Breaks up at 4k-5k rpm Advice please

[nhd1981]

I am currently working on a buddies 79 xs650 special. It was running poorly, hard

starting, with breakup at 4k rpm. I installed a pamco electronic ignition and gave the carbs

a thorough clean, as well as adjusting the float height.

I then started and timed the the bike. Overall throttle response was much improved

along with much, much easier starting. However the bike continues to break up at 4k-5k

rpm.

When I cleaned the carbs the was noticable signs on the jets that they had been

removed before, but I neglected to inspect the jet sizes.

So at that point I was starting to suspect that the main jets had been changed to a

larger than stock size causing them to bleeding over in the 4k rpm range. But much to

my dissappointment when I removed the mains they are 135s which I believe to be

stock.

My next inclination was that the needles had maybe been moved up one or to positions,

so off came the carbs for a second time. And again to my dismay the clip was in the

third position from the top which I believe to be stock.

I have given some consideration to lowering the needle and see If it makes a

difference, but I'm starting to think that maybe its not a carb issue at all.

Any help would be greatly appreciated.

Also a few other pertainent pieces of info: The bike has cheap junky pod fiters on it at

the moment, but will be upgraded to xs pods in the near future. Also I am In NH at

around 1000ft above sea level.

 

[5twins]

I can almost guarantee it's those cheap pods. Take them off and take a test run, bet the 4K break-up is gone. You'll be changing from junky to a little better junk if you go with the XS pods. I recommend UNI pods, about the same price, higher quality, and easier to tune for.

 

[nhd1981]

Do you know what the uni-filter part # is? You think a foam filter will result in better running than say a K&n? I will try running the bike with no filter and see how it goes.

 

[nhd1981]

Well that definitely seemed to clean up the break up. Now I just need to double check the sync and re dial in the mixture screws I fooled around with. I'm suprised to hear that the mikesxs pods are sub-par. I thought he generally sold top quality parts.

 

[5twins]

In the plain black pod, the UNI part # is UP4200. If you want the dual layer for a few dollars more, the part # is UP4200ST. The black foam used on these is very fine so don't use the normal foam filter oil on them - it will plug them up. Instead, just use a 50-50 mix of gas and plain motor oil.

I read years ago that CV carbs don't like a tapered filter or the pleated K&N style element. I didn't believe it because I had always run K&Ns on all my bikes with seemingly no problems. Well, I proved it to myself on my 650. I started out running real K&N pods. They seemed pretty good but I was never able to go more than one size up on the mains without causing upper midrange break-up I was unable to tune out with the needles. The bike seemed like it wanted a bit more on the mains but I just couldn't do it. I tried the UNIs and that break-up magically disappeared, I was also able to go up another size on the mains no problem.

Most of Mike's parts are O.E.M. quality or below. You have to be very selective and well informed when you shop there because some of their items are just plain junk and a total waste of money.

 

[maxwell]

http://www.amazon.com/Uni-Filter-LAYER-FILTER-UP4200ST/dp/B002KONO02

is this what they are? i'm currently running the xs pods, but if i can upgrade i will!

 

[5twins]

Yes, that is the dual layer version. They do look better than the plain black ones and probably filter a bit more as well since they're dual layer. I just have the plain black ones because they're hidden behind my side covers and you can't see them. If you already have the XS pods and the bike is running OK with them, you might want to just run them until they wear out, then upgrade. The thing is, they're so new, there's really no long term use reports yet. They haven't been around long enough to see just how long they will last. There have been a few reports about the rubber flanges splitting but hopefully they corrected that by changing rubber compound (just like on their manifolds).