i have a stock Tamiya/OS FS-18SR engine in my Tamiya TNX. When i start it, and so the engine is cold, it runs very, very well, like wheelspinning on tarmac. then once the engine warms up to temperature it starts to be very hesitant and jerky at low throttle and difficult to get up to speed because of the hesitation, especially when accelerating from idle. I am using 22% fuel, with a duratrax silver sport plug.

Any help is appreciated.

We've not seen an O.S. engine with the designation, "FS-18SR". Are you sure it's an O.S. engine? Perhaps it's one that was made especially for Tamiya, and available only through them.

If the engine has the mixture properly-set, then it's possible that the piston/cylinder fit is starting to erode and you're losing compression as the engine warms. That would be a result of a possible overheat at some time, foreign object damage, dirt ingestion, corrosion, and so on. The engine may need a teardown to check it out.

this engine was made by tamiya for the TNX in collaboration with OS engines. i have not run this much, so therefore i do not think that the internals are worn out. Also to piston/cylinder fit is still tight since sometimes i need to loosen the plug to start it. I would think this is a tuning issue because i messed around with the needles. i have returned them to stock, but now i am using a hotter plug, so should i richen the engine. iF so, should i richen only the HSN, or Both the HSN and LSN?

With most engines, "factory" settings are all on the rich side, and you usually have to lean from there.

The engine you're asking about was never serviced by us because it would have been a "private issue" for Tamiya. We'd also not have parts for it, nor know which regular O.S. parts might work in it.

but would it sound like it is too lean on the low speed?

or a lean HSN?

An acceleration problem is usually a result of a bad setting of the idle mixture.

idle?, i thought that only controlled the carb opening at idle. i thought it was the low speed needle that controlls acceleration from low to mid?

The low-speed needle controls the idle mixture. The throttle opening controls the idle RPM. In your last post, you have the two mixed.