I'm going to service my leaking 34mm forks on my 650b this weekend, as a light person (10.5 stone) and stock forks with progressive springs. I accept I'm fighting a loosing battle with the really but should I use a heavyish weight oil, maybe 15w?
It's not harsh but I can pump the forks to near full travel just sitting on it and pushingWhat is your complaint? It the ride too harsh? Heavier oil will slow the speed of fork travel. If the ride is too harsh, you’ll make it worse.
That’s normal. Check the free length of the springs against the spec in the factory service manual.It's not harsh but I can pump the forks to near full travel just sitting on it and pushing
This should keep you from bottoming out. It works in the last 25% of travel.Yes, the Minton fork mods were developed for the later 35mm forks. The bike Joe Minton based his "Minton Mods" article on was, I believe, a 1978 Special. I don't know if or how they would apply to the older 34mm forks. Personally, I wouldn't do them unless I had a spare set of damper rods to experiment on, just in case they didn't work.
As far as fork oil weight goes, your '75 model was back when Yamaha still recommended 10W30 motor oil, lol. They state that a specialty fork oil can be used but don't give a weight. I'm not sure how motor oil weights compare to fork oil ones. As far as the amount goes, a good rule of thumb is to set the oil level 6" from the top of the tube, springs removed and forks fully compressed. I'd put in the stock amount and measure. If it's lower than that 6", I'd add oil to bring it up to that point. On the 35mm forks, this 6" setting works well. It puts about 6.5 ounces of oil in the fork. The factory spec calls for just under 6 ounces. The extra oil and higher oil level just plain makes the forks work better. They sound "squishy" when you pump them up and down with the stock amount of oil in them, the added extra cures that. They will also dive much less under braking.
I have searched "Minton Mods" in this forum and get only references to the article. Can we assume that the 6" measurement for oil fill is with the fork tubes vertical? Further, I did read the "Minton Mods" article (which I cannot find) and remember he reccommending a 36mm fork seal. Does that still apply?Yes, the Minton fork mods were developed for the later 35mm forks. The bike Joe Minton based his "Minton Mods" article on was, I believe, a 1978 Special. I don't know if or how they would apply to the older 34mm forks. Personally, I wouldn't do them unless I had a spare set of damper rods to experiment on, just in case they didn't work.
As far as fork oil weight goes, your '75 model was back when Yamaha still recommended 10W30 motor oil, lol. They state that a specialty fork oil can be used but don't give a weight. I'm not sure how motor oil weights compare to fork oil ones. As far as the amount goes, a good rule of thumb is to set the oil level 6" from the top of the tube, springs removed and forks fully compressed. I'd put in the stock amount and measure. If it's lower than that 6", I'd add oil to bring it up to that point. On the 35mm forks, this 6" setting works well. It puts about 6.5 ounces of oil in the fork. The factory spec calls for just under 6 ounces. The extra oil and higher oil level just plain makes the forks work better. They sound "squishy" when you pump them up and down with the stock amount of oil in them, the added extra cures that. They will also dive much less under braking.
One of us lazy b-stards should compare that to a spring in place level. Cuz, well who wants to take the springs out and make a huge mess to do a fork oil level change?Worth pointing out, the oil level is checked with spring out and fork fully compressed. Most know this but for those who haven't done the job before . . .
I believe I would try 10w fork oil as recommended in the later bikes. I’ve learned that viscosity changes between brands. For consistency, you have to pick a brand and stick with it.One of us lazy b-stards should compare that to a spring in place level. Cuz, well who wants to take the springs out and make a huge mess to do a fork oil level change?
Used factory spec 30 weight in a set of 70 forks and ouch they are harsh, need to drain and go lighter. Will prolly put 15 or 20 weight fork oil or 10-30 motor oil??? in the 70 set ready for oil now.
Hint; precoating the fork seals with DS2 powder is a no brainer good thing. smearing seals with soap (thick) lithium grease before install a good thing also.
Spring in place will get you a reference for your fork. As springs sag or a different or upgraded spring is installed, the level will change.One of us lazy b-stards should compare that to a spring in place level. Cuz, well who wants to take the springs out and make a huge mess to do a fork oil level change?