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Charles
Charles, Marine Mechanic
Category: Boat
Satisfied Customers: 3370
Experience:  US Navy (Ret) Certified Mercury Marine Tech with over a decade of repair experience.
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2 cycle outboard boggs down under load past half throttle.

Customer Question

2 cycle outboard boggs down under load past half throttle. Rebuilt carbs, and it looks like the fuel in the lower carb is not atomizing the fuel.
Submitted: 1 year ago.
Category: Boat
Expert:  Jason replied 1 year ago.

Good morning, my name is ***** ***** am I speaking with?

When you say it looks like the lower carb is not atomizing the fuel, can you be more specific? What are you seeing that makes you think that?

Also, how did they look when you took them apart? Were they fairly lean on the inside, or in rough shape?

Customer: replied 1 year ago.
They looked pretty good and as if they had already been rebuilt by the previous owner, I'm thinking he actually knew about this problem. I went ahead and rebuilt them anyways. When I put the engine in gear and open the throttle past half I can see a stream of fuel droplets being sucked into the engine through the venturi. The upper carb looks as if there is no fuel because it is all atomized.
Expert:  Jason replied 1 year ago.

Have you done compression and spark output testing on the engine yet?

Customer: replied 1 year ago.
Expert:  Jason replied 1 year ago.

What kind of compression numbers did you get?

Expert:  Jason replied 1 year ago.

Also, on the spark test, there is a right way and a wrong way to do that. What kind of tool did you use to check spark output?

Customer: replied 1 year ago.
One of those adjustable spark testers that the plug wires go onto and you can measure the jump
Customer: replied 1 year ago.
Compression was between 128-130 on 1,2,3 and 4 cylinder was 125
Expert:  Jason replied 1 year ago.

Got it. The next thing is have you run the engine off of another fuel source, a different tank? A well as trying pumping the primer bulb over and over and over?

Customer: replied 1 year ago.
Tried that, as well also just pumping fuel with the primer bulb to keep fuel pushed to the engine. Also worked around the fuel pump to eliminate that possibility and still same problem
Expert:  Jason replied 1 year ago.

If you run the engine and hold your palm flat against the bottom carb, do you feel suction or can you feel any spitting back at you?

Customer: replied 1 year ago.
I can feel and see fuel being sucked, I put a piece of paper about 1to2 inches in front of the carb and no fuel, I looked into the and the reed valves look fine
Expert:  Jason replied 1 year ago.

On your engine are the fuel pumps part of the carbs or is it a separate pump? They make them both ways.

Customer: replied 1 year ago.
The fuel pump is seperate. There is a small hole above the intake on the actual block that would be behind the carb when bolted up. I presume that hole allows pressure or vacuum to affect the carb somehow. I noticed fuel is in that hole, as well as near the fitting that provides crankcase pressure to the fuel pump. I preformed a vacuum test and pressure test on the fuel pump to ensure the diaphragms were not leaking which they were not
Expert:  Jason replied 1 year ago.

The pump has 3 lines on it. Fuel in, fuel out, and pulser. The pulser line runs between the pump and block. Is this the line you are seeing fuel in?

Customer: replied 1 year ago.
It has 4 lines but one is straight into the engine block. The bottom engine fitting is the one I notice a small amount of fuel. But it doesn't appear to be coming from the fuel pump. I replaced all the gaskets in the pump as well.
Expert:  Jason replied 1 year ago.

Do you have a fuel pressure gauge that you can hook up in order to measure fuel pressure while underway?

Customer: replied 1 year ago.
I do not, but would fuel pressure cause fuel to not atomize? I can get one if that could be a reason for the issue.
Expert:  Jason replied 1 year ago.

The only thing that is going cause fuel not to atomize is bad reeds or a clogged carb. we covered the reeds, if it had bad reeds you would feel it spitting back out at you. You said you rebuilt the carbs, I can only take you at your word that you know what you are doing and did the carbs correctly.

Have you ever seen this engine on the boat it sits on now make full power?

Customer: replied 1 year ago.
It looks like when you are sucking up water using a shop vac. Not to that degree but how the water kinda forms a stream of several drops. That's kinda what it looks like if I shine a flashlight into the carb while it's running, I just have to give it gas to really see it. That's the reason it's running rich as well but not sure what causes all of this.
Expert:  Jason replied 1 year ago.

We can test if the carb is flowing or not if you have a good sized compressor. Do you have a good sized air compressor along with a decent blowgun?

Customer: replied 1 year ago.
I have not, I bought the engine and watched the guy run it.... not under a load (lesson one, should have gone out with him on his boat) I check compression and didn't think it wasn't going to run under a load. Placed the motor on my boat and it has never made full power.
Expert:  Jason replied 1 year ago.

If the engine has the wrong prop on it, it might simply be overloaded. What size and style boat is he engine on now? Between yourself and all the gear, how much weight are you putting on the boat? And what size prop is on there now?

Customer: replied 1 year ago.
being overloaded would cause the fuel not to atomize? I can check on the prop size, the boat weighs 740 pounds plus me, fuel, and engine so probably about an additional 450 pounds I'm guessing the engine weighs about 200, me 165 and then fuel and battery.
Expert:  Jason replied 1 year ago.

Sounds good, let me know what you get for a prop size and then I will run all the numbers. .

Customer: replied 1 year ago.
Is there another way to test the reed valves? Not sure if I can do a leak down test of some sort or pressure test. I have access to compressors and blow guns.
Expert:  Jason replied 1 year ago.

The only test on the reed valve is hold your palm over the carb and see if you feel pressure. You should never feel pressure, only suction. And if you feel suction, then the engine would be drawing fuel in from the carb unless the carb is clogged. To see if the carb is still clogged. Take the carb back of, make sure it has fuel n the float bowl. Hold the carb in your hand and open the throttle all the way. And then hold the blowgun about 2 to 4 inches away from the throat of the carb and blow right through it. If the carb is flowing properly, you should see a fine atomized mist of fuel exit the other side of the carb. If the carb is clogged, you won't see anything. Does that make sense?

Customer: replied 1 year ago.
For the reed valve pressure and suction, is that with the engine running or should I be checking the air movement just cranking the engine?
Expert:  Jason replied 1 year ago.

With the engine running.

Customer: replied 1 year ago.
I'm gonna go pull the prop now, now I'm not sure if I mentioned this but I can Rev the engine without it bogging down, however when I did this it was very brief so it might not be very accurate.
Expert:  Jason replied 1 year ago.

Reving the engine in neutral does not mean anything, all its doing there is spinning it's own weight, not making horsepower.

Customer: replied 1 year ago.
Could the fuel pump on the actual carb be causing any of these issues?
Expert:  Jason replied 1 year ago.

I thought we established the fuel pump was a separate unit. Let me get the engines serial number that way I can run it and see exactly what you got there. These engines were made for 20 years and they came configured a dozen different ways.

Customer: replied 1 year ago.
OB368635
Expert:  Jason replied 1 year ago.

Got it. There are no fuel pumps inside of the carbs. Pull the bottom carb and do the test with the blowgun and see if it shoots atomized fuel out of the back of it or not. That will tell you if the problem is still in the carb or someplace else.

Expert:  Jason replied 1 year ago.

Hello, just checking in. The website has charged you but no rating has been put through yet in order to credit me for helping you out. Did you have any further questions on the issue?

Thanks,

Jason

Customer: replied 1 year ago.
Well I still have the problem, I replaced the reed valves got the engine put back together and it's still running the same. Another mechanic said it was the stator so replaced it and still same problem.
Customer: replied 1 year ago.
I went as far as marking TDC on the flywheel of all 4 cylinders and checking with a timing light on all 4 and they looked good. Today I ran a straight fuel line into a tank to eliminate any tank line issue. At this point I'm wondering about possible electrical solutions.
Expert:  Charles replied 1 year ago.

Hello, my name is***** will be handling your question from here.

When you look into the bottom carb, do you see the black plastic nozzle?

Customer: replied 1 year ago.
Are you taking about the venturi?
Expert:  Charles replied 1 year ago.

yes is it there

Customer: replied 1 year ago.
Customer: replied 1 year ago.
Both top and bottom are new
Expert:  Charles replied 1 year ago.

Ok great.

When you saw the fuel pattern in the carb, what RPM was the engine at?

Did you use a timing light or just a flash light to look into the carb?

Customer: replied 1 year ago.
Not sure what you mean by fuel pattern. I used a timing light on the flywheel to verify each cylinder was getting spark at the correct time by using the flashing to see the number of the cylinder I marked at a given spot at TDC.I used a regular flashing to look into the intake of the carb
Expert:  Charles replied 1 year ago.

The fuel droplets that you have seen is what I was referring to.

Customer: replied 1 year ago.
that was using a regular flashlight, but I can see it without as well if outside during day
Customer: replied 1 year ago.
I can put a hand pressure tachometer on it and see the rpm, the max rpm that i can get I think was around 2300rpm. The fuel not atomizing is slightly noticeable around 1300rpm. I can verify the rpms if it helps.
Expert:  Charles replied 1 year ago.

Ok,

I dont think that you are running on all 4. The bottom must not be able to burn that mixture going into it. Take the bottom carb back off and really inspect the area around where the tube come into the carb body. There may be a crack there, or the tube is not installed tightly.

Customer: replied 1 year ago.
Is the tube the one that the bowl bolt screws into?
Expert:  Charles replied 1 year ago.

Sort of, it is the brass tube that screws into the venturi.

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