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Old     (gvb)      Join Date: May 2003       11-15-2003, 4:06 PM Reply   
1989 MasterCraft ProStar 190 w/ Indmar Ford 351.

I bought the boat with 300 original engine hours. Checked compression before buying and they were all around 135psi.

Since then I've changed plugs, switched to magnetic pickup, cap, rotor, coil, both fuel filters, rebuilt carb (then replaced it due to cracked bowls and bad metering block), changed oil twice and tranny fluid once.

It now has about 390 hours on it, and last weekend after riding for about three hours, we were on our way back to the dock, running about 3000rpm, and I lost power (felt like I hit a sand bar). My first reaction is always to turn the key off. After checking depth, I tried to start the boat back up and it would not start.

It took a lot of gas to get it started and we noticed a very small amount of white smoke out the exhaust. It would not idle, or run below 1200 rpm. I was able to get us back to the dock (another 1/4mile or so) and turned the boat off.

Waited about 10 minutes for my trailer and started it back up again, same thing, still a small amount, but more white smoke, and we noticed a small amount of oil slick behind the boat as well.

I just pulled the plugs today and they look like this (check pics below):

Any ideas? I have no idea where to even start. My compression gauge only fits on two of the spark plug holes because the exhaust manifolds are in the way, but the two that I checked were both at 135psi.



(Message edited by gvb on November 15, 2003)
Old     (gvb)      Join Date: May 2003       11-15-2003, 4:19 PM Reply   

Old     (peter_c)      Join Date: Sep 2001       11-15-2003, 5:33 PM Reply   
Are any of the plugs covered in an oil film?

What it sounds like to me is that you blew the power valve with the low end problems but the lack of power at high speed would not be a symptom.

By alot of gas do you mean you opened the throttle 3/4's and left it, then it started?
Old     (gvb)      Join Date: May 2003       11-15-2003, 7:32 PM Reply   
Yes, 5 or 6 of the 8 plugs have oil on them, all of them are black, as shown in the pics above.

This is a brand new carb, I have about 5 hours on it total I think. No backfires or anything like that. So I don't think it is a power valve issue.

Yea, I had to give it about 1/2-3/4 throttle in order to get it started and keep it running.

Old     (csquared)      Join Date: Jan 2002       11-15-2003, 9:34 PM Reply   
Best case is the carb is off and you fouled the plugs from running rich. Was the "oil slick" behind the boat gas or oil? Was the smoke white, blue, or could it have been steam?

Were the 2 or 3 clean from the 2 cylinders you were able to check? Have you tried a leak down test?
Old     (peter_c)      Join Date: Sep 2001       11-16-2003, 1:30 AM Reply   
The plugs do not look fouled, except there seems to be one or two that have a film of oil on them. It is common for the plugs to get a dark haze on them, but a sooty black coat is definetely fouled.

To clear a flood you open the throttle 3/4 too full throttle. This takes the vacuum away from the jets, which equals less fuel. The point here is that you are dumping fuel into the motor. There are only two things that cause this problem. I assume you have a Holley carb?? Float level can cause a major problem. The second being a power valve. I have replaced alot of power valves on Holleys. In fact I did my own just to be safe that it would not blow out on me. How old the carb is does not matter and backfiring is not always the cause. There is no check for a bad power valve other than tear down and I would just replace it anytime a float bowl on a Holley is opened.

Where to go from here? Well......keep posting any finds or facts that you have and I will see what I can do to help. I highly doubt it is an engine mechanical problem.
Old     (vortech347)      Join Date: Aug 2000       11-16-2003, 7:12 AM Reply   
I agree with Peter. It sounds like you may have blown a power valve. But to be safe. Drain some of your oil and look at it. If it's milky then you have a blown head gasket. But it doesn't sound like that's the problem. Just do it to be sure.

Your plugs look like your air/fuel ratio is pretty rich. But that could have all been from the little bit of running time with a ruptured power valve just getting back to the dock.

Oil on the spark plug threads is usually from valve seals leaking a little bit. It doesn't look like it's very bad on your plugs though. Changing valve stem seals is pretty easy if that is the case.
Old     (gvb)      Join Date: May 2003       11-16-2003, 11:40 AM Reply   
Thanks guys... I'll replace plugs, check the carb, and do my 100 hour maintenance a little early.

Is it worth it yet to pull the valve covers and check the seals?

Old     (peter_c)      Join Date: Sep 2001       11-16-2003, 11:57 AM Reply   
You can not check the valve seals. The test for them is start the motor up and see if it burns oil. Unless they are completely shot and broken you can not tell how hard or worn the seals are. A sticking oil ring can cause oil to get into the combustion chamber. Non of this is your most pressing running problem as they would just cause a cylinder or two too miss not a major runability problem.

As for water in the oil it will be the first thing to drain out of the oil pan if that would be the case. If you are not due for an oil change and it is still clean you can just undo the end plug and quickly see what comes out then recap it.

Ps: I like the special A/C Delco plugs you are using :-)
Old     (gvb)      Join Date: May 2003       11-16-2003, 8:28 PM Reply   

I put the plugs back in, but now that I am thinking about it... It is possible that the plugs fouled (while running for 90 hours with that bad carb), and unburned gas mixed with the carbon/soot deposits on the plugs, and it just looked like oil. The rainbow I saw behind the boat could have also just been unburned gas coming out the exhaust due to the fouled plugs. Let's hope so!!

What do you mean by "special" plugs? I sense sarcasm ;)

Thanks for the help!

Old     (yooper)      Join Date: Jun 2002       11-16-2003, 8:46 PM Reply   
I don't know how much difference it makes, but certain plug brands work best on certain engines. Motorcraft or Autolite are supposedly best for Ford. AC delco are for GM typically.

They are all supposed to be interchangeable, but I have mechanic friends who swear it makes a difference.

Try posting your question on

Also you can always call the guys at They answer questions for free, and are extremely knowledgeable.
Old     (peter_c)      Join Date: Sep 2001       11-16-2003, 10:47 PM Reply   
Those plugs you have in there are great spark plugs, I ran them in my Ford motor. They are GM's special marine spark plugs, that is what the "MR" stands for. Motorcrafts are good plugs too but will rust. The GM's are stainless. Autolites are complete crap! Never ever use them in anything.....or so my opinion goes.

The rainbow sheen on the water definetely could have been gas, from the engine dumping fuel. As for the plugs being fouled from the previous carb it is unlikely as they will clean themselves once the motor is running correctly.

I can not tell from the pictures. Are a couple of the plugs covered in oil? They will be shiny.
Old     (peter_c)      Join Date: Sep 2001       11-16-2003, 10:48 PM Reply   
I would also smell the oil for gas. If there is any kind of odor change the oil too.
Old     (fox)      Join Date: Jul 2002       11-17-2003, 1:14 PM Reply   
My guess is powervalve too. I had the same problem this summer. Did you use a rebuild kit for a Holley that was not marine specific? If so, the powervalves are not the same. The marine unit is shorter and has a different vacuum pressure to open.



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