engine stalling

[paceman]

Hey guys I am new to the forum and have an engine question for you. I recently bought a used boat with an 87 40hp johnson. It appears to be a cold starter. Meaning that I have to choke it a bit to keep it running when I first start it. Once I keep it running I fly across the water pretty good, but once I get to my spot whenever I slow down to an idle it dies. It will start right up again with a bit of choking but will not run at an idle. The Motor is in mint condition with very little use from the previous owner. I have messed with the gas can cover with no difference. Is there something I am missing? Can the type of Gas make a difference? I put premium in. How about oil mixture? The manual says 100:1 the previous owner says 50:1.

Any input would be greatly appreciated.

[ChuckN]

Welcome paceman!

I had an '88 Johnson 40 once and that motor never even gave me a hiccup. Great motor. I'd say from your description (motor didn't get used much) your carbs are gummed up.

The carbs need to be thoroughly cleaned, or first try running a good batch of Sea Foam and fresh gas. Give the tank a shock treatment of Sea Foam, say 2+ oz for every gallon.

Replace the spark plugs too.

50:1 gas/oil mixture is what you want. Best to have too much oil than not enough.

For gas, I've actually noticed less performance with 92 octane, especially at idle for some reason. I've always ran 89 and been really happy.

Good luck.

[Gissert]

Good point on the octane, Chuck.

For some reason, there is a belief that high octane means better performance.

All octane is a measure of how resistant a fuel is to premature detonation. On a normal OEM configuration set up to run 89 octane, 92 or 93 is a waste of money. Like you mentioned at idle, your cylinder pressures are lowest, and the high test fuel will not combust as efficiently as it should, causing bogging and plug fouling.

I saw a great example of this snomobiling in the UP a few years ago. We stopped for breakfast at a joint on Lake Gogebic. They had a pump with 100LL av gas. Two of my buddies just had to try it, stating that they were gonna kick my butt since we had similar powered sleds. I tried to warn them, but they filled up at 3 bucks a gallon. They spent the next several hours blubbering out of corners and fouling plugs. Once they put standard pump gas back in to dilute the av gas, they startedc to run better. By the end of the day, they were running good again.