Question about Suzuki GS 400 E Motorcycles

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Hello ,i have a 1981 GS400(not E) with a prominent engine rattle/knock wich appears to be sounding from within the head area but only when engine warm and audable whiles shifting gear and feathering the throttle when cruising,has fresh oil and filter,recently adjusted timming chain.I have also takn the bike to a suzuki dealer but told me itl iether blow up or it won't??.could it be piston slapp maybe.cheers

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There are of course many possibilities as to what the noise could be. It is troubling that a mechanic listening to it could not make a better diagnoses other than piston slap. Early Suzuki four strokes often had some piston slap probably due to larger tolerances. This sound generally will go away or lesson as the pistons get warm and expand. With out being able to hear the noise , my best guess is pre-ignition. Start by doing a compression check, both cylinders should be over 110 PSI and with in PSI of each other. If compression checks out, install new spark plugs and more importantly new NGK spark plug caps (about $5 each). Next take off the carburetors, inspect the intake manifolds for cracks or separation (air leaks), clean remove the carburetor tops and inspect the vacuum diaphragms if damaged or pin holed replace them (about $152 each)(a used set may give you the needed parts for less), thoroughly clean and reinstall the carburetors. If the problem was just a lean condition on one cylinder this may bring the cycle back to its original glory. If the damage is deeper in the motor (crank) perhaps a donor cycle or engine would be the best option. Unfortunately, the only way most of these options will work is if you are willing to put in the sweat equity your self to save the labor rate.

Posted on May 28, 2011


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