Stuttering after full engine rebuild

[Davidae5]

Hey guys! I'm new here and my english isn't perfect
So my '94 125dtr stutters at higher rpm at full throttle, but if i gently pull the throttle it runs perfectly.
My mechanic said it could be bad compression or poorly set power valve.
So we set the power valve pinned fully open and nothing changed, after this we measured the compression and got 3bar.
After all this we rebuilt the whole engine, and now the power valve is working so it's not pinned open always.
Still the same issue with stuttering.
We have no clue what to do now.

[Calum]

Hi there and welcome.

3 bar? How did the engine even run, are you sure you measured it correctly? That is way too low. You should be looking for something north of 10 bar on an engine after the break in period.

Something isn't right. The usual culprit is using non genuine head gasket. The white head gaskets don't work. Use genuine or Athena.

Did you put the head gasket the right way up. It has a little cut on the inlet side that should read UP. Head gaskets are also no reusable so make sure you use a fresh one.

Are you sure you're using the right sized piston and rings? What's your cylinder code? 3MB00 or is it 3BN as the latter has a larger bore.

[Davidae5]

@Calum
Hey after the rebuild, and the break in we measured good compression

[Calum]

Is that 3 Bar? As that is really low and I'd be very surprised if the engine ran at such low compression.

[Davidae5]

@Calum
Yea, before the rebuild it run on hopes. But now it runs perfectly, just that stuttering or missfire is annoying.

[Calum]

@Davidae5 Cool, so what compression is the engine making now then?

What about my other question?

It shouldn't misfire anywhere.

The main points I am thinking is if you've not followed the steps I described above is that you're leaking coolant into the combustion chamber, which at high RPMs is causing problems.

Similarly, it could also be something simple like the coil, HT Lead, or even just the spark plug.

But I want to eliminate the questions I set out above before throwing a million and one ideas around.

[Davidae5]

@Calum
The engine is making around 10-11bar.
Cylinder got a brand new bushing with the stock bore and new piston(wössner) too.
We put everything back together like you said.

Yes, it could be something electrical related just don't know where to start.

The engine code is 3BN. We ordered every part for this engine code.

[Calum]

That's more like it.

The problem is, the bottom end is labelled 3BN, but the cylinder will have something stamped on the side. Very early DTRs ran a different cylinder to the rest of the models (3BN) and the later ones 3MB00.

So just "ordered every part for this engine code" is misleading as the engine code is used to reference to distinct part numbers.

The fact it's now making 10-11 Bar is a good sign that you haven't accidentally ordered the wrong size piston, as I said before, the 3BN runs a bigger bore.

The stock Mikuni TM28S runs something called a "Power Jet". This alters the mixture at wide open throttle.

You might want to ensure that this is not blocked, this could be causing the hesitance at the higher RPMs/Load/Throttle.

[Davidae5]

Whoa.
Yes, it's a good thing we didn't ordered the wrong parts for my bike.

So you're saying i should check the carb?