Super Jet What's up with my piston?

[Waternut]

Had to pull the engine to fix a stripped bolt hole and saw this. Why does the rear (right) piston have an obvious yellow tint on it? The tint is heaviest at the top and is roughly the size of the exhaust port. The tinting covers the exhaust port when the piston is a little below TDC.

A little background... The rear cylinder requires one size smaller low jet to prevent fouling the rear plug if I spend too long in the low revs. The high jets are the same. If I run out of gas, I will either foul the rear plug or come very close to fouling it. If I don't foul it, it will take some time to clear out the engine and requires high revs to do so. Speedwerx engine with dual 44 GRS carbs, total loss, B pipe, and ADA head running 200 psi.

 

[Mark44]

Remember you low speed jet is only used to adjust your engines highest possible idle. I would check your pop off on both carbs and check your needle and seats.

 

[Waternut]

Pop offs are within 1psi of each other as checked with a gauge (34-35psi). Low speed adjuster is only to adjust engines highest possible idle. Low speed jet serves an entirely different purpose.

 

[Mark44]

If you use the adjuster to get rid of a low speed hesitation you will load up at low rpms which sounds exactly like what is happening here. Low speed jet smooth idle only.

Mark44

 

[Freakrider]

Waternut, possibly slight blowby oil stain on that piston ?

 

[Waternut]

If you use the adjuster to get rid of a low speed hesitation you will load up at low rpms which sounds exactly like what is happening here. Low speed jet smooth idle only.

Mark44

Ok, let's pretend this is the case. I'm not understanding what you are saying the problem is? The engine did run a slight bit better with the larger jet but 5 seconds below ~2500rpms would destroy the plug. I'm already at 35psi on the pop off so I can't go much higher and if I go lower, I foul the plug as well.

Waternut, possibly slight blowby oil stain on that piston ?

I don't think it would be blow-by just because it's clean right below the rings.

 

[Mark44]

Ok lets start over what is the motor platform I am guessing 701 with a limited factory pipe? Sounds like its internal carb issue dumping massive amounts of fuel at idle and the only way to burn it off is with high RPM's. Is your coil and wires known to be working properly and is it a stock ignition system?

Mark44

 

[Waternut]

Motor is a 61x big bore at 785cc and yes it has a limited factory pipe. The coil and wires produce strong sparks and I'm running total loss. The carbs ran fine (not perfect but I could do anything without bog or hesitation) before rebuilding the engine. Engine was rebuilt last winter because the piston/wall clearance had gotten too big and compression was dropping. After the rebuild, the engine ran fine if I hammered it constantly while playing in front of my dock but once I started cruising to get across the lake, I noticed a problem. At that time, I realized the carb tuning needed to be changed.

PS - When I say the plugs foul, sometimes they are pure black and oily and sometimes they are only a little black. Regardless, the plug tries to fire from the base of the porcelain. Keep in mind, I've been ripping this engine since Feb-Mar.

 

[Waternut]

Is this just something that's not really a big deal then?

 

[figbob]

long shot- is it a sweat stain from water ingress in the cylinder??

 

[Waternut]

That's actually a good possibility. Might not really be water ingress since I haven't sunk this motor yet but condensation and exhaust fumes may do it if the piston sat in one place for 3-4 weeks...which it has.

 

[#ZERO]

Could be a water stain or from exhaust reversion because the fuel mixture is off on that cylinder. I'd spray that piston skirt with some carb cleaner and see if the stain comes off or if it's something permanently etched into the aluminum.

 

[figbob]

It'll rub off with your finger, its only a stain! Dont think it's anything to do with your fuel mixture! But TBH both your carb's should be set Up using the same components!

 

[ryan]

is it the same piston that you were blowing orings on? looks like it was cooking.

 

[Waternut]

I'm not 100% sure but I believe I was blowing the forward cylinder o-ring and I haven't had a problem with the o-rings since my stainless steel tape fix a few months back. Good memory though...

 

[ryan]

this is y i dont mess with worn out heads. that piston is that color from being cooked. air bubbles dont do to good of a job cooling. it should be fine. but i would pull it apart and check it. to much heat does funny things to pistons.