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Flat Spot at 2100 RPM

Got a customer with a marine 2300. It has a flat spot at 2100 RPM.

The position of the pump arm cam and/or the correct pump cam. He might be using up the total pump discharge to fast.
Low float level. The marine Holley uses an internal Needle & Seat and can only be adjusted with bowl off. The Float should be set to specs held upside down by bending the float tab that touches the needle.
Correct Power Valve (7.5/6.5 hg) They make P/V from 1.5 to10.0 hg.
He should make sure the squirter is working, could be a problem with the inlet and/or discharge check. Turn the engine off, pump the gas while looking down the barrel. You should see 2 good squirts of gas. If not then you have something plugged in the accelerator pump circuit.

Secondarily there could be an accelerator pump problem. It could be it isn't filling up fully. This would assume that we are throttling up at 2100 RPM. If cruising, then no, it's something else.


Holley Power Valve
Ethanol Defense


Engine only runs when I pour gas into the carburetor.

Block part of the carburetor intake with your hand. Does it run better?
Anytime you restrict the air you will get a higher fuel to  air ratio.
So obviously you aren't getting enough gas if it runs that way.

  • 1st check for a vacuum leak. A lot of times a leak will whistle.
  • Spray carb cleaner around the base of the carburetor, all connections, intake manifold and then any vacuum lines coming from the carburetor. If this straightens out the RPM then you found the problem.
  • If your 2300 has site screws in the side of the carburetor remove one to see if gas is up to the hole. (be careful not to catch fire). If not, then you have low fuel quantity possibly from a an incorrect float level.
  • Test your fuel pump (new pumps are especially suspect) for proper pressure. The pressure should be in your motors manual. Probably 4-5 lbs.
  • If the level looks good then chances are the fuel isn't getting from the float bowl to the carb bore. Starting with the main jet, following the passage into the carb bore. Make sure there isn't anything behind the jet that could obstruct the gas flow.
  • Is the choke working. Make sure the choke valve is closed when starting cold. If the engine is hot, then the choke valve should be open.

Gas is leaking out of the throttle shaft

Throttle shaft seal or no seal, the throttle shaft is not meant to seal out the fuel. Any seal if present is used to keep the shaft sealed from a vacuum leak.

When gas leaks out of the shaft it means gas is running down the carburetor bore, hits the throttle shaft and leaks out the ends.

It isn't the throttle shaft you should be worried about, but the excess gas coming down the bore.

Here is a list of things to look at not in any order.
  • Check the fuel pressure. Look at your auto manual for the exact measurement but 5 would be a good bench mark. New pumps are especially suspect. You may have to put a regulator between the pump and the carburetor.
  • Test the needle & seat for leaks. This is the most common problem. 
  • Test the brass float for leaks by immersing in hot water. Leaks will show up as bubbles as the air inside the float heats up and expands. For non brass floats weighing is the only way to test. If in doubt, change the float.
  • Float should sit level with the carburetor upside down.
  • Move the float up and down to determine if there are any flat spots in the float pin causing the float to catch.
  • Is the power valve sitting on the gasket flat?
  • Put the on side of the power valve in your mouth and see if you can blow through it. You shouldn't be able to.
  • While at idle is gas dribbling out of the main discharge? That would indicate a plugged idle vent hole, or a leaking main discharge.
  • There are several air vent passages that will get plugged. Check any passages that leads to the top of the carburetor.

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