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Discussion Starter #1
a mate has just fitted a long throw crank that is supposed to make his bike a 54cc and for some reason it has now lost all bottom end.this bike was very fast before this and it is the bike that i have always been trying to get mine as quick as,but never succeeded,until now that is.
it is fitted with a big bore kit,polini carb,uprated clutch and rocket key.he was expecting to gain bottom end not lose it so anyone know what has gone wrong.the kit came with 2 headgaskets and a spacer as well to stop piston and head contact,but i am thinking that perhaps this could be where the problem lies,and that perhaps it doesnt need all these,or perhaps the rocket doesnt work with the long throw and totally messes up the timing,or perhaps this crank has messed up the port timing.anyone else fitted one of these and got good results,or anyone know what the problem could be,cos this sounds like a total waste of £50 at the moment.
 

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He most likely has timing issues, port timing and ignition timing. The rocket key advances by 10deg, way too much. I've found that the rocket key "seems" to only "work" with a completely stock engine. They are useless IMO. If he used the 2mm spacer and two head gaskets, that's one of the problems right there. Generally, only 3-4 gaskets, no metal spacer, will do the job. Squish should be around .4-.5mm. Also when using the 44mm barrel with the stroker crank, the exhaust port must be re-worked/ported. If you do a search you will find the porting specs. This has been covered many times in the past.
 

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Discussion Starter #3
zero4 said:
He most likely has timing issues, port timing and ignition timing. The rocket key advances by 10deg, way too much. I've found that the rocket key "seems" to only "work" with a completely stock engine. They are useless IMO. If he used the 2mm spacer and two head gaskets, that's one of the problems right there. Generally, only 3-4 gaskets, no metal spacer, will do the job. Squish should be around .4-.5mm. Also when using the 44mm barrel with the stroker crank, the exhaust port must be re-worked/ported. If you do a search you will find the porting specs. This has been covered many times in the past.
cheers mate it sounds easier to just put the standard crank back in,cos he isnt going to be clued up enough to start porting.
 

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I just run the metal spacer with some rtv sealant to achieve the right squish, I think the spacer is about 2mm. I agree with zero4, the squish is probably like 1.5 to 2mm with the 2 gaskets installed as well. With the spacer and some exhaust porting, my transfers are ~122 degrees and exhaust is 170 degrees, within the ballpark of correct port timing
 

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tblumer, does your engine rev better now? i would think with that exhaust duration and a FSC it would? what bore size is it? did you drill balance holes in the webbing?
 

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yeah, revs up nice.44mm bore. I did the old "hack the flywheel" trick to get it balanced out pretty good. I'm not gonna say it doesnt send a buzz up your bum, but the stock motor was pretty close in vibes.
 

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hehe, I'm afraid of the cheap pot metal that they make these things out of is gonna blow up as it is now, couldn't imagine 20k+ rpms, at least not without a cast iron scattershield.
 
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