Happy New Year!
I’ve recently brought my NSR250 back on the road, but I’m struggling with some persistent running issues.
The bike was "pieced together from a mountain of parts," and since I'm using an MC28 chassis with a mix of MC21 and MC28 internals, I suspect major incompatibilities in the intake and electronic setup are causing the trouble.

My Current Setup:
- Engine: Early MC21 wet engine with MC21 flywheel
- Electrics: PGM-III unit (MC21) and full MC21 wire loom.
- Carbs: MC28 carburetors with MC28 TPS.
- Jetting: MJ 145 / Power Jet 80 / Pilot 42. (all on both cylinders).
- Air System: MC28 solenoid system and hoses.
- Airbox: Original MC28 airbox.
- exhaust: original MC28 derestricted

The Symptom: The bike isn't running cleanly, especially in the transition zones and during deceleration (stalling issues when quitting throttle.

The "Back to Basics" Plan: To eliminate the guesswork, I’m planning to revert to a full MC21 intake side to match the PGM-III:
- Switch to MC21 TA22B carburetors (including the matching MC21 TPS).
- Install the MC21 solenoid hose assembly
- Switch to an MC21 airbox (wire splice performed, lid modified/drilled).
- then rejet for new components

I have two specific questions regarding the compatibility of my current "mixed" parts:

1)
TPS Signal: I’ve read that the MC21 and MC28 Throttle Position Sensors have different resistance ranges.
Can the PGM-III actually interpret the MC28 TPS signal correctly? I'm worried my ignition and RC-valve mapping might be receiving garbage data.

2)
Solenoid System: My understanding is that the PGM-III is designed for the MC21 hose routing, which relies on physical air jets inside the tubes.
The MC28 system seems to rely on a different duty cycle/logic from the PGM-IV. Is it correct that the PGM-III simply cannot control the MC28 air system properly?
The solenoid valves themself should be interchangable, since on CMSNL the part number from MC21 and MC28 are same.

Am I on the right track by standardizing everything back to MC21 specs to match the PGM-III?
I'd appreciate any advice or confirmation from those who have dealt with these hybrid setups before.

Kind regards from Germany, Felix

To be perfectly honest, while you could, possibly get it to eventually run, I wouldn't waste my time with the MC28 carbs and air correction. They're great on a stock MC28, but a terrible idea for anything else. You idea to revert it to MC21 is bar far the superior option, in both simplicity and performance.

The MC21 and MC28 TPS is essentially the same, but the initial position may be set differently, and it's all about resistance/voltage from it to the PGM. You can try going through the 87-93 Service Manual to try and get the output the same, but all the air correction and jetting is very "wrong" for the MC21. You also need to consider the expansion chambers. If you are using stock MC28 chambers (to clear the Pro-Arm), they (and the silencers) are heavily restricted, and will also affect setup.
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Andy.
NSR-WORLD.COM

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Hello Andy,

thank you for your thoughts about my plan. Then I will search for a MC21 carburetor with TPS and air correction components, this will surely be the best choice.

One more question on the air correction components differences between MC28 to MC21, because I think i can perhaps directly use the parts I already have:
The solenoid valves and the "+" (X-connector) have the same part numbers on CMSNL for both models. However, the "Y-connector" has a different part number.

Does anyone know if I can just use the MC28 "Y-connector" for the MC21 carbs as long as I follow the correct MC21 hose layout?
Is there a notable difference in the internal restriction of the "Y-connector"?
(alrealy have read some threads about the X and Y Connectors, but I'm not quite sure)

Thank you and kind regards, Felix

I honestly think that is your best course of action, Felix.

As for the air correction, yes I believe the MC21 and MC28 share the same solenoid setup, but the X and Y-pieces are "tuned" to the particular application, so particularly, if you are retaining the airbox, you would be wise to fit the MC21 Y-piece. We have actually mixed MC18, MC21, and MC28 X & Y-pieces, and MC18 & MC21 emulsion tubes to fine tune MC21 fuelling in the past, but that's starting to get really technical and fussy!

If you check the Stock NSR250R Jetting page in the Workshop Section of the main site, you will see there is a large variance between the MC28 and MC21 [and MC18] base settings. As I said previously, the MC28 carbs could be made to work with time and patience, but there is no starting point I can give you or recommend, and honestly believe the MC28 TB type carbs are actually a step backwards performance-wise from the TA series, particularly for a derestricted bike.

While the crankcases, crankshaft, barrels, and heads are essentially the same between MC21 & NMC28, the MC28 ignition is significantly different [as you already noted], and both the airbox and exhausts (both chambers and silencers) are substantially more restrictive. Also, the way the PGM-IV controls the air correction differs significantly to previous models. While the setup works extremely well for a standard MC28 in road trim, and it unquestionable the most refined & smoothest of all NSRs to ride in standard form, its fuelling requirements are very different fuelling to that of the MC21.
_________________
Andy.
NSR-WORLD.COM

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Please DO NOT PM me for technical advice. My time is precious, and you will probably receive a faster response on the Forum anyway.

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