5.0L starts fine on muffs; on water, takes long time to fire; once running, runs well

afurness

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Joined
Jul 9, 2011
Messages
3
Hi Guys,

Summary of this long post:
  1. Bought 1994 Sunesta with Merc 5.0L last August
  2. Boat started and ran great rest of 2011
  3. In 2012, engine will turn over easily, but does not fire. Eventually it will fire if I keep a constant full throttle applied.
  4. Had spark plugs, caps, rotor changed, and timing set and same behavior. Only starts when I crank for a while at full throttle.
  5. Once running, boat has no hesitation, accelerates quickly, runs at any speed, idles well, etc.
  6. HELP!

I have an issue that I would like some help in debugging my 1994 Chaparral Sunesta with a Merc 5.0L. I have some experience with basic maintenance of engines and machines, such as changing fluids, spark plugs, thermostats, etc., but not much diagnosis experience when things are "off".

The issue was first experienced earlier this year. I bought the boat last year in August, and it started and ran beautifully all of last year. I winterized the system myself, and changed the oil, filters, thermostat, impeller, lower unit fluid at that time. I always ran with marine sta-bil in the fuel and stored with about 3/4 of a tank and a winter dose of sta-bil.

The boat starts easily on muffs. It starts nearly immediately (maybe a crank) when I turn the key. However, when I get the boat on the water, it is more difficult to start. My method is the same as was taught to me last year; I pump full throttle in neutral 2-3 times and then put the throttle at the 1/3ish starting position. When I try to start the boat that way, it cranks over well (doesn't seem to be difficult to turn over) but doesn't fire. I get no sputtering or anything. The first time this happened, I tried for about 3-4 ~5 seconds of cranking (with small breaks and pumping between) before it finally started. I assumed that maybe I had old fuel in the carb or something, I ran the boat for a while at varying throttles, and a lot of full throttle, before stopping to go fishing. When I attempted to restart the boat, it did the same thing as before, but was even more reluctant to fire (I first tried without pumping the throttle, as the engine was warm). I removed the flame arrestor and looked in the carb, and I saw fuel spray when I applied the throttle. I only got the boat to start by applying full throttle. Even then, it took a lot of turning over before it fired.

Being busy with work, I took the boat in to have a tune-up and have them look at it. They changed the spark plugs, cap, and rotor and checked timing. They said that nothing looked bad, and when they took it out, it started fine. When I took the boat out the next time, it started up great the first time. Again, I drove around for a while and then stopped. When I went to restart the boat, I had to again apply full throttle and it finally fired up (this happened 2 more times this outing). When I drive the boat, I notice nothing that seems out of whack; it has plenty of get up and go and runs without hesitation at any speed or any acceleration level.

I have lived with this for a little while, but want to fix it to relieve the frustration. Any thoughts of where to look would be greatly appreciated.

Thanks for any help!
-Austen
 

Don S

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Joined
Aug 31, 2004
Messages
62,321
Re: 5.0L starts fine on muffs; on water, takes long time to fire; once running, runs

Your flooding it with all the throttle pumping.

1 pump of the throttle is all it should take to start a cold engine, 2-3 pumps is probably flooding it, and holding the throttle wide open is clearing the flood you created.
 

skydiveD30571

Lieutenant Junior Grade
Joined
Feb 13, 2012
Messages
1,042
Re: 5.0L starts fine on muffs; on water, takes long time to fire; once running, runs

Austen, needing full throttle to get a carbed engine to fire is a classic sign of a too rich condition. When I read that you usually give a couple pumps before starting, I assumed this is why. When the engine is warm (i.e. after you drive around awhile and stop to fish) are you cracking open the throttle? Merc recommends giving it 1/4 throttle to warm start. If you are trying to start it in neutral and it doesn't fire so you give it a couple shots then need to put it at wide open throttle to start, you are clearing the flooded condition you just created by giving it 2 or 3 pumps.

Next time the engine is warm shut her off, wait a little bit like you are fishing, then use the throttle-only button and crack the throttle open about 25% and try it without pumping the throttle. See how that works.
 

ktbarrentine

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Joined
Dec 12, 2011
Messages
1,296
Re: 5.0L starts fine on muffs; on water, takes long time to fire; once running, runs

While you're at it, check the sight-flow tube from the fuel pump to the carb, with the engine running. If you are getting extra fuel into the carb via that hose (flooding), it will make it hard to start when warm.. I had the exact same symptoms and traced it down to a bad diaprhram on my fuel pump.... 1990 vintage 5.0L (Chaparral, too, btw), ethanol fuel side effect....

Anyways, easy to check and rule out.
 

afurness

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Joined
Jul 9, 2011
Messages
3
Re: 5.0L starts fine on muffs; on water, takes long time to fire; once running, runs

:facepalm: Thanks everyone!! I am lean more and more every day with my boat. I will not be so throttle happy with it and see if that makes things better. Also, good idea to check the site tube. Can't hurt with something so simple.
 

afurness

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Joined
Jul 9, 2011
Messages
3
Re: 5.0L starts fine on muffs; on water, takes long time to fire; once running, runs

One other thing that makes me wonder a little is that this is the same thing that I did last year for multiple outings (10 or so, with starts and restarts), and never had an issue... Might just be something slight that changed and now is a little more sensitive. I will try as suggested and see where it gets me.

Thanks again.
 
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