1975 Mercury 110 (9.8 hp) Ser # 4248296

by John Schmitt
(Columbia Heights, MN)

Issue: Won't hold prime while running.

Quick history. Free motor but it hasn't run in 20 years. Checked and had good spark. Ran on non oxy fuel for a short while but lost prime and died. Primer gets hard, set on full choke, coughs on first pull, take off choke, starts and runs fine for a little while. At idle or cruise speed, it runs fine, but eventually loses prime. If I periodically pump the primer bulb, it will keep running fine.

I got two tanks with this and one fuel line with a primer bulb. Because the lines were so brittle, I replaced all the fuel line lines. Primer bulb was fine. Used the same tank and line for all my testing.

Here is what I have done so far.

I put a complete carb kit in it, including the diaphragm kit, and I used the “updated” brown check valve gasket and not black one. This did not resolve the issue.

I have a 1981 20 hp Merc with the same pump on the side of the carb that I recently rebuilt, and the 20 hp runs fine. I took the pump off the 20 hp and installed on the 9.8 hp. Still losing prime. Put back on the 20 hp and it pumps and runs fine.

Are there any mechanical related issues that could cause the engine not to pull a good enough vacuum to keep the pump from operating correctly? I haven't checked compression yet, because this engine idles and runs so smoothly, but if someone knows what the minimum compression should be, I will do a compression test on both cylinders.

Other than compression, is there anything else I should be looking for? This is a nice outboard, and I am not ready to give up on it.

Any suggestions would be appreciated.

Sincerely, John