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Author Topic: RD400 - Back from the Dead  (Read 15246 times)

Offline LaVolpe

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Re: RD400 - Back from the Dead
« Reply #110 on: Aug 27, 2017, 13:58:14 »
Absolutely inspiring, it's nice to see these old bikes being brought back to their former glory. Makes me want to do a full rest on my GT250.

Offline AimlessMoto

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Re: RD400 - Back from the Dead
« Reply #111 on: Aug 29, 2017, 10:55:16 »
Thanks for the responses!

Currently battling 3 gremlins.

1- Rebuilding the Tach.. all the dashpot oil turned to molasses and it was non responsive. Its torn down waiting fluid. (You can see in the picture the clear lid removed form the pot.) 

2- the LH carb keeps overflowing. I checked float height and its spot on, I think its hitting the inside of the bowl. (I just had them off to chase a bad idle issue... check those emulsion tubes!)

3- Slight top end rattle on the RH cylinder. I think the rings are binding causing the piston to rock on the rings... tore it down and there was minor scoring. Going to double check the bore later this week. Hopefully another quick fix.
« Last Edit: Aug 29, 2017, 12:49:10 by AimlessMoto »

Offline jpmobius

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Re: RD400 - Back from the Dead
« Reply #112 on: Aug 29, 2017, 13:12:00 »
A common reason for the bowls overflowing is the float mechanism hanging up.  Make sure the floats slide smoothly on their pins obviously, but closely examine the brass arm.  If there are ANY wear marks on either the arms where the float pins push on them or on the tab that pushes on the needle, smooth them out perfectly flat with a fine file or sandpaper block and then polish them smooth with at least #600 sandpaper.  Both areas tend to wear only where the fuel level is maintained, and the tiniest hump or notch will keep adequate pressure from the needle.  Also a good idea to polish the needle and seat with a Q-tip and rubbing compound as well as the bore for the needle.  Always use OEM needle and seat assemblies as aftermarket parts are often slightly different in assembled length which screws up the geometry for proper fuel level, though unless you see actual wear or damage to the ones you have you won't need to replace them to correct the problem.  Often it is perceived that replacing the assembly cures the overflow issue, when actually the tiny physical difference in the parts moves the set point slightly enough to no longer  hang the mechanism at previous wear point.
Mobius


On a long enough timeline, the survival rate for everyone drops to zero.

1973 RD350 Yamaha build  http://www.dotheton.com/forum/index.php?topic=66498.0

Offline clem

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Re: RD400 - Back from the Dead
« Reply #113 on: Aug 29, 2017, 21:54:14 »
On an unresponsive or erratic tach or speedometer you can soak a cotton ball in kerosene and place it in one of the back light holes. Place the gauge upright for a few days and it will oil it up without disassembly.

Sent from my SAMSUNG-SM-G930A using DO THE TON mobile app

"After every war there are soldiers who refuse to surrender. To this day there are still thousands who cling to their 30+ year old motorcycles, thinking that the war is still on, refusing to concede that the four-strokes have won"

1972 DS7 http://www.dotheton.com/forum/index.php?topic=45886.msg505995#msg505995
1983 CB550sc bobber http://www.dotheton.com/forum/index.php?topic=30599.30

Offline CrabsAndCylinders

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Re: RD400 - Back from the Dead
« Reply #114 on: Aug 30, 2017, 01:46:56 »
On an unresponsive or erratic tach or speedometer you can soak a cotton ball in kerosene and place it in one of the back light holes. Place the gauge upright for a few days and it will oil it up without disassembly.

Sent from my SAMSUNG-SM-G930A using DO THE TON mobile app

Wow, that's good tip!
Lighter, Quicker, Faster.
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Offline AimlessMoto

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Re: RD400 - Back from the Dead
« Reply #115 on: Aug 30, 2017, 15:08:20 »
A common reason for the bowls overflowing is the float mechanism hanging up.  Make sure the floats slide smoothly on their pins obviously, but closely examine the brass arm.  If there are ANY wear marks on either the arms where the float pins push on them or on the tab that pushes on the needle, smooth them out perfectly flat with a fine file or sandpaper block and then polish them smooth with at least #600 sandpaper.  Both areas tend to wear only where the fuel level is maintained, and the tiniest hump or notch will keep adequate pressure from the needle.  Also a good idea to polish the needle and seat with a Q-tip and rubbing compound as well as the bore for the needle.  Always use OEM needle and seat assemblies as aftermarket parts are often slightly different in assembled length which screws up the geometry for proper fuel level, though unless you see actual wear or damage to the ones you have you won't need to replace them to correct the problem.  Often it is perceived that replacing the assembly cures the overflow issue, when actually the tiny physical difference in the parts moves the set point slightly enough to no longer  hang the mechanism at previous wear point.

The second time I had it apart, I thought it was hanging up on the non OEM gasket I had installed. I could place the bowl on, shake the carb up and down, and couldn't hear the float jumping.

The needle and seat are stock, with 0 wear on them. They seal at the slightest touch of the float when the carb is on it's side.

I think the problem is that the floats themselves are hitting the sides of the bowl. The float hinge pin is smooth as silk, and the little spring plunger is working in the needle correctly.

Offline AimlessMoto

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Re: RD400 - Back from the Dead
« Reply #116 on: Aug 30, 2017, 15:12:00 »
On an unresponsive or erratic tach or speedometer you can soak a cotton ball in kerosene and place it in one of the back light holes. Place the gauge upright for a few days and it will oil it up without disassembly.

Sent from my SAMSUNG-SM-G930A using DO THE TON mobile app

Really?! This stuff was STUCK on. I needed to hit it with brake-cleaner to remove it, and the dash-pot was completely empty. It basically puked itself into and around the friction (centripetal) drive. It was dead from 0-3000 RPM, then jumped from 3000 to redline (on a drill) and then hung up around 4000 when letting it drop to 0. The bike was stored at a 45 degree angle for 10 years up against a tree, outside, with a tarp over it with the bars turned. Basically the worst way to possibly store a tach... lol.

I just filled it up with silicone 50000cts oil (RC Car Differential Oil, basically) and it works perfectly! I just stuck it back together last night and re-installed. It's super responsive, drops back to 0 smoothly, and holds steady at all RPM. I was testing it with a cordless drill, as currently I don't want to run the bike until I get the top end sorted.
« Last Edit: Aug 30, 2017, 15:14:47 by AimlessMoto »

Offline teazer

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Re: RD400 - Back from the Dead
« Reply #117 on: Aug 30, 2017, 16:20:09 »
I missed something here.  Were the carbs rebuilt and if so why is there so much corrosion on the needle jet?

How do the floats touch the bowl and stick?  There's not a huge amount of clearance in there but they can't touch unless they are quite badly bent.

Offline jpmobius

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Re: RD400 - Back from the Dead
« Reply #118 on: Aug 30, 2017, 19:02:45 »
How do the floats touch the bowl and stick?  There's not a huge amount of clearance in there but they can't touch unless they are quite badly bent.

Right.  It should be super easy to eliminate this as problem - just put the floats on their pins and set the gasket on the bowl and see if they can touch.  I've never seen this as a problem, but it is totally possible especially if the gasket is old, used, and dried out.  I just looked at one that has been parked on my desk for forever and the gasket has shrunk a bit and curved inward toward one of the floats.  It isn't hanging the float, but I can see it doing so potentially.  Easy to trim a bit off the inside and eliminate it as a problem though.
Mobius


On a long enough timeline, the survival rate for everyone drops to zero.

1973 RD350 Yamaha build  http://www.dotheton.com/forum/index.php?topic=66498.0

Offline AimlessMoto

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Re: RD400 - Back from the Dead
« Reply #119 on: Oct 03, 2017, 14:47:07 »
Quick little update.

Carbs are back to 100%. I was getting a back knock on the right cylinder, took it all apart and it turns out I was out of round. I was hoping the honing would've fixed it, but it seemed to have made things worse.

I took the cylinders off and mailed them out to a machinist friend. He's got them squared up, bored out, and honed, just waiting on getting them back.

The floats WERE touching the sides of the bowls... a quick tweak and re-check before re-assembly and they're 100% now. No leaks.