Spark Plugs

"I'm sure we've covere

"I'm sure we've covered this before, but I couldn't find the thread.
5.7L merc, 2bbl, Thunderbolt IV, 1997.
The books says I should have AC MR43LTS at 0.045".
I bought plugs, giving the engine desription above and not paying close attention to the Delco part number, and got AC MR43T gapped at 0.035".
It had AC MR43T's in it gapped at 0.045" (and ran fine).
The book says the MR43T should be gapped at 0.035" but not specifically for the 5.7L.
I put the MR43T plugs in gapped at 0.035" and it seems to have a little miss at low RPM's.
I attributed this to a bad fuel problem I was battling at the same time. I think I'm through that now, but the miss seems to be persintant and it's been a little harder to start on occasion.
Do I have the right plugs at the right gap in there now?
What's the difference between the MR43T and the MR43LTS? (...besides .010"!)"

Not sure what the "S"

Not sure what the "S" means. But... My manual for that year and model calls for AC-MR43T or Champion RV15YC4 or NGK - BR6FS. Gap is .040.

.005 on the gap shouldn't create a running problem on its own. Just thoughts.

"I found an article that recom

"I found an article that recommends going to the 43LTS for a colder running engine (raw water cooled) to prevent run-on. Don't have that problem, and based on what was in there (MR43T) before I did the fresh water cooling mod, and it didn't ever have a run-on problem, I'm pretty sure the right plug is MR43T. That just leaves the gap question... any other thoughts on if a difference of .005 or .010" would matter?"

"MR43LTS is for Vortec heads a

"MR43LTS is for Vortec heads and may be too close to the piston head when used on non-Vortec motors.The 43 is the temperature range. If you have run-on, the MR41TS (or 42) is recommended by Mercruiser.

The MR43LTS should not be used on your motor is MR43TS were indicated on the flame arrestor cover. What model year is the boat from? Vortec motors have intake manifold bolts that are installed vertically, not square to the mating surface where the intake mounts to the head."

"Also, to reiterate... my merc

"Also, to reiterate... my mercruiser manual states 0.040" and not 0.045" for gap."

"Flame arrestor cover?
Appare

"Flame arrestor cover?
Apparently that part was determined non-essential by the previous owner.
The engine is a '97. I dont' have the serial number in front of me, but I have verifed it to cross to that year.
It's not a vortec.
I'm thinking based on what Jim has posted, my manual (Clymer) needs a revision.
Seems there's some conflicting information out there... especially in the gap... if it really even matters, regarding that, I'm still un-decided.
I've heard compelling arguments from both side of the issue."

"I have never used many afterm

"I have never used many aftermarket manuals but the ones I did use left a bit to be desired. Mercruiser manuals have specs for tune-up parts in them and if they were changed later, it was in a bulletin. I would call a Merc dealer and ask what is supposed to be in that motor, based on serial number. You can also call Mercruiser and give them your motor's S/N and they can tell you whatever you need. Since it's a long-distance call, have a list ready so you can make the most of it."

"Looking at Champion's and

"Looking at Champion's and NGK's websites, I think there is enough conclusive evidence that the spec is 0.040". Like you said above... probably not enough to worry about for now, but soemthing to keep in mind for next time.
I think the coil is going to be the next thing replaced, and see what that does. Otherwise, plugs, wires, cap and rotor are new. If it's an ignition problem, there's not much left to eliminate.
Thanks"

are You sure you dont have a s

are You sure you dont have a sticking valve? old man winter does nasty things.

""Am I sure I don't ha

""Am I sure I don't have a valve sticking?"
- No, but I don't really have any reason to think there is one sticking... no unusual noise, no back-firing, runs fine at all speeds above idle, and even at idle it runs OK, just a little hic-up and dip in RPM avery now and again, and sometimes stalls.
If you have any other insight or experience, please do tell.
Winter is a non-event in Va Beach compared to where I was in Maine, plus the engine has been run routinely throughout. If I don't actually put the boat in the water and go fishing, I at least run it on the rabbit ears at least every two weeks to keep the batteries fresh. It's fresh water cooled and the fuel has stabilizer in it. Carb is clean, fuel filter is new, and the choke works, admitedly slow, but fully opens in due time. The low RPM miss persists even when the engine is at normal op temp."

"Troy:
Here is my $.02 wort

"Troy:
Here is my $.02 worth. "even at idle it runs OK, just a little hic-up and dip in RPM avery now and again, and sometimes stalls". "I put the MR43T plugs in gapped at 0.035" and it seems to have a little miss at low RPM's". You stated you attributed it to bad gas.

Just stick to what worked before. Gap the MR43T plugs at .045". Pre 1990s engines had MR43Ts gapped at .035". The "hic up" at idle can be a failing plug wire--even new wires fail--check the routing of each wire to see if there is any contact with the engine block--good point to wear and cause it to fail.

Have you used a product called "SEA FOAM"? Great stuff. Get three cans, dump one can in the oil and two in the gas and fill the tank. Putting it in the oil will dissolve any varnish sludge that would cause a "STICKING VALVE" which was mentioned as a possible problem. Putting it in the fuel will darn well clean out any varnish or gum in the carb circuits. Run the tank down to at least 1/4 before refilling.

"choke works, admitedly slow, but fully opens in due time" The elec. choke is a bi-metal coil with an elec. heating element to help it to warm up instead of the old exhaust stove pipe type. It wears out over time after repeated heating and flexing and causes the butterfly to get out of adjustment; hence the intermittent wrong choke setting which causes the occaisional hard start conditions. Check for 12+ volts at the coil wire with a digital voltmeter. If the voltage is 12 to 14 volts, the wiring to the choke is good. Consider replacing the choke coil to improve starting performance.

Good luck. Guy"

"The Seafoam idea has caught m

"The Seafoam idea has caught my attention. After looking at their website and reading the product description, what they claim the stuff will fix sure mathces my gripes to a tee.

The plug wires are new, not something I'd think of as a likely problem, but it's worth taking a look.

The choke... it should be repalced, but where it's adjusted to now seems to be working OK. There's no more room to adjust it any further, so if it gets any worse, the element will have to get replaced... probably more sooner than later."

Troy: Your choke sounds like

Troy: Your choke sounds like mine before I replaced it--was only 2 years old. I kept adjusting it and it finally pooped out. I am thinking of putting an on/off switch in the circuit since there is 12 volts keeping the heater on and cooking the coil spring all the while the ignition is on.

My new ignition wire set surprized me when the damn engine would not start one morning--coil wire slipped out of the socket--had to slip-clamp it into place.

Guy

"I've been thinking about

"I've been thinking about just going with a manual choke. The times that I'll actually need it are pretty few, but sure as God made little green apples, I'll use it and forget it's pulled and run around half the day blowing brown smoke and fouling spark plugs!"