I posted this previously in a thread with somewhat similar problems. If anyone has any suggestions, please drop me a line...I've been getting mildly frustrated with this issue.
Symptoms:
-The bike will start easily and run well standing still. I can open the throttle and hold it at any engine speed I like.
-I can drive hard, consistently near or at wide open throttle and it responds well.
-At low speeds in first through third, the bike behaves fairly normally.
-When above around 40mph in appropriate gears for general riding somewhere around 1/4 throttle, the bike acts like it is starving for fuel. The misses last for between 0.5s and 3s before it hits for a few cycles then misses again.
-At any time during the poor behavior, I can open the throttle and it behaves normally. I can also slow down and downshift and it will behave fine.
Things I have tried:
-Fuel - I initially drained the tank from the fuel line entering the carburetors since the previous owner was kind enough to block off the main drain. I found no water here. More recently, while being frustrated with other parts and checking the fuel shut-off valve, I spilled gas and saw water bead up. After draining the fuel the hard way, I recovered about 1l(!) of water. With the entire fuel system dry (carburetors included since they were disassembled at the time), I refueled and saw absolutely no change in performance.
-Carburetors - I have fully disassembled and cleaned them twice with no change in the running characteristics. All jets and internal passages are spotless. The rubber boots on both sides appear to be pliable and leak free. The vent hose is clear. Using a piece of clear tubing, I have checked the still float heights and found them both to be slightly above the gasket. While I don't know which jets are installed, I do know that they did work fine for the past two years and for the first few rides this year. The slide diaphragms are soft, pliable, and leak free. The jet needles show no signs of wear. I toyed with the fuel screws going from too rich to too lean and saw no change in this behavior other than generally what one would expect with those conditions.
-Petcock - I have disassembled this and found it to be functioning well. The vacuum line and diaphragm are leak tight and operating. All three positions operate as they should.
-Fuel Lines - I initially rerouted the lines to remove the petcock from the circuit as well as all of the air space in the line. I also replaced them with a more appropriate material than that which the former owner chose. They are currently installed and routed as designed.
-Fuel Shut-off Valve -I pulled this and the screen assembly and found no obstructions. The screens are clear with the exception of mild sediment as expected. If one opens the valve, fuel will pour from both the primary and reserve take-off points.
-Fuel Tank - The vent is likely working due to the rate at which fuel flows from the ports. To verify, I drove with the gas cap open and saw no change.
-Valve Clearances - Three valves are in spec and one of the intake valves is only marginally out of spec on the tight side.
-Battery - I forgot to remove the battery last winter and consequently had to replace it this spring when I found that it would no longer hold a charge. Whether correlated or not, this is about the time the misfiring problem got far worse. The voltage across the battery when the bike is off is around 12V and at high RPM, it settles to about 14ish.
-Spark Plugs - I have replaced them as a long shot with no effect. A running-kill-check test shows that they are slightly on the lean side but still seem fairly normal. There is a strong spark from both plugs when I check.
Because nothing I have tried has even changed the behavior, I'm leaning toward the problem actually being in the ignition somewhere though I don't know where to go next. Does anyone have any suggestions?
Hi,i`d recomend checking the floats by measurment (carbs,bowls off),i`ve done the tube test and all seemed well yet after measuring correctly a fair difference between the two.The only other thing i can think of has someone done a timing advance on it and it`s perhaps slightly to advanced? Don`t have much to do with ignition timing now they`re electronic so just a wild guess.
Combo Jay.
Quote from: alyndaker on July 21, 2010, 11:46:38 AM
While I don't know which jets are installed ... I toyed with the fuel screws going from too rich to too lean
?
Edit: Those "fuel screws" aren't gonna correct a lean condition. Go for a ride, and when it starts missing - try engaging the choke. Then try running in PRI.
@Jay - Both float heights are the same and within spec. I don't have the manual in front of me to recall the actual number.
@Homer - Yep, understood. It was an attempt to get any information at all. I've tried engaging the choke when it's missing and saw no change. I've also switched it to PRI at the time and for a whole ride with no effect.
Good job., then.
Find the jetting sizes, for good measure. Do you get any popping on deceleration?
Since you seem to have looked at almost everything and tried many different things but is the airfilter/airbox clean or does it have an aftermarket filter like the lunchbox?
Another thing to check might be the exhaust. Are their any spots on the exhaust that have rust or could have a tiny pinhole that when driving around at slower speeds won't affect anything but once you get to higher speeds could affect things. I would try and see if i could find any pinholes (most likely around the bent areas if anywhere).
Other than that maybe a timing advance done by a previous owner but i don't know why that would wait 2 years to cause a problem.
@Homer - I didn't see a marking on a couple of the jets, but I can grab a set of pin gauges to measure them and report. There is some popping though I don't know if it's really the sound you'd be looking for. It is sort of a muffled pop but not a sharp noise like a straight up backfire.
@Bass - I suppose I'm not sure if the filter as aftermarket; the box appearance matches the manual at least. The filter is clean. I'll check the exhaust pipes when I get home from work...I didn't think to check that something may have changed in the exhaust system.
With respect to timing on these bikes...is it set by the angle of the pickup on the side of the engine? From the conversations, it sounds like if it is, in fact, the timing that it is advanced too far. Am I correct in that statement?
There`s info about ign` advance on home page,you can get a vance and hines ign` advancer or you can DIY by drilling the backplate and moving it i think 4 degrees so that the pickups read sooner.Although what Bassmachanics said about exhaust leak makes sense,are you running stock exhaust? My baffles rotted in my old one.
Combo Jay.
If it's all the time, maybe an exhaust leak. On deceleration, sounds like rich jetting. Maybe from a 2 year old dirty air filter.
Not sure what's up with that plug chop.
If it was advanced too far, you'd have significant backfiring while you're trying to accelerate. 3 7mm bolts on the rightside cover. Don't worry about a gasket. If the circular plate inside has been modified, you'll be able to see.
I don't think the exhaust is stock, but it hasn't changed since I bought the bike. The exhaust tone also hasn't changed so I would probably lean against rusted-through baffles. The pipes look fine to me; they're cosmetically, but not structurally rusted.
The timing doesn't appear to be modified though I did notice an old (already rusted over), small ding on the rotating steel plate. I wouldn't expect that to change the timing since they are merely, I assume, Hall effect sensors.
I had a knowledgeable friend take it out tonight and, aside from being stumped, he did offer one more data point. If he got the bike up to highway speed and in this state, upon pulling in the clutch and letting off completely on the throttle, it would rev down completely to a stall without firing again and would restart and run well immediately. Does that give any more ideas?
I still think it's jetted wrong, but that's just me.
Enter somebody saying it's a heat-related wiring problem.
We have a winner.
I had a bit of an epiphany last night in the shower. The one thing I changed before the problem got noticeably worse was that I removed a fresh wasp nest from the vent tube coming from the carburetors. If one draws a slight vacuum on that tube, you immediately see the same symptoms as when riding. The end of the tube was orthogonal to the direction of the wind so all hail Dr. Venturi.
I fixed it by making a small loop at the bottom and tucking the end of the tube behind the engine. Apparently this has been an issue as long as I have owned the bike, but cleaning the tube exacerbated the problem. I was able to climb a very long hill in sixth gear without going above about 1/4 throttle. Usually I would have to be on the throttle in fifth gear to get up it.
Success.
Im having a low idle problem. wandering if its vent hose like your bike. my valves just got done and it was idling fine. had to replace the choke cable so while i had the tank off i cleaned the carbs and put on a new vent hose. i always thought it was an overflow hose so i cut a long piece that goes down to the bottom of the bike. could too long of a vent hose cause some breathing issues and cause this sudden low hard to idle i have? thanks