1997.5 gsx limited bad piston after break in

[Dirk70301]

I have a 1997.5 gsx limited that i put a fresh topend , rebuilt the carbs 165 mains 85 pilot and dull silver spring w/ 25psi popoff beings im at sea level (originally had 145 mains 65pilots and gold spring ) . I also replaced the reeds , all of the fuel line and flushed the fuel system. I deleted the oil injection and running 32:1 ratio for breakin . It ran beautiful for the first tank cruzing around . After the first tank i replaced the plugs , rode for about 2 hours and it killed at WOT .afterwords It ran as if it was on one cylinder . I inspected the plugs with the stator cyl looking lean . I removed the exhaust and carbs and found the top exhaust side of the piston melted . Pto piston looks fine . Disassembled and checked carbs . I found no debris in the carbs , fuel line or fuel filter.
Any advice on what could have it caused to run lean before i replace the piston would be great. TIA

 

[mikidymac]

Hard to say but something caused it to run lean.
Do you have pictures of the piston?

What size needle and seat?
Stock airbox?

 

[Dirk70301]

Im running a 2.3 needle seat . I did clearance the RAVE valve during assembly so that didn’t cause the damage to the piston . Aftermarket filters .

 

[mikidymac]

Looks like detonation and running really lean.

What was your squish?

 

[Dirk70301]

Everything factory specs . Nothing shaved . I’ll definitely check my squish when i go back together with it .

 

[Dirk70301]

It looks just like the original piston before i did the cylinder and piston replacement. Could it be something with the timing ?

 

[GGuillot]

You said "aftermarket filters" you mean aftermarket flame arrestors correct? If so, you should be cracking the HS needles out beyond factory spec, I've heard the 97.5 951 GSXL is lean from the factory as it is. It really can't hurt to richen the top end, it's that 3/4 to WOT the HS needles can tweak, to a degree that is.

 

[Dirk70301]

You said "aftermarket filters" you mean aftermarket flame arrestors correct? If so, you should be cracking the HS needles out beyond factory spec, I've heard the 97.5 951 GSXL is lean from the factory as it is. It really can't hurt to richen the top end, it's that 3/4 to WOT the HS needles can tweak, to a degree that is.

Correct , Arrestors . I had the high speeds turned out roughly 1/2-3/4 turn out .I also removed the diaphragm line and solenoid so it wouldnt lean it out at WOT like it’s supposed to .

 

[GGuillot]

Correct , Arrestors . I had the high speeds turned out roughly 1/2-3/4 turn out .I also removed the diaphragm line and solenoid so it wouldnt lean it out at WOT like it’s supposed to .

Side note, I'm going through a break in on a Group K sleeper mod, it's a 787 not a 951. I went with the 'recommended' 3/4 out on the HS needles. Definitely lean in my case, after a few hours of break in I briefly would go beyond 3/4 to WOT and the RPM would drop. I'm now at 1 and 1 1/4 to get it to clear up. After a few more hours I'm going to tweak more...going a little richer to see if it holds, gains or loses top RPM.. My break in has been a challenge, for the first 5 hrs or so it would seize when pushed, or start fouling plugs, leaving me limping for a while at 5 mph until it cleared. Last couple trips out no shutting off or plug problems, things are getting better. I guess it takes a while for the rings to seat, I've heard 10 hrs +.

I'd start with more and lean in...you're either going to pick up RPM or not, once you don't, stop then back out slightly. On both of my 787 skis I tuned the top end like this after stock rebuilds and neither were shut at 0 on the HS needles like the stock settings would call for.

 

[mikidymac]

You really need to go up in jetting and needle seats with aftermarket FA. Even then you can have a lean spot at 3/4 throttle that can never be eliminated by jetting. It’s due to a loss of vacuum signal at 3/4 throttle that’s caused by the aftermarket FA causing the carbs to just not draw fuel no matter what you do. If you go full throttle then cruise at 3/4 it will bite you.

The stock air box is really the way to go unless you really get wild with engine mods.

 

[Dirk70301]

Here’s the plugs when i pulled the tether at WOT when it began to act up . To me the PTO side looks good but the stator side is definitely lean . Both carbs same jetting and settings .

 

[GGuillot]

Here’s the plugs when i pulled the tether at WOT when it began to act up . To me the PTO side looks good but the stator side is definitely lean . Both carbs same jetting and settings .

I've heard the 951 crankshaft can twist causing a timing advance on a cylinder. Only thing that would explain it if the carbs are set identical, especially since it's the PTO that tends to lean because of the fuel feeding to the MAG carb first.

 

[mikidymac]

What's interesting with the Group K sleeper mod on the 787 is it retains the 1.5 seats but the mains are bumped a little, the pilots are stock, springs are 65g putting the pop off around 30psi. Harry sends a tech sheet recommending 3/4 out on the HS needles which I've found is still to lean. I'm not sure I agree with the carb jetting setup, but I paid $579 so that's what I'm going with. I would have thought 2.0 needle/seats would have been in order. Disappointed on carb boring job that was done, very sloppy, I could barely tell it was touched, and I sent completely bare carb bodies to make it easy. I ended spending hours improving upon what should have been done.

Not really complaining, I'm impressed the way it's performing right now, stronger mid range and about 150 RPM higher on the top end. I think I'll be able to tweak that some once I can get some more hours on it.

Yes you are correct on the 787. The 951 is different though because the engine and carbs by design run really lean.

 

[FL-Rider]

I put in a rebuilt 951 a several years ago in my XP, and it started burning the MAG piston. I thought I checked everything several times over and ended up doing the top end a few times. It turned out to be the head. Although the squish was ok, the cc's were not. I finally ordered a graduated cylinder and checked the volume. After that, I put on a used cylinder head that had never been cut, and its run great ever since. It might be worth the time to check the head volume.

 

[mikidymac]

Here’s the plugs when i pulled the tether at WOT when it began to act up . To me the PTO side looks good but the stator side is definitely lean . Both carbs same jetting and settings .

Yes super lean. And as Gguilllot said typically it’s the rear not the front.

Just because the jetting is the same doesn’t mean you don’t have a bad carb.

 

[mikidymac]

I put in a rebuilt 951 a several years ago in my XP, and it started burning the MAG piston. I thought I checked everything several times over and ended up doing the top end a few times. It turned out to be the head. Although the squish was ok, the cc's were not. I finally ordered a graduated cylinder and checked the volume. After that, I put on a used cylinder head that had never been cut, and its run great ever since. It might be worth the time to check the head volume.

Correct. Not enough squish would definitely cause this.

 

[Dirk70301]

After disassembling the top-end and inspecting everything I’m starting to believe i have a head gasket issue on the pto cylinder . I can clearly see where the head gasket did not completely seal up and was loosing compression but not sucking water . There was very little to no water was in the cylinder . Heres a picture of the piston .

 

[mikidymac]

Nope, that hole forming in the center of the piston and front edge missing is from detonation. Too low octane, too much compression or too little squish causes it.

 

[Dirk70301]

I finally had time to reassemble the engine . My squish on both cylinders is between .075-.079” with the same style gaskets previously used . Im going to rejet my high speed to 170 main and have removed the fuel tank and completely gone through the fuel system and carbs . Only thing i found was the water box valve was out of spec . Ive gone 3 turns out with it and plan on going 1.5 turns out on the low speed and 3/4 turns out on the high speed to start and carbs are set at 22-24psi pop off pressure . Im wondering if i should adjust the popoff pressure any ?

 

[Dirk70301]

After cruising around for about a mile at half throttle i decided to check the plugs . The mag cyl still looks lean or has detonation .

 

[GGuillot]

After cruising around for about a mile at half throttle i decided to check the plugs . The mag cyl still looks lean or has detonation .

Makes no sense based on everything you've described doing. Why is it only the MAG cylinder? when typically lean issues tend to occur on the PTO. I can't help but thinking something is wacked on the timing, like a crank twist enough to advance the timing on the MAG cylinder causing detonation. There's no way it should lean that bad running at half throttle. It's timing or an air leak somewhere to that cylinder causing the lean burn..

Did you go back to the stock airbox yet?

 

[Dirk70301]

No i didn’t get an airbox yet but will get one . I was able to tune in the carbs and run it yesterday and all was good and plugs looked great but after 30 minutes it dropped to 5800rpm at WOT and I couldn’t get it to open up again . I also removed the rave valves to inspect the pistons and the burn pattern looks good . Im almost thinking its a venting problem with the fuel tank but the vent hose and check valves check out good and i even tried running it with the fuel cap off with no change .

 

[FL-Rider]

When you rebuilt the carbs, did you verify the needle valve lever adjustments? Are the diaphragms genuine Mikuni?
Maybe check the pulse line, check valve, and wire harness to the rave solenoid...

 

[Dirk70301]

Yes lever adjusted correctly and all genuine mikuni parts . I believe i have the fuel issue sorted out . Ive done the rave mod . It ran perfect for 30 minutes hitting 60mph , now it will not run past 5750ish RPMs at 50mph . Ive checked the raves and all looks good . 130 psi each cylinder. Tried another MPED and ign coil . Its charging slightly over 13 volts . Heres the plugs at WOT @5750ish rpm