Mid range cut-out

[apolloant]

A few of you helped with my other threads, heres a new one. The bike is set up as such... '79 XS1100 with 4-2 headers, k&n air, 42.5 pilots, 142.5 mains, needle jets in the 4th position, so 1 notch rich. Floats at 25.7 or so close, timing dead on, color-tuned and synched. Octopus gone, running lines and filters from rear ports cuz that's what flows when it's on run/reserve. I bought all this crap and it's paid for itself already. The bike acts like this... Bottom from start to about 4500 rpm, smooth and powerful, 4500 to around 6500, cuts and pops a little, annoying as heck, 6500 up to redline, goes like she's supposed to, bat outta hel. I don't think its fuel starvation cuz top end is good. moving the needle jet from pos 2 to 3 to 4 hasn't seemed to change anything. Float adjust from 23 to 25.7 didn't seem to change anything either. Prev owner said he was having trouble like this. I thought I'd worked it all out.

[randy]

Pickup wires and vacuum slide diaphrams come to mind. Have these been checked?

[apolloant]

Yah, ive done the wires. I'm going to a mech. soon. I can't figure this out. A 12,000 mile bike should be something simple.

[DiverRay]

Check the K&N filters. be sure you have a little clearance between the filter/rubber rib and the end of the carb. If the filter is blocking off the holes at the top/back of the carb, that may be the problem.
Try loosen the filter mount, slide filter back about 1/8", and tighten down the clamps again. Give it a ride and see what happens. If you restrict the vents on the intake side of the carbs, things start to run badly.
Ray

[pggg]

This should be easy to fix, if it's not getting enough fuel at those revs 4500 - 6500, remove the inline filters and she'll run like a champ.

[edgo]

My 79 Special needed 145 mains to fix this same problem. I have 4 into 2 pipes and a K&N filter in a drilled airbox. I'd make sure the brass needle jet nozzles are really clean too.

Ed

[apolloant]

This is what the mech said last night. He checked the diaphram on the timing and thinks it's got issues. So I'm gonna safety wire it full advance and see if that clears it up. i'll try the air filter tip too. About the fuel filters, it did this before I installed those. It cut out with the octopus on, on prime, it didn't matter. The mech had a pretty cool idea to fix the pulse that you get to the vac advance. run another vac line from an intake boot to a "t " in the vac line, so you get two lines, cyls 1 and 2, sending vacuum. Or is that sucking vacuum?

[John]

Don't use a vacuum pulse from any source other than the nipple on the body of carb#2. It is a metered orifice, designed to supply the correct vacuum to the advance mechanism. Using any other source will cause the mechanism to to really osillate.

[raqumup]

mine shudders too

hey checkout my post from 2 years ago and tell me if your bike is doing what my bike is doing. i still have not found the problem
the post is called "shudder at 4000 RPM" posted on 02-24-2004
@9:58 AM. let me know.

[birch]

Same problem with mid range lag

It sure is nice to hear that I am not the only one with the problem. I have a 78 s. It runs great until you get on it from 4500 to 6000. After that--rocket land. I have done everything but check the vacuum advance. This is my next job. If this does not work, i'm going to shell out some clams and bring it in. I'll let everyone know when she's fixed

[Douglas Money]

Mine does the same thing from 4200-5500. I just figured I needed to synch the carbs.

[Ziggy Stardust]

Mine does it from 3000-3500 rpm's. I definately have issues with the vacum advance. Anyone know where I can get a servicable vacum advance diaphram unit?

[youngnip]

mid range cutout

Apolloant,
Have you checked the advance mechanisim behind the timing plate. If the sliding action isnt smooth yuo will experience the cutting out effect. Check that before you start pulling carbs. The heat from the motor over time will cook the lubricant on the slide guides and well it will just stick in one spot until you get the reves up.
Good luck.
Youngnip

[motoman]

If you all would put stock air boxes back on those bikes, your problems would go away, and the bikes would make more and smooth power all the way through the rpm ranges. With single pods your changing the relationship of engine rpm to amount of vaccum drop and carb diaphrams aren't properly lifting needles out of jets, trust me, it's a proven fact, and dyno results will back it up. That's why the dyno charts show the flat spots and not smooth increase throughout rpm range, and lastly, the peak horsepower results show way down from ones using stock exhaust, stock air box. Guarentee Yamaha had the combination of intake exhaust figured down to a nut. Realize with the age and where some of these bike are at stock exhaust is hard to keep on bike, even though they will give you the best performance and power. A decent 4 into 2 system is the best alternative, and definitely either have it done or do it yourself by putting a crossover. This has always proven out to to give general all around performance help on our Muscle cars and anything with dual exhaust. It balances out a more even proper back pressure. ANY dual exhaust system should create approx. 7lbs of back pressure, no more, no less for optimum all around performance..........and you CAN ask me how I know all this, if ya want.............so start pickin up all the air boxes that the ebay gods have, and you'll love what yamaha designed that bike to be back in 78...........THE worlds quickest production bike...and you know the rest.

[GNEPIG]

and you CAN ask me how I know all this, if ya want......

.. yes Sir Motoman of Grand Junction, we are all here to learn so please, do tell.