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Hank F.
Hank F. , Technician
Category: Small Engine
Satisfied Customers: 14352
Experience:  Certified on Onan and Generac generators
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I have two Onan B43G engines. Both run great except... When

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I have two Onan B43G engines. Both run great except... When engine under load for a couple of minutes the engine will cut out and quit if kept at full throttle. When it starts to cut out under load and the load is removed and throttle is lowered it will continue to run. After a minute at idle the engine can be placed under load again. Points are gapped correctly and I installed a new condenser. Fuel lines and filter are all new. Carburetor is clean and in great working order. The engine starts and idles great. My thinking is something is wrong with the fuel pump or the pulse line. My thinking is I am running out of fuel under load while idle the pump is supplying enough fuel. What do you think?

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If the pulse pump is working at all, it is flowing enough fuel.

I have never seen one flow enough fuel at idle, but not at full throttle.

Your problem is most likely in a dirty or sticking float needle.

Even is a carb looks spotless, it is usually the source of the problem.

Another thing that is a possibility is the valves.

If they are misadjusted, it is possible that they are causing this.


The valves should be set as follows:

Intake .005"

Exhaust .013"


Customer: replied 5 years ago.
I replaced the float, needle seat, i also used carburetor cleaner and compressed air to clean the carb. This happened all of a sudden, one engine started having the problem last year and the other engine this last week, in both cases, I saw this happen very rapidly. The one engine I have not touched yet. The only thing that made me wonder about the fuel pump was I could see the filter element was wet, I just started to wonder if the filter should be filled with fuel.

The filter should have fuel in it, but it will not be full.

You will typically see it be between 1/3 and 1/2 full.


There is one other possibility here.

If the crankcase is building up too much pressure, it will place a severe load on the engine.

Try cleaning the breather assemblies and adjusting the valves to try and reduce crankcase pressure.


Have you done a compression check and cylinder leakdown test to see if you have bad rings?


Customer: replied 5 years ago.



Engine compression is 88/90psi

Valve adjustment on both heads checked and is ok

Pump inlet vacuum is 1.6"Hg instead of 2.6"Hg min.


I checked and cleaned breather assembly.


Checked pump inlet vacuum again with no change.


I cannot visibly see anything wrong with the pump inlet vacuum tube. What would you do? This engine has under 150hrs on it.


Compression is good.



To be honest, if this were my engine, I would install an electric fuel pump with a pressure of 2-2.5 psi max.


But the book fix would be to do a cylinder leakdown test to see if exhaust is leaking past the rings, causing the low vacuum.

If it checked out good, I would replace both the pulse line and the fuel pump.


Customer: replied 5 years ago.
One last question, please describe leakdown test. You are very helpful.

No problem.


On a cylinder leakdown test, the spark plugs are removed, and a special tool called a leakdown tester is installed in the spark plug hole.

The cylinder is then pressurized with air.

The guages on the leakdown tester will measure how much pressure is lost, and how fast it loses it.


The source of a leak is determined by where the air is escaping.

If it is bad rings, you will hear the air escaping into the crankcase.

If it is a bad intake valve, you will hear the air escaping through the carburetor.

If it is a bad exhaust valve, you will hear the air escaping through the muffler.


Hank F., Technician
Category: Small Engine
Satisfied Customers: 14352
Experience: Certified on Onan and Generac generators
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