Hi there,
trying to get an old Dt400c back to the road. After getting it running/idling pretty well I have only one issue to solve and I am suspecting a failing Pulser, CDI from what I have read here, maybe out of my own thought something mayor wrong with the carb. Maybe someone had the same issue before.

I can´t (most of the time) rev the bike out to more than about 4-5k. It won´t go higher, especially on full throttle. The problem persists on "half" throttle but seems to be less aggressive in "stopping the revs".

The issue is more interesting when the bike is standing and out of gear. Bike is warmed up, Idels good and starts pretty good. But when I go full throttle with no gear I rev up to about 6,5k for a moment and then (I am just holding the throttle pinned to max) the revs drop down to about 6, then 5,5 and then to 5k. Sometimes it surges a bit inbetween 4,5k-5k. With the Oil Pump connected it starts smoking heavy

Thanks for the Help

The "what have I done yet" section:
-completley disassembled Carb and cleand throughly with ultrasonic cleaner and compressed Air
-Magneto area was free of any debris or corrosion
-Rewired all of the Magneto->CDI->Coil Cables. Resoldered new cables to both coils and the pulsar.
-New Sparkplug NGKB9ES (I tried with others I had laying around too, of course after checking that I had enough clearance to piston)
-Decoupled Oil Pump with it just running to a bottle (and plugged carb port) and running Premix
-Checked Pulser and Ignition Coil for Resistance (don't know the values on top of my head, think pulser was a little above 100 Ohms)
-Checked with disconnected Killswitch and Ignition to not get a bad ground on the Killswitch wire
-Checked engine crankcase, carb area, oil pump area, magneto area with brake clean to see if the engine reacted in any way -> no reaction
-Looked into Piston with cam to see damages -> non to be seen (engine was bored and overhauled 30Years ago when the bike was parked
-Replaced Fuel Petcock with new ones
-tank cleaned from rust and debris and tanksealed afterwards
-Checked without fuel cap to see if there is building a vacuum in the tank slowing fuel flow to carb
-Set carb to standard settings after manual and then went one bar towards lean on the needle
-new membranes
-probably stuff I forgot

One word--exhaust, in particular the spark arrestor deep in the removable muffler. I say exhaust that often i've got to be right eventualy lol. Try it minus the complete rear muffler--the complete thing not just pulling the baffle out of it. Now i'll go read ya story sorry as just responding to the heading.

& to add to that, look where the tail muffler engages into the main chamber & be sure its fill size in there as can also grow where the pipe ended it's engagement. If it 'is' muffler related--like it runs ok without it but is damn loud--then best to burn it out good. Put it in the log fire for the night, or similar.

So run it without Muffler and Pipe from the engine so I only have the cylinder and that's it?
Or do you mean leave the pipe from the cylinder back under the seat and just remove the muffler at the end?

Maybe a translation issue on my side here

just remove the muffler on the end leaving the main part of exhaust.

Yes just the muffler on the end as stated above. Just to be clear, that's the complete end muffler not just the removable parts out of it. Then while that is off, look done where it came out of or slide a garden hose in to see if its clear beyond where the end muffler stops as can build a ring of carbon in there.

I'll try that tomorrow, thanks.

From what have heard the complete exhaust system was at some point many, many years ago opened up and the dampening was removed.
Later there was some kind of steel wool added back in to muffle it a bit.

Could be a good clue there in what you say was added & added where, maybe blocking it.

Tried it without exhaust today. Sadly no improvements, just way louder.

I have to see if I can make a video of the issue

 

Just as an update for anyone finding the thread. My issue was the Ignition, most likely the pickup was worn/bad.

I checked the system with a timing light and I had no advance at all when reving.

Replaced my whole system with one from Rex Speedshop and the bike is pulling way better from the start, up to 7000 RPM with no bogging down at 4000