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?
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Fork oil
- Thread starter Dom
- Start date
Heavier oil only serves to slow high speed damping. To dial into your weight, you install custom springs/shocks.
OK, but what are these forks lacking in? I've literally done 20 miles on it but the fork seal leak is beyond
Have you read about the Minton mods? I'm not suggesting they are the answer you are looking for, but in any discussion of rebuilding XS forks the Minton mods usually get pulled out for discussion.
FWIW, I have changed the oil in Miss November's forks, can't recall grade right now. I used 5T's suggestion and raised oil level. Seals were ok so left alone. Bought a pair of YSS shocks, which have helped the handling a lot. Unsurprising, I doubt the originals were good when installed forty-odd years before.
FWIW, I have changed the oil in Miss November's forks, can't recall grade right now. I used 5T's suggestion and raised oil level. Seals were ok so left alone. Bought a pair of YSS shocks, which have helped the handling a lot. Unsurprising, I doubt the originals were good when installed forty-odd years before.
I have read about the Minton Mod but it wasnt lear if it would work on 34mm forks, ive tried contacting Brookes Suspension but had no response as yet to see if they know if the YSS emulators would fit. I'd then need to new straight wound springs to replace my progressive ones. Maybe I'll just get 15w oil, pull the forks out and see what's what, I think I need to make up a long slotted tool to get the dampers out from reading on here.
The rear shocks are new from Heiden Tuning before I got the bike.
The rear shocks are new from Heiden Tuning before I got the bike.
What 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.
It's not harsh but I can pump the forks to near full travel just sitting on it and pushing
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.
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.
This should keep you from bottoming out. It works in the last 25% of travel.
Thanks @5twins, the lower original oil level and the fact that mine are leaking it out at a rate of knots probably accounts for the dive under braking
I think I will leave drilling them out for now and just try a total strip and damper polish with all new seals and 10w oil, a polish of the fork lowers wouldn't go a miss while I'm at it.
If all this still doesn't improve matters I've got all winter to play!
I think I will leave drilling them out for now and just try a total strip and damper polish with all new seals and 10w oil, a polish of the fork lowers wouldn't go a miss while I'm at it.If all this still doesn't improve matters I've got all winter to play!
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?
I never tried Minton's fork seal recommendation, I've always just used original types. They've always worked well.
Yes, it's easiest to set the oil level with the tube vertical. I have a spare triple tree I clamp in a vice to hold the fork, and a homemade leveling "tool". It's nothing more than a length of brass tubing with an adjustable stop. I make it longer and use it like a dip stick to check the level, then set it to 6" to suck off the excess .....
To suck off the excess oil, I attach a small syringe to the tube or my MityVac with a long hose.
Yes, it's easiest to set the oil level with the tube vertical. I have a spare triple tree I clamp in a vice to hold the fork, and a homemade leveling "tool". It's nothing more than a length of brass tubing with an adjustable stop. I make it longer and use it like a dip stick to check the level, then set it to 6" to suck off the excess .....
To suck off the excess oil, I attach a small syringe to the tube or my MityVac with a long hose.
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 . . .
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.
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.
A local auto store stocks Lucas fork oil 5, 10, 15, 20 what I've been using in all my forks cept those 70's that called for 30 weight motor oil, blech!
Doubt they ever be excellent, "a bit" crude in design...
Doubt they ever be excellent, "a bit" crude in design...
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.
Having done loads of searches I can't find any reference to a minton mod being done to 34mm forks so for now I'll replace seals and add a higher volume of 10w oil and go from there
Before I try to get this thing apart can anyone tell me what size the flattened sides of the damper rod are on a pre 75 so that I can make a tool in preparation.
