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Old 07-06-2012, 11:20 AM   #1
caryder OP
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Looking for 1976 RD400 tuning advice

Just wondering if there are any knowledgeable RD tuners out there that could offer some tuning tips?

I have a 76 RD 400 and thought I was pretty good at optimizing 2-stroke jetting and ignition, but this one proves to be difficult. I have owned and raced RDs in the distant past. This bike was bought as an almost complete project from a fellow inmate in Wisconsin. Had a fresh motor and was fairly well sorted out with new parts as well as a lot of the stock parts.

It has great off idle response and good pull from 3K to 4k RPM with very little throttle. It gets fat at 4500 RPM holding the same throttle position with a light load. Works better in taller gears (purrs along without blubbering) up until 5.5K RPM then will 4-stroke. At 6.5K it starts to get in the power band and works okay in lower gears (rips up to 10.5K RPM at partial and full throttle). In top gear it will fall off the power band if I let the RPM below 7K and then it just bogs. I donít recall my 1975 RD350 behaving the same way with a similar racing setup. The big difference is the 350 had stock carbs/reeds, and either Jemco or DG chambers, and more radical porting.

The annoying part is WHERE it needs to have a little bit of torque is from 5K to 7K because that's top gear on the freeway. It can't hold speed on overpasses. What I am aiming for is the similar pull that I feel at 3 to 4.5K RPM to maintain through 6K RPM and then hit the pipe at 6.5K RPM.

I don't know if it's the wrong combination of parts (32 mm carb bore, with two stage reed and porting/chambers maybe not working together at that RPM)? Or am I just missing something in the jetting/timing? It behaves exactly the same way with a beat up set of factory expansion chambers on it as well but needs a leaner main jet to run proper at full throttle.

The current setup: Running premix currently @ 32:1 (have also been running 20:1). K&N on Y-boot, Moto Carrera chambers, stage-2 pro-flow porting (donít have the measurements but itís not that radical). Stock ignition, two stage Boyesen reeds and 32mm over bore stock VM carburetors. Heads milled and base gasket thickness is set to get correct squash band measurement. Timing is set to 2.3mm BTC (stock), VP110 fuel used straight or cut 50/50 with premium pump gas (because I donít think I need to run straight VP110 for this motor current fuel is 50/50). Stock needle jet and jet needle. Needle is in middle position. 35 pilot jet, I have tried different needles but stock always seems to work the best. Have raised and lowered the clip one notch, but stock position always seems to be the best "safe" compromise (slightly fat mid range). I currently have a 290 main jet installed. I have run 270 to 300 main jet. 300 is definitely fat. I havenít risked going below 270 without having a better idea as to whatís going on between 5K to 7K RPM.

One thing I can do is reduce the gearing (stock 17/38). At least that would move the flat spot from 65 to 75 MPH to a lower speed.

Am I trying to tune something out that can't be tuned out?

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Old 07-06-2012, 12:42 PM   #2
stainlesscycle
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first try moving your timing to somewhere in the 1.8mm - 2mm range. see if that cleans it up.

then: what is squish currently? and at bottom dead center is the exhaust port fully open or is the piston covering the bottom of it?
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Old 07-06-2012, 03:40 PM   #3
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Timing & Exhaust Port lower edge height

The pistons just clear the bottom of the exhaust port. I have about 600 miles on the motor at this point. I haven't rechecked the squash band measurement since everything has taken a set after a re-torque on the heads several times during break in. I should probably do that, but this issue has pretty much been there from the start.

Plugs read slightly rich to normal most of the time. Just did a plug read today after a ride while spending time in the happy spot (above 7K and mid throttle, it's really responsive). Right plug was slightly grey in color (almost perfect), left was very light tan and kind of wet, so a slight mismatch between cylinders.

Re-checked carb sync and it was spot on.
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Old 07-06-2012, 05:05 PM   #4
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A lot of chambers require a pretty lean needle position, my spec II's did. My DG's were best with the needle in the middle, but when I went to the specIIs, I ends up with a much richer pilot, leaner needle position, and slightly richer main.There is also a common carb modification, known as the Dave Friest mod that uses a different style and size needle jet, needle and air jet. The result is a leaner mixture in the mid range. It made a world of difference on my 350. Good luck, you'll get it!
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Old 07-06-2012, 05:06 PM   #5
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you can do a squish check without removing head - just gently feed some solid core solder in with an 'L' bend through spark plug hole.. it works for quick and dirty check (be careful not to cut the end of the solder off on a port!) - then just turn over motor gently by hand at ignition rotor.. pull the solder out and measure..
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Old 07-06-2012, 05:52 PM   #6
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It's important to get the squish right, but having the squish out of spec won't cause the running issues the OP is complaining about. I'll also add that a flat spot around 5-6k is almost to be expected with chambers. If you're jetted right, it's not very bad, but it's still there. RZ crossover manifolds are supposed to help some too, someday I'll get some.
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Old 07-06-2012, 05:59 PM   #7
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Jet the bike for what it wants with the combo you happen to be using. Every combo of pipe, port, and carb will jet differently. That being said, if you have a miss-matched port to pipe combo, it prolly never will run right.
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Old 07-07-2012, 07:07 AM   #8
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Like allready said , if the porting dosnt match the pipe design, you may never get all the flat spots worked out.

I would first do the 1.8 to 2.0 timing and for grins, if you have a set of stock metal yamaha reeds, put those in and do your main jets from there. Leave the stock needles in the middle position for the time being.

I also would just run premium pump gas if your compression is below 140 PSI.

And yes, the intake manifold crossover with the boost bottle helps smooth out the notorious mid RPM flat spot somewhat.

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Old 07-07-2012, 02:58 PM   #9
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Thanks for the responses.

I understand the desire to retard the timing BECAUSE of pump has, but I'm not running pump gas so I set the timing to spec. Regardless changing timing doesn't help it get out of the hole if I fall off the pipe in top gear or if I under shift running in 4th to 6th gear. Timing has been set anywhere from 1.8 to 2.3 mm (on purpose) with no real change regarding the flat spot. I can tell you if the timing accidentally gets set to 3mm BTDC on pump gas it will pre-ignite and ruin a piston in about 30 seconds at WOT. That was a disappointing day. Seizing up at 90 MPH isn't fun, but I avoided any major drama because I heard it coming. The piston was toast but I didn't damage the cylinder (or head) on the offending side that was miss timed so the cost was contained to two new pistons.

The comments about the porting matching the chamber are spot on. If I had the $$ and the time I would have a set of custom chambers made and do a bunch of dyno time with a knowledgeable tuner, but that's not going to happen (at least not yet).

So the suggestion regarding jetting for the setup I have is the target I'm aiming for. I realize it may just be what it is and I'll have to downshift for freeway overpasses. My wife's car requires the same thing.

As a side note, there is a dramatic difference between the construction of the Factory chambers and the Millennium chambers in addition to the Factory chambers being beat to hell. The flat spot exists with either set installed. The only thing that changes with swapping chambers is the jetting. The Millennium chambers allow for a fatter main jet at WOT. With the current porting, the power is between 7 to 10K and then signs off pretty quick at 10.5K (as measured by the stock tachometer).

The Millennium chambers also seem to contribute to the noticeably better pull from about 2.5K to 4.5K. The motor runs really well when jetted right for that RPM and throttle range. Makes for nice mellow operation at commuting speeds on surface streets. But then from 4.5 to 6k it's too rich and 4-strokes at part throttle unless I'm in a taller gear. It can pull itself past the flat spot with more throttle up to 4th gear but will bog and lose speed with more throttle in top gear. If I nurse the throttle I can sometimes increase speed but generally more throttle results in less speed on an incline.

I might try the crossover tube. The manifolds aren't that expensive. I guess that first started with the Banshee motors and has been found to benefit the air-cooled RD's as well.

I've also read about Dave's mod, although I thought that was aimed at R5 motors and carbs (no reed cage models). I feel if I went so fat on the pilot it would ruin how well the motor runs below 4K at cracked throttle but maybe the different needle makes up for it. I did buy a leaner needle set, but installed I couldn't even open the throttle without an immediate stall. Maybe with 55 pilots that would solve that problem?

I do also have the stock steel reeds and have thought about reinstalling them to see what the effect would be. Yesterday I read that modified YZ 125 reeds are a better alternative to the Boyesen two stage reeds. $46 for a set of four pair on Economy Cycle, enough for two motors.


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Old 07-07-2012, 07:18 PM   #10
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No, you misunderstood the Dave F mod. It's not meant for R5s, it's meant for the RDs. It uses primary type needle jets, instead of the RD's bleed type. This is how R5s were originally jetted, so basically, you end up with R5 carbs on your reed valve motor. It just happens that it helps to get rid of the flat spot you get with chambers. You wouldn't necessarily want to do it on a bike with stock pipes. When I got my bike, it had DG pipes, and big mains and pilots, stock needles and needle jets. It ran just like what you're talking about. Good down low, blubbery in the middle, and it screamed above 7k. It was hard to ride, and annoying as hell. Once I did the mod, it was like a whole different bike. I ended up with 25 pilots 169 PO needle jet 5dp7 needles on the middle clip, and 180 mains, down from 220s before the mod. I could ride it slow, moderate, or fast, and it always ran well. Now that I've got the SpecII's, I went up to 35 pilots, leaned out the needle a notch, and went up to 190 mains. Runs great. When I hear people say how great their piped RD runs with un modded carbs, I just think they don't know how much better it can run. I won't keep belaboring this point, but I'm a believer in it.
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Old 07-08-2012, 06:20 AM   #11
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+1 on the Dave F mods. After a zillion jetting combos and plug chops, I went back to the drawing board after doing the DF mod and started over from there. Couldn't be happier.

Have you done a plug chop after holding it at 6K for an extended period?

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Old 07-08-2012, 10:48 AM   #12
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Negative on the plug chop

I assume you mean have I done the plug chop in top gear with the bike in bog mode.

The circumstances required to get there are on the freeway. I don't really have an opportunity to do that.

If I were to find an out of the way surface street with a long enough run I could attempt it.

I will reread the thread on the Dave mod. It sounds like my memory of the context is ass backwards.

It took me all morning looking at 2SW and USA 2 strokers to find this. Too many interesting tangential threads.
http://justyamahard350.com/articles/david.htm

And then more reading on the crossover mod seems to yield not using the boost bottle and just a crossover tube.
http://www.economycycle.com/servlet/the-1/RD-RZ-Banshee-Manifolds/Detail
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Old 07-08-2012, 09:34 PM   #13
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Ordered $170 worth of jets and crossover kit from Economy Cycle. We'll see where that gets me. I'm going to try the crossover kit by itself before I drill holes in my carbs but I'm pretty certain the Dave F. mod will be the biggest improvement. Seems Dave has lung cancer and is going through chemo.


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Old 08-27-2013, 02:16 PM   #14
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Quote:
Originally Posted by caryder View Post
Ordered $170 worth of jets and crossover kit from Economy Cycle. We'll see where that gets me. I'm going to try the crossover kit by itself before I drill holes in my carbs but I'm pretty certain the Dave F. mod will be the biggest improvement. Seems Dave has lung cancer and is going through chemo.


Chuck

For $100 + shipping you could have got 2 x 32mm OKO flatslides, which are very easy to jet, and would have likely have cured your problems if they are carb related.
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Old 08-27-2013, 09:31 PM   #15
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Cool! Here's my 76: (was mine anyway...)



Ya, I know.... 77 bodywork. The original was painted with a brush by the previous owner.

I was lucky enough to have a Yamaha two stroke race mechanic living in my neighborhood.
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