Problem solving guru's!! My 75 70 HP Evinrude seems to have issues with idle. I replaced 2 cracked coils. Replaced the water pump impeller. The previous owner said the carburetors were rebuilt. It is as though the motor is loading up due to loss of spark at idle. Never stalls. Starts easily. Please watch the videos and chime in! I just do not want to chase too many rabbits on this old motor. Cream puff. Never seen salt. Hopefully its nothing major
Check for spark at idle. You can use a timing light for that. It'll be bright enough to see on the water. You should be idling in gear to simulate the load. Or idle in a barrel if you have the facilities, but in the drink is better.
Don't assume the P.O. (or whoever serviced the motor) knew how to work on the carbs, they may have been improperly assembled. Here's a diagram:
#27 in the diagram is the idle jet (aka "orifice"), .030". #28 is the optional size, .033", which I'd assume would give a richer mixture. You can pull the idle passage screw (#26) and washer, then the idle jet just unscrews all the way out of the carb. Take care to use the correct-sized flat-blade screwdriver and use caution when unscrewing, so as not to damage the brass jet.
Pull the airbox lid and spray pre-mix into a carb throat while idling in gear, and see what happens. If that cyl picks up, it's not getting enough fuel. If it dies out, then recovers, it's probably ok. If it does nothing, you got problems!
You may have to pull the carbs and check for proper assembly; leaking float needle, bad float height, missing gaskets (like #39 "nozzle" gasket; seals the float bowl & emulsion tube and if missing will cause Big Time running problems!), etc.
Leaking or bad reed valves can cause issues, place a mirror (or equivalent bright shiny object) near the carb throat and if the carb is spitting fuel on the mirror you'll see it. That cyl has a reed valve sealing problem. You can pull the carbs & remove the entire reed block assy on this powerhead. Lucky it's not an old Merc!!
These don't idle well if cylinders are worn. Check compression but better yet do a leakdown test. Relatively inexpensive leakdown testers are available or you can make your own.
If a compression check looks good (all cylinders within 5 psi of each other would give good confidence), I wouldn't spring for a leakdown tester unless you've exhausted all other possibilities. If the compression is off quite a bit, it's not gonna idle well regardless.