Nikki carb OE4215 jet size for Generac 27hp

LMPPLUS said:

Wouldn't the jets being loose create the over fueling problem? With many Nikki carbs the main jets are held in place by the mini o-ring when they get hard the flow of fuel moving thru the nozzle will actually lift the jet out of place and more fuel flows around the bad o-ring causing the over fueling.

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Auto Doc's said:

Hello T,

Going a little rich will keep the cylinder temperatures down.

A basic rule to remember is fuel cools down and air heats up the combustion process.

The ideal scientific mixture is 14.7 parts air to 1 part gasoline. (Stoichiometric mixture)

Don't "hog out" your jets dramatically. A .002 change above what you currently have may be all that is needed.

When people use the term "rich mixture" it means there is more fuel in the mixture than can be ignited during the combustion cycle. It does not create damage, but it does carbon foul the spark plugs quicker.

The term "lean mixture" is too little fuel, and it burns super-hot. That super heating creates damage over a period of time.


"Reading" the spark plugs is something I learned as a kid (and later riding iron head Harley's) when I was starting out, and it applies to all spark combustion engines. Here are some examples:

Too Rich= The plug electrode end will be very dark tan or black and produce black smoke when it runs.

Too Lean= The plug electrode end will be white or near white, and engine will get very hot quickly and "spark knock" from fuel mixture self-ignition will happen. No smoke just high heat and potential for damages.

Ideal mixture= The plug electrode end will be a faint light tan to sand color. Engine will only lightly puff a little smoke during a hard acceleration. At a steady run, no smoke.

Too much oil= A shiny, wet black plug electrode end and blue smoke when it does run.

I will also try to read the screw in jet numbers in my Generac 30HP generator parts carburetor if you need me to.

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StarTech said:

Yes normally they do but in the OP case the jets were completely missing as apparently someone else had already been in the carburetor. Now he was trying what the correct jets are size wise as Generac does not provide them, just complete carburetors.

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RevB said:

Too rich means you're washing down the cylinder with excess fuel. Too rich also means excess carbon build up and that is reflected in the deposits and the really black oil that results.

Too lean does not burn hotter as the highest temps are achieved rich of peak temps, not lean of peak. Lean is better for the engine. Less deposits, cleaner oil. You can cool with fuel or with air. Which one is cheaper.

https://www.savvyaviation.com/wp-content/uploads/articles_eaa/EAA_2012-12_red-box-red-fin.pdf

Yeah....these aren't aircraft engines. But the principles remain exactly the same for all gasoline internal combustion four stroke engines. If too lean "burns" valves and destroys engines the lean burn Hondas would have littered every road.

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Auto Doc's said:

Hello RevB,

Gasoline aircraft engines do have the same operating principles as automotive and small engines.

The big difference is that most automotive engines from the early 70's to present use an EGR system and/or specialized computer timing and fuel injection controls to prevent excess in cylinder temperatures.

The optimal fuel ratio for any gasoline engine is 14.7 parts (grams) of air to 1 part (gram) gas. (Stoichiometric)

Fuel injection and timing modules for larger mower engines are gradually being introduced, but they are going through some "growing pains" due to environmental impacts and component reliability. Along with that is the added expense of digital components to make them function. New equipment cannot take near the neglect and abuse of the older equipment and still be made to run economically.

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