4.3 Bogging down at 3/4 throttle

[BlueandGreen]

Hey guys, I'm new to the forum but have been reading on here extensively. Bought a 1990 Celebrity 180 4.3 mercruiser with Alpha one outdrive in December 2015 and set to work restoring her.

The issue I'm having is that she will idle and accelerate fine, decent hole shot, and getting up to about 35 MPH at 3500-3700 rpm at 3/4 throttle, but then wants to bog down if i push the throttle any further. Runs and handles fine. Doesn't bog down in turns. Pulls a tube or skier great. Gets up on plane fine. Just won't get to WOT or as fast as she should. Wondering what is wrong.

Here is what I have done:
Completely rebuilt interior including floor and all seats,
Rebuild top half of engine:
New valves and springs, resurfaced heads, new marine head gaskets, new plugs and wires, new intake gasket, rebuilt carb, cleaned and flushed gas tank, new fuel filter, new electric fuel pump and new fuel lines, set timing properly at 8 degrees before TDC, checked compression on all cylinders (within 10 psi), new oil in engine and lube in outdrive. New main bearing, all new bellows, and new lower shift cable. New cast iron exhaust manifolds and gaskets.

Prop is a 14 1/4 x 20, 3 blade stainless steel. Came with boat, looks like it was dented on the edge and hammered back out at some point. but runs without any chatter.

Any thoughts or help would be greatly appreciated. Thanks guys!

 

[Rick Stephens]

Carb rebuild time. Pay particular attention to float height.



note; the most often job not quite done right is the carb rebuild. If at first you don't succeed, join the crowd.

 

[harleyman1975]

Sounds like you need to raise float a tad. also make sure main jets are clean. fuel pressure should be 5-7 psi.

 

[alldodge]

The issue I'm having is that she will idle and accelerate fine, decent hole shot, and getting up to about 35 MPH at 3500-3700 rpm at 3/4 throttle, but then wants to bog down if i push the throttle any further. Runs and handles fine. Doesn't bog down in turns. Pulls a tube or skier great. Gets up on plane fine. Just won't get to WOT or as fast as she should. Wondering what is wrong.

I suspect your fuel pump or fuel lines, think you have a restriction or weak pump.

When was the last time the fuel filter was changed?
Check anti-siphon valve
What do your plugs look like?

 

[Bondo]

checked compression on all cylinders (within 10 psi),

Ayuh,.... Welcome Aboard,..... Within 10 psi of What,..?? 90 psi,..?? 150 psi,..??

What carb,..??

Has it Ever run better/ faster, since ya did the work on it,..??

 

[BlueandGreen]

Hey guys. Thanks so much for the quick replies. A couple follow up questions.

Harleyman1975:
Would a low fuel float cause the engine to be running low on gas? It bogs down the same whether i accelerate rapidly up to 35 mph or slowly crawl up there. Would it drain the fuel bowl that fast (within a couple seconds) at that speed? Should i be measuring 5-7 psi in the fuel line going into the carb?

AllDodge:
Plugs look ok. I put a new fuel filter on after flushing the fuel tank about mid summer. It has had this problem since i took it out the first time. Thought perhaps a fuel restriction, so i changed the filter again, and detached the fuel line from the carb and ran the pump to make sure i had good fuel flow. Anti siphon valve is new and i checked it as well for clogging and it appeared to be clear. I have not done a fuel pressure check though.

Bondo:
Within 10 psi of each other. please correct me if i am wrong, but i have read that gauges can vary in accuracy, so it is most important that the cylinders read within 10 psi of each other on a particular gauge. With that said, they ranged from 110-120 psi. They ranged form 0-118 before the upper end rebuild. damaged/ worn valves causing lack of compression. The carb is a mercarb 2bbl. and no, it has had this problem since i first got it running and took it out.

 

[achris]

If it's an LX, then it'll be a Rochester 4 bbl carb, easy to get them wrong. If it's an L, then a Mercarb 2 bbl (which is a Rochester knock-off anyway) and the 2 barrels are quite simple.

But I agree with Bondo, we need more information about the engine. Serial number would be a great start....

I'd also check the timing advance (I always like to check the timing advance ) and ask about the lifters. Did you replace the lifters, and if so, with non pump-ups....

Chris........

 

[BlueandGreen]

But I agree with Bondo, we need more information about the engine. Serial number would be a great start....

yeah, about that serial number, I tried to find it a while ago. I believe the serial number for the engine should be on a plate near the starter, correct? Well, i found the plate location, but it is bare cast iron, like the identification has been removed completely. Is there a second location for the engine serial number?

It is a 2bbl mercarb for sure. Got into the carb to properly identify before ordering the rebuild kit.

Did you replace the lifters, and if so, with non pump-ups....

no i did not replace the lifters. should I have? Forgive my ignorance, but what potential issues could the old lifters be causing that would lead to these symptoms?

here are a few pics of the engine if that would help.

 

[alldodge]

With that said, they ranged from 110-120 psi

I hope your gauge is bad, but if it's not then your engine may need rings. Manual shows it should be 180 psi and I would expect in the 150 range. The minimum it needs to run is 100 and your a tad above it

 

[achris]

no i did not replace the lifters. should I have? Forgive my ignorance, but what potential issues could the old lifters be causing that would lead to these symptoms?

The pictures helped plenty, thanks.
For identification, that engine is a non-balance shaft engine, I'd say of a similar year as the hull, 1990-ish.

As for the lifters, my concern was if you'd put new ones in and they weren't 'anti pump-up'.... What can happen is at sustained higher engine revs the lifters don't 'collaspe' as the cam lobe starts to push on them and they don't allow the valve to close properly, giving a sympton similar to what you're getting. But if you didn't replace the lifters, ingore that suggestion.

I agree with AD, those compression numbers are a tad low, but I wouldn't think they would lead to a hesitance, more just a lack of power, and maybe some excess crankcase 'breathing'.

I'd still be thinking to rule out any ignition system problem, then look more closely at the fuel system. I see a pressure regulator in the fuel line, I'd be looking closely at why that is needed.... (with a view to getting rid of it)....

Chris..........

 

[jimmbo]

I would say you have dirt/gum blocking the power enrichment circuit in the carb, or the actuator and/or power valve is stuck shut.

 

[thumpar]

I wouldn't suspect a power valve since it is used at just above idle to get going. A clog in the carb or some other fuel restriction is what I would be looking for first.

 

[harleyman1975]

If problem was there before rebuild then I am going to still say to try raising float level. low float makes the carb shut needle too soon and it would surge/starve at high end. 5-7 psi at the carb. fuel bowl holds about 4-6 ozs fuel and at W.O.T. that engine is using 6-7 gals per hour so 4-6 ozs of fuel would be gone pretty fast.

 

[BlueandGreen]

thanks again for the great feedback! I feel like I might be able to get to the bottom of this. ok here is what I've got.

No mechanical fuel pump was installed with the boat, so i purchased and installed an electric fuel pump (from the local marine parts store) that i was told would supply proper pressure and volume for my engine. Tested this morning. Pump is putting out just shy of 4 P.S.I at the carb with the fuel pressure regulator removed. I looked up the model fuel pump that is installed and it says it will deliver 2.5-4.5 psi. So from what you guys are telling me that is insufficient to run the engine under full load and WOT.

I ordered another marine fuel pump that says it puts out 5-9 psi. Do you think I need to put a regulator on it to keep it below 7 psi at the carb? I'm somewhat worried that if the pump does put out 9 psi that it could overpower the needle valve and flood the carb. thoughts?

I would say you have dirt/gum blocking the power enrichment circuit in the carb, or the actuator and/or power valve is stuck shut.

A clog in the carb or some other fuel restriction is what I would be looking for first.

as far as the problem being a blockage in the carb, it is possible, but I find it unlikely. I never had the boat in the water before the carb rebuild so I don't know if the issue existed before then or not, but I did take the carb completely apart, soak all the parts in berryman's chem dip and then blow out all the passage with compressed air. It is certainly possible i missed something, but I was very thorough, and the problem has been present from day 1. I also set the carb float to exact specs using a machinist ruler. And I purchased a quality rebuild kit, not one of those super cheapo ones.

fuel bowl holds about 4-6 ozs fuel and at W.O.T. that engine is using 6-7 gals per hour so 4-6 ozs of fuel would be gone pretty fast.

Thanks, that is good info. Based on that:

7gals x 128oz/gal = 896oz/hr
896/60min = 14.93/min
14.93/60sec = .249oz/sec
4oz / .249 = 16.06 sec.

so if my math is right it would take the engine 16.06 second to drain the fuel bowl at the lower 4 oz level running at WOT. However, my engine has never made it all the way to WOT, and no matter how quickly I get there it always seems to bog down at the same RPM/MPH. with the fuel pump refilling the carb at 4 psi does it seem likely to you guys that the float potentially causing low fuel level in the carb is the issue?

This is seeming to me more like a mechanical or electrical issue. Do any of you have a good resource or page i could read regarding the steps/ specs for checking/setting the timing advance?

Thanks again for all the help!

 

[alldodge]

I ordered another marine fuel pump that says it puts out 5-9 psi. Do you think I need to put a regulator on it to keep it below 7 psi at the carb?

Yes, you need to keep the pressure under 7 psi

However, my engine has never made it all the way to WOT

Need clarification:
Will the throttle go to full WOT, meaning the crab butterfly is straight up and down?
Or when the throttle is moved full WOT, the power is reduced?

If power is reduced what does it do? Cough, spit, spudder or die

 

[achris]

...No mechanical fuel pump was installed with the boat, so i purchased and installed an electric fuel pump (from the local marine parts store) that i was told would supply proper pressure and volume for my engine. Tested this morning. Pump is putting out just shy of 4 P.S.I at the carb with the fuel pressure regulator removed. I looked up the model fuel pump that is installed and it says it will deliver 2.5-4.5 psi. So from what you guys are telling me that is insufficient to run the engine under full load and WOT....

That to me is within spec. But the proper test is to check the pressure when the engine is running under load. That ensures it can produce the flow.

 

[BlueandGreen]

ok, good clarification on fuel pressure. Thank you.

I can move the controller all the way forward and the carb will open all the way to WOT. When I do that in the water with the engine running the engine seems to loose power and if i push it too fast between 3/4 and WOT it will just die. If I inch it up from 3/4 (where it will run fine all day) towards WOT and it starts to bog down i can bring it back to 3/4 and it will pick back up and accelerate back to 35 MPH every time. It has never coughed, spit, or sputtered. The engine just looses power and acts like it wants to shut off right at that same spot every time.

 

[Rick Stephens]

I would still say redo the carb rebuild.... carefully.

 

[Baylinerchuck]

I would also check each cylinder by pulling a plug wire off each plug one at a time. You should see an rpm drop every time you pull one. It's possible you have a dead cylinder, or plug wires mixed up. Don't laugh it's more common than you think. I had similar symptoms as you and found I had 3 & 6 swapped. Also use a timing light to set base timing and ensure you are advancing properly.

Hope you get it figured out quickly.