1978 Suzuki GS1000 top speed 70ish

So i recently purchased my first motorcycle a GS1000 and as you are aware this is a big bike and should easily do 100mph but its topping out at 70ish mph, what could be causing this? I did notice it backfires al ot also. I have changed the plugs and cleaned the K&N air filters well i did last night and still have not had a chance to see if that helped, so I will when i get home, but I am looking for suggestions on that issue.

thanks, great site you have here very informative.

Note from Reader:
As you open up the throttle, the gas/air mixture in the carbs changes.  You have 4 carbs and it sounds like you need to get them synchronized.  The backfiring indicates too rich a gas/air mixture.

My Comments:
If you’re running a K&N filter and especially if you are running an aftermarket exhaust, it may just be running way too lean. Do a plug chop. Find a gentle upslope, run the bike in 5th gear, full throttle up the hill. After 10 seconds at full throttle pull in the clutch, kill the engine and coast to a stop. Then pull out the plugs and look at their color. I bet they are white (lean).

Though it could also be a synchronization issue as mentioned by millknee – though you would also have erratic idle if this were the case. It could also be an air leak in the manifolds, or valves that need adjustment.

Evan Fell

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