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One for RT325

  • shyted
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One for RT325 was created by shyted

RT 325,did i read you short stroked a 360 motor by putting a 250 crank in 360 cases.
If i did, how did it perform? Did it run okay?
Was there a good amount of torque ?
Regards 
Dave.
26 Jul 2022 00:09 #1

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Replied by RT325 on topic One for RT325

Gosh, i shouldn't have had a wine on an empty tummy before tea [dinner]. Ok where to begin. It started with someone saying "Ya can't do that"--so i did lol. Hang on i'll get another wine [got some free bottles from a business that got free bottles & sitting covered in dust & i don't drink hardly at all]. So ok the real question. I knew the crank would fit so easy 'as'. 250 stroke is 6mm shorter so took 6mm off the top of a 360 cylinder which ended up giving it about 180 degrees exhaust duration. I had to use a shorter rod [311 comes to mind] or shorter than a 214 dt1 rod as i think there's a difference in gudgeon height in the piston between 250 & 360 [it's a blurr now]. Cylinder with 6mm off the top came very close to the dercomp hole by memory. Inlet was compromised but probably drilled a hole high up on piston. No, thats the one i used an RT1 piston & set at bottom dc then felt penned the inleft shape onto the piston & cut it out. It never broke but always had me worried till i parked it. She ran sweet though & silky smooth & a bit pipey/camy & came on strong. Main problem [& i might try again] was compression was trials bike low--too low & could almost start it by hand on the flywheel. Used to stop it with foot on the flywheel. I'd forgotten but i had a YZ250A crank in it so slightly lighter than a DT but didn't do that on purpose it's just what i had. Woulda had a big carb on it too. Also an old procircuit pipe off something adapted. Oneday, if i live long enough & stop injuring myself haha i might rebuild it & squeeze some good compression out of it. Ask away if there's things i've not thought of. Be good to see someone do it properly. I just cut X amount off in the cutoff wheel then had to shorten the fins a bit to spin it i my little lathe as hit the bed. I'm sure a had a video of it running. I'll go look.
Ok, found the video, Bit of a hard watch. Clutch slipping like crazy & think it jumps out of 3rd or 4th. Cylinder has horse & car clearance which you hear rattling. Done properly ie spend money rebore & stiff plus high compression, she'd go well. A closer ratio trans would help too. Plus i see at the end ov the video that i have my heavy weighted flywheel on so a small rotor YZ/MX stator would be good. Plus a pipe to suit--where does it end. Bugger its private, i'll alter it if i can remember how. Was 4 years ago looking at the date but actualy seems longer.

 
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Last edit: 26 Jul 2022 01:44 by RT325.
26 Jul 2022 01:23 #2

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Replied by shyted on topic One for RT325

Thanks RT. I have looked at several different things on the MX / RT motor i've got here. 
When i finish what i've got on here now, i'm going to look at experimenting with that. I ask this after reading what Turbo dan did with his DT360 motor.
When you look at the piston position in relation to the exhaust port at BDC, the RT is exactly as described by him on his DT 360.
As you have said, you can cut away the piston crown as long as you leave 0.8 mm over the top ring untouched. I couldn't swear to this but after reading the Clymer manual, its stated in with the GYTR mods on the porting map section for the CT1. So don't see that the 360 would be any different. 
I do wonder what could be done with the head. I also think an 06 KX 125 reed cage would go in. It's pretty close as it is. 
The other thing is rather than doing the head, the gasket area of the cylinder could be dropped and thus the liner. I did that on my LC 350 by .08mm and cleaned the gasket area of the head up. Big difference with it after that.
So ill gather some info as time goes by and see what happens.
The clip is good and despite the bike sounding like a cross between a YZ, Woody Woodpecker and a Daimler auto, it just demonstrates what's possible.
Thanks for the response RT.
Dave.
Last edit: 28 Jul 2022 03:44 by shyted.
28 Jul 2022 03:34 #3

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Replied by RT325 on topic One for RT325

All good & have fun experimenting. Not sure where i said about 0.8 above the top ring but whenever that was i might've been talking about when i reverted back to a 360 crank but was stuck with the shortened cylinder. So juggling different rods & things i got it to where the piston just came out the top. So to get around that hitting the head i curved the piston crown to the combustion chamber shape as closely as i could, which raised the comp pretty high, i guess. Not an ideal shape but ran ok although never used it in anger as cylinder was pretty loose on clearance. That motor is parked up nowdays as sold the twin frame but have a terrible video somewhere of it running in that frame. Was partly when i was experimenting with funny ignition ideas. Just cdi stator coil & no trigger but used an RG50 plug coil with box combined, one wire from source to coil/box--& away it went.
Found the video of that but its almost unwatchable haha.
Last edit: 28 Jul 2022 04:28 by RT325.
28 Jul 2022 04:19 #4

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Replied by shyted on topic One for RT325

You didn't mention 0.8 RT . It's in the GYTR stuff and shows the crown chamfered away by each port and specifies no closer than 0.8mm to the top ring. That video made me laugh. Probably the best use of the parts especially the TZ tail piece .
Last edit: 28 Jul 2022 15:40 by shyted.
28 Jul 2022 15:40 #5

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Replied by RT325 on topic One for RT325

Oh, ok haha. I'll get a pic of the piston as it ran with the 250 crank. Used a non reed RT1 piston so had to cut a skirt hole but way overdid it by felt pen the port shape onto the skirt. Stupid idea but held together.

I probably mentioned i went the other way on another motor running a 360 crank in a 250. That worked good too. Might have a video of that, in the same twin frame. Getting away from your question sorry.
Found the video. Bit hard to watch & dodgy sound. Has a DT1MX top end & 360 crank. Makes a lazy but long legged fast motor with good power from zero to infinity & revs out of sight. Carburation is dodgy as had been parked for a long time & the usual blocked up varnishy fuel mess inside.
Last edit: 28 Jul 2022 15:59 by RT325.
28 Jul 2022 15:45 #6

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Replied by shyted on topic One for RT325

Did you space the barrel up ? Did you go up carb sizes too. I notice the 360 had a flat slide.
I'm thinking of going up 2mm for my RT1B.
28 Jul 2022 16:19 #7

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Replied by RT325 on topic One for RT325

Its all a bit of a blurr nowdays but on this video 360 crank in 250 i can see a plate under the cylinder which makes sense due to 6mm stroke increase but recal the ring 'just' showing at the top of the inlet port but safe. Being an mx cylinder it'd be safer on a std cylinder with a smaller port or just run the top ring. MX has a single ring piston & possibly slightly further down the piston than a two ring top ring guessing a bit. Carb is my TS400 carb i keep passing around bikes as like it. Heavy brass chrome slide. 34mm by memory off the first model TS400 in about "72. I like the long filter side of carb as easy to keep the filter hose on.
The 360 earlier video has a flat slide EI carb that i've had for years & never used. Carburation is a bit hit & miss but close. Only adjustment is via needle height & different flat taper needles if which i have none damn it. Well, have one someone had filed down probably for using methanol fuel. Just a bit of a novelty carb i thought i'd better try incase i want to sell it--carb that is. Only a 32mm. Talking bigger carb on an RT1, i used to ride to work on an RT1 with a 38 bing stretched into the biggest manifold i could find to fit the studs. Then mate took his 38 bing back after many years & left me with a his 36 bing with one float missing. So that worked just the same & never flooded with one float. Sorry lost track of where we are. Things to do--till later.
28 Jul 2022 16:56 #8

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Replied by MarkT on topic One for RT325

 The method of piston modification to alter port timing is an old one...  played with that as a kid.  Several manuals and magazines described the technique.  I think even some of the Clymer and Intertec manuals had "hop up" sections that discussed it?

Gordon Jennings had a good discussion on it in his "two stroke tuners handbook" which you can find in the tech library.

yamaha-enduros.com/index.php/other-manua...oke-tuner-s-handbook

I know those methods are archaic and modern two stroke design has come a long way...  but I won a lot of races in the 80's and 90's with my home-brew porting and modifications that followed Jennings' recommendations...  I was also a member of the SAE and had access to the published Yamaha time-area engineering studies Jennings presents in his book.  I spent many hours carefully mapping the porting on fine grid graph paper, identifying the existing time-area, and modifying the ports on the graph paper to achieve the time-area I wanted.  Then I simply transferred the port dimensions to the cylinder using a manila folder template.  No computers back then!

My budget home-built engines were fast enough to compete with the high dollar teams in the Mickey Thompson Stadium Off Road Stadium series and in many desert  races with several wins.  So I believe the Jennings info is valid...    that's not to say the new computer porting and pipe software isn't better...  but I also believe that what allowed the big advancements in two stroke power (modern YZ85 has more power than a DT400!) are the advancements in engine cooling.  I doubt an air cooled engine would survive long with modern high-power porting...  at the very least the heat problem would limit the output.  So Jennings info is still pretty valid for the old bikes. 

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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Last edit: 28 Jul 2022 21:48 by MarkT.
28 Jul 2022 21:45 #9

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Replied by RT325 on topic One for RT325

 
28 Jul 2022 21:49 #10

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