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Carburettor Question


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shmokica

 
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1990 Honda NSR250 MC21
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Carburettor Question

Fri Mar 21, 2008 8:08 am » Post: #1 » Download Post

Can anyone tell me what the difference is between the MC28 and MC21 Keihin's? I have three carbies that came with an MC21 project, two are the same and have two A's in the code and one has a B, guessing the B is MC28 from a search of the forum? The B carbs look cleaner, and I have a HRC jet kit so need to decide which carbs to use long term. Are the HRC kits different for each? Thanks in advance,
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Andy
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Fri Mar 21, 2008 2:58 pm » Post: #2 » Download Post

Easiest way to tell is the venturi.

The MC18 and MC21 is U shaped, and the MC28 is round. U or O, 18/21 or 28! Very Happy

Jet kits for both types are significantly different. MC21 kit will fit an MC18 (and MC16 to a degree) but MC28 only fits MC28.

Technical Resources: Standard carb settings
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montsta56

 
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Fri Mar 21, 2008 7:42 pm » Post: #3 » Download Post

I HAve a similar issue, Are the 28 carb's any better the the A Series?? Apart from being newer?
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shmokica

 
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Sat Mar 22, 2008 5:19 am » Post: #4 » Download Post

Thanks for the reply, they all look like U's, and a recheck of the codes has two BA's and 1 AA. Guessing the AA is an MC18 carb then. Anyway fitted the HRC kit to one carb, and noticed the other already had the tubes, adjustable needle and bigger jets fitted. Interestingly it had 42SJ and 170MJ but still with PJ's. I've gone with the base HRC setting so I'll see how that goes. Will be awhile before I can road test, but seems to pass the stationary test recomendations in the tuning section. One other Q, in the additional PJ blanking pack I noticed a ball bearing, do you know where that goes?

Pic so far

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Andy
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Sat Mar 22, 2008 1:06 pm » Post: #5 » Download Post

The #42 slows are really only for pre-mix setups.

The ball bearing is a blank for the oil feeds. You are supposed to loop one of the 2T feed pipes between the oil injector unions on the carb bodies, and press the ball bearing into the tube to prevent any bleeding across the carbs.

Use the other 2T pipe to loop the two outlets on the oil pump to blank it off, or remove the pump and fit the HRC blank plugs/plate to the casing.

Of course, if you keep the injector pump, keep the ball bearing in the bag! Very Happy
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