SOHC.co.uk Forums > CB350/400
Need larger main jet?
Mikep328:
When at WOT accelerating up a long, straight, slightly uphill road in 6th gear, the RPM will max out/hold at around 7500 RPM. Reducing the throttle position slightly does not reduce the speed. That indicates to me that it could use a bit more fuel under those conditions. Or is my thinking in error?
If not in error, what is the next jet size up from the standard 75 and are they available anywhere?
Nurse Julie:
400/4's don't do any type of incline in 6th gear. Sounds like you just need to change down a gear. 6th really is only for cruising on motorways and even then, they aren't really happy if you need a bit more speed to overtake something. They are bikes that run at their best being ridden at high revs and never under load.
K2-K6:
--- Quote from: Mikep328 on September 09, 2024, 07:46:47 AM ---When at WOT accelerating up a long, straight, slightly uphill road in 6th gear, the RPM will max out/hold at around 7500 RPM. Reducing the throttle position slightly does not reduce the speed. That indicates to me that it could use a bit more fuel under those conditions. Or is my thinking in error?
If not in error, what is the next jet size up from the standard 75 and are they available anywhere?
--- End quote ---
I'd agree with that view :)
A delicate balance on these as they dont produce the torque in sufficient quantity to pull that sixth to get themselves accelerating at that point and given any sort of gradient to climb.
As Julie notes, you definitely have to favour revs to make use of it properly.
Increased main jet is not suitable as it'll skew all of the running to its detriment and increase oil dilution ultimately from expelling the excess unburnt past piston rings.
Concisely, when it can't exert enough torque to further accelerate against that higher ratio gear, then opening the throttle does make it go leaner as venturi negative pressure /vacuum drops away with slide open further and no increase in engine speed to keep adding volume (a function of swept volume at x rpm) which reduces the amount of fuel pulled from main jet. It's not the size of jet but the total vacuum in play here.
Ultimately , if driven like that for substantial period, it will detonate from being lean and start to damage the piston crowns.
There's no way round it really, it just doesn't make the torque to pull that ratio. Increase in capacity is the answer ;D
Mikep328:
Thanks! Good info! Maybe I'll hop back on my 900 Kawasaki to go up that hill! ;)
No, not really. The 400 is a lot more fun to ride on the twisty back roads which is pretty much the only type of riding we enjoy though we endure short runs on the motorway if necessary. I haven't ridden the Kwaker since I got the 400 running well AND braking well last year.
BUT...I am curious that in road tests I've seen from the period, virtually all of them show a top speed of 103-104 MPH. Based on what I've experienced at motorway speeds, I don't believe it could reach the ton unless it was downhill with a tailwind or tossed out of the back of a C130! :)
TrickyMicky:
Don't forget that when these bikes were new, the scribes from the motoring press, (and manufacturers), got away with quoting some amazing figures. The likes of you and I will probably never reach the ton under normal riding conditions. Max. speed testing years ago involved involved such things as unbelievably high tyre pressures, rear chain de-greased and given a light dose of 3 in 1 oil, the rider wearing tight leathers and laying flat on the tank, feet on rear footrests, left hand holding top of left fork leg, and peering between the clocks. If you want to see what your 400 is like, nice level road, 4th gear, take the revs up to about 7,000, then give it the works up to about 9,500 and a quick upward change to 5th. If it's all set up right, it ain't bad for a 50 year old 400. Well this 77 year old rider thinks so anyway!! See you all at Leiston, end of September hopefully. Mike.
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