have to run it later today to see if it loses any pressure towards top end. Is 5 psi of pressure enough to feed this carb at full throttle?

I also went ahead after and pulled the carb again and spent a couple hours going through it the best I could.
I ended up getting g the power valve out with a impact screwdriver. The outside around threads was pretty cruddy but all the passages were pretty clean so I'm sure that probably wasn't the problem.
I did go online and get the specs for the float height though and mine was out of spec. Not sure enough to cause my issues but we will see I guess.
It had just under an inch for float drop. I set that to the 1 3/32 it called for and set the closed height at 3/8. I did notice while blowing air through the carb some air is able to pass through the linkage rod that operates the butterfly valves on bottom of carb. Is this normal would it cause a vacuum leak? I don't see how that could be an airtight area since the rod needs to be able to move and rotate.

I also drained all the fuel out of the tank again just to be sure I didn't miss any water or anything.

Then I gapped my plugs they were at .30 so I gapped them to the .35 it calls for.

Added some seafoam to the new fuel.

I don't know what else I can do to this thing. Really hoping I put it in the water today and it runs great

 

[Eric 3.7LX Alpha 1]

I was actually thinking float height as I read this and you finally did check it... You gonna have to check the fuel PSI when it acts up at WOT... see if it remains steady or drops off.