ANOTHER burnt piston, HELP!

[workaholic]

OK, 1994 SL 750. Rebuilt top end (.50mm oversize). New topend gaskets. New triple out fuel pump. Installed 3 car kits.

So..... At the end of last season when i did all of the work mentioned above i took it out for a ride and burnt a hole clean through the #1 cylinder. I told miself maybe its a fluke (yah right). SO i went and installed BR9EYA plugs instead of 7's or 8's along with a new piston. Bang!!! Again, same cylinder.

Now, i am wondering what could be causing this to keep happening. Crank seals?? oil pump?? I dont think its the oil pump though, because i also mixed the gas when i installed the 3 new pistons and it still happened.

This is my bosses machine and i do the work on it. I had him take it apart tonight because i had no time. He also mentioned that it "looks" like there was some little water splashes in the dome of the head??? This might be when he took it apart.....

How can i do a leakdown test on this engine?? Any other ideas??

Thanks!

[ph2ocraft]

Check the crank index, if it's out you will keep holing the piston. Did you change the fuel lines and fuel selector switch?
How were the plugs looking and what were your max RPM's when the engine was running.

[xlint89]

Leak down test for sure. Seal up carbs and exh pipe air tight. Apply 9 lbs of vacuum or PSI to the pulse fitting on the block. It should hold for a few minutes, if not, you've got a leak. Spray soapy water around suspected areas, like crank seals and intake gaskets.

[workaholic]

Alright, I finally got the machine all put together ready to go friday night. Just to recap what i did, i installed a new crankshaft, new seals and all new gaskets. Carb kits installed last season along with triple out pump. New piston and rings on the MAG side due to my previous hole, (and the one before that one and before that). I even moved the cylinders and pistons around (keeping the pistons with their same cylinders).

Took it out for a ride and now it bogs then cuts down to 2. It goes fine for roughly a couple hundred feet then down to 2 she goes. But! If i pull on the choke she kicks back in. Still seems to be the mag side. Also i mixed a 500ml bottle of oil in the tank along with the oil pump still working. The plugs still seem golden LITE brown. should they not be dark brown if not really black from running rich??

I am thinking something in the carbs maybe.

[workaholic]

this might be a stupid question but here it goes. Does the height of the fuel pump and length of the ouput lines make any difference?? When i installed the triple out pump i simply just let the pump float around nere the bottom of the hull (i know i need to remount it). Also now that i installed the triple out pump the new output line going to the MAG side carb is longer than the other 2.

Also, I was just reading up on the past posts regarding the fuel pump upgrade. One of the posts mentioned a fuel restrictor in the line?? near the MAG side??? did i have to remove this when i installed the triple out pump?? also what does it look like??



Thanks

[xlint89]

You definately need to mount the pump as close to the pulse fitting as possible. You want that hose to be no more than 10" long. (as short as possible) The fuel hoses should be as short as possible too, but don't cut them short and try to keep them as close as possible to each other in length. The restrictor is in the return line nearest the PTO carb. Squeeze the hose about 1/2-3/4" past the cartbs. It's a solid piece of brass with a TINY hole. I'd go through your carbs even though you did them last year. You may have something blocking the jets.

[workaholic]

I will take apart the carbs and check them again. Sould i remove that fuel restrictor??? Also, i will try and match up my lines as close as possible.

Would my pump being low as it is cause whats happening now??

[xlint89]

Keep the restrictor in as that's what gives you fuel pressure at WOT. The pump should be mounted low for a few reasons, 1 being shorter pulse hose, and 2, easier to prime the pump. Gravity helps to fuel the pump vs. pushing gas up to it. Check your fuel selector valve also, if it's restricting flow, you'll hole a piston in no time.

[Keddano]

Here's were the restrictor is on mine with stock fuel line.It has a zip tye on it holding it in place.

[CrazyA]

Definately make sure crank index is good.

Definately check the fuel selector switch.

Definately mount that pump. Vibration & heat are the two killers of those. Mount it low like these guys are saying, but not so low that fuel and oil residue from the crankcase can make it's way up the pulse line. You always want it to drain back to your crankcase. For sure you don't want it just laying on the hull bottom.

Once you've done everything you can and you're convinced that it's fuel related, don't be hesitant to check inside the tank. Sometimes the problem can be in there. See pic.

Also, remember that almost always, when a two stroke starts losing power under WOT, it's bad and usually doing some type of damage. (but not always)

Let us know what you find.