RD400 first run after rebuild, very low power

[ElFuegoBlanco]

Correction:
In neutral, it won't rev over about 5k rpm. If I hold the throttle open for a second, it drops to about 4300 rpm and won't go any higher.

When I first started it and revved it in neutral, I didn't rev it too much and the throttle response gave me the impression it was going much higher. I don't think that's changed, I think I just wasn't paying close attention to the tach. It's also loud as hell, even at 5k rpm.

Speaking of loud, there is no baffle material in the exhaust. It all burned away years ago.

The tach also bounces about 500 rpm at idle. It mellows once I open the throttle a little.

[m in sc]

i think your timing is way off or something.

[ElFuegoBlanco]

i think your timing is way off or something.

I certainly can't rule it out, because something is still wrong. I'm using a buzzetti micrometer to set it. I'll ask around town to see if anyone has a dial gauge or I'll go ahead and buy/modify one.

[m in sc]

can you get a timing light? to me, easiest and most repeatable way.

[SoCal250]

After you get the timing confirmed, below for your reference is my current RD400 configuration, which aside being a Daytona is very similar to your setup. My Daytona has the standard 76-78 carbs on it. We may even have the same pipes. I'm a little on the rich side but I prefer safe tuning.

Elevation: Sea level
RD400F
0.25 over pistons
Autolube pump operational
New OEM points, Timing at 2.0mm
Aftermarket vintage chambers (unknown brand, Coffmans?)
1A1 carbs
Y-boot with K&N
And if I remember correctly, here's what I'm running after swapping out the stock Daytona carbs for the standard 1A1 RD400 Mikunis:
250 main jet
30 pilot jet (thought I was running 32.5 but the last tuning notes I have says 30)
stock NJ & JN (175 P-2 & 5L1) - clip #4
Air screws: 1 turn out
Floats set at 23.5mm

[ElFuegoBlanco]

After you get the timing confirmed, below for your reference is my current RD400 configuration, which aside being a Daytona is very similar to your setup. My Daytona has the standard 76-78 carbs on it. We may even have the same pipes. I'm a little on the rich side but I prefer safe tuning.

Elevation: Sea level
RD400F
0.25 over pistons
Autolube pump operational
New OEM points, Timing at 2.0mm
Aftermarket vintage chambers (unknown brand, Coffmans?)
1A1 carbs
Y-boot with K&N
And if I remember correctly, here's what I'm running after swapping out the stock Daytona carbs for the standard 1A1 RD400 Mikunis:
250 main jet
30 pilot jet (thought I was running 32.5 but the last tuning notes I have says 30)
stock NJ & JN (175 P-2 & 5L1) - clip #4
Air screws: 1 turn out
Floats set at 23.5mm

I gotta say, that does sound just like my setup. I think my air screws are 1 1/2 out, though. I don't know the needle jet or jet needle, but I have to assume they're either stock or they were replaced at some point while the bike ran.

can you get a timing light? to me, easiest and most repeatable way.

Working on that now. I picked one up from hf, but I've never needed to use a timing light, so I'm off to YouTube haha

Thanks everyone for your help. Hopefully I'll get it sorted soon.
Rob

[irk_miller]

Have you done a leak down test?  I am reading through the thread and I am not seeing where you have. I would start there and work up.  You'll be chasing your tail if you're sucking air somewhere.

[ElFuegoBlanco]

Have you done a leak down test?  I am reading through the thread and I am not seeing where you have. I would start there and work up.  You'll be chasing your tail if you're sucking air somewhere.

I haven't. I don't have a leakdown test setup for a 2 stroke. I've seen some homemade stuff out there, though.

[Striker1423]

Possible but unless the air leak is massive, you won't be limiting revs. You'd be idling at 4-5k rpm consistently though lol. Saw a buddy's old Yamaha Phazer snowmobile motor running at like 4k at idle and when we finally convinced him to look, the right cylinder base gasket was hanging out

[irk_miller]

Have you done a leak down test?  I am reading through the thread and I am not seeing where you have. I would start there and work up.  You'll be chasing your tail if you're sucking air somewhere.

I haven't. I don't have a leakdown test setup for a 2 stroke. I've seen some homemade stuff out there, though.

I am a novice at 2 strokes, so I can only speak from the issues I have been having, which have been similar.  Two major culprits were timing and an air leak under the left reed cage.  The bike idled extremely well.  The plugs were even a nice toasted color as though the mix were pretty damn near perfect. I chased my tail for a solid week trying to figure it all out.  Did a leakdown and the air went straight through.  It seems crazy bad to leak like that but idle normally or even well.  However, it was a combination of retarded timing and the air leak.  Once you got on throttle, there was no power.  It would eventually take off once you hit 5k, but not half the power it should've had.  Just food for thought.  Air leaks are often the culprit for inconstent issues, as variables tend to be wild.  My kit is made from pvc parts, two 1 1/4" freeze plugs and a low psi gauge from Home depot with a schrader valve in one of the carb plugs.

[ElFuegoBlanco]

Okay, I've thoroughly confused myself. Can anyone refer me to a good writeup on how to check the timing with a timing light?

I guess my confusion is this-
I set the timing 2.0mm btdc.
1. Should the timing mark on the stator(?) match the mark on the plate at TDC? Or 2mm BTDC?
2. When I aim the timing light, should the marks line up?

I think I'm overthinking this whole thing.

[ElFuegoBlanco]

I am a novice at 2 strokes, so I can only speak from the issues I have been having, which have been similar.  Two major culprits were timing and an air leak under the left reed cage.  The bike idled extremely well.  The plugs were even a nice toasted color as though the mix were pretty damn near perfect. I chased my tail for a solid week trying to figure it all out.  Did a leakdown and the air went straight through.  It seems crazy bad to leak like that but idle normally or even well.  However, it was a combination of retarded timing and the air leak.  Once you got on throttle, there was no power.  It would eventually take off once you hit 5k, but not half the power it should've had.  Just food for thought.  Air leaks are often the culprit for inconstent issues, as variables tend to be wild.  My kit is made from pvc parts, two 1 1/4" freeze plugs and a low psi gauge from Home depot with a schrader valve in one of the carb plugs.

This definitely matches what I'm seeing. I posted on the local vintage mc fb group asking for help troubleshooting. Hopefully someone nearby has a 2 stroke leakdown tester so I can go down that route.

[SoCal250]

Okay, I've thoroughly confused myself. Can anyone refer me to a good writeup on how to check the timing with a timing light?

I guess my confusion is this-
I set the timing 2.0mm btdc.
1. Should the timing mark on the stator(?) match the mark on the plate at TDC? Or 2mm BTDC?
2. When I aim the timing light, should the marks line up?

I think I'm overthinking this whole thing.

"Timing Light" can have two meanings. It depends on how you're setting/checking timing -- static (engine off) or dynamic (engine running)
--- With a dial gauge you are doing static, and the timing light used is a simple test light that shows when the points open (the light turns on). You can also use a digital mutli-meter or a buzzer.
--- With an automotive timing light (inductive timing gun) you are verifying that timing is set at the spec you wanted while the engine is running.

When you set the timing with a dial gauge and test light (or DMM), first you find TDC and then you want to adjust your points so they just open at 2.0mm BTDC (or whatever the desired timing setting is that you want to meet). So in the case of an RD400 on today's fuel this would be something like 2.0mm or 2.1mm BTDC.

When you have located TDC and then rotated the engine back to 2.0mm BTDC you will want to adjust the little timing pointer so the lines match up at your desired timing setting. If the adjustable timing pointer on the ignition is still in it's stock position then it would be aligned at 2.3mm BTDC (since the factory spec was 2.3mm for the RD400C/D/E)


If you're looking for a cheap and easy test light setup for static timing, here's what I've been using for 15 years or so:
Gardner Bender GET-202A low-volt tester & a pair of test leads with alligator clips to use as extensions.
With this I can just lay it on top of the engine case or wedge the test light housing between the cooling fins for easy viewing while I'm turning the crank.
And I use a Central Tools #6491 dial gauge kit. It's made specifically for two-strokes and the gauge face is small enough that I can do most bikes without removing the tank.

And then, afterwards I double-check with a Craftsman inductive timing light while it's running to verify that the marks line up.

[pidjones]

Don't confuse 2mm of rotation (not what is set) with 2mm BTDC piston movement (what we set timing to). Mine was set while building the engine, still on the bench, and haven't touched it since.

[ElFuegoBlanco]

Thanks for the great writeup. This is exactly what I was looking for.

I went through all of it again just now:

On the left cylinder, set the gauge at zero at tdc
Back off the motor moving the crank clockwise
Set the gauge at 2.0mm deeper
Move the crank counterclockwise until it stops (using a buzzetti micrometer, my attempts at making a dial gauge fit have failed, badly. maybe time to break down and buy one)
Set the point (orange wire) so that a test light illuminates just at this position
Set the marker on the rotor

Repeat the process for the rh cylinder

I did find that somehow the lh cylinder was a bit out. I've set it three times in the last week. I don't know.

Fire it up.

Connect an inductive timing light to the left plug wire.

The timing mark on the rotor is way off to the right and bounces quite a bit. I let it warm up a bit. Move the point plate until the marker lines up.

Repeat for the right cylinder. This one is more stable, but still to the right of the plate marker. Adjust until it lines up.

Similar result. It runs smoother, but still won't rev over 5k. It's also idling at 3k now. If I press the choke lever down, it calms down to about 1800 rpm. Lift the choke lever, and it stays there until I rev the motor, then idles at 3k again. All of this is new today. It was idling much lower yesterday.