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Makotosun

Performance question

  • Don Dieago
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Performance question was created by Don Dieago

So lets say on a DT250 with a 28mm carb other than the manifold size if a person went to a 30mm or maybe a 32 and jetted perfectly do you think any noticeable if any power gain would be seen? On both my bikes when were stripped to frame I drilled 4 3/8 holes in air box on outside air filer area and out of site to let breath a little better whether it helps or not IDK but I know didnt hurt seem like they run better without lid on so. I also have read a couple times where exhaust port side runs best ported to 70% of bore diameter anyone know anything on this first hand? They also stressed opening width not height unless a professional !
Last edit: 01 Mar 2020 18:05 by Don Dieago.
01 Mar 2020 17:24 #1

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Replied by RT325 on topic Performance question

32 won't hurt it but they're pretty mildly ported & restricted in the exhaust system in factory condition. I know on the airbox of my RX125 it had a restrictive noise baffle for induction noise & was down in where the air pipe to the carb meets the airbox itself. Made quite a difference without that fitted.
YZ top end & exhaust--that'd fix it.
01 Mar 2020 18:37 #2

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Replied by RacerClam on topic Performance question

First of all Is Your 250 reeded or piston port what year ? Second a 28mm is way too small for a 250 to make any good power , So a jump if would be a good thing , how much depends on reeds ore not and other modifications . The carb shown has venturi dividers that I fabricate often the mane a big carb behave like its not . I makes it like openong 4 small carbs in series so air velocity stays high through all slud positions making linear power , no blah sags . A Ufo in the slide ir a great mod also even in a stock carb. Do NOT! widen the whole port or raise the whole port . The best port design is trapizoid shaped like in the picture this is a Honda MR 250 it started as a Rectangle shape and I altered that , If you want to know specifics about this stuff give me a call 702-858-9177

Rich
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01 Mar 2020 18:39 #3

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Replied by MarkT on topic Performance question

Widening the port will change things... and if not done right you can end up with drastically shortened ring life. Really, really short ring life if you go a little too wide or don't get the shape right... like "ring catches in the port and breaks locking up the engine" short. (Don't ask me how I know!) :Ugh

I've experimented, studied everything I could on the topic, and learned from experience... and had some great pros give me some tips years ago... and I still suck at porting compared to the pros. Especially these days. Porting has come a long way from the 1970's. Doesn't stop me from still doing it... I recently spent many weeks slowly porting an engine for a speed record attempt in the 175 class... I've resisted the temptation of taking things to the max... stayed a little smaller on the exhaust than I think I can go... have to run it it and see... then maybe I'll take it to the limits of what I think I can... might end up junking the cylinder and starting over. (I have five more cylinders.... hopefully that will be enough!)

If I was smart, I'd leave my porting to the pros... but I've always done my own since I was a kid and I know I'm probably going to screw up a cylinder or two or three... Plus nobody has ever accused me of being "smart" :ROFLOL

That's one of the mysteries of the two stroke... they appear so simple, yet what goes on with the ports is incredibly complex! For good power, especially a wider, usable, powerband... you really need to know what you're doing. Port timing and sizes need to be balanced with each other. Indiscriminately grinding away can (and often does) end up with little gain and likely a loss at least someplace in the power. Just a single mm "too much" can really hurt the powerband making it "peaky" and not so fun to ride. (Again, don't ask me how I know)

A smart man will find and use a trusted professional for porting changes... it will be money well-spent. :Buds
1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
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01 Mar 2020 19:16 #4

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  • Don Dieago
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Replied by Don Dieago on topic Performance question

its reed valve engine also I wasnt going to hog out the port just maybe widen it up3-4mm on each side and polish and at least match the cylinder to case. Also I have no shortage of carbs be it keihin or Mikuni laying around Im not looking for race numbers mainly because I dont really want to mess with the exhaust pipe I dont want a super loud exhaust because Im on good terms right now with my neighbors. Im pretty ok with the low end but both bikes run good they just seem to be lacking up around 4500-6000 area. I already have fiber reeds and a perfectly lapped cage. I had a MX donor for my 400 and know they made a 250 but that just seemed way to radical for street use after I thought about it a while so dont really want a yz top end and pipe. mediocre improvement yes. race bike no
01 Mar 2020 19:58 #5

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  • Don Dieago
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Replied by Don Dieago on topic Performance question

Rich Ive never seen those very interesting! What do reed stuffers do? low end? I know years ago on new model watercraft there was a issue and factory sent us a kit with oval shape stuffers but cant remember much more that was back in 1992



Last edit: 01 Mar 2020 20:05 by Don Dieago.
01 Mar 2020 20:04 #6

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Replied by RacerClam on topic Performance question

Reed stuffers are great if they can be used , they kep the air from expanding and slowing down when entering the reed cage , But if using v force reeds then the game changes . My venturi dividers keep air from expanding when passing the slide keeping velocity up they work.

Rich
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02 Mar 2020 09:04 #7

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Replied by RTD2 on topic Performance question

Rich, you just hurt me with the MR250 example. I may need to send you the cylinder for mine as soon as I get to Flagstaff permanently in mid-March. I LOVE that bike and to make it better really excites me. And yes I am the bad guy who showed up at Endurofest on a Honda...Sorry.

Mike
68 DT1, 70 DT1C MX, 70 Husqvarna 250 Cross; 71 CT1C, 71 RT1B, 71 DT1E MX, 73 AT3MX, 73 MX360, 76 Honda Elsinore; 2021 KTM 500 EXC-W; 2001 Honda XR400R; 2007 Harley Davidson Sportster 1200
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02 Mar 2020 19:36 #8

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Replied by RacerClam on topic Performance question

Mike , Nice bike. I showed up on my Hodaka 250 a couple of years ago . Give me a call when you want to talk about your bike .

Rich
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03 Mar 2020 08:35 #9

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Replied by Bigdog302 on topic Performance question

Sweet MR250 there! You can use 1975-76 CR250 parts like the Head, Cylinder and Pipe to pep it up a bit and these parts are a bolt on! Honda did make a GP cylinder for the 73-76 CR250 as well. the 1975-76 cr250 I'm told basically has the 73-74 CR250 exhaust port replicated in the 75-76 cylinder. not sure whether it is the 73-74 GP cylinder exhaust port or the Basic 73-74 CR250 Cylinder through, should had asked when being presented this information.

The GP or Grand Prix Cylinder is mostly a very meaty midrange and extended top end revs but you loose the spread of power somewhat on the 73-74 and not totally sure on the 75-76 CR250. the 75-76 CR250 stock has a better spread of power but 1 hp less on peak hp compared to a stock 73-74 CR250, no where the mid range 4500 rpm explosion as found on a 73-74 CR250.
2 1968 DT1s,1970 RT1M,1970 DT1C,1971 RT1B,1971 Honda Z50K2,1974 DT100A 1974DT250A,1974 Honda MT250,2 1975 DT400Bs,1975 Honda CR250M1,1978 DT175E,1979 Honda CR250R.
Dave
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Last edit: 30 Mar 2020 21:26 by Bigdog302.
30 Mar 2020 21:23 #10

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