Yamaha superjet burned pistons.

[CFW]

Harry,

Just got done with a basic freeride superjet and I have destroyed two new wiseco pistions.

It seemed to be running perfect then seized at wide open throttle

the head was normal temperature while running it against the sea wall

They are both burned on the exhaust side toward the front of the ports looks as if you ground them on a bench grinder. Also it seized at the trasfer port on the intake side of the cylinder.

Basic setup is as follows:
New hot rod crankshaft and new seals
82 mm wiseco pistons in a 61X cylinder set at 0.0045 clearance
Stock porting with minor clean up on transfer ports and exhaust ports
Single SBN 48 MM Blowsion/Riva carb on Power Bomb manifold 150 pilot 155 main
62 T Case
Factory B pipe modified version
61X electrics and MSD enhancer with factory yamaha flywheel 61x type
Static timing on stator is set to factory setting

What have I done wrong here ?? why are the piston tops burning by the exhaust ports?

Thanks

Andrew Katros

[ski4]

Re: Yamaha superjet burned pistons.

ouch

[CFW]

Re: Yamaha superjet burned pistons.

yep got that sinking feeling when it stopped, knew just what happened, but not why.

The burned tops of the pistons indicate high temps over 1200 F at the exhaust-- cant figure it out but seems lean and tight, dont want to make the same mistake twice. seems like I always used t set them up with more clearance before, Wiseco reccomends .0040 I added another 1/2 thou. but it seems that I had better results with .0050 to .0055 clearance

[ski4]

Re: Yamaha superjet burned pistons.

i have heard with wiesco .005 is where you should be, i am guessing you leaned out too? any air leaks?

[T-Bone]

Re: Yamaha superjet burned pistons.

i have always heard never run a 62T wide open with a single carb, it will end up running too lean. From the pictures that looks like what happened.

[Group K]

Re: Yamaha superjet burned pistons.

Andrew,
Based on the pix and your description, it looks like you just experienced heavy detonation. The gereral causes are too much ignition advance, too much compression, too little octane, or too lean a mixture.
Shooting from the hip, I would strongly suspect too lean a mixture is very likely you "biggest" potential problem. We run a 160 main in the stock 44 carb on that same setup. I strain to imagine that a 155 is enough for a 48mm carb. One of the top orders of business during break-in should have been to clearly establish that you are able to set the high speed circuit too rich....then gradually lean down to the best setting (preferably using a digital tach on smooth water).
It bears noting that the Riva carbs are (as I am made to understand) the same atomizer design as the old Buckshot BRM carbs. While these carbs had good CFM capabilities, they did not have very strong signal. (for info on signal, go to www.groupk.com/carbs99.htm ). The inlet setup you have chosen is one that does not play well with a carb that has weak signal characteristics (as this design has). One quick-fix that can improve the signal (and response to mixture adjustment) would be to install a flame arrestor that is not so free breathing.... it sounds backwards, but it works.
As a footnote, I take strong exception with the conversion of 62T case engines to a single carb because the very large interior volume of the inlet manifold can weaken inlet signal by very a big margin. The problem of weak signal becomes bigger yet, if a "larger than 44mm" carb is used. Personally, I would opt for a stock 760 inlet setup using unmodified 44mm carbs (blocking off the crossover in the stock inlet manifold). Such a setup has much lower inlet manifold volume than what you have (resulting in much stronger signal and more responsiveness to adjustment). If you were firm about retaining your single carb setup, I would recommend to go to a stock throat 44mm carb to recover more responsive tuning.
Just my opinion.
Respectfully, Harry Klemm

Harry,

Just got done with a basic freeride superjet and I have destroyed two new wiseco pistions.

It seemed to be running perfect then seized at wide open throttle

the head was normal temperature while running it against the sea wall

They are both burned on the exhaust side toward the front of the ports looks as if you ground them on a bench grinder. Also it seized at the trasfer port on the intake side of the cylinder.

Basic setup is as follows:
New hot rod crankshaft and new seals
82 mm wiseco pistons in a 61X cylinder set at 0.0045 clearance
Stock porting with minor clean up on transfer ports and exhaust ports
Single SBN 48 MM Blowsion/Riva carb on Power Bomb manifold 150 pilot 155 main
62 T Case
Factory B pipe modified version
61X electrics and MSD enhancer with factory yamaha flywheel 61x type
Static timing on stator is set to factory setting

What have I done wrong here ?? why are the piston tops burning by the exhaust ports?

Thanks

Andrew Katros

[CFW]

Re: Yamaha superjet burned pistons.

Harry thanks a lot,

I thought it might be a lean situation, but I value your opinion and wanted to see what you had to say about it.

Blowsion offers this set up and I don't recall the BRM buckshot carbs, but this power bomb 48 is a "Bulls eye" type set up with great response. I will call them and see why they reccomended this jetting with my set up.

Like you said, I think I should have known better and tried to establish the known rich base line first.

I guess the .0045 clearance on the wiseco pistons didn't bother you.

Thank you for your time

Andrew Katros