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]

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?

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 

[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.

[Nurse Julie]

I got Hettie up to 95, that was after the engine rebuild and the engine was fully run in. But, I agree, they aren't long distance cruising bikes (the hard seat is a killer on long distances for a start) , they are at their best on twisty roads, using high revs and being ridden like you stole it. That is when they are very, very happy and run at their best, they just love it.

[McCabe-Thiele (Ted)]

I agree with the previous comments, in many ways my 400 reminds me of my old 1960's Honda  250  Super Sport - a decent head wind meant dropping a gear to maintain any sport of speed.

[Athame57]

When at WOT accelerating up a long, straight, slightly uphill road in 6th gear,

I hope I'm not going to sound daft as a brush, but wot does WOT mean?

[Mikep328]

WOT = Wide Open Throttle

[Mikep328]

" 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."


I guess this would do it...https://silodrome.com/wp-content/uploads/2018/07/Rollie-Free-Vincent-Black-Lightning.jpg

[K2-K6]

" 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."


I guess this would do it...https://silodrome.com/wp-content/uploads/2018/07/Rollie-Free-Vincent-Black-Lightning.jpg

Theres a book about the man in the photo "Rollie" in going into that speed attempt .... review here https://thevintagent.com/2017/09/02/flat-out-the-rollie-free-story/ that seems an interesting read.

My parents were personal friends with Vincent head of engineering and his wife when the bikes were contemporary, with many stories around them and racing them too.

[McCabe-Thiele (Ted)]

I am not advocating this as an answer Mike - if you are running the standard air cleaner system there are adjustments that you could make with the needle rather than changing any jets - raising all four  needles on the sliders would richen the mixture at any given throttle opening. (i.e.one groove lower than the mid point effectively raising the needle in the jet)

There must be a reason why they put five grooves in the needle - might be for those living at high or low altitudes.

[K2-K6]

Thanks!  Good info!  Maybe I'll hop back on my 900 Kawasaki to go up that hill!

As the American saying goes "there ain't no substitute for cubes" as a certain amount of torque will only be able to overcome a set amount of resistance.

You're just seeing the finite tail out of tge 400 here in this example, the 900 and anything else will do it too, just at much higher velocity.

Also a characteristic of slide type carbs, in that if the torque available can't pull it of of the "hole" then further opening of the slide ultimately decreases vacuum in the carb throat, such that they would be forced lean under these circumstances.
 
The engine rpm " following" the slide up, think big torque motors, is keeping that equation in check. As soon as that breaks down and the engine cannot keep adding torque, then the "gap" between the two gets progressively wider, dropping the absolute vacuum as the hole is bigger in the venturi but air speed through it is not increasing, in reality air speed there drops under this scenario. Bernouli principle I believe .... http://hyperphysics.phy-astr.gsu.edu/hbase/pber.html 

CV  carbs seek to avoid this, their biggest attribute in this circumstances as properly designed they should keep the vacuum level to match the intake "suck" the engine is presenting to them.

When an engine is run on a dyno too, they don’t use a "terminal" gearing ratio generally, for example 3rd gear often engaged for a recorded pull as it then shows in graph terms the whole power curve, up to peak output and then on past that to observe the decay of dynamics as it goes over peak efficiency and starts to drop again.

[K2-K6]

I am not advocating this as an answer Mike - if you are running the standard air cleaner system there are adjustments that you could make with the needle rather than changing any jets - raising all four  needles on the sliders would richen the mixture at any given throttle opening. (i.e.one groove lower than the mid point effectively raising the needle in the jet)

There must be a reason why they put five grooves in the needle - might be for those living at high or low altitudes.

Its indolvable Ted as its not the jetting doing this.

Its too high a ratio for the torque available to pull, but if you sent it downhill in that gear (removing some resistance effectively) then it would carry on rising in revs.

Raising the needles will just make it rich for all of the general running scenarios other than this one. It's set correctly as it is.

Needle adjustment is there for scenarios you described though, primarily altitude. It would also be unacceptable to produce a carburettor with no adjustment too, then expect a long service life out of it. It just must have some level of compensation built in to cover reasonable eventuality in its usage.

[McCabe-Thiele (Ted)]

Not sure where Mike lives - not on top of K2 for sure.(pun?)