Reply To: Carburetor jets

[David Stanton]

I agree – increasing jet size usually doesn’t get you anything, except maybe a performance degradation by running too rich. The snowblower engines where I increased the jet size, I did because they were running too lean, making for very hot combustion that was turning the muffler red. The timing could not be changed, or I would have backed that off. I tried one and two numbered drill sizes bigger. One size reduced the red hot muffler temp to barely noticeable. Two drill sizes and it didn’t run good, so I bought a new jet and went back to one drill size bigger. My local small engine shop said they had a lot of problems with some makes/models with fixed jets and federal law prohibited them from retro-fitting older adjustable carbs on.

I read awhile back an article in the Pontiac collectors club magazine where a person was setting up his car for racing, and didn’t want to buy the expensive high octane racing fuel, but got nearly the same performance by switching to E-85 and increasing jet size by trial and error. Note that today’s cars that can burn E-85 do so by sensors increasing the amount of fuel injected when E-85 is run. I don’t recommend E-85 modification for outboards because the ethanol can cause other problems, and, I don’t believe 85% ethanol will mix well with the 2-stroke oil.

The only other jet size changes I’ve heard of is for running cars at high altitudes – like in Denver, but I believe the jet sizes become smaller for that – please correct me if I’m wrong.

I’m still curious why Mr Hewitt asked about doing this – we may learn something!
Dave