1983 XS400R Won't pull over 7000RPM in gear , in neutral revs fine

[KEvin83xs400r]

1983 XS400R Won't pull over 7000RPM in gear , in neutral revs fine, can't get it over 90klms!
Carbs were just cleaned and synced with manometer. New plugs , idles fine , starts ok
anyhelp would be nice thanks , i am new here

 

[Wolfe_11B]

Gotta be slightly more specific. What exactly does it do at 7k RPMs? Does it just sit there still running, like hitting a rev limiter? Does it bog down? Does it cut out? Is it in every gear this happens, at the same point? What throttle position are you at when it happens?

Normal culprits are-
carbs(just because they are clean, doesn't mean they are good)
plugs(even new plugs could go bad from other things, pull and check)

Hows the plug wires? Coils ok? Have you done a valve adjustment yet? Done a compression check? Any carb manifold leaks? Some have even had issues with the TCI box causing RPM problems.

Basically you need some more info, and then need to trouble shoot a little to narrow things down.

 

[KEvin83xs400r]

hello
and thanks you for suggestions
the bike idles kinda hard to start if cold ,, Drives fine until about 6000rpm then it starts bogging, but not stalling full throttle might make 7000rpm . engine revs fine when not in gear . gear seems irrelevant , happens in all gears Plugs new, carbs cleaned , float levels set,bench synced and mano-metered, airbox hooked up , intakes are new . all electrical inspected cleaned etc . full air/mix set at3.25 turns . i checked for leaks with propane none found .

The story
I got this bike from a friend who was using it everyday 2 years ago and then it sat in heated storage . once i got it i removed the carbs cleaned them and did all the rest above .it have 40th klms on it. since the bike was operating fine before i did not do a compression test

 

[Wolfe_11B]

I would do the compression test and check the valve clearances, that needs to be done no matter what. I'm going to venture a guess that it's the carbs still.

 

[KEvin83xs400r]

is there a clear set of instructions on how to adjust the valves? and do you know what the compression should be ? If carb issues do you have any suggestions ? it almost feels like the bike is starving at high rpm .

 

[Wolfe_11B]

I don't know the specs for the DOHC models, sorry. There should be a manual for them with all the proper numbers, as well as the valve procedure, in the 'manuals' thread. Yamaha XS400 Manuals

Dave here on the forums might be able to guide you further.

 

[KEvin83xs400r]

how do i reach Dave?

 

[Wolfe_11B]

16VGTIDave is his user name here, PM him or just wait for him to read the thread and respond.

 

[16VGTIDave]

Sorry for the delay, I'm catching up...

- Getting the carbs clean is an art. If you are not frustrated by them, you haven't cleaned them enough yet. Carb cleaner isn't good enough these days. There are small passages in the Zinc alloy bodies that get plugged with corrosion. A mild acid like lemon juice, vinegar, CLR, etc is often required to remove the corrosion. Pine Sol works, but is very aggressive, it will dissolve the alloy if left too long! Heating the solution helps (get a pot from the second hand store and simmer the parts outside!). An ultrasonic cleaner is helpful. A fine strand of wire can be poked through the passages to help breakup the corrosion.

- 6000 RPM the needles and float levels are controlling the fuel. Was the fuel level in the carbs checked, or just the float height? Were the diaphragms checked?

- 7000 RPM is that magic RPM on a DOHC where the Main jets become operational at wide open throttle. What Main jets jets are installed? They should be different sizes, and installed in the correct carbs...

 

[KEvin83xs400r]

hey Dave

i am not sure what jets are in it , i got the bike complete not running with electrical grounds off. then took the cards apart and cleaned them i cleaned them with a varsol bath in a parts washing bin. i am not sure how to identify the parts your referring too

 

[sprocketeer]

I have the same problem. In nuetral or clutched it will rev to 10k, but in any gear It will bog at 6500-7000. If I work the throttle a little I can surge it passed 7k, but it's wonky. I'm wondering if I I can move the needle up a postion to let more fuel in? or possibly a larger main jet.

 

[sipma02]

Having the same issue over here. I plan on swapping in larger jets, but my P.O. has 125 left and 115 right (1 size down from stock) so I think thats my issue. Also, correct me if im wrong, but:

...7000 RPM is that magic RPM on a DOHC where the Main jets become operational at wide open throttle. What Main jets jets are installed? They should be different sizes, and installed in the correct carbs...

I was under the assumption that it was throttle position that affected jet circuits, not RPMs? I could be completely wrong about this - I only know enough about carbs to be dangerous . However, sometimes RPM's and throttle position are hard to separate, especially when you get used to accelerating, clutching, and riding a certain way!

 

[BBS360]

Having the same issue over here. I plan on swapping in larger jets, but my P.O. has 125 left and 115 right (1 size down from stock) so I think thats my issue.

Don't the DOHC bikes come stock with different jets between left and right?

Also, correct me if im wrong, but:
I was under the assumption that it was throttle position that affected jet circuits, not RPMs?

With CV carbs it's a combination of the two.

 

[sipma02]

Don't the DOHC bikes come stock with different jets between left and right?.

Yep. Asymetrical airbox, loony design of you ask me. Stock sizes (per manual) are 127 and 117 though.

Makes sense about the CV carbs. So much work to tame those little guys!

 

[sprocketeer]

On my 1981 xs400 special 2, I believe the main jet is 135 and rpm bog and slight surging happens at 6500rpm or so, should I up the main jet to 140, or try adjusting the needle up a clip?
again it revs completely fine when clutch is pulled in or at neutral.

 

[sprocketeer]

Oh and I completely found the rubber plugs that are suppose to cover the pilot jets missing (on order now) but that shouldn't affect high-end bog, right?