Powerband issue - flat spot and surges

[IanFrancisco]

Hello: Everyone knows how when you buy an old bike you have go from one area to the other trying to get it up to snuff. For me it was brakes & tires first, then the charging system and now it's tune up time. Of course I've had 2000 miles of fun riding in 6 mos. so I can't really complain. However - the bike was running great last weekend, had a smooth broad powerband from 3k to 8k rpm as always. The PO (previous owner) must have done some jetting or carb work to get rid of the EPA-induced factory lean settings because there were none of the driveability issues reported in mag reviews of the time. I adjusted the slow circuit myself, going from pilot screws 1 1/2 turns out to 2 1/2 turns out and backing off the throttle stop screw and had reliable idle at 1000-1200rpm. I actually did some pretty hard riding over the weekend, had a new charging system and fresh Mobil 1 and was a happy camper.

So Tuesday after work I went out and installed a new stock oiled-foam air filter. The PO had some funky unoiled white stuff in the stock airbox that looked like it came from under his xmas tree. Went for a spin and was disappointed to find I now had a crappy running motor. Sounded like it was strangled and was way down on power and misfiring at around 6k. Turned around and yanked out the new filter and put the old stuff back in. The problem went away somewhat but not completely. Ok, maybe the plugs are now fouled. Pulled them and they still look tan, not oily. Have since ridden about 50 miles after work, just mild cruising around town between 4k and 5k to see if whatever happened will go away on its own. It has to a certain extent. Now the misfiring is gone but there is a huge flat spot from 5k to 6.5k. More like a canyon, the bike falls on its face, has no power in that rev range. If I rev thru it the power comes back somewhat from 7-8k. I haven't redlined this motor yet, havent seen a need, seems to make most of its power at 8k. When I close the throttle I get lots of engine braking then a surge. Ruins my normally (somewhat) smooth riding technique.

I figure it's time for new plugs, maybe caps, and a carb clean. Maybe sometime this summer I can get to a point where I can do more riding than tinkering! It would be nice to be able to just check the tire pressures and oil once in a while and not have to do major grease monkey work every weekend.

Any comments/suggestions? The new air filter had what seemed to be only a light coating of oil straight out of the package. The old air filter was very clean as was the inside of the airbox. I'm wondering if this problem is caused by something else like maybe a cracked CV diaphram that just happened to coincide with the air filter change. Or maybe some dirt got in the carbs during the filter change.

Is there anything I can do to get my power and driveability back while avoiding a complete carb teardown/rebuild? That sounds like a winter project to me. It's Riding Time !!

[beby99]

Sounds like you did get some dirt into the carbs
during the air filter swap....it don't take but a spec of dirt
to foul up the pilot jet.....maybe a spec of glitter from the
x-mas tree skirt

[gspaul]

I still have an American mag from that period that pointed out the lean carb prob from that period (cycle I think ).
OK ,don't think about what rpm the prob's happening, think about the position of the throttle.
Generalisation, first 1/4pilot,1/4-3/4 needle/ last 1/4 main jet ,no matter what the rpm is.
There is overlap ,but it's a good place to start.
What do you mean the original , did not look right?

[IanFrancisco]

What do you mean the original , did not look right?

Didn't find the word "original" in this thread so not sure how to reply here.

Thanks for the replies, gents. I understand that the pilot/slow circuit works for the first 1/4 throttle, the needle for 1/4 thru 3/4 and the mains for 3/4 to full throttle. Unfortunately the problem is rev-related. If I keep the revs under about 5.5k there is no apparent problem, throttle response and power seems normal. If I try and wind the bike up to 8k, either at 1/2 or full throttle, the power drops way off at about 5.5k to about 7k, enough that the forks dive! The bike falls on its face and it feels like I hit quicksand or something. Maybe a vacuum leak? The needles aren't rising properly? Could the carbs have sucked in enough oil to gum them up without it showing up on the plugs? I will do the WD40 test for boot leaks this weekend and maybe pull the tops off th carbs to make sure there are no cracks in the diaphrams and the slides move smoothly etc.

Then there is the surge problem when I close the thottle at 5k or higher, the bike turns into a rocking horse. Again it seems like the slides are gummed up and not opening/closing quickly enough.

More suggestions? It's extremely helpful to have two or more experienced brains thinking about the same problem.

Many thanks!

[gspaul]

OOPS! Sorry I was talking about the old air filter!
Check for air leak's with a water mist ,seem's to work better.
Before you think of getting in to the carby's check your ignition timeing,new plug's, etc.
Did you oil the new filter to much perhap's!

[gspaul]

Something that is rev related, rather than throttle position related,smell's like ignition.
Check all conection's,look at the colour of your spark.
Do you have one or two Ignition coil's?

[IanFrancisco]

Ok, here goes, the old air filter is some white gauzy stuff that looks like it will allow low flying jets thru, ie, not filter much at all. Then the PO had stuck some blocks of foam on top of that presumably to restrict the air flow somewhat as the gauze would be equivalent to no filter at all in terms of restriction. No oil, everything was bone dry and clean as a whistle. The new filter was pre-oiled, enough to get my fingers slimy, not enough to drip though. I will dry it off somewhat before reinstallation. I will also check both coils for blue spark and also the advance mechanism and install new plugs this weekend. Would ignition issues explain the surging?

I assume if I use water mist to check for leaks instead of WD40 the motor would slow down rather than speed up if a leak were present?

Thanks again for the replies...

[bwringer]

My 850 was pretty sensitive to an overly restrictive foam filter earlier this year. It turns out the foam had swelled slightly over the years (it was about 6 years old), and squashing it back into the filter frame after cleaning created just enough restriction to make the bike run badly.

I trimmed about 1/2 inch off the bottom of the filter and blotted more of the oil out with paper towels, and it's been perfect ever since. I guess I need to get a new foam filter at some point, but I'm not in a rush.

The oil in a foam filter should be a very very very light coating -- saturate, then blot out as much as you can.

[gspaul]

Ignition prob's can cause surging,and yes the water mist makes the motor slow down and misfire.
It's easy to hear.