would a 20mm carb on a 72cc make it more or less reliable?

[student]

I heard that a slight excess amount of fuel helps cool the engine, if a 20mm carb upgrade works by adding more fuel/air I guess that implies that a 18mm carb can't fill the bore enough with fuel/air to fully fill/ cool the bore and piston, but wouldn't more fuel/air also increase overall power so would this be an upgrade that improves or decreases reliability? 

[GrumpyUnk]

Generally speaking, an under-carbureted engine will last longer than one that has a larger carburetor which flows more fuel:air mixture. The larger will also develop more power. More power puts more load on the internal components, and by general physics, apply more stress which generally leads to more failure given equivalent components. The additional fuel:air mixture would be either rich or lean or stoichemetric. A rich mixture in this case would leave excess HC in the exhaust, and likely cause a build-up of carbon on the piston crown, the valves, and the inner surface of the cylinder head. All that will insulate against heat transfer... to the fins... but also to the parts. But it could cause higher compression, could cause pinging due to glowing carbon particles lighting off the mixture before the spark fires.
In general, run at the appropriate mix, and feed it as much as you want. In general(as usual) a bigger carb won't have a huge effect on longevity unless the engine is run at WOT for long periods of time. An example is a chainsaw engine. It is NOT run at WOT all the time by most homeowners. But is it run at WOT when cutting. It then rests as the wood is arranged, as the operator moves along, and as the cuts fall and so on. It gets rest periods. Without those rest periods, it may get too hot to operate properly long term. I.e., melting the piston, cylinder, etc. A 2T will take extra fuel more readily than a 4T in that it uses the fuel:air mix to ALSO lubricate the moving bits and pieces. The 4T uses oil in the pan/sump to lubricate AND carry heat away from the moving parts, diffusing the heat into the rest of the lubricant, and radiating from the pan. (that's why they add finnage to transmission oil pans).
If you want longevity, don't do anything to increase power. A 39mm bore will last a long time as it keeps the stress least(50cc). 63cc and 72cc bores add more stress respectively, along with a higher likelihood of failure. But most handle the 63 & 72cc upgrades without problems.
tom

[Petro]

Apr 9, 2022 11:34:32 GMT -5 student said:

I heard that a slight excess amount of fuel helps cool the engine, if a 20mm carb upgrade works by adding more fuel/air I guess that implies that a 18mm carb can't fill the bore enough with fuel/air to fully fill/ cool the bore and piston, but wouldn't more fuel/air also increase overall power so would this be an upgrade that improves or decreases reliability? 

If You need to cool the engine with the fuel You add more of it with a larger main jet. A larger carb will not cool the way You talk about since You adding more fuel AND air, the opposite actually. It will how ever increase overall power but my experience is that one looses torque at low RPM.
Larger carb is an upgrade if one want to go faster with high RPM but decreases reliability in the long run.
Personal opinions!