'77 Yamaha RD400 "Lost and Found"

It's a fact. RD's and all two strokes have a reversion wave as the piston descends and that wave is reflected back from the hard end on a filter to pass through the carb a second time picking up more fuel on the way through.

Contrary to popular opinion, reeds do not close instantaneously and much of the wave continues to oscillate back and forth in the intake tract. Reeds do reduce that reversion and resulting fuel stand off behind the carbs. For some reason RD's seem to be more affected than some other two strokes, so for decades now, people have avoided fitting hard backed filters to RDs.
 
Just don't plan on anything like stock on jetting. Any diversion from the stock airbox and filter will create the need for jetting changes.
 
teazer said:
It's a fact. RD's and all two strokes have a reversion wave as the piston descends and that wave is reflected back from the hard end on a filter to pass through the carb a second time picking up more fuel on the way through.

Contrary to popular opinion, reeds do not close instantaneously and much of the wave continues to oscillate back and forth in the intake tract. Reeds do reduce that reversion and resulting fuel stand off behind the carbs. For some reason RD's seem to be more affected than some other two strokes, so for decades now, people have avoided fitting hard backed filters to RDs.
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That correlates to what I read. Thanks for confirming my decision.
 
Well with the motor all back together and a few things nipped in the butt the bike is in one piece. Words cannot express the excitement for having my old RD back to a loving riding condition. See below!

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20200530_202424.jpg

20200817_172827.jpg
 
pidjones said:
So nice! Here's to many miles and much smoke!
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Thank you, to be honest it's been close to done and on\off the road for a bit now. The build thread was a bit behind the actual build. With that said the sega will continue, there is still work to be done.
 
New issues with the RD, only firing on one cylinder when I first start the bike. Fires and idols on the right side, after about 30 - 45 seconds of nursing the throttle the other cylinder will kick in. Any ideas? Oh, I tried swapping the coils left to right, no change.
 
Correct side parts in the correct side. Choke float bowl on the left side (they are different). Throttle cable adjustment, idle adjustment. Usually I read of people have problems with the right cylinder because of a leak sucking in transmission oil to that side through a bad seal. Check float levels, too.
 
pidjones said:
Correct side parts in the correct side. Choke float bowl on the left side (they are different). Throttle cable adjustment, idle adjustment. Usually I read of people have problems with the right cylinder because of a leak sucking in transmission oil to that side through a bad seal. Check float levels, too.
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I really appreciate your time in responding. All good ideas. Once it starts to fire it's good to go. I can shut the bike off and re start it 15 min later and both cylinders will fire. The float bowls are marked left and right, as I've taken the time to do so. The right cylinder fires along happily, and the bike has new crank seals. I'm going to re check the float height this weekend when I have the carbs off for cleaning. The idol adjust seems bang on as per the manual and serviceability. My bets are on Maritime's suggestion of a possible blocked pilot jet. When I sort her out I'll be sure to update.
 
Have you removed your balls and cleared the pilot air jet? Another thing to try would be to turn on fuel with cold bike, tap the left carb/float bowl lightly with a srewdriver handle, then start. If the left cylinder behaves then, you probably have a float valve hanging up that vibration loosens up.
 
pidjones said:
Have you removed your balls and cleared the pilot air jet? Another thing to try would be to turn on fuel with cold bike, tap the left carb/float bowl lightly with a srewdriver handle, then start. If the left cylinder behaves then, you probably have a float valve hanging up that vibration loosens up.
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I have tried the carb tap, no change. Removing the pilot air jet is something I have yet to do. I'll look into doing that this weekend when I have them apart as well. I also have another set of carbs on my other RD I could try as well. If that solves the issue I know it's 100% carb related. I really do appreciate your insight, it gives me somewhere to start when I rip into it rather then starring at it wondering what to check first. Not that I don't live starring at it. haha
 
I like dealcoholized beer, the PC brand blonde haha. That way if I decide I want to see the speedo at 60mph I can.
 
I find the red a bit too sweet, I do like a dark beer, but the red tastes almost like syrup. I'm good for one, but then I'm back to the blonde.
 
NSNickName said:
I have tried the carb tap, no change. Removing the pilot air jet is something I have yet to do. I'll look into doing that this weekend when I have them apart as well. I also have another set of carbs on my other RD I could try as well. If that solves the issue I know it's 100% carb related. I really do appreciate your insight, it gives me somewhere to start when I rip into it rather then starring at it wondering what to check first. Not that I don't live starring at it. haha
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You don't need to remove the pilot air jet - just clean it (not very removable). But, you do have to remove the ball to get to it. Check one of the RD specific sites like aircooledrdclub.com or 2strokeworld.net for details. Not hard, just touchy.
 
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