Can't past 6500/7000 in 6 gear

[niko]

hello,

my problem is still valid

since i 've build my bike i can't really past 6500 /7000 rpms in 6 gear.
In fact the bike go over 6000 rpm on the first gears and more i go up the gear more the area where the bike can't go over is down:

i can't go over: 8000 in 4 gear / 7500 - 5 gear /6500 - 6 gear!!!


I do a lot of testing, but nothing has solved the problem.

It's a 4G5 model with BMC pods and megathon

Carb are very clean, no problem, diaphragm are new
timing is good (tune with flash light)
synchro is good
fuel level is good (clear tube method)
the ignition advance is free and work
I've try 125 (stock)-135-145 main jet but nothing different for this problem
i've try other coil
i've try new spark


The best carb configuration for me (under the wrong rpm) is 135 main jet (10 point more than stock) and needle 3th groove

the bike is very pleasant and powerful under this area

I'm desperate and I do not know where to look

Help!!!

 

[HoughMade]

Sounds like the intersection of friction, aerodynamic drag and horsepower to me. What speed are you at in 6th gear at that RPM?

 

[drewpy]

are the brakes dragging?

 

[bcware]

Slipping clutch? Throwing it out there!

 

[ThePsuedoMonkey]

Could be an aftermarket sprocket. Swap to a smaller front or a bigger rear sprocket if you want higher RPM at WOT.

 

[niko]

What speed are you at in 6th gear at that RPM?

around 110/120

are the brakes dragging?

no problem the brakes are good

Slipping clutch? Throwing it out there!

The clutch is good it's seems not slipping. "Throwing it out there!"??

Could be an aftermarket sprocket. Swap to a smaller front or a bigger rear sprocket if you want higher RPM at WOT.

i've the stock sprocket 36/16 . normally with this model it's a 16" wheel but I replaced with a 18" wheel.

The bike seems to go out this area but not

 

[bcware]

"Throwing it out there!"??

A wild suggestion.

What happens when the revs won't go any higher? Is the tach steady and locked at 6500, etc, or does it bounce? Does the bike suddenly lose power or does it stay strong and steady?

What size tires are you running?

 

[niko]

A wild suggestion.

i just understand !

What happens when the revs won't go any higher? Is the tach steady and locked at 6500, etc, or does it bounce? Does the bike suddenly lose power or does it stay strong and steady?

the bike seems to go over but the tach steady and locked around 6500 un 6. The bike lose power as breaks.

What size tires are you running?

120/80 -18

 

[bentwrench]

That happens to me too. I figure I'm hitting the limit of my horsepower vs. drag.

 

[HoughMade]

That happens to me too. I figure I'm hitting the limit of my horsepower vs. drag.

Well that sounds familiar ...which is why I asked the speed at which this is happening.

 

[Lou Ranger]

i've the stock sprocket 36/16. normally with this model it's a 16" wheel but I replaced with a 18" wheel.

Check out the other threads on here discussing sprockets. Based what people have said there, I thought the standard sprockets for the 6-speeds were 16T & 39T. If so, then 36 is not stock. Those of us with 5 spd Maxims have 35T already.

More significantly, wouldn't using a 2" bigger wheel do way more to lower revs than a few teeth on the rear sprocket? I would think so - to the point where the engine can't push any harder against such a tall overall final ratio.
Maybe a rear sprocket with over 40 teeth would be the solution for an 18" rear wheel and tire (my rear tire is a 120-90-16). There would be a huge difference in rolling circumference to compensate for.

 

[niko]

for the 4G5 model (xs400se 1981) the stock sprocket is 16 / 36 whith 16" rim.

 

[Lou Ranger]

for the 4G5 model (xs400se 1981) the stock sprocket is 16 / 36 whith 16" rim.

There must have been a lot of spec differences between various 400 models and maybe changes from one year to another.

226dark said the final drive on his bike is 16 and 37. At 60 mph the tach reads 4600.​

HoughMade said his '79 was originally 39 and he went to 37. Reduced RPM at 60 mph by around 500rpm.​

xschris said he runs a 16/37 on his and in 6th gear it turns at 4800? Stock is 16/39
​

But I think all the above results, including my 16-35T 5-spd (6100 rpm @ 100kph), are with 16" rear wheels - all with a much smaller rolling circumference than your 18".

 

[niko]

I can not believe that the xs400 engine lacks power to take a gear ratio of 16/36 with a wheel 18.

may be a slightly insufficient flow from the tap would be an explanation?
I do not have the original tank and then a different petcock. It's a 125 dtmx petcock!
I feel like it flows well but actually the 125 dtmx was smaller and less powerful!

I timed the flow to 1 liter: 2min10sec

it looks good but can be just a little limit?

can test your flow and timed like me to compare?

 

[HoughMade]

How fast are you going at 7000 rpm in 6th gear?

 

[niko]

110-120 km/h

 

[HoughMade]

OK, that speed is a little low, but there may not be much more left. What is your compression like?

 

[niko]

OK, that speed is a little low, but there may not be much more left. What is your compression like?

I've changed rings and just after a friend took cylindre compression: 7

this friend said it was normal because the ring were not honed ( do you understand because I'm not sure about my english! sorry!)

 

[niko]

I did a few tests.
I put tape around my filter but no real change!
The only change is that when I arrive in the limit zone (110/120 km/h) I have no small saccade like before but is completely linear (as restrained).

I find it really strange that the tape does not change more!!!

this is how was my two filters:

 

[bentwrench]

Hmmm... that should have significantly reduced your hp...