RD Myths And Legends Revealed

Bullshit or Truth ?

This page is intended to reveal the darker mysteries that surround the RD. Not the sort like when you take your RD400 to be MOT'd and the mechanic says to you "I used to have 250 when they first came out. It was great, I used to be doing 80 in 5th and it'd pull a wheelie when I opened the throttle up" There is no mystery to that - the guy is a fruitcake, spent too much time sniffing exhaust fumes. No, this is more about those mechanical mysteries like:

What are those pipes connecting the carb floatbowls to the airbox ?

Well firstly, they don't actually connect the float bowl to the airbox. They are not overflow pipes or vents to pressurise the float bowl. You can follow the drillings in the carb body and find that they actually are the feeds to the air corrector for the needle jet. Picture 1 show the sectioned carb from the Club's cutaway engine. Highlighted in green are the drillings that the air passes throuh en route to the air corrector. From the large inlet casting on the float bowl, the drill passes acrosss the carb and meets with a vertical pasage (highlighted). This passes through a hole in the gasket into a matching vertical pasage in the carb body. this then meets a drilling passing across the body and travelling downwards at about 45 degs (highlighted). This in turn meets the drilling that is at the 6 o'clock position in the carb bellmouth (picture 2, a brand new, never been on a bike, 2R9 carb for an RD400E) and is goes through to the needle jet. As you can see from picture 2, the 6 o'clock drilling is blocked by a brass ball. The actual air corrector jet is behind this ball and the carb mods described by Dale Alexander involve drilling out this ball and the jet behind it. So, you can see the black pipe from the airbox is the only way air can get to the air corrector jet on the standard, unmodified carbs.

Swapping float bowls so the vents are on the outside is a no-no as the drillings won't match up and so no air can be fed. Also blocking of the vents will have the same effect, resulting in bad mid range richness.

How can you get those damper bolts out of the bottom of the fork legs ?

The problem: You're trying to assemble your forks and as you turn the bolt in the bottom of the slider to rellease the damper rod, the rod spins with the bolt and you get no where. The solution is easy one you get the damper out and can inspect it, but if you can't get it out, you're in a Catch 22! The picture below shows a damper sans fork slider. From the left (the end that the damper bolt screws into), you can see the spacer sleeve, the bump spring, the damper holes and finally the key to success... the flats on the end of damper. These flats are accessable from the top of the slide once the springs are removed. You can make a tool to lock in on the flats to prevent the damper rod turning as you try to release the allen bolt from the bottom of the slider, or as some have done, wedge a wooden pole (ie a broom handle) down on the flats to prevent it turning.

The diameter of the bottom of the damper is 21mm, the diameter of the protusion with the flats on is 14mm and the distance across the flats is 12mm.

What RPM do I time my RD at ?

0 RPM is the best answer! Strobe guns should only be used to check timing after you have set with a dial guage.
The easiest RD's to time are the electronic E and F models (USA guys and gals, skip this, you never had this luxury!!). With the dial guage set at 1.8mm BTDC for a 400 and 1.6mm BTDC for a 250 align the mark on the pickup with the mark scribed on the rotor. Tada! All done! Because the system works with a dual wasted spark, both barrels are timed simultaneously. You can even do this with the motor out of thee frame and no electrics rigged up!
Next up we come to the earlier models with Newtronics boxes replacing the points. Once set, you can leave this forever, or until your engine gets worn out 25k miles later. Insert the dial guage in the LHS plug hole and find 2.0mm BTDC for a 400, 1.8mm BTDC for a 250. Now slacken off the adjusting plate for the optical trigger that fires the left side (I think it is also to the left when facing the timing plates) and with the spark plug inserted in the HT lead and earthed somewhere on the bike, move the trigger so it just clears the back of the shield on the end of what was the points cam. This should cause the plug to spark. If it doesn't, go and fetch the ignition keys and turn the ignition on (been there, done that). Gently tighten the trigger plate screws and using a 12mm spanner or a screw driver (spanner/wrench is easier to control fine movement) move the engine backwards to about 3mm BTDC and then forwards again, checking at which point on the dial guage the plug sparks. it should be pretty close to the required value. You can now tweak the plate position to accurately set the timing. As you rock the crank back and forth around your timing area, you should see the plug sparking. Repeat for the other side. Should take about 10 minutes to do. You can now set pointer on the upper right of the alternator housing to line up with the mark on the rotor inside at the desired timing mm. Only then should you check with a strobe, although it's not worth it, it won't show you anything new. NEVER use that marker to time up with a strobe gun if you have not set the marker up with a dial guage first.
Timing points up can be one in a similar manner, don't forget to set the gap correctly.