View Full Version : AC MR43LTS or AC MR43T

08-03-2007, 11:02 PM
I purchased a 1996 190 Sammy Duval, Ran great for a few weeks and then on occasion would loose power at WOT, would not go over 3000rpm or sometimes 2000, engine would just bog down.

Did the usual fluid changes and put a new fuel filter on but am still having issue with power loss… I think there is also a screen on the fuel pump?… Do I need to to take the tank out to get at it?

Also thinking maybe spark plugs and/or wires so I went and picked up AC Delco MR43LTS as per the Indmar manual, went to put them in and the boat has MR43T plugs in it? these plugs have a much shorter reach. So now I'm concerned about damaging the piston heads with the longer MR43LTS.

Does anyone know which plugs I should be using? There were no manuals with the boat… I'm just going on my downloaded manual from Indmar.

The motor is the standard 350 motor with throttle body EFI. This is a boat that has had a rough first 10 years with the previous owners (too much money not enough brains) but the next ten she'll be babied!

Any help would be sweet as it is a long weekend up here in the great white north (Canada)… actually it has been 85-90 degrees for the last week! :D

:firejump: kids are going to freak if we're not on the water :firejump:

08-04-2007, 02:00 AM
Owners Manual http://www.mastercraft.com/manuals/1996_OwnersManual-PS190-PS205.pdf



08-04-2007, 09:18 AM
PCA- the fuel pump is on the motor, not in the tank. There is a screen at the inlet, however, and that can become clogged. You don't remove the screen, just remove whatever is in it. There's a fuel pickup tube in the tank and that should have a screen, too. If you have more than a quarter tank full, you may want to siphon some of the gas out to make it easier to slide the tank out, but given the boat's history, I would check the tank. You'll need to detach the gas filler hose and possibly the vent line. While the vent is removed, make sure it's not clogged. Once the tank is out (you can slide only the end with the hoses and the plate on top), remove the black plastic plate and look at the pickup tube, as well as in the chamber where it lives. You may see some debris and this should be removed. There may be another filter near the tank. Change it. You may see a valve- make sure the tab is in line with the fuel line. These sometimes move and close off the flow slightly, making it run like a stove. Remove the fuel line from the fitting and either raise the open end or squeeze the line to keep the gas that's in the line from siphoning out. Remove the fitting that's inserted in the aluminum block and inspect it. It should have a spring-loaded ball in it, but it also may have some debris in it. If you have an air compressor, use a blow gun from the barb end to blow into it while pushing the ball in. Wear safety glasses- it can spray.

If you have a fuel pressure gauge, check the pressure before you start working on it. The gauge should have a bleeder valve on it- while it's idling, bleed the air from it into a rag or better yet, a clean, clear bottle (take about 6 oz sample). Put the cap on the bottle and let it sit. If water settles to the bottom or the gas is cloudy, you need to address your fuel quality problem. The fuel pressure test should be done be done at Key ON, Idle, 2000 RPM and WOT. You should see about 30 lb at all times with a slight dip that is corrected on hard acceleration. If you see the pressure drop when you go from 2000 to WOT (usually while passing through the 1500-3500 RPM range), it sounds like a clogged fuel system.

Go through every fitting in the fuel line and make sure they're clean and tight when reinstalled.

08-04-2007, 09:27 AM
As rspiecha said, MR43T. The LTS are in the LT-1 and Vortec motors.

You talk about changing the plugs and wires but not the cap and rotor. Looking good doesn't mean working good. Don't buy a cheap cap and rotor. Don't buy cheap wires- get good ones (these came with Taylor 8mm). I have had good luck with Borg Warner and they have a series with a lifetime warranty. They're more expensive but since they have that warranty, it's the last cap/rotor you'll need to buy. Change them every couple of years (keep the receipt and box) and you should never have tuneup issues caused by these parts. The old rotor may be hard to remove. If you need to, cut the plastic top off, then split the sleeve to remove it. Once it's off, use a Scotch Brite pad to remove any rust on the rotor shaft and coat it with grease.

08-04-2007, 11:45 PM
The Indmar manual (1994 to 2002 per their web site) specs the MR43LTS for 5.7 TBI and the AC41-931for the LT1. The '94 Mastercraft manual specs the MR43T for the TBI and the MR43LTS for the LT1.

Very confusing!

08-05-2007, 09:43 AM
Thanks for all the good info… I'll go through the fuel system to try and narrow down.

I'm going throttle up the boat underload and spray some extra gas into the TB (with some extra help) if the boat recovers then I would think it would be a fuel supply issue.

Rotor and cap all seem good… boat seems to run good up to the point where it starts to bog down usually around 3000rpm but then seems to be less rpm as the boat gets warmer, around 2000rpm.

Could it be a timing advance issue… Timing advance is electronic? Is this controlled by the ECM?

I spent about two months this spring working on the boat one of the things I did was redo all the gauge plates in mirror stainless… looks awesome.
Process was…
- Scanned in old gauge plates to create CAD file.
- Sent out to get laser cut and engraved.
Put all this time in getting the boat looking great… sure hope we can use it soon.

Spark plugs are confusing although clear now… I found the info here in this forum regarding the vortec engines and which plugs to use. I put in the 43T's!

Thanks again for all the help I'll post what I find out to be the problem.

08-05-2007, 09:55 AM
The plug issue should only be confusing to someone with an LT-1, since there's no conflicting info regarding the TBI. You need MR43T.

If it's a timing issue, I have never heard of this particular symptom. When a motor loses power, it's usually a fuel issue. If you do check your timing, it's going to look odd because the timing mark will jump around due to the use of 'Spark Stabilization'. You would need to put it in diagnostic mode to go to base timing, which should be at 10º BTDC.

An empty mustard bottle works well for adding fuel. You can squirt it into the flame arrestor and stay safe, too. If it improves, you'll be looking into the filters and fuel pump inlet screen, which has been discussed in several recent threads.

08-05-2007, 09:49 PM
Found the problem to be the screen on the pump, cleaned it off and all is good. Thanks for all your help!

It is great to have a resource like this forum, especially to new owners!


08-05-2007, 09:52 PM
Once the stuff dried, was it grey and fuzzy?

08-05-2007, 09:54 PM
ya… how did you know? what is it?

08-05-2007, 11:06 PM
What color is your carpet?

08-08-2007, 08:13 PM
The plug issue should only be confusing to someone with an LT-1, since there's no conflicting info regarding the TBI. You need MR43T.

I have a 2000 PS195 w/ TBI, my MC manual does call for MR43LTS for the TBI. JimN are you talking specifically about the 96? Should I be using the MR43LTS?


08-08-2007, 10:28 PM
In post #4, I said the MR43LTS is used in the LT-1 and Vortec motors. Vortec heads were introduced in '98, IIRC. If your intake manifold bolts are installed vertically, as in aligned with a vertical centerline on the motor, it has Vortec heads. The older, non-Vortec has intake manifold bolts that go in perpendicular to the head/intake mating surfaces.

05-19-2016, 03:02 AM

I have been wondering the same now.

I replaced my MC ProStar 190, 1995, base engine two years ago (First Mate Marine rebuild). Last autumn I noticed the most rear, right side spark plug tip had been melted. Noticed the spark plug is MR43LTS, it came with the rebuild engine.

The engine is (should be!) exactly same as original, which used MR43T spark plugs, and what the user manual suggests also.
(GM350??, non-Vortec, flat tapped cam if I remember correctly (correct for the TBI computer), etc.)

This what manuals say is interesting.