Hope this helps:
Engines that are not tuned/cleaned/adjusted correctly and subsequently have start/run issues can create a problem within the starter-relay itself. The solenoid when activated, pulls down a heavy copper bar across two equally heavy contacts thus completing the high current circuit to the starter motor. Cranking the engine over for "lengthy" intervals in an attempt to get an engine running will create high surface temperatures on the starter-relay contact areas. This leads to arc-pitting, corrosion and high resistance which can eventually result in no power at all to the starter-motor. Intermittent starter operation can be a result of many hits of the starter button which will literally "hammer" the corrosion/pitting down enough to make electrical continuity. However, with the contacts surfaces now already damaged, corrosion and further pitting will continue to create intermittent(and high resistance)starter operation to a point where electrical continuity is no longer possible no matter how many times the starter-button is pushed. I had this exact problem many years ago. Eventually the relay would activate (click) but no power to the starter-motor.
(The starter-relay is easy to disassemble.) I cleaned the damaged contact surfaces and "brazed" some silver (high conductivity-low electrical resistance/better anti-corrosion properties)to these same areas. I filed and polished the (new silver) contact areas flat and smooth and this "fix" has lasted about ten years without any further problems. On the other-hand I have no Carb/fuel/ignition-system issues so the solenoids internal-contact time from "push to run" is merely 1 to 2 seconds. This is not significant time to build any damaging heat or related problems to the contacts of the relay.
As a (unique?) foot-note to electrical-switch issues: 1980 - Mt. St. Helens !!!!! Volcanic-ash was a real bitch for almost every one of my switches contacts. Weeks after the event I started having problems. To the naked eye everything appeared clean, shiny and electrically viable but to the electrons it was a "no go". The Volcanic particulate was so fine that (apparently) even the slightest current (arc)) melted it which coated the contacts with a microscopic layer of "glass" which as we all know is an insulator.....Thus creating a no-contact situation. It took several complete dis-assembly and cleanings to finally rid all the switches and housings of this foreign material.
My personal experience over-all has shown that the slightest corrosion and/or contaminate (grease-oil-coolant) between electrical contacts and connections increases electrical resistance potentially to a point of inadequate to NO current flow.
