Odd problem with Mercruiser 3.0L

[ezjam]

First, thanx to all who respond.

I have a very odd problem with my 1994 Bayliner with a Mercruiser 3.0L. It is a four piston engine with EST ignition system.

At any rate, here are the symptoms I'm experiencing..

Engine starts right up every single time from cold. If I never accelerate and leaving idling, it will shut down and start right up ever single time. So far so good. The oddity starts when I push the tachometer to 3000 RPM for a few minutes. After I do that, when I bring it back to idle the following will happen...

1. It might run rough.
2. It might die after running rough for about 30 seconds.
3. If I kill the engine, it refuses to die and will chug along for a while. When it finally dies, it does so roughly.
4. When I try to restart, it has a very hard time starting.

After further investigation, I have found that the carb is leaking gasoline into the chambers but only after I have run it at high RPM for a while. It will not do this if I do not run at high RPM regardless of how long the engine is run at idle.

At first I was suspecting a timing problem. But after finding out the gasoline leak, I guessed that the needle and seat in my carb were bad so I replaced them. No joy. So now I'm thinking the float angle might be off and there is just too much gasoline in the carb chamber. Just a guess though.

Anybody have any ideas what the angle of the float should be? Should I just play with different angles?

Any other suggestions?

ezjam
3.

 

[Bondo]

Re: Odd problem with Mercruiser 3.0L

Should I just play with different angles?

No,.... You should get a Carb Kit,+ Rebuild it.......

 

[ezjam]

Re: Odd problem with Mercruiser 3.0L

No,.... You should get a Carb Kit,+ Rebuild it.......

Thanx for your reply.

I forgot to mention that the previous owner rebult the carb with a kit back in January. So this has already been done.

ez

 

[Zackman]

Re: Odd problem with Mercruiser 3.0L

Thanx for your reply.

I forgot to mention that the previous owner rebult the carb with a kit back in January. So this has already been done.

ez

It sounds like it probably wasn't done correctly.

 

[jamyers]

Re: Odd problem with Mercruiser 3.0L

It sounds like it probably wasn't done correctly.

Yup, lots of people "rebuild" carbs - but apparently using rocks and rubber bands instead of screwdrivers and rulers.

(matter of fact, I haven't seen a "remanufactured" carb come out of a parts store in ten years that wasn't a complete wreck out of the box. )

Take your time, and be obsessive about measuring everything. Twice.

 

[ezjam]

Re: Odd problem with Mercruiser 3.0L

Yup, lots of people "rebuild" carbs - but apparently using rocks and rubber bands instead of screwdrivers and rulers.

(matter of fact, I haven't seen a "remanufactured" carb come out of a parts store in ten years that wasn't a complete wreck out of the box. )

Take your time, and be obsessive about measuring everything. Twice.

Thanx for the suggestion. My biggest problem though is that I have never rebuilt a carb and I have heard horror stories. Even though I am very mechanically inclined, I have some reservations about tackling something that complicated.

Do the carb kits come with detailed intructions on how to rebuild?

ez

 

[jamyers]

Re: Odd problem with Mercruiser 3.0L

Thanx for the suggestion. My biggest problem though is that I have never rebuilt a carb and I have heard horror stories. Even though I am very mechanically inclined, I have some reservations about tackling something that complicated.

Do the carb kits come with detailed intructions on how to rebuild?

ez

Carbs (especially the Rochester 2-bbl, one of the simplest carbs ever) aren't all that hard, as long as you take your time, lay out all the parts as they come off, and don't force anything. I can have my 2-bbl completely apart in about 30 minutes (but I've got practice). You ought to be able to rebuild yours in a couple of hours. Basic hand tools, some carb cleaner and rags, and a good clean flat place to work is all you'll need. (I've put newspapers down on the kitchen table - but then my wife is very understanding.)

Most kit instructions don't have a step-by-step, but don't worry. All you're doing it taking it apart, cleaning it *completely* (cleanliness IS godliness with carbs), replacing the float valve and its seat, a couple of checkballs, the fuel accelerator pump, and all the gaskets. Most kits cover a number of carbs, so there'll be some extra parts you won't need.

What the kit instructions DO have is the adjustment specs, and how to set them. Float height, choke rod length, etc. Just follow them and you'll be set.

Seriously, it's not that hard as long as you take your time, get everything CLEAN, and double-check all of the settings.

 

[ezjam]

Re: Odd problem with Mercruiser 3.0L

Seriously, it's not that hard as long as you take your time, get everything CLEAN, and double-check all of the settings.

Thanx, now for the million dollar question... what are the odds that this is a bad carb and a carb kit will not fix my problem. Given that a shop had already done a carb kit on it, I would think there is a small chance that the carb itself is faulty. Would you not think so?

ez

 

[jamyers]

Re: Odd problem with Mercruiser 3.0L

...The oddity starts when I push the tachometer to 3000 RPM for a few minutes. After I do that, when I bring it back to idle the following will happen...

1. It might run rough.
2. It might die after running rough for about 30 seconds.
3. If I kill the engine, it refuses to die and will chug along for a while. When it finally dies, it does so roughly.
4. When I try to restart, it has a very hard time starting.

After further investigation, I have found that the carb is leaking gasoline into the chambers but only after I have run it at high RPM for a while. It will not do this if I do not run at high RPM regardless of how long the engine is run at idle.
...

Looking back at your symptoms, it sure sounds like it's flooding out after the pump gets up to full speed / pressure.

The only thing that doesn't fit with that is #3, which is usually caused by an idle that's too high - the throttle plates aren't closed enough, allowing air (and fuel) to be drawn in and ignited by the heat of the combustion chamber (dieseling).

I'm going to say that either there is some trash in the float valve / seat, keeping the float from closing completely, or the float is too high, or that the float valve and seat are the wrong ones for your carb (there are different sizes).

Plus you might double-check the idle speed setting.

Not knowing who did the carb rebuild, I'd say that there's a better than 80% chance that they did it wrong. There just aren't enough carbs around anymore for most shops to pay attention to doing them right - unless they're a shop that specializes in carbs, and even then I'm skeptical.

Actually, since your carb is flooding out, I'd say it's a 95% chance that they did it wrong. The other 5% I'll allow for some trash in the float valve.

 

[ezjam]

Re: Odd problem with Mercruiser 3.0L

I'm going to say that either there is some trash in the float valve / seat, keeping the float from closing completely, or the float is too high, or that the float valve and seat are the wrong ones for your carb (there are different sizes).

Plus you might double-check the idle speed setting.

Since I started this post, I've replaced the seat/needle with the original one since the new one had no effect.

I will now do the following...

1. Make sure there is no debris in there.
2. Lower the float a bit. (Wish I had some guidelines rather than guessing amount.)
3. Not sure what you mean by idle speed setting. Currently when it is happily idling, it does so at 950 RPM. When it idles rough, it does so at about 500 to 700 RPM.

ez

 

[jamyers]

Re: Odd problem with Mercruiser 3.0L

Since I started this post, I've replaced the seat/needle with the original one since the new one had no effect.

I will now do the following...

1. Make sure there is no debris in there.
2. Lower the float a bit. (Wish I had some guidelines rather than guessing amount.)
3. Not sure what you mean by idle speed setting. Currently when it is happily idling, it does so at 950 RPM. When it idles rough, it does so at about 500 to 700 RPM.

ez

I'll dig around this weekend and see if I've still got the float height settings from my rebuild kit.

That idle seems high...mine calls for a 650-rpm idle (but it's an OMC 2.5 liter).
(Most any well-tuned engine ought to be able to idle down to 400-450 before it simply dies.)

 

[Don S]

Re: Odd problem with Mercruiser 3.0L

Actually, your problem is not that uncommon with a 3.0L engine. Your biggest problem is the "Dieseling" or "Run-On" after you shut it off. When that happens water gets sucked back into the exhaust and into the engine. That in itself is hard on an engine.
You probably need a tuneup, and a compression check just to make sure everything is ok. Then get the idle down where it belongs. Trying to shut off a 3.0L at anything over 750 rpm will cause it to diesel.
Also, here is a link to a service manual for your engine. USE it.
http://www.boatfix.com/merc/Servmanl/13/13covr2.pdf

Also, here is a copy and past of a Mercruiser service bulletin on dieseling.


No. 97-17 = Revised July 1999
Engine ‘Dieseling’ or ‘Running-On’
Models
MCM 4 Cylinder, V6 and V8 with Carburetors.
Situation
Under certain conditions, carbureted engines may ‘run-on’ when the ignition key is turned off. Normally, it
will take a combination of several of the following conditions to make the engine ‘run on’. These conditions
include but are not limited to:
1. Type and formulation of gasoline available in the area.
2. Type of boat and its engine compartment ventilation system.
3. Outside air, inside engine compartment air and engine coolant temperatures.
4. How quickly the engine is shut off after running at cruising or higher RPMs.
5. How the boat and engine are operated.
6. Engine idle rpm set too high.
7. Wrong heat range spark plugs or spark plugs with deposits on them.
8. Deposits in combustion chamber.
Carbureted engines, unlike EFI, do not shut off the fuel supply to the engine when the ignition key is turned
off. As the engine is coasting to a stop, air is still flowing through the carburetor into the manifold. This air
flow can still draw fuel from the carburetor, through the intake manifold and into the combustion chamber.
A hot spot in the combustion chamber will cause this fuel/air mixture to burn. The engine now ‘diesels’ or
‘runs on’ although the ignition system is not firing. EFI engines do not have this problem because the injectors
do not inject fuel once the ignition key is turned off.

Suggestion to Minimize ‘Run On’
Items number 1 ,2 and 3 can correct or reduce most of these ‘run on’ problems. Letting any engine idle a
minute or two before shutting it off is desirable. This helps the engine to normalize at idle rpm before being
shut down.
1. Be sure engine idle mixture, idle rpm and ignition timing are correct. On 4 cylinder and V6 or V8 engines
with Thunderbolt IV ignition, setting idle rpm as low as possible can help stop ‘run on’.
2. Adjust throttle cable barrel so that the carburetor’s throttle lever returns against the idle speed stop
screw every time the remote control throttle lever is returned to neutral, idle.
3. Idle engine 1 to 1-1/2 minutes before turning key off after running at cruise or higher engine rpm.
4. Try a different brand of regular fuel or try some premium fuel.
5. Increase idle mixture by 1/8 turn ‘richer’.
6. Go to next colder heat range spark plugs.
7. V6 and V8 only: Install a 140 ?F(60 ?C) thermostat.

 

[ezjam]

Re: Odd problem with Mercruiser 3.0L

I'll dig around this weekend and see if I've still got the float height settings from my rebuild kit.

That idle seems high...mine calls for a 650-rpm idle (but it's an OMC 2.5 liter).
(Most any well-tuned engine ought to be able to idle down to 400-450 before it simply dies.)

Did you find the float height info? I'm have the new needle and seat in there and the height is a little different than how it was.

Thanx

ez

 

[ezjam]

Re: Odd problem with Mercruiser 3.0L

New developments....

So I adjusted the idle speed to 650. This is what I am seeing now..

If I kill the engine (in neutra) when it is happily idling at 650... no more dieseling. Perfect. However, if I restart it right up... it will idle at about 300 - 450 and putter along for a while unless I give it more gas. Once I rev it up and then back to neutral, it will once again go back to happy 650.

Note that I did not touch the throttle at all between the time I killed it and the time I restarted it. Yet it idles at completely different RPM's. What gives?

NOTE: I do have some play in my throttle cable which I will address next. But as noted above, I did not touch the throttle at all between kill and restart.

ez

 

[Don S]

Re: Odd problem with Mercruiser 3.0L

It's a carburator! Not EFI like your car. It takes a little playing with the throttle to make them work properly.
Disconnect your throttle cable and readjust the idle speed and mixture on the carb. Then adjust the throttle cable so there is a slight preload on the cable towards the idle position.

I also gave you the link to the manual in my previous response. If you go to section 5B, it gives you ALL the information on your carb. The information for a 2.5L OMC Rochester carb isn't going to help you at all since you have a Merc 3.0L with a Mercarb.

 

[jamyers]

Re: Odd problem with Mercruiser 3.0L

I couldn't find my float settings - but Don's right, they'd likely be different from your 3.0 Merc.

But I'd say that you could try lowering the float height by 1/16 of an inch at a time until it stops dripping out the nozzles. Just be sure to watch the temps and check the spark plugs to see if it's going too lean. What I've been seeing in carb kits in the last few years is that because there's a wide latitude in float height settings for the same carb across applications, and the carb kit guys are consolidating kits and info, the kit specs are getting pretty broadly generic, and it's not unusual for them to differ from the OEM specs.

Your boat must have been talking to mine, because I was out on the lake saturday and noticed that my carb is dripping fuel from the nozzles into the throttle bores when the engine is off. Since everything is clean, set properly, and all - I'm going to go in and lower the float height as indicated above.

 

[ezjam]

Re: Odd problem with Mercruiser 3.0L

Hi Jamyers,

I adjusted the float and I no longer have any leaking and no more dieseling. I also adjusted the idle speed though, so I"m not sure which fixed the problem but at least that problem is now gone.

Now if I could find the reason why it idles at different speeds just before I kill the engine and just after I start it when the two events are a mere 10 seconds apart.

Thanx for your help.

ez

 

[jamyers]

Re: Odd problem with Mercruiser 3.0L

Glad it's working better! The lower float height fixed the leaking, and the lower idle fixed the dieseling for sure. I'll bet your throttle cable needs lubing, and probably a tiny adjustment to put a little "pull" on the carb throttle when it's in the idle position.

Any idea what your float height is set to now? I found my kit instructions yesterday..here's what I mean by generic info for consolidated applications.

Hygrade carb kit #1625, application book shows it correct for a Rochester 2bbl on a 1984/5 OMC 2.5 engine.
* But the spec sheet inside doesn't show an OMC. Instead, there are automotive applications from a '74 Chevy 350 to a '78 Pontiac 305, and under Marine there is: Mercury Marine, Chris-Craft 4 and 6 cylinder, Crusader 225 V6 and 283 V8, Grumman 300, Revley 225, Universal Motors 198, 225.
* Float heights range from a low of 14/32" (7/16) up to 28/32" (7/8), a range of 14/32 or 7/16".

I originally went with the closest thing, the Chris-Craft 4-cylinder, which calls for a float height of 5/8".

Think I'll drop it down a bit and see.

 

[bwood1]

Re: Odd problem with Mercruiser 3.0L

Hi

My '98 Larson with 3.0 volvo is pretty much exactly the same
.starts / runs fine when cold
.dieseling
.floods when hot and left sittting awhile
.idles slower after restart - then if you rev it a bit, goes back up to normal idle

I have adjusted the idle using the needle valve adjustments (650 rpm idling in gear) this fixed the dieseling.

I will try adjusting the float down - perhaps thats my re-start problem - fuel dripping into the engine when hot.

like you I don't know what to do about the re-start idle issue

best luck!

 

[brodie6]

Re: Odd problem with Mercruiser 3.0L

Take the carb of and have a mechanic rebuild it. Save yourself tons of heart ache. Me and my very knowledgable neighbor rebuilt it and followed the manual to the letter. NO JOY.
Brought it in to a pro. JOY.