Rd 350 6th gear top speed and crankshaft

[karol128]

Hi

I'm Karol. I racing in supermoto competitions. I'm not from UK. Also my English is not very good;)
This year I bought rd 350 YVPS 1987 N2. This is my 5th motorbike in garage. I love two strokes. 20 years ago I had Honda mbx 80.
Cut the shit;)

I replaced pistons to Vertex, changed ignition coil to banshee (made in china). Rest is stock setup.
Last week I tested top speed and I have 183kmh speedometer, 173 gps (115mph speedo) on 5th gear and 10000rev. On 6th gear RD always slowing down. Not enough power.

1.Is this behavior normal fo RD?
Can it be cheap igniton coil? Motor is working fine, only on 6th gear is lack of power. Spark plugs colour is ok (light brown).

I'd like reach the 189kmh GPS. Like in Yamaha specs. 15kmh is the hudge diffrence.

2.Is Hot rods crankshaft with Taiwan bearings good? or maybe I should stick with OEM?

[kpke]

Welcome Karol.

What is your sprocket gearing?

Are you still running the stock exhaust system and jetting in the carburetors?

I can't speak to hot rods crankshaft. You probably can't go wrong with a Yamaha OEM crankshaft. I now have 2 engines with Vitos crankshafts. They may come out of the same factory as hot rods. One I ride a lot and one in a bike getting restored. The rider runs very good. No problems after about 2500 miles.

[karol128]

What is your sprocket gearing?

17/39

Are you still running the stock exhaust system and jetting in the carburetors?

Jetting and carbourator and exhaust stock, 185 main 60/65 powerjet. I removed rubber snorkel from airbox.

[kpke]

It seems like as soon as you modify the airbox like that it would require jet changes. I have never run one with a modified stock airbox. Maybe someone here will comment on this. I use uni filter pods on modified engines. The one I am building now will use the 2 into 1 with a uni filter.

Ditch that 17 tooth countershaft sprocket for a 16 tooth ASAP for fun factor. Not sure what this will do though for your goal of 189kmh. The stock exhaust should be dumped for aftermarket, then more jet changes.

[karol128]

Yeah, OEM exhaust is not great but I've spent so much money on my bikes in the last five years (only modifications and tires) that I could buy a house with it;) I have to stop:)

[brrrappp]

Your original observation: not pulling red line in top gear might be because top gear is an overdrive.
Going down 1 tooth on the front will help the engine pull the gearing but you may not actually increase the top speed if you don't have enough power.

I'm going to say something really obvious -

If you are doing top-speed runs with jetting that you are unsure of ---- BE CAREFUL!

[karol128]

This time jetting is not the main case. Two days ago I seized piston because of lack of fuel (max rpm + low fuel level). Lean mixture melted piston - bad luck.

After rebulid I will mount EGT gauges

[MK]

All kart tracks that I use, wouldn't require full speed. As a consequence I usually gear down to be in 6'th gear with around 120 kmh top speed @ max rpm.
(I guess you'd like my kart track RD https://youtube.com/shorts/KcYMNBZAtFE)
If you race and require 180 kmh or more, then the RD may be the wrong bike at all.
You'd at least need to reconsider the budget and add some Banshee aftermarket cylinders (like CPI offers).
Some years ago I assisted in building this one with a 421 Serval/Wampus kit.
https://youtu.be/PbPisDuDFl8

Nevertheless, if it doesn't make around 185-195 the engine lacks performance. Stating that top speed is in 5th gear adds to this suspect.
Gearing down to 16/39 or shorter would make it better but not deal with the underlying problem.

The EGT will help with the diagnosis.
But as poor top end can have a couple of origins you'll need to check all of them one by one.
Pipes, carbs, compression, PV operation & valves fully open on both sides,....

[karol128]

I have modern bikes with more performance for race. Rd is for fun on the street.

Compression is good, PV fully open on 8000rpm (tuned) and works fine. Maybe carbourator or chinese igtion coil. On 6th gear bike instantly slowing down to 170kmh from 183kmh.

Some years ago I assisted in building this one with a 421 Serval/Wampus kit.
https://youtu.be/PbPisDuDFl8

I like it:)

[abar121]

The lack of a faring doesn't help above 160kph.

Go up two teeth on the rear sprocket, wear tight fitting leathers and tuck in like a Moto3 rider. Or get a jockey to ride it :) As above, check the PV alignment when open. They get sloppy with age / wear too.

Then it's down to some decent pipes, set the squish / compression correctly and jet to suit. After than, porting, a Zeel or Ignitech and 28mm PWKs. Mix and match to your preference. Have fun!

[dinamos]

Been a while since I've posted but here I am.
I bought a new CDN 86 RZ from a dealer, put 5,000km on it and one early Sunday morning took it on a local highway and opened it up. I saw 200kph on the speedo before I dialed it back. That's stone stock.
I know better now not to do that shit on the highway but I'm pretty sure my stock 89 RZ with after market pipes and minor jetting can do better.