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Discussion Starter · #1 ·
Hello all,
This post is really out in the weeds, be forewarned. The CRF230F is the CRF150F’s big brother so maybe I’ll be spared for my question?!
I am restoring a 1983 ATC200X, the original engine that came in the bike was not a good candidate for repair. Instead, I’ve purchased a CRF230F as a donor bike to swap its engine into the ATC frame. It’s a bolt in affair less some gentle massaging on the factory Honda mounts.

On to my question for the Planet Mini’s experts, I’m contemplating installing a Yoshimura MJN TMR26 (or similar MJN carb) on this engine for the ATC.

The original 192cc Honda ATC engine came with a Keihin round slide 24mm carb, this engine swap has been successfully completed using the stock ATC carb mounted to the 230 engine using an XR200 manifold, or some folks use an XR200 carb and manifold to feed the engine.

I’ve found more MJN info on PM than any other site out there, so I’m taking a shot in the dark to see if anyone has experience using these carbs on engine with more than 150cc of displacement.

In the interest of keeping the CRF 230 engine reliable I’m thinking of bumping up the compression with a stock bore Wiseco HC piston, maybe doing some high velocity porting on the intake side and depending on the results I may add a camshaft later down the road. No insane triple digit HP ambitions, just a solid reliable trail bike.

To those who have used them would a TMR-MJN26 make sense for this build? Not looking for a 125cc 2t power band; more like a wide, torquey, linear 2 valve head trail bike should be, but with some extra snap and throttle response to break the rear end loose to slide around corners when desired.
Thoughts? Comments? Flames? Bring ‘em on!

I’m open to other MJN carbs, would love an accelerator pump but am not ready to drop $750+ for a MJN28 pumper. If the pumper option is a better route, I’m not opposed to a regular Mikuni TM28 pumper with the standard needle.
I’ve found zero info on the 230 forums about MJN carbs so here I am. Many of those guys are sticking PWK 2t carbs on the 4t 230 motor!?!
Thanks in advance
Tire Wheel Vehicle Plant Automotive tire
 

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195 Posts
Discussion Starter · #3 ·
Great insight, in the case of the ATC the frame really limits the carb dimensions due to the right side downtube wrapping around the carb. I know I’ll probably have clearance issues with the stock carb as the few people who have done the swap either use the PD 24mm that came on the ATC or an XR200 carb. Haven’t heard of anyone successfully fitting a 230 carb yet. I’ll certainly explore that option when I get the bike back together and can see any clearance issues for myself.
Thank you very much for your time and expertise! I really appreciate the reply!

Hopefully this picture illustrates the narrow mounting clearances for the carb.
 

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Discussion Starter · #4 ·
i don't have experience with an MJN on displacements you are talking about, but I do have experience with the CRF230 engines and 150f upgrades. I have a 230F that I built with a wieseco 240 piston and aftermarket cam and stuffed it into a CL125 frame for a custom bike. When it came time to choose a carb, the right choice, IMO, was the stock carb for the reasons you have already stated. It only required a main jet bump by two and runs perfect bottom to top with a pod filter and relatively open custom exhaust.

I also have a 150F that's been converted to a dual sport. And the person who had it before me had Honda swap in the crank, cylinder, piston, etc to convert it to a 230F. It does only have the 5 speed in this case, and is only kickstart (and they had to update the kickstart gears) but this one is basically a stock 230f otherwise. This is mated with the stock carb from an xr200 and runs perfect top to bottom. I have never considered swapping out the carbs since the stockers perform so well.

Lastly, I had purchased a xr200 core engine at a swap meet and rebuilt the whole thing, but added a mild mid-range cam, but otherwise had the cylinder stock. This one, too, i used a stock xr200 carb on it and again, it ran perfect.

I have MJN carbs on several of my bikes ranging from 110cc-175cc and they are amazing. They are also a real PIA to tune them because the atomization is so good that you might think that it's "good enough" when it's actually not optimized.

For your application, I'd recommend the stock 230f carb...
To get the bike up and running I’m going to use the stock PD24 that came with the 200X. But in time, I’ll add either an MJN or a TM28 to this bike if I can make it fit without having to chop up the frame.
Notwithstanding my application, I’d love to learn more about your experience with the MJN carbs. I’ve set up and jetted about any standard combination of carb and 2t or carb and 4t engine, even some not so standard stuff, but I have zero experience with the MJN style carbs. I love the principal of how the work and how even the fuel delivery is comparatively speaking.

I’d be willing to start a new thread dedicated to the MJN carbs themselves and how they vary (tuning wise) from standard tapered needle/needle jet carbs.
Specifically, they obviously work better than standard carbs with a weaker vacuum signal if people are overcarbing their minis like I’m reading about on PM. Do you optimize the jetting with a wideband O2 or do you have another baseline reference?
Thanks for your time!
 
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