Small Engines (Lawn Mowers, etc.): Tuning a Sumec Carb for Altitude, air fuel ratio, hp generator

Question
My father recently bought a Coleman ProForce Powermate 2500 5.5 HP generator for use at his cabin. The 11HP B&S he had before was a heavy pull and proved to be difficult for him to start. The new generator starts and runs fine at lower elevation, but at about 9500 Ft. it will run for a short time and then pulse from run to stall and back again. My assumption is that it needs to adjusted to run a little richer, but this thing does not appear to have an adjustment screw for mix. There is one spot on the carb near the top that appears to have a brass plug and I wonder if there is a mix screw hiding behind it. My question is this: should I try to find a different carburetor for this thing or is there a way to adjust it? The user manual is garbage, and I can't seem to find anything on line. I hate Chinese stuff...
Ben

Answer
Ben
The optimal air:fuel ratio is around 14:1. This must be lower for a cold engine and thus a choke plate or other means to increase the richness of the mixture is usually provided. A choke plate restricts air intake forcing more gas to be sucked into the cylinder. A primer bulb effectively squirts gas into the intake pipe to augment the normal carburetor action. Some carburetors have no choke and no primer but incorporate a small gas reservoir which fills when the engine is off and provides some extra when starting.
To much air results in a mixture that is too lean, burns too quickly, and can result in engine damage over extended periods of operation.


Check that any choke is not stuck in the open position and not doing its job.

The carburetor may need adjustment or cleaning.
Too little air results in a mixture that is too rich - there will be loss of power and possibly black smoke from the exhaust. This could be due to several factors:


Check the air filter. For testing, it can usually be removed to see if the engine will start. However, do not run it for an extended period of time without a properly functioning air filter in place. Some are designed to be washed and reused while others must have their elements replaced.

Check that any choke is not stuck closed. Though needed to start cold, if the choke remains closed, the engine will not restart and will quickly stop (truly choke!) due to an overly rich mixture.

A defective carburetor may also cause the mixture to be too rich or too lean.
The adjustments are not made on the needel but with the choke plate