Mercruiser misfire

[TCROfreak]

I need some help; I have a 1981 Glastron with a Mercruiser 470, 3.7 (224ci) liters 4 cylinder.
My boat has a bad misfire any time I put the boat under a moderate to heavy load.
If I slowly give the boat gas it is possible to get the boat on plain but if at any time while cruising or trying to get on plain quickly causing a load that engine will misfire until the engine is slowed down to an idle and a slow acceleration is achieved. I have been unable to duplicate this problem while on land.
I am a ASE Master Automotive Technician, so I have tried to address this problem just like I would in a car, first I inspected the spark plugs, distributor car and rotor, points and wires, the plugs and points were the only items to be changed, so of course I replaced the wires, distributor cap and rotor, and plugs. I have also rebuilt the carburetor due to a small fuel leak at the bowl gasket, (Rockchester 2jet). As a further attempt at fixing my misfire and making the boat more reliable I replaced the points with an Ignitor II electronic system and added a heavy duty 45kv epoxy filled coil, again no fix. I have performed many other test to try and pin-point the problem and have found no answers.

Fuel pressure is 5 psi
Adjusted timing to 8
Fuel filters replaced
New tune parts
Electronic ignition conversion and new coil
Rebuilt carburetor
Inspected plugs after boating and found to be normal

Any help would be greatly appreciated; I have tried what I can.
If you have any questions I will do my best to help

 

[Haut Medoc]

Re: Mercruiser misfire

Welcome to iboats.....
Missfire or backfire?........
I did not see a vacuum, compresssion or leakdown test on your list......
These are the basics to test your engine's overall health........
I would also check the ignition advance.....
My guess is that is a bad valve, possibly the head gasket.....
The compression test should rule out a bad valve....​

 

[TCROfreak]

Re: Mercruiser misfire

I am pretty sure it is misfiring I hear no backfire through the intake; I guess there could be a backfire in the exhaust under the water. I have not performed a vacuum test (no ports) but I can do that today. I have checked the mechanical advance on the distributor and it moves freely. I have not performed a compression check all plugs indicated firing and the boat runs smoothly and starts fine I will perform a compression check today when I get to work and post my results.
I have thought of the head gasket also I have heard of the overheating problems with overheating on this motor. The boat has a closed cooling system and I have checked the coolant level before every trip out on the boat and the coolant level is always full, I ruled the head gasket out because of this I would have expected to see a loss of some coolant. I have made decisions based on my knowledge of car engines if they are incorrect in the boat please let me know. I will perform a compression test and vacuum and post the results today. Thank-you for your help

 

[bhammer]

Re: Mercruiser misfire

Double check your timing as well. There is a merc bulletin for the engine that says nowa days, it should be 4 BTDC (see below) becasue fuel quality is much better. Not that this will fix the probelm though. I run mine about 5 or so to get as low as possible with a good clean idle. You may want to head over to http://www.breezeworks.net/cgi-bin/MBoard/forum/display_forum_topics.asp?ForumID=1 as they are all about the 470. I'd hook up a vacume gauge and check compression settings.

Brian

Initial Timing Specification
Change For All 3.7 Litre
(224 CID) Engines
The quality of todays fuels have changed considerably
over the last 5-10 years. Because of this, the 3.7
Litre and 3.7 LX Litre MerCruiser engine timing will be
set (by the factory) at 4 degree BTDC. The timing
specification will be unnoticeable in performance for
all practical purposes and allowing for more tolerance
to todays fuels.
It is suggested that when a dealer services one of the
following engines, the ignition timing be reset to 4 degree
BTDC instead of the original 8 degree BTDC. All
service manuals will be changed in the future to reflect
this timing change.
MIE 470
MCM 470
MCM 485
MCM 488
MCM 165
MCM 170
MCM 180
MCM 190
MCM 3.7 Litre
MCM 3.7 LX Litre

Taken from a Merc.TSB 87-17

 

[Bt Doctur]

Re: Mercruiser misfire

Pull the riser and check for rusty looking stuff or a wet exaust .Might be leaking water into #4 from the hole in the riser.

 

[TCROfreak]

Re: Mercruiser misfire

I took the boat to the shop and performed a compression check this morning and found #1@145 psi #2@140 psi #3@145 psi and #4@145 psi. I also adjusted the timing to 4 and checked vacuum at idle I have 16 in hg and at 2000rpm I had 20 in hg. I have replaced everything on the ignition system so I moved to the possibility of a fuel misfire, installed an aftermarket electric fuel pump for added volume.
I took the boat to the local dock and strapped it down good and performed a load test while strapped to the trailer, I was able to duplicate the problem quite easily I checked my spark kv while the boat was acting up and found it to be ok. I them powered up the electric pump and was able to duplicate the problem again eliminating my spark and fuel volume as a problem. I had a can of brake cleaner I used to spray into the carburetor while the boat was acting up, this temporarily smoothed ok the engine running, while under load I could spray in brake cleaner smooth out the engine stop spraying and the engine would return to running poorly, I repeated this process 4 times and each time came up with the same result. I think I am having a fuel problem in the carburetor at the metering rods but I am not sure. Hopefully this n new information is helpful.

 

[Bt Doctur]

Re: Mercruiser misfire

Pull the riser.

 

[TCROfreak]

Re: Mercruiser misfire

what is the riser??

 

[johnnyd]

Re: Mercruiser misfire

I had the same exact problem. It boggs down when you give it gas, if you don't back off you will stall it. If you slowly give it gas you can get it up to speed. It was my accelerator pump in my carburetor. I rebuilt it just like you and still had the problem. I bit the bullet and bought a new carb (old one was worn ) . problem solved. I don't know the condition of you carburetor even though you rebuild it. This may be your problem. Good Luck

 

[Bt Doctur]

Re: Mercruiser misfire

FINALLY, getting the hang of this
the type in the photo

 

[TCROfreak]

Re: Mercruiser misfire

When I pull the heat riser where do I look for a leak. Also a note that may be of importance is when I built the carb I used an auto carb kit using the numbers off of the carb.

 

[Bt Doctur]

Re: Mercruiser misfire

in the exaust cavity, it should be black , carbon look to it, wet or rusty colored indicates water.You have to be careful with those kits, they may not use the marine gasket to prevent fuel or fume leakage.
The "miss" could be
cross firing
cracked dist cap,rotor, dist button
a lean condition caused by the carb or the rebuild
water intrusion.
Do any of the plugs look sort of a "pure white" on the tips.
water will vaporize carbon deposits.

 

[TCROfreak]

Re: Mercruiser misfire

I will check the heat riser, the plugs look a medium brown color, the distributor cap and rotor are 3 hours old and show no signs of tracking under the cap. Will I need a new gasket on the heat riser

 

[Eli mashmoor]

Re: Mercruiser misfire

hi there,

I have been having problems with my boat for 18 months now, i have twin 4.3 litre petrol mercruisers, the problem sounds similar to yours.... in that the starboard engine makes a load banging noise at high speed and jolts the boat at the same time (shunting feeling) i have had the boat (regal 2760) in and out of the water and the mechanic just cant seem to find the problem, the engine has been stripped down and put back together but still when we accelerate the banging and shunting returns. we have even taken the outdrive apart but that was clean with no sign of problems..... could this be down to the fuel injection system? i would appreciate any advise and please let me know if you find a solution

 

[Don S]

Re: Mercruiser misfire

I will check the heat riser, the plugs look a medium brown color, the distributor cap and rotor are 3 hours old and show no signs of tracking under the cap. Will I need a new gasket on the heat riser

1. It's not a "Heat" riser. It's an elbow of sorts that raises the height of the water passages that cool the exhaust to a higher than the manifold height.
If the gasket surface, the gasket, the manifold or the riser (elbow) leak water into the exhaust gas passage, the when you shut the engine off, the water gets into the cylinders. If it leaks bad enough, it will cause a misfire.

2. RULE OF THUMB. Anything you take apart that has a gasket, oring, or seal. That gasket, oring, or seal needs to be replaced.

I would also look for faulty plug wires, or correded ends. They may look ok and ohm out, but could still be a problem. How old are they.
Might try disconnecting the gray wire on the coil (goes to tach) and verify it's not a tach problem.
Champion spark plugs. That's all I will say on that subject.
Just incase you are starting to develop a lean condition, might want to try this test.
http://forums.iboats.com/showthread.php?t=167561

 

[TCROfreak]

Re: Mercruiser misfire

I took a little talking and some persuasion but I was finely able to get the local Glastron dealer to work on my old boat. It took almost 300 dollars to find the problem but they found it. The misfire was cause by one of the parts on the ignition system I did not replace, the TACHOMETER it was shorting out under a load and causing the ignition to ground out. I personally have never seen this in a car and would have never thought of it. While I was at the dealer I decided to look at all the new boats and was hooked, my wife and I decided to buy a newer boat, a 2004 Glastron SX-175 with a Volvo Penta. The new boat has a aluminum prop and I would like a stainless should I get a 3 or 4 blade prop with the same pitch or does the pitch change when you have more blades?

 

[Don S]

Re: Mercruiser misfire

Might want to post your prop question over in the Prop forum for best results TC.

 

[SEARAY0001]

Re: Mercruiser misfire

i am having a similar problem with my 2007 sea ray mercruiser 3.0.
things i have done so far.
carby removed and cleaned carby (lots of gunk in the bowl)
fitted water sepperater filter.

took boat out and it started and ran well all through the rev range..
then after 45min its started to splutter again and hard to idle.

?? should i drain the tank
?? tappets do sound a little noisey

 

[love1u0]

Re: Mercruiser misfire

You might want to post your own thread, this one is over a year old.

 

[Bondo]

Re: Mercruiser misfire

You might want to post your own thread, this one is over a year old.

Ayuh,... And about a Very different motor...