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Old 07-06-2012, 11:20 AM   #1
caryder OP
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Joined: Nov 2007
Location: Phoenix
Oddometer: 217
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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Chuck
2012 Honda NC700X
2006 Gas Gas 300 Pro
2003 Kawasaki KDX220R
1976 Yamaha RD400C
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