RD400 first run after rebuild, very low power

[SoCal250]

If you are usng the correct procedure with the dial gauge you shouldn't need to move anything when you check it with the engine running. The timiing marks should line up when checked with the inductive timing light.

Not sure what you mean about "Set the gauge at 2.00 mm deeper" but maybe this will help. I hope my explanation below makes sense


When looking at the ignition points plate, the contact point on the left is for the left cylinder and the one on the right of the plate is for the right cylinder.
The engine normal rotation is anti-clockwise when viewing the points plate. So to turn the engine forwards, turn the crankshaft bolt anti-clockwise or to turn the engine backwards go clockwise.

Here's the quick rundown on using the dial gauge to set timing events:
1)  Insert the dial gauge into the plug hole (make sure it can't move or shift)
2)  Rotate the crank around until you find TDC. (You will see the sweep of the needle change directions, where it stops between changing direction is TDC)
3)  Set the gauge to 0.00 at TDC
4)  Rotate the engine backwards, watching the dial gauge until you are passed 2.00. I usually go back to about 3mm and then approach the 2.00mm mark as this will eliminate any potential error from crank slop. When the gauge is exactly on 2.00mm away from 0.00 the points should open for that cylinder (the test light will turn on). You will want to loosen the points screws and adjust so the light comes on exactly at that position.
I usually repeat this several times until the light turns on exactly where I want it. It will take several tries because very slight movements of the plate will move the timing event.
5) I also make sure that my timing point marks line up when I'm at 2.00mm BTDC
6)  Repeat the process using the light for the other side. No need to adjust the timing pointer on the other side because it should already line up since you're crank is 180 degrees.

NOTE: Since we went BEFORE TDC, after you have zeroed out the gauge at TDC you will be subtracting 2.00mm On my gauge that is two full sweeps of the large needle beckwards.

[ElFuegoBlanco]

If you are usng the correct procedure with the dial gauge you shouldn't need to move anything when you check it with the engine running. The timiing marks should line up when checked with the inductive timing light.

This is the point of my confusion in the process, since I am getting such a different reading with the light.
I've gone over the buzetti gauge steps over and over. They're really not hard.
Insert the gauge
Set it at zero
Rotate the crank counter clockwise until the post reaches its highest point.
Tighten the set screw
Back off the crank
Rotate the gauge dial two full rotations for 2mm (1/10 rotation=.1mm)
Rotate the crank counterclockwise until it hits the gauge

It really is that simple, as I understand it.

So why the timing would be so far off when running, I don't know. Bad plug wires or caps? They're undoubtedly original. Bad coils? I ohmed them out and they're within spec, but again, likely original. I don't even know if coils or plug wires would cause the timing to be off when running vs not. I do know I've had bad performance problems with bad plug wires, but I don't know enough about it to say if it would cause what I'm seeing.

So maybe a bad gauge? That's possible. They have a great reputation, but who's to say?

[m in sc]

when the marsk line up riunnign the timing is good enough to diagnose the rest.

its on the carbs i think since that is sorted. what is the exact carb setup? jet tubes, needles (stock? rebuild kit ones?) etc.

[ElFuegoBlanco]

when the marsk line up riunnign the timing is good enough to diagnose the rest.

its on the carbs i think since that is sorted. what is the exact carb setup? jet tubes, needles (stock? rebuild kit ones?) etc.

As far as I know, everything is stock with these exceptions:
Main jet: 250
Pilot jet: 35 (I completely forgot I changed the pilots to 35)

I haven't replaced any other jets or the needles.

I'm read up on building a diy leak down tester, so I'm off to the hardware store in the morning.

[m in sc]

did you pull the jet tubes completely out and clean them and the air jet?

[ElFuegoBlanco]

did you pull the jet tubes completely out and clean them and the air jet?

That's an excellent question. Honestly, I don't remember if I did that when i did the carb work in 2020. I'll give it a go in the morning.

Thanks!

[ElFuegoBlanco]

Unfortunately, no progress today.

I gathered the materials for a leak down test, so hopefully I'll run it in the morning.

One thing I found funny is I put a plug cap in my pocket so I'd remember to pick some up from the auto parts store while I was out. I reached into my pocket and found insert had worked itself out of the cap. I'll be pretty disappointed in myself if a shoddy cap is the root of my problems.

Once I get the leak down tester together, I'll post some pictures for anyone who needs them

[m in sc]

the cap is likely. i had one take a shit on me one time, temporarily fixed it with a part of a cigarette wrapper, the tinfoil (years ago).

[ElFuegoBlanco]

the cap is likely. i had one take a shit on me one time, temporarily fixed it with a part of a cigarette wrapper, the tinfoil (years ago).

That's awesome.
I was in the parking lot of the hardware store and reached into my pocket and pulled out the threaded insert of the plug cap. I stood there for a solid minute before I realized what it was. It's like I've definitely seen this before but what is it, where did it come from, and how did it end up in my pocket.
Hopefully new plug wires and caps will do the trick.

[ElFuegoBlanco]

Leak down test has finally been performed.

There was a leak in the left side reed cage. Cleaned up the surfaces with fine san paper and it's good to go.

What's not good to go is the left side crank seal. There's a small amount of air leaking out.

Where do I need to go from here? The seal was replaced when the engine was rebuilt. I don't suppose there's some weird trick that doesn't require removing the motor and splitting the case? Right?

[ElFuegoBlanco]

Over fifteen minutes or so, the pressure dropped from 5psi to 2. Still very slowly bubbling out of the seal.

[pidjones]

Leaky fork seals can sometimes be sealed by sliding a piece of 35mm film GENTLY under it and cleaning out bug guts. I doubt you have debris fowling the crank seal. More likely a bur on the shaft or carelessly installed.

[ElFuegoBlanco]

Leaky fork seals can sometimes be sealed by sliding a piece of 35mm film GENTLY under it and cleaning out bug guts. I doubt you have debris fowling the crank seal. More likely a bur on the shaft or carelessly installed.

Careless installation is the story of my life. I'm wondering now if the seal was installed backward, but I've seen a lot of conflicting information out there on it. Either way, it's looking like I'm splitting the cases. But for what it's worth, the spring on the seal is on the outside of the case. Along with the printing. I'm 99% sure I verified it was correct when I installed it. It was from the "Sudco/ARS Engine Oil Seal Kit for Yamaha RD400/350/250 R5/DS7" from Economy.

[m in sc]

glad you caught the leaks but i seriously doubt that the cause of the running issues you are describing.

[ElFuegoBlanco]

glad you caught the leaks but i seriously doubt that the cause of the running issues you are describing.

I'm in inclined to agree, but I'll add that the leak at the intake was pretty bad. So at the very least, that was a problem I'm sure.

So what now, then? Put it all back together, hope for the best on that seal, and continue looking for the root of the performance problem? Yank the motor and replace the seal, focusing on the performance problem along the way?

Thanks for your help everyone. I'll be owing some homebrew beer or home roasted coffee to people here by the time this is done.