Volva Penta 5.7 "Dieseling" after shutdown, sucking in water

G

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I have a buddy of mine has a 1998 Volvo Penta 5.7 GS SX, He rebuilt the engine, stepped up the compression a bit.

The engine has been is service for a year with no issues, and runs great. (I also know that stepping up the compression increases your risk of water ingestion.)

Recently, after winter storage, he launched the boat and now she has a habit of "dieseling" after he shuts her down.

The engine has sucked water into the cylinders as a result. Forunately , he's on a lake, and removed the plugs and blew out the water. The engine is fine.

My question is, what causes the dieseling, and what to look for to prevent it?

Anyone with experience with this problem, I'd sure like to hear from you.

Thanks!
 
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alldodge

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Dieseling is caused by motor running rich (carbon builds up in combustion chamber), incorrect idle and or timing, wrong spark plugs

Reversion is caused by increased valve over lap (cam), increased compression has no effect
 

Scott Danforth

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Dieseling or run-on is caused by the items that AD posted. that little cough at the end is the motor actually running backwards and intaking from the exhaust and blowing out the carb.

running rich does two things. first it carbons up the combustion chamber. the carbon then glows cherry red (like a glow plug in a cox airplane motor). as the motor ignition is turned off and dieseling (compression ignition ignited by the hot carbon), it is also drawing fuel from the carb, which is why some motors will diesel/run-on for minutes. however as the motor momentum slows, the diesel combustion event can happen at the moment to then fire the motor backwards for a few revolutions or more. then the motor is basically sucking water from the wet exhaust.

increased compression coupled with a cam on the edge with overlap with increased base timing will lead to reversion. at least it did with my BBC. I was running 14 BTDC, ended up having to re-curve the dizzy to have less base timing below 1200 RPM (base of 8 BTDC)

On your bud's GS, what is the base timing? stock carb, or did he re-jet?

Verify timing is correct, and verify fuel settings for idle (read the plugs)

One of the things I used to do with my old 3.0 which would diesel if I was pulling skiers was to shut off in gear.
 
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Scotty,
All I can tell you is he bolted up an 850 double pumper on an Edelbrock mid-rise. He then had the engine installed at the glass shop , where his transom was reworked to ft the Volvo. Thier in-house mechanic got it running, timed it and tuned it.
Ran like a top all last year (his first year with it)

He did report that after the reversion, one of his spark plugs "crumbled" after he removed it.

Has anyone ever heard of that?
 

alldodge

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Don't hear about that very often, but take a real hot plug and splash some water on it, things can happen
 

Scott Danforth

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Scotty,
All I can tell you is he bolted up an 850 double pumper on an Edelbrock mid-rise. He then had the engine installed at the glass shop , where his transom was reworked to ft the Volvo. Thier in-house mechanic got it running, timed it and tuned it.
Ran like a top all last year (his first year with it)

He did report that after the reversion, one of his spark plugs "crumbled" after he removed it.

Has anyone ever heard of that?

an 850 double pumper is what they bolt on big blocks with over 420hp.

largest carb on a small block from VP ever was a 715CFM with vacuum secondaries - on the AQ271s and the AQ310s. (and I am using it on a big block). after that they dropped down to a 650 CSM

a 350 in a boat cant use more than 650 CFM (most get by with 600 CFM or less)

my guess with a 850 Double Pumper is the thing is running super rich, and carboning up the combustion chambers thick enough you would need a chisel to get thru it.
 

Grub54891

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I agree with Scott. Seen it happen to much over the years, they put huge carbs on motors that can't handle that much fuel. Then wonder why they have problems. Can't tell ya how many I've switched back to Quadra jets, tuned properly and viola! Issue solved.
 

Scott Danforth

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Doesnt matter if the carb is a Holley or Webber, or Rochester. It has to be matched to the application

Eric, if you or your bud wants to swap carbs, my 468 could use an 850 as the 715 is limiting the BBC to 6000 rpm
 
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