engine sputtering

dgs119 · **Posted:** Sun Aug 04, 2013 8:06 pm

Hello my engine is sputtering when I go to turn it off. It is like it doesn't want to shut down. Any ideas?

Toddely · **Posted:** Sun Aug 04, 2013 8:12 pm

Add ethanol treatment, Mine does the same thing, I add the treatment everytime if get fuel and it stops. Sometimes you will get this effect if you are running hard/ pulling tuber or skier and then quickly shutting it down then turning it off. Its a pretty common problem.

LouC · **Posted:** Sun Aug 04, 2013 9:22 pm

Idle too high, carbon build up in cylinders, spark plug heat range too hot, fuel octane too low.
I had heard that some carbureted 4.3s tend to do this but mine (with a Rochester Quadarjet) never has dieseled (run on when shutting off). I do run premium gas all the time (all they sell at the gas docks is 93 octane and I've never had either detonation or dieseling problems.
You want to get that fixed because when an engine diesels at shut down it can suck water up the exhaust into the cylinders (not good can hydrolock the engine).

dgs119 · **Posted:** Sun Aug 04, 2013 9:51 pm

Thanks. I will add ethanol treatment. Should I add octane boost also? Thoughts

pickledboater · **Posted:** Sun Aug 04, 2013 10:50 pm

Mine is prone to do the same thing. I find that letting it idle in neutral for 30 seconds or so before shutting down helps a lot.

deafwish · **Posted:** Mon Aug 05, 2013 3:43 am

LouC wrote:

Idle too high, carbon build up in cylinders, spark plug heat range too hot, fuel octane too low.
I do run premium gas all the time (all they sell at the gas docks is 93 octane and I've never had either detonation or dieseling problems.

Excellent response!

Lean runaways can be a frightening thing on an engine & they're generally something that lives with you forever! Lol! :lol:

Amazingly, opening the throttle or choking the intake (only with a manual choke) will shut it down, fast.
OP: What octane fuel do you run?

ric · **Posted:** Mon Aug 05, 2013 12:35 pm

First world carb issues. In a world of fuel injection people forget this wasn't all that rare 30 years ago.

What's happening is there's a hot spot in the combustion chamber igniting the fuel, and since carburetors don't care if the key is on or off, they will continue to feed the engine air/fuel to run.

I would first try to cure whatever is causing the hot spot: usually the wrong heat range spark plug. Go down a number on them, and make sure they're gapped correctly.

My buddy recently changed the plugs in his old 80's ski boat and didn't gap em and loved running around at full throttle. $1500 and a new engine later he gaps his plugs now.

dgs119 · **Posted:** Mon Aug 05, 2013 9:54 pm

Hello I put 87 octane on the last fill up. It sounds like I should have put premium. Should I add octane boost. If so any suggestions on brand? Thanks

ric · **Posted:** Mon Aug 05, 2013 11:44 pm

dgs119 wrote:

Hello I put 87 octane on the last fill up. It sounds like I should have put premium. Should I add octane boost. If so any suggestions on brand? Thanks

No it doesn't need premium. It might actually hurt performance. When was the last time you've done a tune up?

Toddely · **Posted:** Tue Aug 06, 2013 7:35 am

Wait,, premium gas can hurt performance in a boat? I thought it would help it run better? Just trying to figure it out , Getting ready to fill the boat up tonight. Wanna know what i'm putting in her.

ric · **Posted:** Tue Aug 06, 2013 12:06 pm

Premium gas isn't "premium". It's higher octane gas. What that means is, is that the higher the octane fuel, the more you can compress the fuel before it self ignites (it raises the ignition temperature). It has nothing to do with it being a more "powerful" gas, because it's not. In an engine designed for 87 octane, using 93 octane can lower the horsepower because of those effects. It doesn't get really noticeable until you start putting 100 octane in an old engine like what's in our boats. They get really hesitant.

It doesn't "clean" anything out. The only way premium fuel could "give" you more power would be to advance your ignition since the fuel doesn't ignite as fast as the lower octane fuel. Advanced ignition is what gives you the power, not the gas.

Putting 93 octane in our boats is a waste of money.

LouC · **Posted:** Tue Aug 06, 2013 1:04 pm

Unless you have an old boat with carbon build up and then the octane requirement of the engine (maybe designed for 87 or 89) has now increased. Also running E-10 fuels tends to lean out the mixture which also exacerbates this problem, leaner mixtures make engines more prone to spark knock, detonation and run on. If you run 87 and your engine never pings and never runs on it is OK. If it does ping (harder to hear on a boat) then you can go up a grade or two. Older boats are very prone to this because they are set to run rich and they run cool which makes the chokes stay engaged longer than really needed (seen this on my Q-Jet even with a new choke spring). Newer boats with EFI are not as prone because of the better fuel mixture control.
I've run 93 in my boat the whole time I've had it (11 years) and it was 14 years old when I got it. No ill effects other than more $$.

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