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Discussion Starter #1
I'm contemplating putting the airbox back on my 92 Wildcat project sled. I had a piston melt down last year on a wide open lake race run. One piston was completely washed to bare metal, the other had no wash, the top was partially collapsed, and the exhaust side top edge of the piston was eroded away so that the compression blew under the top ring.
Both carbs were rebuilt and cleaned before running the sled before running it.

What happened? Why one cylinder rich and the other super lean? The sled has twin fuel pumps with a shunt between them. Would one would cover both carbs if the other failed?
Could a bad head gasket let water into the cylinder and made it run lean and cause
the exhaust side piston meltdown? Could the lack of an airbox let one carb get different
air and run totally differently than the other?

A quick summary of the engine mods from exhaust in:
Stock can, PSI twin race pipes, cylinders bored to 790cc total, porting, including added exhaust boost ports (ie triple exhaust ports), Wiseco pistons, high compression heads modified for the extra cc's, 2 stage reeds, stock carbs with psi custom needles, Z5 or Z6 needle jets, 400mains, 50 pilots, kick### pod filters. Clutched to run 8200 to 8300 rpms.
 

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Discussion Starter #2
OK- no takers on this one?

Well, I just got done putting the re-nikisiled cylinder and new piston in the sled, and took
the carbs and one fuel pump apart and inspected. I re-synched the throttle cables and decided to check the choke cables too. The chokes were about 1/3 of an inch out of synch. I lengthened the
short one to match the long one, since longer would engage the choke (enrich) circuit later,
and be less likely to be enriching the carb when it shouldn't.

My analysis is that the sled was jetted too lean on the main jets, causing the one cylinder to
melt after the wide open lake run. The rich side had the choke/enrich hanging on and ran rich.

Remedy: lengthen the short choke cable, move mains up to 440 from 410, and watch plugs
more carefully before doing any more racing.

Going to leave the pod filters on for now- don't think that was the cause of the problem.
 

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Discussion Starter #4
Checked the crank seals?[/b]
In a word: no.

I assume a bad seal would allow air in, leaning out the affected cylinder. I have not read that
seals are an issue on that motor, but honestly, I never checked them and really don't know
what to look for as far as signs of failure.

The motor had previously melted a piston on the rich side piston (Mag side) which had just been replaced. It was the PTO side that had not been replaced that burned down the second time.
 

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Sometimes there might be some oily or greasy residue around the seal, sometimes it's dry. You can do a leakdown test when the engine is together, also could spray a shot of ether on the seal when the engine is running and see if the rpm increases. Just something to watch, would hate to see it burn down again over a 3 dollar seal.
 

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your lookin in the wrong place as far as the pilots...their long out of the picture at wot. if you dont come up with somthin make sure you make passes at 500, 800 1200 and 2000' reading the plug wash each time. theres losta stuff...carb boots, base gaskets, plug mains/needle problems, crank seals..so much, does that moter have a rave type exhuast? be carefull on this one.
 

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Discussion Starter #7
your lookin in the wrong place as far as the pilots...their long out of the picture at wot. if you dont come up with somthin make sure you make passes at 500, 800 1200 and 2000' reading the plug wash each time. theres losta stuff...carb boots, base gaskets, plug mains/needle problems, crank seals..so much, does that moter have a rave type exhuast? be carefull on this one.[/b]
Just threw the pilots in for completeness. Mains were actually 410's. Replaced with 460's today.
Will work down from there when I get a chance to test on grass or snow. Not sure what crank seals everyone is talking about. The crankcase seals, or the seals on the crankshaft itself that separate the areas under the cylinders and the ends of the crank? Motor is as stated, overbore 700-790 with fairly extreme porting and racing pipes. It has cylinder reeds(Boyesen), and no exhaust valves. All boots are new and sealed with rtv. Needles were specially designed by PSI to work with the pipes with smaller than stock needle jets. Thus the relatively small Z series needle jets.

I read the plug colors and the port wash after several runs before I melted the piston the last time.
One cylinder was rich and the other lean. Just couldn't hold myself back on the lake.

Thanks for the replies. Will look into testing for seal losses.
 

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Discussion Starter #8
Just adding some pics of the project.
[attachment=40089:triple_ports_2.jpg]
[attachment=40088:transfer...nlarge_2.jpg]
[attachment=40086:wildcat_...y_conv_2.jpg] Secondary Roller conversion
[attachment=40090:wildcat_...engine_2.jpg] Engine showing Pod Intakes
[attachment=40087:psi_pipe_logo_2.jpg] Psi Pipe-not shown-matching needles
 

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