Super Jet How easy is it to seize a piston - newbie carb tuning?

[menatep]

How easy is it to seize a piston by being too lean while tuning carbs WITHOUT a tach and WITHOUT a pop off valve checker?

Two carbs are Mikuni SBN 46mm. I believe all the specs are stock. low speed jet 120, high speed jet 150 and needle valve is 2.5 mm. spring is supposed to be 80 grams. 80 grams is black colored spring, but when i took apart the carbs the springs looked like they were silver looking. Could the color have been changed after use?

Engine is stock, bored out to 83 mm with R&D intake and R&D wet pipe.

Today I started my motor with low speed adjusters at 1 turns out (stock setting) and high speed adjuster at 1/4 turns out for now (stock is 1.5 turns out). Adjusted the idle so it idles fine and throttle response seemed fine half way through.

Tomorrow i'll take it for a spin and use watcon instructions. With the high speed adjusters not opened to stock specs yet I should see stalling so i'll just keep opening them up gradually till I have enough fuel in that throttle range.

But, i'll be lean for sure....so what are my chances of seizing a piston if i try tuning things like that? Or should I approach from the rich side just to be safe? I think it's much cheaper to replace fouled plugs than seize a piston on a new engine.

This is a new engine, so maybe it's safer to run it a bit too rich at first during the break in period....

What do you guys think?

 

[Matt_E]

Spring colors don't change with use.

 

[easyrhino]

I use a Bend-a-Light (form Sears) down the sparkplug hole at the end of the day or during adjustments to scope the carbon wash. Me thinks the oil's flash point has something to do with how much carbon is acceptable on the top of the piston. Runnig Yamahalube, carbon almost to the edge of the piston is ok. Using Caster927 if the carbon is to the edge, the piston is melting.

Comments welcome.

 

[ryan]

you need a pop off tester. to tune . your guessing with out one.

 

[JonnyB]

you need a pop off tester. to tune . your guessing with out one.

I agree you need it to check pop-off PSI and to make sure it doesn't leak. I had a tuning nightmare few years ago because I didn't check the pop-off and had a leaking fuel pump gasket. You can make a pop-off gauge.

 

[kevbo]

Go find a leaf blower and learn the basics of carb tuning. Make it fourcycle at full throttle and slowly lean until it runs clean on the pipe. Make sure you have a low pitched prop. Stock flame arestors will probably help.

 

[menatep]

I agree you need it to check pop-off PSI and to make sure it doesn't leak. I had a tuning nightmare few years ago because I didn't check the pop-off and had a leaking fuel pump gasket. You can make a pop-off gauge.

Your carbs were fully rebuilt and leaked fuel?

 

[SUPERTUNE]

How easy is it to seize a piston by being too lean while tuning carbs WITHOUT a tach and WITHOUT a pop off valve checker?

Two carbs are Mikuni SBN 46mm. I believe all the specs are stock. low speed jet 120, high speed jet 150 and needle valve is 2.5 mm. spring is supposed to be 80 grams. 80 grams is black colored spring, but when i took apart the carbs the springs looked like they were silver looking. Could the color have been changed after use?

Engine is stock, bored out to 83 mm with R&D intake and R&D wet pipe.

Today I started my motor with low speed adjusters at 1 turns out (stock setting) and high speed adjuster at 1/4 turns out for now (stock is 1.5 turns out). Adjusted the idle so it idles fine and throttle response seemed fine half way through.

Tomorrow i'll take it for a spin and use watcon instructions. With the high speed adjusters not opened to stock specs yet I should see stalling so i'll just keep opening them up gradually till I have enough fuel in that throttle range.

But, I'll be lean for sure....so what are my chances of seizing a piston if i try tuning things like that? Or should I approach from the rich side just to be safe? I think it's much cheaper to replace fouled plugs than seize a piston on a new engine.

This is a new engine, so maybe it's safer to run it a bit too rich at first during the break in period....

What do you guys think?

Those spec's are how they come from Mikuni...made to fit everything and are too rich for a stock ported Superjet.
No way you'll be even close to lean! Put at least a 135 mains in before you even put it in the water.
Set the screw high screw @ 1 1/4 turn out.
Low jet is close.

 

[JonnyB]

Your carbs were fully rebuilt and leaked fuel?

Didn't leak fuel. The carb didn't retain the pressure. The pump side gaskets/o-rings did not seat right letting the pressure out.

 

[menatep]

Chuck, my engine is not ported. Just bored out to 83mm.

The main question hasn't been answered though.... during the time of break in and carb-tuning is it possible to seize a piston if i become too lean?
I won't be doing any WOT runs for extended periods of time (longer than 5-10 seconds) and for the first couple of tanks i'll be very very conservative and careful.

 

[SUPERTUNE]

Chuck, my engine is not ported. Just bored out to 83mm.

The main question hasn't been answered though.... during the time of break in and carb-tuning is it possible to seize a piston if i become too lean?
I won't be doing any WOT runs for extended periods of time (longer than 5-10 seconds) and for the first couple of tanks i'll be very very conservative and careful.

Stock cylinder (un ported) is what I meant.
Not to likely...
This is the main reason aftermarket 46's work very well, compared to stock lean burn carbs.
Dual cooling (cold water/un restricted) and not enough piston to wall clearance would be the cause if this happens. This is called a cold seizure.