EvolvD

So if you didn't read my last post, I was having trouble with my super sport xi 750. It was bogging down and dying after 20-30 mins. So i did a compression test and found the front cylinder to be dropping to 90 psi when warmed up and boggin out.
So now I romoved the engine and tore it down, found a pretty fried piston and an exposed ring on the front cylinder..mild scoring on the skirt, but the cylinder looks and feels smooth. wrist pins, bearings, rod bores, all look ok. crank seems to be fine.

Questions are:

1.why and how did this piston get like this? is it the octane booster, lean condition, carb issue, etc.?
2.will i be ok to measure and hone it myself? or should i do a cylinder exchange kit?
3.why only one cylinder? engine doesn't appear to be oil starved
4. could the crankcase drain cause this? (blockoff in the mail)
5. is this piston wear from detonation? if so what causes it?

Keep in mind we've run high octane in this one for years..and dunno if they've had previous work..we were told by previous owners 6 yrs ago this one has vortex carbs and needs high octane, but thet have the stock duel keihin carbs that were JUST serviced last summer by kawasaki..they look brand new

So look at the pics.. am i gonna be ok with just a simple upper end rebuild?







bad piston with exposed ring: (note it is deteriorated on the exhaust side of the engine)







good cylinder (140 psi)




intake side view:




carbs (you can see the fresh gaskets still)






coupler bushing looks new


really worried about this oil leak from crankshaft oil seal (behind the coupler on the engine)..should i replace this? will it cause a lean condition if its leaking at all? if so, should i fork up for the coupler tool or it there a safe trick to get it off??

topbloke

im not famillier to this model but i geuss all 2 strokes work the same way .. looks to me that no oil got to the engine or a cooling problom ie blockage .. convert to pre mix and one less problom to worry about if you allready did the pre mix i dont think oil will be the problom... it may be even a carb problom {lean] so i would clean and rebiuld and re set the pop off pressures ... how bad are the scores in the block becouse i would really have it bored and cleaned up then put the correct over size pistons to match and yes your right they are keihen carbs but how clean or dirty on the out side doesnt matter... its the inners of the carbs that really count ... not to sure if high octane caused this problom when done re check compresion... higher the compresion higher the octane needed

EvolvD

Thanks for the advise Topbloke. The only thing i'm not familiar with on the carbs is the high/low adjustment screws...do i need to mess with those at all? It ran, idled, and accelerated great before it started boggin out and dropping compression..

EvolvD

looks like my rear crank seal and crankcase drain is leaking, could that cause this lean condition also? Got the seal and blockoff here..just need a tool or trick for the coupler..

topbloke

the leaks could be a problom for a lean condition so i would get this fixed. best thing to do is hunt on the net or ask around for a workshop manual. when i rebuilt my engine a had the crank in a vise and got the coupler off with a big set of stilsons {pipe wrench]. my crank was buggerd any way so i wasnt worried about what damaged i caused so be care full when getting them off .

note that all carbs have different settings for different engines

turn them out = rich screw them in = lean

a work shop manual should have your desired factory settings known as pop off pressures.
my 900 it has keihen carbs i screw in the screws and lightly seat thems then i wind them out to my settings and always start out rich and slowly lean them in. my current settings are screw all the way in lightly seat and 1 turn out on the lows/ and highs are half a turn out i did use a pop off gauge to set them up and you can do this on a bench its much easier. once you no what pop off presure and how many turns for that presure you can easly tune in the water .

allways keep notes right them down so you had a before and after setting . copy and paste this address in your browser it gives you a better run down on the heihen carbs

http://www.shopsbt.com/pdfs/keihincdk2.pdf

http://www.groupk.com/tec-carbs98.htm

keep me posted on how you get on

EvolvD

wow! thanks for all wonderful info topbloke! I will definitly keep ya posted...

tiagobaracho

Evolvd,
High octanage may help but it should be lean condition in one of carbs.. or clogged carb.
And always port the crankcase on two strokes