Reply To: 1960 Merc 400 won’t idle

[Robert Bourque]

I really appreciate the reply. The carbs and mag came from the old block that had the bad plugs. They ran fine then. The belt between the flywheel and mag is good. The fuel pump was rebuilt.

As I said, it runs fine with the throttle pushed up to the neutral stop. When I bypass that stop and push the throttle further, it still runs fine. It’s just when I try to drop back to idle, just a little, that it suddenly quits.

I replaced the carbs with another set that were thoroughly cleaned first. I get the same result – no idle.

I suspect air is somehow getting into the new block behind the carbs. The leaning out from that leak will have a bigger effect with partly closed throttles rather than open.

Today I will run it again in the water tank. I will spray starter fluid around the block to see if that affects operation. If it does, I’ve found a leak. I will tape vacuum cleaner hoses to the carbs so the air they bring in with away from the starter fluid. That will also tell me if the reed valves are bad. If they are, the air would just slosh back and forth in the tubes.

Again, thanks. This is still a big mystery that I’m determined to solve.

Bob