my bike starts and idles and cruises properly but missfires under medium/high load

[apsolus]

2002 with kn filter and kerker slip ons with dyno jet stage 1 kit installed,
i parked the bike a week and a half ago. started it up today to take it to a friends house for a pool party. the bike would often spit and sputter a bit when cold after pulling out of the drive way but would clear up pretty quickly and off i go. i always figured i have the carbs set to rich or something and off i went. well today the same thing happened but felt worse and 5 miles later it didnt clear up. the bike will idle and take off from a start just like it should but as soon as you ask any power out of it feels like a light switch turning off one of the cylinders . no power but when i back off its fine. first thing i did was the peashooter carb clean. it didnt fix it. so i pulled the plugs and no surprise the bike runs rich as the plugs were on the darker side. but one of them was very dark. sitting on the bike the right front cylinder is the culprit. very dark plug. im changing plugs in the morning. hopefully that clears it up. any recommendations on plugs to use? the plugs i pulled out are NGK IRIDIUM DPR8EIX-9

 

[Fire-medic]

Did you try draining the float bowls into a receptacle to see what's in there? It could be some water. That appears to be a 'lens' of fluid floating in the gas, due to their differing viscosities.

FYI, the Dynojet Stage 1 kit is a known problem from members' input on-here. I'm not saying that this is your problem here, but the only kit from Dynojet a lot of enthusiasts use is the Stage 7, and though it calls for tubes in-place of the VBoost flapper-valves, it can be tuned to retain use of the VBoost.

Right front is #4 piston/cyl. You might spray some water on the header pipe and see if it's evaporating as-quickly as the cylinders which seem to be firing normally. If the #4 cyl 'cuts-in' beginning to run at say 4K rpm + then you probably have an obstructed pilot jet on #4 carburetor.

If you have a heat gun (the Harbor Freight one is OK, they have a couple different ones, remember that the 'cone' of infrared that it uses to sense heat, widens quite a bit, with the gun's distance from the heat source, so you need to hold the beam of the gun close-to the source to-be read, say, < 1 ft. ) use that to compare header temperatures.

Plugs are inexpensive, though the Iridium are more $. If you switch the plugs does the 'miss' follow the plug to that cyl?

 

[apsolus]

hmmm. i installed the dynojet kit probably 2 years ago when i bought the bike. i had to rebuild the carbs so i figured why not. ive been told by fellow riders the bike belches smoke under WOT. ive sense removed the air snorkle and im told its cleared up. i dont recall the directions saying anything about messing with the vboost. ive still got the box it came in so ill go see what version i installed. i did drain the bowls on the left side when i was verifying the fuel pump was still working and i did get a little something in there. ill take a closer look tomorrow when i get out there.

 

[MaxMidnight]

Are you saying that the problem started after you installed the S1?

If it was a pilot jet issue then I would have expected the problem to manifest itself at low r.p.m. and clear up or become less noticeable as the revs rise.

Can we assume that the carbs are synched and that the fuel filter has been changed within living memory?

Until you know the issue is sorted I'd fit standard NGK plugs.

IMO the best way to establish what the fueling is doing is with a plug chop or better still getting a dyno baseline run.

 

[apsolus]

the problem developed all on its own. i installed the jet kit years ago. im sure the carbs arent synced because the bike will idle a bit higher unless i pull the grip shut then it idles down. that tells me a few of the butterflys are cracked open more than they should. another none issue i have with the bike. im headed down to autozone to pick up some new plugs now. i hope thata all it needs i see on the forum coils like to go to ****. seems like a spark issue to me . oh and yes the fuel filter has been changed

 

[Fire-medic]

My experience isn't that the 'coils like to go to ****,' but more-often than that, the pilot jets collect sediment. Also I've had two pick-up coils go bad.

The coil cases can and do become cracked, but somehow, it's more cosmetic than it is a problem resulting in their lack of proper function. Swapping them around to see if the 'problem moves, too,' would indicate that the issue is with the coil, its high-tension wire and/or the plug cap.

Many members have found green corrosion on the end of the spark plug wire at the cap, trimming it back a bit to shiny wires usually will fix that. I'd suggest a bit of dielectric grease on the wire end before re-installing the plug cap.

 

[apsolus]

Did you try draining the float bowls into a receptacle to see what's in there? It could be some water. That appears to be a 'lens' of fluid floating in the gas, due to their differing viscosities.

FYI, the Dynojet Stage 1 kit is a known problem from members' input on-here. I'm not saying that this is your problem here, but the only kit from Dynojet a lot of enthusiasts use is the Stage 7, and though it calls for tubes in-place of the VBoost flapper-valves, it can be tuned to retain use of the VBoost.

Right front is #4 piston/cyl. You might spray some water on the header pipe and see if it's evaporating as-quickly as the cylinders which seem to be firing normally. If the #4 cyl 'cuts-in' beginning to run at say 4K rpm + then you probably have an obstructed pilot jet on #4 carburetor.

If you have a heat gun (the Harbor Freight one is OK, they have a couple different ones, remember that the 'cone' of infrared that it uses to sense heat, widens quite a bit, with the gun's distance from the heat source, so you need to hold the beam of the gun close-to the source to-be read, say, < 1 ft. ) use that to compare header temperatures.

Plugs are inexpensive, though the Iridium are more $. If you switch the plugs does the 'miss' follow the plug to that cyl?

the bike idles just like it should on all 4 cylinders the problem cylincder actually cuts out with rpm and throttle. i drained the fuel and while it looks cloudy and generally crappy there isnt any water seperation going on. i verififed i instaled the stage one dynojet kit as well. i can get the offending cylinder to cut out if i give the bike throttle while riding it. it cuts out like a light switch. im gonna just remove and redo the carbs i guess. they were never perfect to begin with and 2 of them weep fuel from somwhere onto the intake boots. so im going to just install new kits and retune them a touch leaner. i did find that one of the coils is cracked but its the front left coil , not the right with the black plug. so im guessing for now the coil is ok. ive got a spark tester so if redoing the carbs doesnt resolve the issue ill check the coils . ill check for corrosion for sure. i went back in with the standard plug and it made no difference. bike still starts and idles great and cuts out under acceleration and med to high load
does this bike use waste spark? im used to waste spark using 1 coil for 2 cylinders and this bke has a coil per cylinder

 

[DreamV4]

Is it back fire or exhaust popping? Exhaust popping is very common sign of lean mix or exhaust leak.

 

[apsolus]

no popping at all. no exhaust leak either. im a mechanic by trade so ive def got the basics understood

 

[02GF74]

Note that it is common to find a crack in the plastic covering the coil body, if you compare yours to others, the crack follows the same line... but it does not stop the coil from working.

The HT lead is a push connect into the coil. Unscrew the plastic nut and check that the lead is pushed in far as possible and that the wire is not showing signs of corrosion. If it is, cut off 5mm, push the lead into the coil and do up the nut.

You could try swapping coil lead going to and from a pair of coils (note, the leads may not be long enough so I don't know) to see if the problem moves.

New plugs as per OEM spe. (NGK) is never a bad move.

 

[Woody]

After reading everything you've done.I only seen that you drained.2 of the 4 float bowls.
The problems of cutting out.When getting frisky with the throttle. Acts just like moisture in the bowls.you said you checked for water.but what you drain out.needs to sit so it has time to separate.then you can see what's in the fuel.
Also , And I'm sure you have thought of this.check all clamps that hold the carbs on.air leak under throttle.can cause cutting out.
Good luck.
And keep us posted.

 

[apsolus]

later on i drained all 4 together and let the fuel sit overnight. no seperation but i did notice the fuel turned cloudy yellow. i tested another batch of fuel from a jug and it came out clear then yellowed just the same. so i think i can rule out fuel quality. i think the yellowing is just the south texas humidity right now. im going to check the spark plug wires next

 

[Fire-medic]

I'm not familiar w/gas turning yellow unless one of your buddies is relieving his bladder into the gas you have.

 

[DreamV4]

Spark plug replacement sometimes eliminates backfire, but I have no explanation why.

 

[Pighuntingpuppy]

I'm not familiar w/gas turning yellow unless one of your buddies is relieving his bladder into the gas you have.

Yellow dye is used for gas station fuel, red dye for agricultural fuel. Thats what I remember from back in the day. What it is today...no clue. I dont really study the color of fuel anymore. I have seen all colors of fuel in my tenure. Blue was some super high octane race fuel, I seen green which I was told was AV fuel. Best fuel I ever used was AV fuel in my Virago. Leaded, oxygenated, high octane, and super expensive. When I used it, couldnt pump directly into the bike. Could pump to a can, then can to bike. Was like $10-11 a gallon.

 

[DreamV4]

Attach clear hoses to bowl drain nipples and duct tape them to bowls. Get your bike vertical (centerstand?) and check levels at idle and with throttle up to 5k(?) RPM.
If it is ok, you can:
ride it at full power, then stop as fast as it is still safe, turn off engine and check gas levels.
another way:
ride on highway in right lane and your friend driving a car with passenger to the left from you. Passenger takes pictures of clear hoses. Then on anothe side.

 

[El Canadiense]

How are your rubber diaphragms? Check for cracks, holes, etc.

 

[MaxMidnight]

Attach clear hoses to bowl drain nipples and duct tape them to bowls. Get your bike vertical (centerstand?) and check levels at idle and with throttle up to 5k(?) RPM.
If it is ok, you can:
ride it at full power, then stop as fast as it is still safe, turn off engine and check gas levels.
another way:
ride on highway in right lane and your friend driving a car with passenger to the left from you. Passenger takes pictures of clear hoses. Then on anotheR side.

Not sure what you are trying to achieve by doing this dynamically as there are no specifications.
The Service manual gives the following method:

ADJUSTMENT 1. Measure:

Fuel level​

Out of specification - Adjust by the following measurement steps.​

​

• Fuel level measurement steps:
• Place the motorcycle on a level surface.
• Use a garage jack under the engine to ensure that the carburetor is positioned vertically.
• Connect the Fuel Level Gauge (a.k.a. a length of clear hose) to the drain pipe using a level gauge adapter.
• Loosen the drain screw and warm up the engine for several minutes.
• Measure the fuel level with the center mark.
• Repeat the above procedure for other carburetors.
• If the fuel i s incorrect, adjust the fuel level(s)

 

[DreamV4]

Not sure what you are trying to achieve by doing this dynamically as there are no specifications.

I forgot to say!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
PS!!!!!!!!! If fuel pump half dead, engine can starve at higher speeds

 

[MaxMidnight]

I forgot to say!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
PS!!!!!!!!! If fuel pump half dead, engine can starve at higher speeds

So you are not trying to establish float height?

I drained the fuel and while it looks cloudy and generally crappy there isn't any water separation going on.
I can get the offending cylinder to cut out if I give the bike throttle while riding it. it cuts out like a light switch. I'm gonna just remove and redo the carbs I guess. they were never perfect to begin with and 2 of them weep fuel from somwhere onto the intake boots
I did find that one of the coils is cracked... not the right with the black plug.
Does this bike use waste spark?

From the above there are several issues that need to be addressed.

• Fuel - unless you have fresh fuel I'd be inclined to drain what you have and fill with fresh. Also have a butchers inside the tank and check for corrosion. Unless I've missed it you haven't confirmed when the fuel filter was last changed. You seen to be assuming that the carbs are synchronised but IMO you need to do that properly with the requisite tools.
• The problem is isolated to one cylinder? If that is so then the fuel pump is not the issue and if the plug is black then that cylinder is over rich. Have you checked the float heights? Have you also checked that the offending cylinders slide moves up in concert with the others? Also check that the right jets are installed in the right position and that the need les are installed correctly.
• As you have indicated you need to sort the leaking fuel which makes me wonder if the were re-assembled correctly.
• Your spark tester will give an indication if the coil is an issue. If all four show a similar result then don't worry.
• It is a wasted spark system.