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Spark plugs confused

D

dhyams

Regular Contributor
"It's time for a spark plu

"It's time for a spark plug change, and my manual recommends AC-MR43T or NGK BR6FS plugs, which I can find reasonably easily.

I would like, to go with some colder plugs, however, since I've had dieseling problems and I've tried everything else. Which has me looking for AC-MR42T or NGK BR7FS. Problem is, I can't find these to save my life.

Do any of you guys have any tips on where to find these, or perhaps a alternative plug to use (e.g., I can find a NGK BR7ES, but I don't know if those will be OK to use)?"
 
how do you relate diesling to

how do you relate diesling to spark plug heat range???

that symptom usually is due to to high of a idle or to advanced on the timing.
 
Have you ever decarboned the e

Have you ever decarboned the engine? If you have been using octane booster additives they make the carbon problem worse and turn red.
 
"I've had the dieseling pr

"I've had the dieseling problem ever since I've had the boat (only happens after a skiing run, but it's awkward to have to wait a minute to let the boat cool down while there is a skiier wanting to get back in). I tried a little decarb with SeaFoam, but it probably wasn't that good of an effort...I didn't want to smoke out the neighborhood.

Re: Kghost...Mercruiser service bulletin 97-17 gives seven things to try to cut dieseling:

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

I've checked idle speed and timing, and they are both correct. Tstat is already 140, and I don't want to buy expensive gas ;)"
 
"did you use the dash tach to

"did you use the dash tach to check the rpm? They are known to be inaccurate. Try an optical one as 100 rpms can make a big difference.

You can try cutting off the engine in fwd, too. just remember to go back to N so it starts when you need it."
 
"""""but it&#3

"""""but it's awkward to have to wait a minute to let the boat cool down while there is a skiier wanting to get back in""""

"""I don't want to buy expensive gas """


What is hot??

Whats one tank cost???

ever since you had the boat???

Well we are just dieing to know how long you have had it and if you bought it new or used and maybe how old is it?? how many hours are on it?

Please do not reciet service bullitens to me about diesling engines. That type of bulliten is general at best and not complete.

Bad gas or low octane gas and high idle and advanced timing and running HOT ARE 9999.999% THE CAUSE!

I personally have NEVER heard of dropping a heat range on a plug on a used "stock" boat period!

Tstat is 143 not 140 just so you know. engine temp should be less than 180 after a hard run pulling a skier.

See guys post.

OH and please include as much info as possible( boat, year, engine, stock or modified anything, hours if possible, any and all data so anyone of us could come to a reasonable conclusion.

My original question had to do with Why!!!

If we can solve the why then maybe you wont have to buy better gas!"
 
"Sorry Kghost...my post above

"Sorry Kghost...my post above had a highbrow tone that I didn't intend...the only reason that I copied the text of the bulletin in there was to save you the trouble of looking it up.

The coolant temp really never gets above 150, even pulling a skier. I can let it idle for 30 seconds to a minute, and it won't diesel when I shut it off. Where I run into problems is when a skier is trying to get back on the boat (dangerous w/ the engine running), or when someone else is driving the boat, and they click the key off as soon as the skiing run is over (chug chug chug).

Boat: 1996 Sea Ray 175 OB/XL
Engine: Engine Serial Number: 0F781989 [4.3L GEN II 2bbl carb, 1996]
haven't modified anything.
Bought: September 2007, and it has always dieseled. There have been a host of problems that were solved through persistence and the help of you guys here, and this is really the last nagging one.

It's not that I'm completely opposed to buying a tank of expensive gas, but it made sense to me to try something else (a cooler plug) if that would solve the problem, especially when I'm wanting to replace the plugs anyway.

I have checked the timing and the idle speed, and both seem OK. The timing is at 8 BTDC, and idle speed at 700-800 in the water, forward gear (checked with a shop tach, not the dash one).

I pulled the most accessible spark plug just now (man those things are HARD to get to!), and there is no buildup on it, but the plug looks black/fouled. On top of that, the marina that put them in in Oct 2007 put in automotive plugs, and not marine ones. Grr..I didn't know that.

If dropping the plug heat range is a bad idea, I certainly won't do it....it's just that it was one of the things on the list in that bulletin, so I figured hey, why not..."
 
"Sound like a over rich condit

"Sound like a over rich condition possibly and also let me ask this,

when you pull the throttle back to neutral ( any time when in the water not specifically when pulling a skiier) does the rpms go right to 650-700 rpm idle immediatly or does it take some time?
say 20 to 30 seconds?

You may have a vacuum leak?

You may need to increase your throttle linkage pressure at the carb so it always returns to the exact same idle no matter what.( adjust the trunion to apply more positive pressure on linkage) You may have the idle set to a non end of travel position therefore it never settles all the way down?

Just some guesses here......."
 
The plug type auto vs marine d

The plug type auto vs marine does not affect the perfomance and also if you have a rich/oil/age issue the norm would be a hotter plug to help in combustion. (not going into the expanaition as to the hows and whys)

A colder plugs are typically used with higher compression or sometimes highly elevated temps are seen and of course crappy gas with a carbed motor.etc.etc.
 
Cant realy see it the electrod

Cant realy see it the electrode is facing down.

Also if you are diesiling then they may look rich.
 
"earlier is was 700-800, in th

"earlier is was 700-800, in the water, in forward gear.

Kghost asked
when you pull the throttle back to neutral ( any time when in the water not specifically when pulling a skiier) does the rpms go right to 650-700 rpm idle immediatly or does it take some time?

then answered:
When I pull the throttle back, it goes right back to idle; no problems there.

I think 800, in gear, will get you a bit more RPM in neutral - that's fast enough for the runon."
 
""and idle speed at 700-80

""and idle speed at 700-800 in the water, forward gear"

I have the same engine and 550-600 RPMs in gear is plenty of idle; 650-675 RPMs per specs. in neutral.

Merc. makes a carbon removal product for the engine. Yours could probably use it."
 
"OK, so I have three items on

"OK, so I have three items on my plate, and I'll report back as soon as I can make the adjustments and test for a bit:
1) recheck and adjust idle speed
2) decarb the engine
3) replace spark plugs

So, is it OK just to use the automotive spark plugs? AC-R43T instead of AC-MR43T?"
 
"Hey Guy do you have a name fo

"Hey Guy do you have a name for that product?

Here is something interesting I found while in Canada I had a dieseling problem which I never had before.

The fuel up there at the dock is all mid grade 89 octane. Down here I usually use a good quality 87 octane and never had a problem. I never had the problem after I filled up next either.

I adjusted the idle air mix screws out one full turn and it seemed to fix the problem. Turning them in made the idle worse. After that I was able to adjust my idle rpms down to 700 in nuetral which everything was huncky-dorey after that.

What fixed it? Not exactly sure, maybe a combo of changing idle and new tank of gas.

As far as plugs here is what I've read about them. Hotter plugs are better for stopping and starting such as city driving or water skiing. Colder plugs are better for highway driving and long distance cruising. This is all I remember but if I can find that technical website I'll post a link, it was very informative about plug types."
 
AC-MR43T vs. AC-R43T

The &

AC-MR43T vs. AC-R43T

The "M" is for Marine environment. Exposed metal is treated to reduce rust and corrosion in a moist environment.
 
"Guy:

Right, but does it re

"Guy:

Right, but does it really matter? Should I insist on M plugs, or is it something that doesn't really matter?"
 
"Courtesy of Bondo: Posted on

"Courtesy of Bondo: Posted on Thursday, February 14, 2008 - 07:04 am:

--------------------------------------------------------------------------------
"Ayuh,......

Just to narrow this arguement, to defined parameters,...

The SBC is the most common motor ever used in Boats,+ Automobiles,...
Up til 1996, the SBC used the R43T for automobiles,+ the MR43T for the Boats.....

The Only Difference between those 2 part #s is the "M", which means the plug body is made of an Alloyed steel,+ the other is just plain ole Steel.....

The Working end of these plugs is Exactly the Same,..... Period.....

You can run the R43T in a boat, but the chances of Rusted in solid plugs is much greater,.....

But there's No difference other than that.....""
 
"OK, sounds reasonable ;)

"OK, sounds reasonable ;)

Even though plain automotive should be fine for me (as I'm a freshwater boater who stores his girl in a nice dry garage), I just found some MR's in town for 4 bucks each...gonna go grab some now."
 
"It will take years and I mean

"It will take years and I mean years for those to rust if they ever do with the way you do your boat, (fresh water and dry storage).

I think my auto plugs in my boat are over ten years now as I put a small amount of copper antisieze on the threads. $1.85 a piece if I remember."
 
"Just in the driveway (my

"Just in the driveway (my tow vechicle is currently out of town), I lowered the idle speed from 750 to 670 or so. Here's hoping it will do the trick ;)

When I take it out on the water, I'll check the idle speed in gear and make sure that it's OK. 670 might be a little low for in-driveway."
 
"A High idle speed sounds like

"A High idle speed sounds like the logical cause but until the problem is solved...can you simply turn the motor off while it is still in gear??? If the dieseling isn't too severe, this will usually do the trick."
 
"Oh, and another question rega

"Oh, and another question regarding spark plugs....the service manual for my engine lists the spark plug gap at 0.040", but the splash guard mounted on the carb lists the gap at 0.035". Which one should I go with?"
 
"Readjusted the idle speed in-

"Readjusted the idle speed in-gear from 750 or so down to 630; did my best to decarb the engine, although I never saw a ton of white smoke coming out (just some, but not clouds of the stuff).

Still diesels
"
 
"Try a seperate tank 5 gallons

"Try a seperate tank 5 gallons worth ( outboard or similar) of 93 octane. Take it out and run it and see if the issue stays.

I ran some 87 octane in my motorcycle last year when the price was high and believe me it had diesle issues and starting issues!! 93 octane and it was back to normal.

Also did you try to back the timing down to 6 or 4 btc?

If none of this works for you then You may have a vacuum leak.

Also one more thing, Can you get ahold of a infared thermometer? you said the boat never gets above 150 degree. Most boats run between 160 - 180 when under extra load or high speed. If you temp gauge never see's over 150 it may be a good idea to make sure it realy is 150 and not 190 or 200."
 
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