Super Jet 701 Melting pistons

[kswiatk]

I'm having issues my squarenose 701 melting the rear piston, the engine is running very hot for whatever reason and i cannot figure out why.

Setup:

• 61x/61x with top end ported by CPT @Nuklear6 for bottom end.
• 1.50 prox pistons with .0035-.0045 clearance
• bored out manfiold to accept 46 sbn.
• Engine porter reccommend 117.5p and 135 main but i ran 117.5p and 150 main. Both screws were out 2 turns, i had not yet tuned, 17 psi pop off
• Factory b pipe and manifold.
• Dual cooling, 1 line for engine 1 for pipe with fcv
• stock timing with higher rev limiter

After the first time the piston melted i swapped the cooling lines so both on the engine and one leading from the engine to the pipe.

Conditions:

• 80- 90 degree water
• 80-90 air temp
• Run time: 15 minutes with no full throttle over 5 seconds.

Pictures to come. I'm not sure what my next steps are to cool down the engine are. Triple cooling? Up the fuel?

 

[kswiatk]

Forgot to add.
33CC domes with ada girdled head.
93 octane fuel from shell station
40:1 amsoil interceptor

 

[Quinc]

That low jet looks really small. I think FFP recommends a 125 low.

 

[kswiatk]

I did what my engine porter told me to do, but I'm starting to think its not right due to other peoples suggestions, but it ran very well.

 

[jetx]

That's pretty lean on the low jet, coupled with high compression and only 93 octane...those jetting specs of 117.5 and 135 are probably for dual oem 44's I can't imagine he'd recommend such low pilot for a single

How did it run for those 15 mins?

 

[Quinc]

My similar setup with coffman pipe, same porting, and single 46 I ran 130P and 155M with 24psi pop off.

 

[kswiatk]

Ran great, no hesitation, it was light switchy though.Thats what he reccommended for his porting, i asked 3 times

 

[Blue]

If im reading this correctly you said it has 15 minutes on it with no WOT over 5 seconds? If its that new you shouldnt be WOT until at least the second 5 gallon tank. And 40.1 is not enough oil for breakin. And pro x pistons need to be closer to the .0045 at assembly. Check your squish too that could help figure out your problem.

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[kswiatk]

If im reading this correctly you said it has 15 minutes on it with no WOT over 5 seconds? If its that new you shouldnt be WOT until at least the second 5 gallon tank. And 40.1 is not enough oil for breakin. And pro x pistons need to be closer to the .0045 at assembly. Check your squish too that could help figure out your problem.

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The first time i blew the piston was maybe 25+ hours of ride time. The second time I had broken in the piston after a hone the previous day with 8 gallons of fuel at 32:1, maybe an two hours or so of ride time. The piston blew 15 minutes into riding the next day at 40:1.

My mechanic measured the bore and the top was at .0045 but the bottom of the piston was at .0035

First seizure

 

[Blue]

Ok that sounds good. But .0035 is to tight and will usually sieze. If it was that tight after the first piston failure that means it was even tighter when it was new. Hone it out to .0045 and try again.

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[Roseand]

Ok that sounds good. But .0035 is to tight and will usually sieze. If it was that tight after the first piston failure that means it was even tighter when it was new. Hone it out to .0045 and try again.

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What clearance do you usually recommend?

 

[Blue]

Smaller motors 81mm to 83mm are .0045 and larger 84mm and up are usually .005 to .0055

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[tor*p*do]

I run 120/150 in my ported 61x/61x with a 46 oem. 1 3/4 hi & lo
b headpipe, C4 chamber, MSD cdi.
But I have 35 domes, on 93 no etoh.
over 200 hours, runs WOT. great throttle response
Oh, and a 2.5 n&s. what size are you running?

 

[Pro-pulsion]

Looking at that damaged piston and seeing the damage right on the wrist pin boss makes me wonder if the wrist pin is a little too short allowing the piston to wander over that way on the power stroke. If the issue is recurrent no matter what you have done then it may not be what you think. It could be what you do not think about, shorter than required wrist pin, smaller than required diameter wrist pin circlips, when it was bored was it bored according to the center to center distance of the connecting rods or just tossed onto a mill and bored according to the center of cylinders, is there a missing shim on the crankshaft at the side of the connecting rod...lots of things to consider, when it repeatedly does the same damage on the same cylinder it means something was not done correctly.

 

[Blue]

Shims are not need with pro x pistons. And when the bore is too tight they typically seize on the 4 corners front and rear of the wrist pin.

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[Dartman]

too tight
especially with 1.5mm oversize thicker pistons
run .005
quit screwing around with separate cooling lines
just run standard dual cooling and the lower fp b pipe screw at 3/4-7/8

 

[Nuklear6]

@kswiatk sorry I just saw this.

We have at least 5 of these 61x's with a 46sbn in our group alone. All are 117.5-120 pilot (keep in mind the difference in jet diameter between the two is only .001"-.00175" which is almost an undetectable change)and 135-145 main. No doubt a 150 main will work but I would suspect you'll have the high speed screw mostly closed when tuned optimally.

That being said there is not much evidence in the pics to support lean mixture is to blame. There is Def evidence to support the bore is too tight.

I did not bore this cylinder and do not know who did. But .0035 at the bottom of the bore is too tight for even a cast piston in a watercraft. .0045 -.00525 would be best. Keep in mind the piston is largest at bottom especially at start ups/ short runs.

As a favor to a patient good paying customer I'll fix the bore taper and set the piston to cylinder wall clearance properly for just the cost of return shipping. I'll turn it around in one day to get you back on the water. Let me know if I can help. Thanks

Chuck Palmucci
CPT Enterprise LLC
609 510 3062

 

[Nuklear6]

Above is a clearance issue of one sort. Improper bore clearance/ cooling issue/etc (notice top edges are still intact and don't look to have started to molten)


This one is a lean failure. Small jets/bad fuel pump/bad fuel filter/etc. (Molten aluminum evidence everywhere)

 

[Lumpy]

Customer service at its finest right there.

 

[Nuklear6]

Sorry for so many posts but also keep in mind too much cooling can cause issues. I try to remeber to suggest that all my customers run a temp label on the head. If your running an o ring head with added compression and your head temp isn't at least 110 120 Def F your cylinder is too cold. (Don't forget to check as soon as you stop from a hard run as temp will actually go up for a few minutes after cooling water stops flowing). I control cylinder temp with adjustable pisser. There are other ways though.

Chuck Palmucci
CPT Enterprise LLC
609 510 3062