Author Topic: Yam DT, RT, MX fork dilemma.  (Read 385 times)

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Offline John Orchard

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Yam DT, RT, MX fork dilemma.
« on: April 26, 2021, 09:30:27 PM »
Ok, standard DT2 forks, standard fork oil quantity is quoted at 175cc BUT as the forks get close to topping-out the damper head becomes uncovered of oil, hence no damping for the top aprox inch of travel. So l add an extra 15cc so that the top of the damper rod is in the oil all the way to the top of the stroke. But with thicker springs (taking up volume) and the extra oil, l am getting hydraulic lock about 30mm before normal bottoming.

I guess l have to make up cannisters, for the top of the forks, of about 50cc volume to fix this, what say you chaps.
Johnny O - Tahition_Red factory rider.

Offline sleepy

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Re: Yam DT, RT, MX fork dilemma.
« Reply #1 on: April 27, 2021, 09:15:46 AM »
I have an RT3 with stock forks and with standard oil quantity there is rebound all the way to the top. Are you sure everything is original?
Are you sure the heavy spring isn't coil binding?

Offline John Orchard

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Re: Yam DT, RT, MX fork dilemma.
« Reply #2 on: April 27, 2021, 04:34:13 PM »
Yeh "coil binding" was my first thought, I measured the coil spaces & assembled the forks without oil and could compress the full stroke EDIT .... right side leg.

My oil height is 160mm AND I fitted a balance hose between the forks (I have fitted air-valve stems to each fork-cap) to negate any additional pressure rise from the added oil with a little extra volume.

I had fitted a stiff half of an RD350 fork spring, maybe the fork is just using up the softer travel up to the heavy spring, and maybe it's hitting a wall of spring, rather than what I think is a wall of oil?

I was running 20wt oil, I will change to 10wt and reset back to std quantity (175cc) of oil and see what happens.

I may even try straight air to rule out the springs.

I hope you guys don't mind me bouncing my ideas here, sometimes typing it out gives me ideas ;-)
« Last Edit: April 27, 2021, 08:04:40 PM by John Orchard »
Johnny O - Tahition_Red factory rider.

Offline John Orchard

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Re: Yam DT, RT, MX fork dilemma.
« Reply #3 on: April 27, 2021, 05:35:25 PM »
Found it, bent left lower slider :-)
Johnny O - Tahition_Red factory rider.

Offline Hoony

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Re: Yam DT, RT, MX fork dilemma.
« Reply #4 on: April 27, 2021, 06:59:39 PM »
well done, happy days
Long time Honda Fan, but all bike nut in general, Big Bore 2 stroke fan.    http://www.youtube.com/watch?v=XJoKP6MawYI
1985 Honda CR500RF "Big Red"
1986 Honda CR250RG
2005 KTM 300EXC "The GruntMeister" ( I love that engine)

Offline sleepy

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Re: Yam DT, RT, MX fork dilemma.
« Reply #5 on: April 28, 2021, 04:21:28 PM »
That will do it. I had a customer that thought it would be a good idea to get the outers machined down like the YZ forks to save weight and make it look special but after a couple of ride they both bent to a point the forks jammed down and ended up as scrap metal. The YZ forks must be made out of better alloy.

Offline John Orchard

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Re: Yam DT, RT, MX fork dilemma.
« Reply #6 on: April 28, 2021, 06:55:28 PM »
I got another lower, funny thing is, to get a similar damping rate (measured by pushing each one down on the floor before fitting) from eack fork, one is running 10wt, the other 20wt !!!!

That's the way it has to be, l'm cool with it.
Johnny O - Tahition_Red factory rider.

Offline sleepy

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Re: Yam DT, RT, MX fork dilemma.
« Reply #7 on: April 28, 2021, 10:17:41 PM »
Try swapping the oil and see if the damping changes. Trouble with different weight oil is pushing up and down on the floor doesn't simulate hitting a dump at 60kph.

Offline John Orchard

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Re: Yam DT, RT, MX fork dilemma.
« Reply #8 on: April 29, 2021, 10:29:16 AM »
Yep, tried them each with the different oil, the recently acquired leg must have some wear, with 10wt it feels like no oil in at all, with 20wt it's sweet.
Johnny O - Tahition_Red factory rider.

Offline sleepy

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Re: Yam DT, RT, MX fork dilemma.
« Reply #9 on: April 30, 2021, 08:56:52 AM »
Hard to believe a 50 year old Yamaha fork would have some wear. Warranty claim!

Offline John Orchard

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Re: Yam DT, RT, MX fork dilemma.
« Reply #10 on: April 30, 2021, 01:01:43 PM »
Hard to believe a 50 year old Yamaha fork would have some wear. Warranty claim!


I'll see if l can find the original receipt, dealer probably not in business anymore ;-) lol
Johnny O - Tahition_Red factory rider.

Offline pokey

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Re: Yam DT, RT, MX fork dilemma.
« Reply #11 on: April 30, 2021, 06:03:25 PM »
Sounds like someone may have "adjusted" the valve holes for them to feel the same with different weight oils.