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My '77 GS750 is mushy in the front. I think it needs fork oil, but I don't know how to put it in. There are some valves on top of the forks that look like air valves on a tire. What are they for? Thanks in advance for the help.
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The valves on top of your forks are air valves. You can change the stiffnes of the ride by inflating them. You'll have to ask someone else how much air they can hold. If you're not leaking fork oil, it's still all there. If you add oil, you need to drain both tubes completely first. It is VERY important to fill them both with exactly the same amount. If one has more than the other, you can put uneven stress on the alxe or front wheel bearings.
Tim -
Run your forks between 7 and 12ish p.s.i. The psi is preference; more, generally, for heavier riders. DON'T go too much!!! You could really damage the seals. Do NOT use a large air compressor. Use a little bicycle tire pump or little 12v air compressor. They will fill immediately under too much pressure (like 1/2 sec.). Good luck
-SeanComment
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Re: Fork Oil
Here's a question: Do you know when the oil was last changed?
If you don't then its time to do it.
Most late seventies/early eighties bikes suffered from too much spring and too little rebound damping. I'll bet the only adjustment your bike has is the air pressure. that ok, thats all mine ever had too. 
Are you the tinkering type? Good, lets get dirty!
Get yourself a shop manual and get to it! At the least, drain out the old oil, your shop manual will tell you how much goes back into each leg - most bikes used ATF, thatsa little thin, try a 10w or even a 15w fork oil. If your bike doesn't have rebound damping adjusters the only way to change the ride is to increase the oil viscosity. Top up the air pressure and try 'er out. More air gives a firmer ride it helps with compression only. Too much pressure can blow a seal.
If you're really a grease monkey, the forks could do with a cleaning to get any sludge out and check the condition of your springs. If they've sacked some new aftermarkets will do nicely - they make plastic coated ones that are quieter.
I've done this to my gs450 and an old honda cb650 - good results both times. To really go nuts, check your back shocks too. some cheapish aftermarkets from progressive will ride better than your old stockers.
hope this helps...Sam
PS: any questions, lemme know
Originally posted by TheNoseComment
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Thanks for the help. I think I will try to put some air in first. I've only had the bike about three weeks and all I've done is work on it. I'm ready to ride, so I think the fork rebuild will have to wait. I will take your advice and rebuild in the future.
I have a shop manual for a 1981 GS750, but it really doesn't tell me how to drain the old oil or how to replace it. It shows the forks completly dismantled and tells you to pour oil in while putting it back together. Is there another way to drain the old and put new back in?Comment
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There should be some small phillips screws near the bottom of the forks on the outside. These are the drains, it will speed things up to remove the top caps, and you have to anyway because thats how you get the fork oil back in. The top caps unscrew just like a bolt, put the bike up on the centerstand first and raise the front tire with a jack under the engine. I used 15W oil in Karens bike when I replaced the seals, stiffened the front up very well, haven't heard any complaints about too stiff so I must have did OK. Safe Riding, BillComment
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I think it's important to use the recommended oil in forks. If you get it too stiff, or compressing and expanding too slowly you risk not compressing quickly enough in a high speed corner. If that happens your center of gravity won't lower and you're in a bad position. Of course oil weight only controls the responsiveness of the fork. Springs and-or air controls the firmness. Just my .02 cents.
TimComment
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