Vintage Dirt Bike Q & A

Ok, i am building a new race bike on an XL350 chassis and i am planning on swapping a set of old CR250 forks i have laying around on the front of the XL. I met this guy once who was showing me some drawings he had of modifications that where done to the dampening rods on these forks (weld up existing holes, drill new ones) to make them handle much better. That guy is long gone now and i was wondering if anyone on here knew of the tricks i mentioned? He seemed to think it was quite common to do back in the day.

Thanks!

As I understand it, the welding/redrilling mod changes the balance between compression and rebound damping.

If a fork had too much compression damping but good rebound damping, you could weld up the existing rebound holes and drill smaller ones, then use lighter-weight oil; this would give you the same rebound damping as before, but less compression damping. The Race Tech cartridge emulators use adjustable compression-damping valves to accomplish pretty much the same thing.

I believe another mod was to machine flat areas on the damper rod shafts, which would reduce both compression and rebound damping in the middle of the stroke.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

There is another option. I've read that the dampener rods from the xl350's can be put into the cr250m forks to improve action and that this was a popular mod from way back. This is internet information though, and I wasn't involved with these bikes in 74 so I can't vouch for anything. But if you have the xl forks laying around, you might spend some time to confirm this tip. Anybody know for real?

Well, the fork piston ring, rebound spring, bottom rod bolt, and oil lock piece is the same for the '74-'78 XL350's and '73-'74 CR250M, so the damper rod should fit OK. I don't know about travel, but I'd think the heavier XL would have greater damping, and probably smaller rebound holes. The '74-'75 XL350 damper rods are different from the later ones.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Do the XL350's have the same lower fork tubes as the CR? What i have laying around are two complete sets of XL350 forks, and one complete set of MR250 forks. Quick question, are MR250 forks same as CR250? Maybe the best bet would be to put the dampening rods form the XL forks into the MR250 tubes and then run the XL350 springs for the weight? My other option is to run a set of 76 IT400 forks i have which look super lightweight. But i cant mention that here, LOL.

I was told by a guy on ebay who builds and sells bikes that the xl350 had slightly thicker fork tubes than the cr did and they are less likley to bend/warp than the cr fork tubes. I dont know if this is true but its what I was told.

I have both XL350 & CR250 forks in two of my XL350's & fork the action is better on the CR forks, the XL forks blow through the travel under hard braking & big down hills even with up to 20 weight oil so I fitted YSS PD valves which helped some but they are still not as good as the CR250 forks with extra 20mm pre-load spacer add & 15 weight oil.

I should say both bike are raced in motocross.

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If you are not making dirt you must be eating it

I've been doing a lot of experimenting and i have some OK results and some ?????

First of all I have one grade heavier springs from Vintage Iron and I have tried many different oil grades and have ended up with using 15-40 Castrol deisel oil!! Sounds crazy but Henry Gref who was a factory CZ mechanic back in the 70's told me to try it and it really works. He uses it now in his CZ's and they go really fast!! (Probably just Henrly going fast!! HAA) I was using 30 wt. BelRay fork oil but the 15-40 is better. I just installed some air valves in the caps and this is a big help!! Releasing the air before a Moto will help a lot. I have those in some 74 Maico forks on another bike and there a must. I still think the compression dempening is too fast and the rebound is also so I'm continuing to find out if anybody has any real specs as to which holes are the compession and which are the rebound and which ones need to be welded up and what sizes and where to re-drill new ones!! If anybody has any info that would be great and if I find out anything new or develope some type of formula by trial and error I will post it up!!
Missouri VMX races start this weekend and Diamond Don's is next week so I'm off to the races!!

I am redoing my 74 CR 250 forks with the 78 XL350 forks.

I was wondering how this modification has been working.

Did you use the XL 350 damping rods?

How is that Diesel Oil working out for you?

Any other modifications I need to do to make these work decently?

Are you using emulators? If so which ones. I will be using the XL 350 tubes since my CR's are pitty.

Thanks

Joe