mazuullaa Posted September 14, 2009 Report Share Posted September 14, 2009 My brother inlaws 91 banshee just got some work done by a local shop. They put in new pistons T6 pipes and jetted it. The new top end has 15 hours on it and the other day he just got into 6th gear at the dunes and it made alot of noise and he shut it down. He asked me if I could look at it so I did a leak down test and it passed. The main jets he had in it where 330's and the plugs where black. The left piston is toast. What I did find is on the left carb top there was no rubber washer could that be the air leak? I am going to rebuild it for him with a new crank because the crank in it is damaged now. Just trying to figure out how it died. Quote Link to comment Share on other sites More sharing options...
swrbansheeboy Posted September 14, 2009 Report Share Posted September 14, 2009 no rubber on the carb top would probably be your air leak. your leak down passed because a leak down test doesnt check for a seal of the rings or how intact the piston's are, , it's a test to show the integrity of the seals and gaskets that are in the intake and combustion area. as long as your head gaskets, base gaskets, intake gaskets and crank seals are all good everything in the middle wont effect your test. to check piston ring and just general piston condition you need to preform a compression test. you cant realy just do one test of the other as they kind of go hand in hand. if you have an in service engine then you generaly want to do a compression test followed by a leak down test. for a newly rebuilt engine you generaly want to do a leak down test while it's on the bench followed by a compression test once it's installed in the frame.... Quote Link to comment Share on other sites More sharing options...
mazuullaa Posted September 14, 2009 Author Report Share Posted September 14, 2009 I thought a compression test would be a waste of time with a motor making that much noise. So I did the leak down test and then got into it. Carb tops couldn't be air tight because of the cable but no rubber washer would give it a chance to suck alot more air I would think. Just trying to find the real cause. He was using 50/50 blend of 91/110, 4 degrees of timing, and 20cc domes so I think he was safe there. Thanks for your thoughts Quote Link to comment Share on other sites More sharing options...
halamkajohn Posted September 16, 2009 Report Share Posted September 16, 2009 My brother inlaws 91 banshee just got some work done by a local shop. They put in new pistons T6 pipes and jetted it. The new top end has 15 hours on it and the other day he just got into 6th gear at the dunes and it made alot of noise and he shut it down. He asked me if I could look at it so I did a leak down test and it passed. The main jets he had in it where 330's and the plugs where black. The left piston is toast. What I did find is on the left carb top there was no rubber washer could that be the air leak? I am going to rebuild it for him with a new crank because the crank in it is damaged now. Just trying to figure out how it died. I would inspect the cylinder. Then buy the same size piston and rings. I dont believe that the crankshaft was damaged during this blowtorch meltdown. Too much heat for a bit too long. Put it together and dont break the transmission or remelt another piston. Quote Link to comment Share on other sites More sharing options...
sleeper06 Posted September 16, 2009 Report Share Posted September 16, 2009 Chech piston/cylinder clearance in both barrels with a bore guage if accesable,check if piston skirt collapsed Quote Link to comment Share on other sites More sharing options...
mazuullaa Posted September 17, 2009 Author Report Share Posted September 17, 2009 The bore on the cylinder has to be re done three good sized scratches. The left rod end looks like it got hot. And there is alot of play on the rod on that side. I just did a 4 mil on my bike and have a good crank just needs to be welded I am going to give him. Thanks for the help Quote Link to comment Share on other sites More sharing options...
dajogejr Posted September 17, 2009 Report Share Posted September 17, 2009 No rubber boot on the carb will not cause this. I've run mine for years without a rubber boot. 6th gear in dunes/sand? My guess is it ran out of fuel. Stock carbs? Stock fuel petcock? Stock fuel line? Just because the jets are getting it enough fuel under normal riding doesn't mean there's enough fuel in the bowl after a long WOT run to supply the motor. I'd be checking the screen on the fuel valve, possibly checking the gas cap check ball assembly as well. Quote Link to comment Share on other sites More sharing options...
shanYE west Posted September 17, 2009 Report Share Posted September 17, 2009 Yep, I'll agree with Dajogejr. That rubber boot will not cause a lean condition. It would pull more air from the cable going through the cap. I'd be looking else where. I'd be leaning towards fuel supply. Depending how far you want to investigate.. also check the crank for any twisting if it hasn't been welded. Quote Link to comment Share on other sites More sharing options...
mazuullaa Posted September 18, 2009 Author Report Share Posted September 18, 2009 The rubber washer (gasket) that sits inside the carb top was missing on the left side not the boot. I am going to go over the whole fuel system. The bike leaned out some how. The crank is getting checked out and welded this week. Thanks Quote Link to comment Share on other sites More sharing options...
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