Best i can tell the carbs are fairly interchangeable.
Different engines definately have the different types.
Ive noticed that the 4HP have a steel main jet and the the 3.5's have a brass one. Not sure if this is fact or just an impression i got.
I had one with a badly scored needle the other day, and searching through the spare ones i had they all appear to be the same profile.
Nothing makes a rough runnning motor like a needle with a big croove around it from being bottomed out way to hard into the jet.
Short of model and type and checking part numbers theres no way to tell what parts are the same.
A sure bet is, get a tank/carb off a 4Hp and write that on it in marker. Clean it out new diaphragm etc, test it and its a known good carb.
Do the same for a 3.5HP. (not sure if you would need one for a 3Hp as well)
Once you take it off a motor it should be within a bulls roar of being right on the next one should you suspect carb issues.
At $15 a throw i dont like to put new diaphragms in every carb. Clean them out and back together, got caught out the other day with one that looked ok but it wasnt. Holy crap, had no muffler on the the thing, just a lang 1/2 gal elbow. Pumpin black soot, shootin flame and the backfires. Awesome stuff.
