motor running on tooo long?

The engine now stays running (in neutral) at 1500 RPM?

Out of curiosity, what did it run at before?

What if you disconnect the throttle cable from the carb? Could that be hanging up and keeping it high? Maybe play with the throttle linkage by hand?

I am by no means an expert on carbs, so I might even be using some wrong terms here... but, if it's running that high, I would think something's got to be going on with the throttle valve or the jets/venturi's?

Wouldn't your worse case scenario be having a carb job done... or replacing the carb?
 
No expert here, but if it's idling too high despite backing off the adjustment, perhaps you have a vacuum leak.

Leak could mean lean, which means hot.

Also, what about running a cooler thermostat?
 
These engines are pretty simple...if the mechanic says the way to fix run-on is to replace your motor with a V6 I would highly suggest finding another mechanic. You may indeed need a new engine for other reasons, but if he is at a loss for ideas and that's his best shot....geesh!

1.)Do a compression and leak down test.

2.)Check the plugs and post some pictures of what they look like, are they the right length and temp? Gapped properly?

3.)Bore scope the cylinders and look at the combustion chambers for deposits.

4.)Carb needs to be checked/rebuilt along with inspecting cable adjustment and condition.

5.)Verify timing is set to factory recommendation.

Do you have a service manual for your motor?
 
Mark RSA2AU said:
Hi Guys

Spent this week end on the water, after trying the water sprayed into the carb trick. I also tried a carb cleaner, the aerosol type you spray into the carb.

Sadly the problem is now worse. The idle is now arround 1500 - 2000 rpm. Turning the idle screw ( what I believe to be the idle setting) completley out does nothing. Screwing it in increases the idle rpm. To stop the motor running on I now have a water bottle with a flip top lid which I spray into the carb, otherwise it does not stop. Nope not at all, it will run on untill it runns out of fuel. If I close the choke it backfires, not pretty!!

Retarding the timing only looses power but does not stop the run on.

My local mechanic is also at a loss, he suggested replacing the motor with a V6 wich I am keen on except for the $$.

Any pearls of wisdom?

Thanks
Click to expand...
Can you tell if your carb throttle plates are completely closing?
 
My thoughts are with the others who suggested that the throttle cable is holding the throttle slightly open and to also check to see if the throttle plate is closing all the way.

If your throttle cable is adjuted just slightly too tight with no slack at idle, it will hold open the throttle plate and give a high idle. If you completely unhook the throttle cable at the carb fitting and retest you may find your answer. If the cable is unhooked and you can now fully adjust the idle with the screw on the carb, the throttle cable is adjusted too tightly. If the problem still exists with the throttle cable disconnected, the problem most likely is within the carb itself.

Make sure you are not adjusting the carb "mixture" screw by mistake. The mixture screw is the screw at the bottom, middle of the carb on the port side. If this is the screw you have been adjusting, re-adjust it by: While at idle speed and ending warmed up....slowly turn screw counter clockwise until engin speed slows. (note position of screw) Then turn screw clockwise until engine speed slows. (note position of screw) Finally turn screw so it is in the approx. middle of the two noted screw positions.
 
THANKS Scottbenson!!

I have changed the money grabbing good for nothing idiot that calls himself a "Marine Mechanic", and applied some logic as well as your advice. The throttle cable is holding open because of the cable. It seems the cable is not returning the throttle body back to the closed position. If I wriggle the cable at the Throttle handel just as it comes out of the back of the controll box the idle returnes sweetly back to normal.

I now need to find out why it is holding all of a sudden, but it might just need adjustment. The running on has reduced to under a minute which is great.

Next is to try different fuel grades, as well as adjusting the idle screw and the mixture once I have reset the cable.

Will keep you up to date.

Cheers

Mark
 
mercruiser has a kit for the run on condition. If you want pm me and and I'll find the part number for the kit tomorrow at work. I've done a few of them.
 
Mark RSA2AU said:
Hi Guys

I have in the passed discussed this issue but have been unable to solve it, so my questions are will this do dammage to the motor, and can anyone think of another solution.

I have a 1999 Sea Ray 180 with a 135 hp 3lt Mercruiser and Alpha 1 leg. The motor frequently runs on after the ignition has been turned off, some times for over a minute, even after allowing it to idle for a minute or two.

I have had it serviced, and lowered the idle speed to just above stalling, which it now does painfully often and does not idle properly when cold.
I have used hotter and colder spark plugs ( now using Irridum tipped), better but not stopped.
I have used a fuel system cleaner which is also supposed to remove carbon deposits.
I have tried different fuel grades, changed the air filter ( K&N), fuel filter ( Merc)....
and I have danced around the boat in small circles ( after I burned my hand)

And still I cannot stop the motor running on.
This is especialy irritating when out sking as I do not feel it is safe to clime aboard untill the motor stops.

So will it dammage the motor or should I continue to look for a solution???

Cheers
Mark
Click to expand...


http://www.themarinedoctor.com/cgi-bin/YaBB.pl?num=1156939357
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.
97–17 JULY 1999 - 2 -

Available Colder Spark Plugs
Original Spark Plugs in the Engine: Short Reach
Spark Plug
AC-MR43T
Long Reach
Spark Plug
AC-MR43LTS
Replacement Colder Spark Plugs: Short Reach
Spark Plug
AC-MR41T
Long Reach
Spark Plug
AC-None
NGK-None NGK-TR6
Champion-RV12YC
OR
Champion-RV91MC
Champion-RS9YC
NOTE: Quicksilver does not stock these colder spark plugs. They will have to be ordered from a spark plug
distributor.

TMD

Then Also just to be safe you could take a look at these two links to help insure the engine timing is right on, And you are not using bad information. See these two links.

(Side Note) to get these to links to work you will need to fill in the ******* with the word B O A T F I X. For some reason this forum thinks this is a bad word and it will not let you use it.:smt009

http://www.*******.com/merc/Bullet/99/99_11.pdf

http://www.*******.com/merc/Bullet/98/98_14.pdf

To adjust the idle speed on a MerCruiser 3.0 engine you really do need a lot of time and patience to get it just right.
 
Hi Guys

Thanks for all the advice!! I have checked the timing, spark plugs, adjusted the idle air fuel mix, and adjusted the throttle cable. So far so good.

With the hose on and boat out of the water it seems much better. We are heading out for a day on the water tomorrow and I realy hope we have sorted this out.

Will keep you informed!

Cheers
 
Yipee for you! Now that you have got everything on the right track, if you still have any run on, try lowing the idle speed a bit at a time. It will make the most difference. And.......it's easy and doesn't cost a thing.
 
Sorry for the delay in posting, I was away on Business.

I have since purchased the Service manual for this motor so that I can see the pictures...

I reset the idle mixture as well as the idle screw its self. I adjusted the throttle cable so the the throttle body returned to closed as well.
I also replaced the spark plugs with Irridium ones. I also put a bottle of liquid carb cleaner into the tank.

Then after reading the manual I adjusted the accelerator pump to allow for more fuel on the initial charge.

The result, well the motor runs much better and seems to have a better hole shot, but the running on was still there.

I believe it is all sorted now as I have to replace the throttle cable as it seems to be sticking, but the motor is finally running ok.

Thanks for the help.
 
Glad to hear that you are getting somewhere with the issue. I am doing the same with my boat but think that it is probabaly related to stale fuel.

Will let you know after i have run a few tanks through it.
 
After having my 2007 175 check by two dealers 1st under warrenty and the secound charged me 250 bucks with no improvement even Mercury customer service was USLESS, I started using SEAFOAM as a gas additive and guess what no more run on. Wallmart, pepboys, i use it in every thing check out their website.
 
Did you get it all sorted before you have to put it away for winter?

Mine is about to be winterised and put away in the garage for a while.

Cheers

Mark RSA2AU said:
Sorry for the delay in posting, I was away on Business.

I have since purchased the Service manual for this motor so that I can see the pictures...

I reset the idle mixture as well as the idle screw its self. I adjusted the throttle cable so the the throttle body returned to closed as well.
I also replaced the spark plugs with Irridium ones. I also put a bottle of liquid carb cleaner into the tank.

Then after reading the manual I adjusted the accelerator pump to allow for more fuel on the initial charge.

The result, well the motor runs much better and seems to have a better hole shot, but the running on was still there.

I believe it is all sorted now as I have to replace the throttle cable as it seems to be sticking, but the motor is finally running ok.

Thanks for the help.
Click to expand...
 
I cannot winterise my boat, I would go into withdrawl if I could not get out on the water at least once a month, even in winter.

Jokes aside, I cannot put my finger on exactly what the problem was. I have serviced / replaced every part that could be a cause. The last thing to do is to replace the throttle cable, as it does not always return to idle. I have attached a string to the cable as it enters the hand controll and just give it a pull when it sticks.

So far so good.
Cheers
Mark
 
Here’s a new thought on this for you and others:

Providing the the engine is in tune: I still feel the root cause of this run on business is due to glowing hot combustion chamber deposits becoming an ignition source after the ignition has been cut off.

Providing the engine isn't totally shot and lacks its own ability to control oil usage. Perhaps running some straight weight oil may reduce the crankcase fumes. Having said that, look at how your fumes are vented. Do they vent to your flame arrester?

Now before selecting a oil, do a little research. Some things to watch out for are lubricant additives to protect a slipper/flat tappet cam.

When running straight weight oil: Don't drive it like you Stoll it rite away! Let the oil come up to temp so it flows good then drive it like you Stoll it.

Spooky posted a how to on decarbonize the combustion chambers that works very well. When performing this task, one has to really pour the water to it to where the rpms are basically controlled by the flooding of water into the engine. Also the revs need to be up there around 2500 ish. This is a whole lot easier that pulling the head!

OMT: it's important that the throttle plates close to idle. I'm sensing your still having trouble with this. Some times when a carb gets hot the throttle plates will bind. This can be anything from a bent shaft or misaligned butterflies on the shaft. Or just too much heat to where a spacer block/heat sink is needed.

Keep us up to date on this, and best of luck with it
 
Have to say that I agree with the idea that there are deposits in the combustion chamber contributing to this issue.

How many hours on your boat? and do you know its history? (how, where it was used etc).

Have you tried some upper cylinder cleaner?

I would like to keep using the boat over winter and even though you can get out for a quick blast but it is too cold to play. In this area you can block out from Anzac day till sometime in October and even then the water is still really cold.

At least I can hit the snow if I feel like skiing!

Good luck with it and keep us informed.

Cheers
 
Thanks Guys

I have done the water decarb thing as well. I have still to replace the Throttle cable but when I pull the string attached to it, the butterflys in the carb return to closed and the idle returnes to normal. Before all the previous work this trick had no effect.
Now when the finance minister (Wife) gives me more money I will replace said cable.

Cheers
 
As a old mechanic from the dark ages...... We put tranny fluid down the carb,,,engine hot- untill the engine stopped. let it sit for a while the start it up and run the hell out of it.WOT until all the smoke clears. See what happens. If it works, cheap fix. And I agree with the timeing settings with the octane that you use.

Rick
 
You must log in or register to reply here.

Forum statistics

Threads
113,164
Messages
1,427,650
Members
61,075
Latest member
Jrrwsu1
Back
Top