Smoked #1 Piston

[Crater]

Ok I am starting a new thread on this motor cause my last thread as it turns out is off what is now a new problem.

1)1994 Merc Sportjet 90hp (Force power head) in a 18' Tracker Pro
2)No serial number as the sticker is worn off--unless theres another way to Id
3)Condition this happened--WOT, just lost top rpms, will run around 3K RPM currently
4)After lose of RPM's I rebuilt carbs
5)Already have the Red stator and Ohms ok (see below)
6)Trigger Ohms out of range (see below)
7)Pulled head off and this is what I found (see pics)
8)Switchbox bias test ok
9)New CDI Regulator last year
10)Install new fuel pump diaframe after lose of rpms
11)Installed new coil assemblys after lose of RPM
12)Installed new plugs and wires all after lose of RPM

Stator:
683 on 2k scale -- Green/white to white/green

Trigger:
1232 Brown to white/black
1601 white to violet

So my question is
1)Could the Trigger going bad cause this type of damage?
2)Is this motor worth rebuilding? (from what you can see)
3)What should I look for to dertermine if the block is ok?
4)Appox what is it going to cost to rebuild based of what you can see, and I will replace the trigger at the same time obviously. I'm not sure about cost of machine work to the block?
5) What is the best rebuild kit? I can get one on e-bay or maxrules.com, both are $750.00 although it look like I get rod bolts also from max. Biggest problem here is no serial number to order by. My opinion a hugh problem.
6)If I do rebuild, would you bore all cylinders over, or just as needed?

Oh yea as everyone else I will be working with a limited amount of $$. I am mechanically capable of doing this rebuild (I think) with help I'm sure, so that will help cut some cost also.

 

[CharlieB]

Re: Smoked #1 Piston

You need to finish taking it apart. Lightly run a hone thru there and see how well it cleans up then mic the cyl in at least 3 places to find out if it is still within tolerance to put a new piston in it or if you will need to bore and fit an over-size.

Plan on one piston, rings for all cyls, gaskets and seals.

You need to determine what caused the burn, it looks more like a lean fire than a lack of cooling.

Order a fuel pump rebuild kit, replace any old fuel hoses and primer bulb, make sure the fuel tank pick-up tube and fitting are clear and not restricting fuel flow. Clean all carbs, install new gaskets.

 

[Crater]

Re: Smoked #1 Piston

Thanks for the reply,
I had rebuilt the fuel pump last summer (not that it couldn't be that) but doubtful.
When I rebuilt the carbs (after the fact) I noticed the idle by pass tubes were all lower than what my Clymer Man. says they should be. Could this have caused the lean burn?

 

[Crater]

Re: Smoked #1 Piston

New BAD news;
I pushed the burnt piston back in to look at the exhaust ports, middle two ports have cracks running from the cylinder through to the exhaust manifold side. #2 piston exhaust port has 1 crack going all the way though. #3 no cracks. Guess the block is trash??

 

[Crater]

Re: Smoked #1 Piston

so does anyone think I could just hone the cyls throw in some new rings and one new piston and get it to run decent with the cracks in the ports? Just how important are the cracks? seem to me in the exahust side shouldn't be too much of a problem.

 

[CharlieB]

Re: Smoked #1 Piston

A competent machine shop will pull out those cyl liners and re-sleeve them back to original dimensions.

Call a couple machine shops in your area for estimates, Replace the pistons, add all this to your original estimate.

 

[Faztbullet]

Re: Smoked #1 Piston

That piston looks water damaged from the erosion on the exhaust side of piston. The cracks in exhaust bridge can allow water to seep around sleeve and into cylinder. As 6killer posted these are cast in sleeves and a solid sleeve would have to be installed and them port matched on exhaust/intake. I charge $395 for a repair of this kind.

 

[Crater]

Re: Smoked #1 Piston

hmm I'll check the #'s on the switchbox tonight. I did try a bias check, but with a digital multimeter it was all over the place, but all the numbers were somewhat in range as far as I could read. Maybe I was doing the bias test wrong though.
Yea ouch @ 395 a cyl x 2 cyl a reman would be a better choice + The head is pit'd, not much but never the less something else to worry about. So a reman looks in order when I can afford it anyway. Or a outboard conversion .
Without doubt I need to find the cause, so I'll test all electrical's over and over before a rebuild or a reman installation anyway.
Those port cracks, I thought they were caused by a overheat situation usually? However this motor was running fine at WOT then all the sudden a loss of RPM's. So I think it was either fuel (I just found out that our local gas station had there tank water alarms going off ) Or maybe it maybe eletrical, I have found that the trigger tests out of range. So I'm guessing trigger caused it. Any other ideas? I'm seriously thinking about throwing in a piston, new rings and seeing what happens. The middle cyl exhaust port bridge is cracked also and it seems to be running fine (although I don't know how long it's been that way)

 

[Crater]

Re: Smoked #1 Piston

well I found a macine shop that does this type of repair. New sleeves, welding of the block. I'm gonna send it in to get a estimate on the repair. Will post results for everyone to see. They do not disassemble or reassemble 2cyl motors though.