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RT3 with yz400 piston

06 Aug 2017 20:17 06 Aug 2017 20:49 by turbojeep95.
turbojeep95
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RT3 with yz400 piston #1
Had the first start on my new rt top end today! Had a bad experience a couple months ago when my first attempt at rebuilding the 360 ended in failure. I had a circlip fall out and go through the engine, resulting in a top end seizure and junked a standard bore cylinder.

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I had another cylinder laying around but was at at 81.5-81.75 bore so it was pretty worn out. There's a post around here that I can't seem to locate again about a member using a 79 yz400 piston in his 360 due to lack of availability of overbore pistons. Here the standard bore is 82mm and its only 5mm longer than an rt piston. Only other differences I can see is that the stock rt piston has 2 small windows for intake ports and the yz has a big moon shaped cutout.

I picked one off ebay and had Bolland Machine in Beaver Falls match the new piston to the cylinder and take 5mm off the bottom. Threw it all together this morning and took my first ride! First impressions are its really lean and it kicks like a mule. Plug is light brown and the bike when warm surges at idle. Has a good bit of power but I'm hesitant to open it up without more fuel. I'd appreciate any jetting or tuning wisdom I can get.

Current mods and settings:
Boyesen dual stage reeds
Torque engineering pipe
Pod filter
Premix fuel, injection pump disconnected
240 main jet/ 45 pilot/ needle one setting from richest setting

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06 Aug 2017 21:24
Meis
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RT3 with yz400 piston #2
My recommendation would be 280 main jet 60 pilot..32:1 premix.

You want the bike running rich at this point to break it in...

"Get Sideways ".....Meis
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06 Aug 2017 22:05 07 Aug 2017 20:15 by Enduronut.
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RT3 with yz400 piston #3
.

Part of your issue may be coming from a substantial boost in compression
Assuming the Yz400 piston you bought was the standard size, you have increased engine displacement from around 352cc's to 370cc
I don't know the combustion chamber volumes of these two bikes off hand, but besides re-jetting . . . you might want to re-machine your head to better match the larger bore size, and/or raise the exhaust port 1 ~ 2 millimeter, and lastly, consider retarding the timing a smidgen.

.

Control your thoughts . . . or your thoughts will control you!.
You can't accomplish . . . what you haven't dreamed!
Don't aim . . . and you'll hit your target every time!
A wise man knows . . . he isn't!
Failure squared . . . equals success!

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07 Aug 2017 17:40
turbojeep95
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RT3 with yz400 piston #4
That's a lot of jetting! I did order a 245 and a 250 main with 50 and 55 pilot jets. Probably going to grab a set 255-280 and work my way down from what Meis recommended. Also, that timing is killing my foot. There's a new kickstart rubber that's coming apart from hitting my calf!

An interesting note about this current cylinder is that someone before me went to it with a dremel. All the transfer ports are smoothed out and the intake has been opened up slightly. Exhaust port just looks smoothed out. What does raising the exhaust port do? I've rebuilt a few engines but never played with the porting.

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07 Aug 2017 18:31
RT325
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RT3 with yz400 piston #5
Might've been me talking about the 400 piston as have one in one of mine now but forgot to shorten it & working ok but will shorten next time it's apart as gets pretty close to the flywheels. I can't remember now but would have machined the edge of the head out a few mm to be sure to clear the piston. I never thought to lower the compression as are pretty low in std form & my theory is "more is better"--to a point. Pity they never made a center plug nicely shaped head for these. I've got a 36 Keihin flat slide carb on mine but it's way to rich at the moment--goes good though but might go back to a round slide 32mm Mikuni i have as i understand them better & have some jets. The Keihin was just a lucky buy for $11 at auction & is off a mid "90's KX125 by memory.

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12 Aug 2017 15:03
turbojeep95
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RT3 with yz400 piston #6
RT325 your avatar looks familiar so it probably was your post that motivated me to do this project. I'll be looking to you for advice!

I put some new jets in the bike. Went from a stock 45 pilot to a 55 and a 230 main to a 250. I also tweaked the timing a bit. Went to start the bike today and fired right up with no kickback. However, the bike screams to the moon on choke. Scared the crap outta me! It runs ok off choke but doesn't seem to smooth out until about half throttle. What the heck happened? It seemed to run better with the lean jets. I'm trying to trouble shoot and the only thing that comes to mind is my timing is off or it could be my recent discovery of a reed petal that is ever so slightly lifted. For now I'm going to jet down and see what happens but I could use some advice.

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12 Aug 2017 15:58
turbojeep95
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RT3 with yz400 piston #7

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Also, this is the plug after 5 minutes of gentile riding. Nothing over 1/2 throttle so I'm thinking the pilot is too rich. Idle air screw is at the factory 1 1/2 turns out setting.

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12 Aug 2017 21:03 12 Aug 2017 21:05 by Enduronut.
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RT3 with yz400 piston #8
Since no one has responded, I'll jump back in the game.

You have changed one of the things that was making your combustion burn hotter . . . too advanced timing
The other thing (not yet addressed) is the slight increase in compression.

Retarding the timing has fixed your kick-back situation, and is making the engine run cooler . . .
there's a happy spot, I hope you didn't go too far with the timing retard.

So, the point I'm getting at, is . . . your engine is running cooler and probably doesn't need quite so much increase in jetting now.
Maybe even close to stock now ???

If it were me, I'd do as you suggested, and drop the pilot back one size, and see how your plug looks then.
You're on the right track.

I have no answer for the run away choke setting, except that maybe you have a bad air leak somewhere ??

.

Control your thoughts . . . or your thoughts will control you!.
You can't accomplish . . . what you haven't dreamed!
Don't aim . . . and you'll hit your target every time!
A wise man knows . . . he isn't!
Failure squared . . . equals success!

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12 Aug 2017 21:51
Enduronut
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RT3 with yz400 piston #9
.

So I've thought about your run away choke setting, and I know this is a long shot . . .
but most of these Mikuni carbs have a small brass tube that feeds the enricher circuit from the bottom of the float bowl.

If it fell out, or is missing, then every time you activated the choke lever, it would suck air instead of gas.
I would speculate that such a senario might cause the revs to run away as you had described.

Worth checking I suppose! :Coffee


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Control your thoughts . . . or your thoughts will control you!.
You can't accomplish . . . what you haven't dreamed!
Don't aim . . . and you'll hit your target every time!
A wise man knows . . . he isn't!
Failure squared . . . equals success!

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13 Aug 2017 02:50
RT325
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RT3 with yz400 piston #10
If i'm on the right track with the story-- the fuel to the brass tube is metered by the fixed jet in the bottom of the hole where the tube fits down in the bowl, so get something small down there & give it a prod. Is only minute [small] like about wire brush strand wire but is right at the bottom so get in the right light to see right down. Gets fuel to that from an angled sideways drilling in the bowl. Needs to be clean to its full size [small as it is] to richen the mixture properly otherwise all you're doing is opening an airway & that'll make it rev up [more air]

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