Xj1100 cutting out at 4500rpm

Alright, I'm getting a little frustrated with this bike. From 1000rpm to 4500rpm it pull nice and strong. As soon as I pass 4500rpm it cuts out on all cylindres....the more I twist the throttle passed this point and the problem just gets worst. Its fine for driving around town but top speed is 100km/h (60mph)....no fun for highway driving. I'm running the stock airbox but have a 4in1 Mac exhaust. Carbs are clean and synched. Any ideas?
Thanks for your input.
Seb

 

Are you 100% sure the carbs are clean? Float levels set correctly, slides move free and no holed in the diaphrams.
Do you know the size jets installed?
When you reach 4500 rpm does it spit and sputter or cut completely out

 

carbs are spotless and have been carefully inspected. Not sure of the jet size though. To answer your question, it spits and sputters. Thank you

 

could it be a vaccuum issue?

 

It could.
My thought are a collapsing vacuum line to the petcock, or a partailly clogged gas cap vent that's restircting fuel flow anough to cause starvation, but not enough to keep the bowls from filling enough to run well at lower RPM.
Fuel height in the float bowls should also be checked.

Does the engine act like it's fuel starvation (as if you were about to need to switch to reserve), or does it fall flat like a vacuum advance distributor not working correctly would cause?

 

Are you still using the XS1100 carbs on your XJ1100? They may not be jetted properly or even compatible with the XJ1100 engine; for example, the jetting on some XS1100 carbs is way richer (or leaner) than on the XJ1100 carbs:

MIKUNI MAIN FUEL JETS:

#110 is used on:
1980-81 XS1100 LG/LH/SH models (*****see note below)

#112.5 is used on:
XJ1100 all models

#115 is used on:
1980-81 XS1100 standard models for cylinders #1 and #4 (*****see note below)

#120 is used on:
1980-81 XS1100 standard models for cylinders #2 and #3 (*****see note below)

#125 is used on:
XJ650 Turbo carbs for all cylinders (**see note below)
1980-81 XS1100 SH model (*****see note below)

#130 is used on:
1978-79 all XS1100 models (****see note below)

#137.5 is used on:
1978-79 all XS1100 models (****see note below)

#140 is used on:
1978-79 all XS1100 models (****see note below)


NOTES:
**** depending on model of carbs used, these bikes could have been equipped with either the #130, #137.5, or the #140 main fuel jets. The #137.5 size seems to be what was equipped from the factory on North American models.

***** there is quite a bit of discrepancy between the service manual and parts diagram specifications for the correct size(s) of the 1980-81 XS1100 main fuel jets:

Service manuals:
1980-81 Standard models: #115 jets on the outer carbs and #120 jets on the center carbs.
1980-81 Special models: #110 jets on the outer carbs and #120 jets on the center carbs.
1980-81 Midnight Special models: #110 jets on the outer carbs and #120 jets on the center carbs.

Parts books:
1980-81 Standard models: #115 jets on the outer carbs and #120 jets on the center carbs.
1980 Special models: #120 jets on the outer carbs and #125 jets on the center carbs.
1981 Special models: #110 jets on all four carbs.
1980-81 Midnight Special models: #110 jets on all four carbs.




MIKUNI MAIN AIR JETS:

Alas, the Mikuni BSxx series carbs used on XJ models---unlike their cousins the Hitachi HSCxx series carbs---did not have replaceable main air jets. The main air jets on these models are in the carb throat, facing the airbox, and are a small, brass pressed-in tube that is non-replaceable, and which we suggest you do NOT try to modify by drilling or enlarging.

And just in case anyone is wondering, here are the sizes of these non-removable main air jets in the various models:

#140: XJ1100 and all XS1100 models




MIKUNI PILOT FUEL JETS:

#42.5 1979-81 XS1100 all models***
#45 1978 XS1100 models
#47.5 XJ1100 models

NOTES:

*** although all 1979-81 XS1100 models uses the same size jet, the jet type differs due to the presence (or not) of the fuel inlet holes in the shaft of the pilot jet. It is critical that you verify which style carbs you have:

http://www.xs11.com/forum/showthread.php?t=35964

Carb sets that require the use of the bleed holes in the pilot jet shafts will have a drilled passage from the main jet boss in the carb body straight thru to the pilot jet boss in the carb body. Look at the 6th picture down on that page, it shows the “early” style carbs that have the drilled passage hole between the main and pilot fuel jet passages, and thus requires the use of the “shaft with holes” style of pilot fuel jet (this style of carbs also requires the use of the HCP7376 rubber block-off plug that seals up that pilot fuel opening at the bottom of the jet).


NOTE: ensuring that your pilot fuel circuit is internally clear and free is one of the most difficult tasks to perform on these carbs due to its incredibly small size (so fuel varnish will accumulate and solidify in there over time):

http://www.xj4ever.com/inside your carbs.pdf

To make sure that your internal pilot fuel circuit is truly “zestfully clean” after your clearing efforts:

1. Drill out an old pilot fuel jet so that the straw from the carb cleaner fits snuggly in the hole (or some other suitable method of “sealing off” a straw into the pilot fuel jet passage where the jet screws into the carb body).
2. Leave the pilot air jet in, but block it off with a wooden q-tip that is covered with a bit of heat shrink tubing (or use your finger to firmly block it off).
3. Install the pilot screw without its o-ring, and set it to about 2.5 turns out from soft bottoming.
4. Blast the carb cleaner through the modified pilot jet and it should look something like this:



By the way, when using the spray-stream method of checking the passage, don't even THINK of putting your eye or face anywhere even NEAR the outflow path, unless you like a painful and potentially serious trip to the emergency room


MIKUNI PILOT AIR JETS:

#170 XJ550 Seca, XJ550 Euro, XJ650 Turbo*, and XJ1100 models
#180 1978-79 XS1100 models (**see note below)
#185 1980-81 XS1100 all models.



Main jet needles are also different:


MIKUNI MAIN JET NEEDLES:

5GLZ: used on all XJ1100 carbs.
5GZ6/3: used on all 1978-79 XS1100 carbs
51Z7: used on all 1980-81 XS1100 Standard models
5GL16: used on all 1980-81 Special and Midnight Special models




Be aware that even if you have a "perfectly" original, functioning set of XS1100 carbs, they still may not work properly on an XJ1100 engine, since the jetting, rate-of-rise of the vac piston (and thus the main fuel jet exposure and needle size differences), etc. are all tuned for the specifics of individual engines, which differ in flow characteristics, compression ratios, valve size, cam lift and timing, head volume, and other features. I always cringe when people ask (or state) that they're going to use carbs off of some other model on their bike, because there are many (some small, some large, and sometimes lots of small differences that become a large difference) that, while they may allow the engine to operate --- that only requires that the carbs are capable of vomiting some reasonable amount of fuel into the intakes --- can result in tuning nightmares...........and, the only answer to the question "well, can I (and how do I) make them work?" is: find someone who has already done the particular swap and ask them:

a) if it worked, and
b) what changes were needed?

 

Thanks again guys, it seems I have a bit of work to do here, Ill work on this and I think im going to try and find another set of carbs and see. (yes I'm running xs carbs on my xj)
Ill keep you posted thanks.

 

@Sebastian, Please try to keep all this in one thread.
Thank's Chacal for catching that bit of important information.