Burned piston on 235

[Streamin101]

Well this is my stock 2.6 GT the motor has been giving me trouble all summer. It would pull hard up to 6000 and then bounce up and down between 6500-6000 and on a good day it would run ta 7000. I have done throttle chops and checked the plugs constantly as it acted up (i assumed a cloged carb) and all plugs showed the normal golden brown burn, and a couple weeks ago #5 and 6 showed rich (on the plugs) so then i figured i had a couple coils going bad, so i started to swap coils and thought i would pull the plugs and have a look and #2 showed a burned piston:mad: pulled the head and here she is, What do you guys think? I am going through the carbs as we speak and will keep you posted.

http://i283.photobucket.com/albums/kk297/Streamin101/100_1707.jpg

[racer]

That looks more like an ignition problem, generally when it is caused by a carb problem the intake edge is burnt.

[Streamin101]

Okay is it no spark or something like a double fire, the head is washed clean.

[ChrisCarsonMarine]

low level detonation,too much advance,too lean,too low octane,too big a prop,or an ignition issue,possibly timer base,but probably one or more of the first five,chris

[Streamin101]

http://i283.photobucket.com/albums/kk297/Streamin101/100_1711.jpg

[Streamin101]

93 octane and 26 degrees timing

[ChrisCarsonMarine]

Looks like a little lean,then,would be my guess.The rpm flux was a sign or fuel starvation,at high speed.Check fuel line,bulb,filter,anti-siphon valve,look for kinks or pinched lines,possibly add a vacumn guage before the pump.

[JWTjr.]

With these engines, fuel quality is extremely critical. Don't think because the pump sticker says 93 that you're really getting 93. Countless fuel tests have proven that what says 93 on the pump may be 93, 89, or even 87. You need to either back the timing off and run thick gaskets, or use 100LL aviation fuel, or some type of octane booster mixed with "93".

In addition, these older blocks were prone to water leaks around the head and head gasket area. Surfacing the heads and ensuring the block is flat is the only way to guard against leaks.

In today's environment, these are finicky engines at best unless they're run with thick head gaskets and greatly reduced timing.

JT
JT

[Streamin101]

Looks like a little lean,then,would be my guess.The rpm flux was a sign or fuel starvation,at high speed.Check fuel line,bulb,filter,anti-siphon valve,look for kinks or pinched lines,possibly add a vacumn guage before the pump.

If there is a restriction before the fuel pump then i would think all of the pistons would be burned.

[ChrisCarsonMarine]

Uh,it has been my experiance[quite a lot of it]that a fuel restriction results in the top carb going lean,this hint,along with the fact that too much timing ,low octane,or undersized jets normally results in your aluminum throwoff,but on the bottom cylinders,not the top....i'll go with fuel restriction.I guess we'll wait and see,chris

[FlowMaxx]

If there is a restriction before the fuel pump then i would think all of the pistons would be burned.

He may be right..... Gravity would make the top cyls go lean first. Pull the other head.....

[ChrisCarsonMarine]

Low level detonation is a condition reached when a motor is not quite able to give up heat quite fast enough,resulting in aluminum puddleing on the dome of the piston,add being carried away in the combustion process.Dramatic detonation usually results in the piston sticking to the cylinder wall so hard the motor pulls the wrist pin from the pin bosses,bad news.Anyway,the pistons hold up well right up to the point they start to melt,and almost always one goes before the rest,i doubt the other top piston is damaged,though its possible,chris

[BeefKid]

id say double fire if i had to guess...

[ChrisCarsonMarine]

Crossflows are not prone to double firing,loopers yes,especially 60 degrees

[Baja16]

When the problem first started i watched a fuel pressure guage installed between the fuel pumps and fuel rail and while brett was driving it was showing 4-5psi at full throttle so lack of fuel to the carbs isnt the problem...

[Baja16]

Crossflows are not prone to double firing


:confused:

[EMDSAPMGR]

Will be interesting to see what did you find when you pulled the top carb and check the jets and check the flow in the carb passages. Check for any air leak around the intake bypass cover on that side. What does the fuel recirc hose look like to that cyl?

[ChrisCarsonMarine]

You may have a piece of debri sitting on top of the inlet needle seat,restricting fuel to that carb.pull a main jet access plug,squeez the prime bulb,note amount of flow,repeat with other carbs,should be the same flow.Fuel lines deteriorate,and shed chunks of rubber these days,common problem.

[Streamin101]

Heres the other side.
http://i283.photobucket.com/albums/kk297/Streamin101/100_1716.jpg

[Streamin101]

Here is the head from the unburned side and notice the plug from the #2 cylinder
http://i283.photobucket.com/albums/kk297/Streamin101/100_1723.jpg

[Riverratt]

Everytime I blew up my XP2.6 it was the powerpack double firing. I would also back the timing back if the motor is stock to about 24. I never noticed any performance loss going back those 2 degrees.

[ChrisCarsonMarine]

We found reducing the timing did virtually nothing to top end,but effects accelleration drasticly,you could certainly go to 24 degrees with nothing but a slight holeshot difference,once up to rpm weve run the timing in to 18 degrees with no drop in dyno hp,but after slowing the motor it would not re-accelerate at that low timing.I still think you have a fuel problem,but index the flywheel,get out your timing light,and find out!

[racer]

Looks like ignition or water leak

[Streamin101]

well i will definetly be changing the ignition system.

[FlowMaxx]

Here is the head from the unburned side and notice the plug from the #2 cylinder
http://i283.photobucket.com/albums/kk297/Streamin101/100_1723.jpg

Was that plug from #2 or #1?

[Forkin' Crazy]

I'll agree with racer on this one. I'd say it was a double fire or ignition issue. I had .63s in mine and stock is 65s. That was marginal if I got a bad tank of gas, and I did, it did this:

http://i120.photobucket.com/albums/o168/forkincrazy/smoked.jpg




In today's environment, these are finicky engines at best unless they're run with thick head gaskets and greatly reduced timing.

JT

Not true. They are great engines as long as they get enough fuel and the timing is not set too high. I have been running mine for over 20 years. Even have a couple high ringers that are still going strong, even though almost every one tells me that I'll sling a ring. :rolleyes:


Crossflows are not prone to double firing,loopers yes,especially 60 degrees

I don't agree with this either.

[Baja16]

forkin is that motor a stock 2.6

[Streamin101]

Was that plug from #2 or #1?
the alum coated plug is from#2 sorry to confuse you with the pic the head is from the right bank and the plugs are from the left bank.

[Forkin' Crazy]

forkin is that motor a stock 2.6

Yep, all stock with thin gaskets. :)

[FlowMaxx]

doubt it is timing or bad fuel. I would think there would be some evidence in another cyl. Doubt water either, would be more widespread on the piston - had it happen to me once and it looked totally different.

Metal burns when it gets hot..... which IMO is either predetonation caused by a bad pack OR a lean situation in that cyl only which would be either a restiction of fuel to that cyl OR an air leak allowing additional air in thus leaning the mixture to that cyl..

Since you have addl electronics, I would set those aside for now and start with a new set when you rebuild.

Like the others said though in the future: timing at 24, best fuel available and I would run it rich too. one or 2 MPH is not worth wrecking a motor. If you are drag racing, it is only 1000 ft or so and then you can afford to run it on the edge because it is less likely for a burn down since you aren't holding it wide open for long stretches of time

[ChrisCarsonMarine]

Forkin,notice you burned a bottom piston,oni top,bottom is consistant with lean.I would index the flywheel

[Steve2ManyBoats]

Particularly the ignition system. Looking at your piston, I'd really suggest a double-fire condtion is a strong candidate for a cause. Happened to me on my V4 140 crossflow. And yes, crossflows absolutely can and do suffer from double fire conditions as the packs age. They can be a PITA to diagnose, too. My problem looked a lot like a fuel flow problem to me at first. I caught it before much damage was done (just felt a funny WOT RPM drop so I backed it way down and putted in). Cleaned up the bore, popped in a new piston, broke it in and on a plug test after the first WOT run, I get way too white on cyl 1.

Posted the question on here and someone suggested a multiple fire problem. Easy test. Use a timing light and look for multiple fire conditions on each cylinder. Sure enough, cyl 1 was double firing.

The problem that supposedly causes is for the fuel mixture to flash off and the cylinder compresses and tries to fire a charge that's already been fired and is hot to boot.

Swapped in a known good ignition pack and it ran just fine.

Funny thing. My 2.0 Merc showed a similar problem a few years later, though this time it was a lumpy idle. Turned out one of the two ignition packs was multiple firing, not just double firing. The timing light on one of the cylinders looked like a laser light show - firing at random times all over the place. Replaced one of the packs and it ran fine too (still does.)

I'd suggest that you go through everything and get it back together but don't start replacing too much before you can diagnose it. You really want to nail the problem and be sure of the fix.

Steve.

[Riverratt]

My last double fire was not showing up at idle. When checking the timing on all 6 it was fine. The motor ran fine until you got to 6000 rpms then it would blow.

[DavidW]

I just today found a bad trigger to be causing a crossfiring condition on the #1 cylinder of my old 200 crossflow. I think I cought this before it did damage. It seemed to be missing bad at an idle and going faster I noticed the one head was running 20 or 30 percent more hot han the other. When I checked I thought I would find no spark or weak spark on a cylinder but it was without a doubt firing the #1 spark plug at least three times per revolution.
I remembered reading this thread a while back and thought I would add what happened with my motor to this discussion.

[powerabout]

I remember a mate at another OMC dealership had a 235 burn a few pistons and it was the trigger when it got hot.
Marked the flywheel and once you removed the cowl it cooled down enough to go back to std.
You could see it advanced on one cyl just as you removed the cowl with the timing light then back to normal as quick as that.
Glad it wasn't me trouble shooting that one.

The Merc School guys taught me to use the Merc heat lamp to heat electrical stuff as you ran the engine on the dyno or test prop.
It always amazed me how just at over operating temp stuff would fail.

[Forkin' Crazy]

Forkin,notice you burned a bottom piston,oni top,bottom is consistant with lean.I would index the flywheel

Flywheel is indexed, also timing checked on both cylinder banks. I was 2 sizes lean from stock on an XP motor with 325- 638 and 639 heads. I ran several tanks with out and issue. Got one tank of bad gas, ran it about 5 min at wide open and it nosed over, I let off and it stuck. I am sure it would have been a lot worse if keep my foot in it. It did run like hell before it stuck though! :D

Note to self, get some pyrometer probes. ;)

[Forkin' Crazy]

The Merc School guys taught me to use the Merc heat lamp to heat electrical stuff as you ran the engine on the dyno or test prop.
It always amazed me how just at over operating temp stuff would fail.

That is some good advice right there!!!! Thanks! :) I guess a heat gun would work good too.

[Jeff_G]

Heat gun works but you have to be very careful not to get the component too hot. A hair dryer works well without getting too hot.

[ChrisCarsonMarine]

I kinda thought it was a fuel problem...