Hi Graham
Based on quite a number of Märklin steam locos I have, I suggest you check (by decreasing order of probability):
- the distance between the tender and the loco (must be in extended position for usual radii). Some locos may have a weak adjustable link (on my BR 06 002) so I had to add some temporary glue (Noch Hin und Weg Kleber) to prevent the link to snap to the closer position and prevent normal operation.
- do not install some part from the super-detailing bag (e.g.: the protection tubes on piston cylinders)
- the spring pushing the pony truk and the one pushing the front bogie. Sometimes, it may be a little weak, so you bend it SLIGHTLY using a pair a long-nose pliers. This is delicate work and you don't want to ruin the spring. Sometimes, the re-assembly may be tricky (proper orientation as the front bogie is fitted with an elongation mechanism.
- on a recent Märklin BR 18.5 I have, they have extended the floor in the cabin or in the tender and when installing on the rails; there is a possibility to install the extension over / under so one of the loco or tender is supported by the other and its clearly not working fine.
- the inner distance between flanges of wheels. It must be 14.1 to 14.2 mm

When in doubt, check an enlarged picture of the explosion diagram about the orrect position of parts


BTW: NEVER EVER use normal smoke liquid (Märklin or others) with dynamic smoke locos like this one as it will ruin the smoke unit, use ONLY the LIGHT version 02421.
I learned it the hard way with a trip to Göppingen.

Sorry for all these things you know already
Cheers
Jean

Yes, my newer br 38 also.

It will be fine by use ESU smoke fluid too.
Both ESU and Märklins smoke unit are same process so to use light version.

"Have you tried with thin small lead weight to add on the front wheels truck?" I thought about that but there really wasn't any place to add weight and then there might be the issue of the weight causing even more problems. Increasing the pressure of the spring solved the problem beautifully.

Hello All,
This is a problem that I also had with an s3/6 (BR18). One switch was causing this problem. Fixed it with a bit of tweeking -- slightly bending the metal spring on the front bogie. I also added some weight in the front of the loco, which did help with the balance.

Just my $0.02.
Andry

Yes I am having problem with my new Class 18 323 on C track. My other 7 locos run fine forward and reverse on my layout. I have dissembled the front bogey of the 18 323 and nervous to try and alter the spring (bronze?) - Its quite stiff and concerned I might snap it. I would expect anyone with this new loco would see the same problem.

Thank-you. I understand your posting and tried this - about 3 times (a little bit more each time). I'm still getting problems with this loco whereas my other 7 locos run well on my layout. I will talk to the NZ dealer this week and ask him what next. I'll send him some videos showing the problem - I reckon there is a minor design issue with this model