4.3 diesling

guy48065 · Thursday at 10:24 PM

2000 Bayliner Capri 1850LX
4.3L carbureted engine, A1 G2 outdrive.
Only had this boat for 10 days so far. Starts right up, runs smooth... In fact during the test drive I was surprised to see a carb & a distributer on it.
No hour meter so I have no idea how many hours. It's pretty clean & this being Michigan--likely lower hours than average for her age.
My only quibble so far is diesling on shutdown. I'm old but I've never had to deal with this issue on old cars. What should I be looking into here?

Mc Tool · Friday at 3:24 AM

1st thing to do would be to pull the spark plugs and make sure they are not carboned up and that they are the right ones

Scott Danforth · Friday at 4:38 AM

dieseling is caused by a few things

running rich, causing carbon buildup
timing being too far advanced
motor running hot

check timing., use an IR thermometer and check temp

also, if you shut them down hot (just coming in from a long run), they will diesel. let it idle for a few minutes.

cyclops222 · Friday at 5:36 AM

A choke that does not fully open. When engine is fully warmed up.. Do check ALL the sparkplugs for black gook. Also if they are the correct part number. and correct gap.

guy48065 · Friday at 6:52 AM

Scott Danforth said:
...also, if you shut them down hot (just coming in from a long run), they will diesel. let it idle for a few minutes.

It might just be that. It's normal?
It doesn't diesel after I idle back to my dock. Only out on the lake when I shut it down after cruising awhile, or running hard.

kenny nunez · Friday at 6:58 AM

Also check that the throttle closes all the way and the idle speed is at the correct rpm.

cyclops222 · Saturday at 5:57 AM

H switched to Marine Hi Test gasoline at the commercial work boat dock. Never anymore after run on a 2002 Mercruiser 5.0L Carburetor engine.

Mc Tool · Saturday at 4:09 PM

Just a thought but if your engine has projected nose spark plugs maybe change them for normal ones ......after checking out all the good ideas above .
Im thinking something is staying hot enough to cause compression ignition if you dont idle it down , and the most likely thing is the earth electrode on the plug and by pulling it back out of the combustion chamber a bit it might help .

Jmunk · Saturday at 9:19 PM

My old boat with a 4.3 would diesel if I ran it mildly and shut it down shortly after coming to idle speed. My fix - run premium gas or if it was easy to get at a fill-up ethanol free.

Pmt133 · Sunday at 3:18 AM

I haven't experienced dieseling yet... I am only running 89 octane too. My 3.0l used to do it occasionally when it was ungodly hot out though.

Lou C · Sunday at 4:11 AM

Ive always run mine on 89 or higher octane & it’s never dieseled….

Bt Doctur · Sunday at 8:31 AM

I tun 87 with a touch of octane boost, all readings normal ,temp is constant at 145 and after long continuous running I slow to idle speed and let the motor get rid of all that hot water by letting it idle for about 10 min. never any dieseling at shutoff

Mc Tool · Sunday at 2:47 PM

How do you guys measure octane ron or mon ? just that we have 91 , 96 and 100+ octane . My Capri sounds like a diesel on 91 , bearable on 96 and loves 100+ .My 07 toyo will run on 91 but there is a noticeable drop in power , I think the ecu must pull a bit of advance out of it .
I use the 100+ for any 2/ mix because apparently it doesnt go off so quickly .

Scott Danforth · Sunday at 3:48 PM

Here in the US we use AKI (Anti Knock Index) which is the average or RON and MON or (R+M)/2

Mc Tool · Sunday at 8:27 PM

Scott Danforth said:
Here in the US we use AKI (Anti Knock Index) which is the average or RON and MON or (R+M)/2

Thank you Scott , makes more sense now

guy48065 · Sunday at 9:02 PM

I'm now stopping & letting it idle for a minute before shutting off. No dieseling.
I'm running ordinary 87 or 89 pump gas. No marina on my 600 ac lake and not many stations in a reasonable distance. It's ½hr drive to closest station with non-ethanol gas but it's $1+ more per gallon and I see little benefit to using it in a 4-stroke Chevy engine.

cyclops222 · 2025-08-11T05:33:58-0600

You do want to remove any carbon spots in the combustion chamber. Seafoam or other SLOW carbon disolvers would be a big help. Have you checked your sparkplugs for hard black deposits ?

Mc Tool · 2025-08-11T14:14:51-0600

guy48065 said:
I'm now stopping & letting it idle for a minute before shutting off. No dieseling.
I'm running ordinary 87 or 89 pump gas. No marina on my 600 ac lake and not many stations in a reasonable distance. It's ½hr drive to closest station with non-ethanol gas but it's $1+ more per gallon and I see little benefit to using it in a 4-stroke Chevy engine.

Generally I try to let engines idle down a bit , specially if it had a caning . I remember watching Bathurst 1000 and when the cars came in if they stalled they would pretty much instantly lunch the head gasket . Im talking ford 2 litre turbo YB cosworth turbo motors putting out about 500 hp but I guess the idea is the same

jimmbo · 2025-08-11T14:23:31-0600

I found Retarded Timing to increase the Chance of Dieseling, as the Throttle is opened more to maintain Idle Speed, and the Retarded Timing causes the Engine to run Hotter. As mentioned above, Deposits on the Piston, the Head can create Hot Spots that can ignite a Combustible Mix
While running, using Squeeze Bottle pour some Water into the Air Horn, that can often decarbon a Motor. Don't feed the Water too fast cause you don't to Hydrolock the Motor

guy48065 · 2025-08-11T17:18:51-0600

jimmbo said:
I found Retarded Timing to increase the Chance of Dieseling, as the Throttle is opened more to maintain Idle Speed, and the Retarded Timing causes the Engine to run Hotter. As mentioned above, Deposits on the Piston, the Head can create Hot Spots that can ignite a Combustible Mix
While running, using Squeeze Bottle pour some Water into the Air Horn, that can often decarbon a Motor. Don't feed the Water too fast cause you don't to Hydrolock the Motor

May I practice this on your car first?

4.3 diesling

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