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I think I'm finally on track with this rd400 project I've been working on. I had to take a few weeks off for sanity's sake.

New coils
New carb setup:
Went from 175-p2 tubes to 169-p2
Went from 5I4 needles to 5dp7 on center notch
Went from 35 pilots to 32.5
Dropped from 220 mains to 210

And what probably made the biggest difference, and I'm embarrassed to say this:
The PVC sleeve in the reproduction rd350 y-boot wasn't seated in the filter, so it was very likely blocking the air flow.

It pulls pretty nice through around 3/4 throttle, but still not 100%. 3/4 to full throttle is worse. The plugs are evenly grey.
So there's still some tuning needed. I have 200, 210, 220, 250, and 310 mains to choose from. When I get some time, I need to ride over to my FIL's garage where I can do a plug chop on flat road. Now that it will actually make it over to the other side of town.

Thanks everyone for helping me through this adventure.
Rob

I picked up some new coils from Economy and the product description states:
"We recommend removal/bypassing of ballast resistor on RD400's"

I just want to make sure I have this right. I'm going to connect the existing male red/white wire from the loom to both black wires on the new coils, leaving the three resistor wires disconnected. Then connect the yellow wires on the new coils to the corresponding orange and grey wires on the loom.

Can anyone confirm or correct me?

I've been neglecting my '78 xs750 for far too long. I rode it about 150 miles on Saturday and it ran *fine* but not great. I had a vacuum leak on cyl 1, which normally I realize real quick (because it runs on 2 cylinders lol) but this time it was just running slightly sluggish, especially on hills. Replaced the vacuum cap, fixed an exhaust muffler clearance issue with the center stand (750d 2-into-1 exhaust on the 750e), and cleaned it up for the first time in a while.

I still have an oil leak on the left side that I've been dealing with for a few years. I'm fairly certain it's from the base gasket and I have a replacement timing chain ready to go, but I've just been putting it off. Because what's a little oil on your boot, really?

I posted before about the troubles I've been having tuning a 1976 RD400.
Before I start up again, now that I have the case sealed up, I'd like to get some jetting advice. I've seen the Google doc on what others have done and I'm seeing conflicting information.

My setup:
1976 RD400
65.5mm pistons
Aftermarket pipes of unknown origin
RD350 y-boot with k&n filter
Brass balls removed from carbs, left open

220 main jets
35 pilot jets
2.5 slides
5i4 jet needles, clip on 4th notch (I believe, 2nd position from the bottom)

I just read the 5i4 on the needles a few minutes ago. It looks like maybe they're 350 needles? I'm not really sure, other than that I don't see 5i4 needles for the 400.

If there's anything relevant I've left out, please let me know. It's looking like rain for the next week, so it's a good time for me to get some parts together.

Thanks

I finally got some time to get back at it. I replaced the crankshaft seals (76 RD400) with genuine Yamaha seals. Engine is reassembled and I did another leak-down test before installing it in the frame.

It's still leaking at the left side seal. It goes from 5psi to 3psi in about 20 minutes, then to zero some time later. Where do I go from here? When I manually turn the crankshaft, the leak remains at the same two places on the seal (around 2 and 6 o'clock).

Is there anything else I can check or do? Or am I just looking at at crankshaft rebuild here?

Thanks, Rob

My RD400 autolube pump isn't acting the way I think it should. The right-side is barely putting out any oil. I mean barely barely. In fact, it wasn't enough output to push through the tube. I had to manually fill the tube for it to put out any oil at all. Left side seems fine.

It was much better last time I looked at it, when I installed it and bled it. I hoed it again but it didn't make any difference. I also rechecked the gap and increased it .1mm to be on the higher side. I don't know it that's the right direction to get more oil or not.

Any suggestions?

I restored a '76 RD400 (US) back in 2020 and never got around to running it when I was done (I ran into winter and 2021 was a mess).

Some info:
oversized pistons 1 size over
unknown aftermarket exhaust (someone said Coffmans, but I couldn't confirm it)
RD350 y-boot
K&N filter
brass balls removed from carbs
35 pilot jet
310 main
new points

The motor started after the third kick or so. It wouldn't stay idling without keeping on the throttle a little bit. When I put it into gear, I really had to keep on the throttle to keep it running. Away I went. Very little power through full throttle. Into second gear, the engine died. Started again, same thing, very little power throughout the turn of the throttle. I managed to get all of three houses down before riding back with a sad face.

Also, there's oil leaking from the exhaust in cyl 2 (between the pipe and the part that bolts to the engine, connected with springs). I'm really hoping it's just part of the break-in process, but I thought it was important to mention.

Any advice on where to start?

thanks,
Rob

This may be a dumb question, but I can't find the answer anywhere.

I installed an RD350 Y-boot in lieu of an airbox on my RD400. What do I do with the air vent tubes coming off the carbs? Just send them to atmosphere? Leave the tubes off?

Thanks,
Rob

I somehow managed to install the side cover without taking note of the oil pump drive gear on my RD400c. When I pulled the cover, I found the nut and pin in the bottom of the case and the washer destroyed.
Some meat removed from the drive gear as well. Of course it's no longer available. Are there any options for a replacement? Or am I boned and my only option to fork over the $65 for the one I found on eBay?
Or should I run it as is?

Rob

After running into problems installing a chain with what turned out (thanks to the awesome people here) to be an extended swingarm, I went ahead, bit the bullet, and replaced it with a standard length swinger.

Now I'm left wondering if the brake tension bar is standard or not. Right away, I realized the caliper bracket would need to rotate to install the tension bar, which led me to this question. Will I need to replace the tension bar as well? The bar is 13 1/8" long. Is there anyone out there who can take a measurement of their RD400 tension bar? Or maybe someone can tell me if I even need to worry about it.

Thanks,
Rob

I rebuilt my front master cylinder a few months back. It took quite a long time to build up pressure in the brake and finally bleed them. All was (apparently) good. Fast forward to today when I pulled the front brake lever and had no pressure. The MC reservoir was bone dry. The bike has been on a lift since I rebuilt the MC and I don't see any evidence of a leak. I'm baffled.

At any rate, I'd feel more comfortable just replacing the MC. I was skeptical it could be rebuilt based on the condition it was in when I got it.

What is a decently priced replacement with a LH mirror? I've seen plenty of reproduction units, but they all have RH mirror threads. I normally wouldn't care, but I just bought mirrors. I'm not looking for anything 100% period. Just something that would bolt up and help me stop.

Thanks, Rob

I ordered a replacement 530 chain for a '76 RD400 cut to stock RD400 length from Economy Cycle
The chain was cut to 96 links and it's too short. The chain I replaced is 98 links. Can someone help me figure out where I went wrong here? The rear sprocket is has 38 teeth, if that helps at all. Even with the rear wheel all the way forward, I'm 2 links from being able to connect the ends of the chain.

Rob

I'm in the final stages of reinstalling the motor on a RD400c.

When I press the gear selector all the way down, the neutral light comes on. There's no more going down from there. Lifting the selector, I get only three clicks before there's nothing else left.

Give it to me straight. Did I reassemble the gears backward or something foolish along those lines?

Rob

I finally got my motor buttoned up and need some jetting advice. The bike is a 78 RD400 barn find.

Expansion chambers of unknown make or model
RD350 y-boot with K&N filter (airbox is long gone)
Stock ignition
Stock porting
Cylinders bored to 65.5

When I got the bike, it had 220 mains. I don't know the pilot jet size as they were trashed in the removal process.

Any advice would be great. Lyn recommended 250-310 for the mains, so that's probably where I'll start.

I'm in the process of restoring a '76 RD400. Early on, I managed to destroy one of the carburetors. It was a stupid thing to do, but basically I thrashed the pilot jet threads beyond repair. I've been told that there's nothing that can be done for it. I'm embarrassed by the whole thing.

So I'm left with one working carburetor and need some options. It looks like a replacement from e-Bay isn't really one of them, from what I'm seeing. I'm not looking for additional power or anything like that. Just a running bike. I'd like to keep the oil pump and keep everything as simple as possible.

I see Economy sells a replacement set with oil ports, replacement cable, and jetting for $349.95 – $379.95. That's a little on the pricier side for my budget. What else can I do?

The details- Expansion chambers (unknown brand), currently no airbox (planning on using a 350 Y-boot), everything else is stock.

Any advice would be great.
Rob

My RD400c came with these pipes. I'm hoping someone can help identify them. I don't want to go through the work of cleaning them up and painting them if they're trash. I'm not looking for anything high performance. Just reliability, really.
I know a few pictures isn't much to go on, but I figured I'd throw it out there anyway.

I came across this piece when disassembling the crankcase on a '76 RD400. I set it aside in a bag, but never did figure out what it went to. It's not quite a half circle. It's slimmer than a half moon clip. It's not bendable and doesn't appear like it snapped. I've checked all the washers and circlips on the gears and I'm stumped.

I'm getting ready to begin reassembling my RD400c motor. Aside from gaskets and oil seals, what should I replace as part of the bottom end refresh?

I don't want to get halfway through and realize I probably should have replaced that $2 part I didn't think of.

Any advice would be greatly appreciated.

Rob

I replaced the rear wheel bearings on my RD400C.

After much aggravation with incorrect diagrams, I think I have the components in correctly.

However, the dust seal on the sprocket side doesn't fit flush with the hub, like I thought it did before I pulled the bearings.

Can anyone look at these pictures and tell me if this looks right? And just to confirm, the hop hat spacer "brim" is on the inside of the bearing, against the spacer tube.

Thanks,
Rob