Rd400F Daytona

[rearwheelslider]

Hi guys! New member here. I recently shared my RD400F build here.
https://www.2strokeworld.net/forum/index.php?topic=6115.0

I have a few technical questions I hope you all can answer!

-I am on another forum(aircooledRD) and they were quite nasty with me when I told them I am going to run the YPVS/Banshee pistons! I spoke to a rep from Wiseco, and a good buddy that had tens of thousands of miles on a 400 with Prox pistons, and they both said no issue after you trim the center tab off the bottom of the skirt. My machinist is bore matching the pistons at 20 thousands clearance. Is this acceptable?

-I am looking into some exhaust pipes/chambers for the build and it seems there is only one option for the 400F, which is Spec2. Are you aware of any other aftermarket pipes that would fit my bike? I like the spec2 F1 pipe with its stinger type silencer, but $1050 is a bit steep for me.

-I will be premixing with the stock carbs and a y boot/foam uni filter. Exhaust valve removed and oil pump removed(with block offs plate). Stock engine/porting, 1.0mm overbore. What general jetting guidelines can you provide? I am unsure if I will try the stock pipes or shell out a grand for some chambers.

Thanks in advance!

[m in sc]

none of that rivet counting here. welcome! as far as exhaust, specII is always an option or JL. (lomas i think sells them for the f)

would recommend better carbs.

[rearwheelslider]

Better carbs? Can you please educate me? Are the stock 400F carbs not ideal?

[m in sc]

TM32's vm30s, etc. the stock daytona carbs, IMHO, are garbage.

[SoCal250]

My recommendations:
Since your Daytona looks to be a pretty unmolested bike I would keep the updates simple. And make sure you keep all the stock parts that are removed! The resale value will be impacted by modifications. Stock bikes bring much better money than modded ones.

I agree with Mark, get better carbs. I swapped out the crappy OEM Daytona "2V0" carbs for the superior 76-78 "1A1" Mikuni VM28 carbs. They bolt right in and are easy to tune. I'm running them with a Y-boot and K&N.

I would retain the oil pump. It's a simple task to rebuild the pump.

[Hawaii-Mike]

I agree regarding the carbs.  When I had a Daytona with stock carbs, any adjustment to the needles required re-synching them. A set of older RD carbs or aftermarket carbs such as TM's VM30's PWK's etc. will be nicer after you have the jetting figured out.

[Kawtriplefreak]

I concur. Putting TM32s on my RD350B was one of the best mods I ever did.

[rearwheelslider]

Gotcha. I am considering other carbs now... Should I stick with the stock earlier vm28's or go up to 30mm? What would you recommend?

[rearwheelslider]

Will 32mm carbs affect my low end performance much? I know that with dirt bikes that I tune, the larger the throat size, you give up low end for better top end. Have you notices an effect?

[m in sc]

with a stockish motor, not much. i run 32s on mine, but honestly, 30s are fine.

[rearwheelslider]

Cool, thank you, i am learning a lot. I would like to use a stock throttle cable, if possible. Will the VM30 and the TM30(flatslides) take the stock cable ok?

[rearwheelslider]

Ugh, so I am looking into this and apparently, if I run an aftermarket carb larger than the 28mm, I will need y boot adapters($62!) as the intake bell is larger. Also, the dual throttle cable(no oil injection) is $62! I am thinking stock 400 carbs for best fitment. Is the dual cable necessary or can I use a stock 400 cable and just remove the third oil pump cable?

Adapters
https://www.economycycle.com/shop/yamaha-rz-350/fuel-and-airintake-yamaha-rz-350/air-filters/y-boot-adapters-for-larger-carburetors/

Cable
https://hvccycle.net/yamaha-performance-throttle-cable-for-yamaha-rd-250-350-400-no-oil-pump/

[SoCal250]

Some advantages of running stock 1A1 carbs:
 - One choke lever (Aftermarket carbs have a lever on both carbs)
 - Stock RD400 1A1 cables work
 - Stock RD400 (2V0 or 1A1) or RZ350 (31K) Intake manifolds work
 - Stock RD350 Y-boot can be used to run either a K&N or UNI filter

[rearwheelslider]

Okay, great! I am seriously considering some TM28 flat slides. The bells of the carb, both engine and filter side, are the same as VM28's. They are nearly the same price. I assume jetting numbers would be very similar, too.

I do not mind two choke levers.
I am gonna buy the dual throttle cable anyways.

If you can think of a reason for the VM over the TM, please let me know!

[m in sc]

none really, tbh. also, you can get cable pull chokes, i have a snowmobile setup on mine with the tm32s
nice flip lever setup but im fussy like that.