miss above 4000rpm

[Anonymous]

just put my GS on the road last week, I have a bit of a miss above 4k rpm and the plug for #3 cylinder black. rest of the plugs brown(maybe a little darker then they should be but its better than black or white).

I will be moving the plug over to a different cylinder tommorow and going for a ride after that to see if its the plug itself or related to that cylinder other than that though im not sure.

[KEITH KRAUSE]

You can also try swapping the leads of #2 and #3, to see if you have a bad connection creating a poor spark. Or just check the leads where they connect to the coil and plug cap. Make sure you don't have any cracks in that lead and the spark is arcing to the frame, etc.
Check compression if you can. Also, if carb related, a poor carb synch could cause a rich mixture.
Other rich mixture causes for that # carb: float out of adjustment, float needle valve worn or not seating because of dirt in the valve seat, bad o-ring/gasket for the valve, loose jet, air passages clogged inside carb.
I believe your petcock vacuum line is attached to #2 carb? So a leaking petcock diaphragm would not be your problem.
Let us know how it goes.

[Anonymous]

moved plugs around today and something related with #3 cylinder.

the bike currently lives at the airport I work at so I only tinker when I get a chance during the day. the plug wires are permantly attached to the coils so i'll try swapping coils side to see if its related to that or not. I just put the carbs back in a few weeks ago after cleaning them so I dont feel like pulling them unless I absolutly have no choise.

if the rain goes away long enough for a quick ride i'll reply again tommorow.
im currently working on aqquiring a compression gauge so thats not done yet.

[Anonymous]

change plug leads around it was the same, did a compression test and the readings were #1: 150, #2: 150, #3: 145, #4: 150 and that was done hot.

and I realized its not a miss above 4000rpm, its a miss under load. if it was a car id replace the plug wires but since there permanently attached to the coils I cant do that so i'll play with carbs more and cross my fingers.

but when I had them appart I tripple checked float level using a digital caliper to get all float levels the same and within spec and put compressed air through every passage of each carb. the bike only has 3800km on it now so carb parts are not worn. GRRRR. its not taking em out or working on the carbs I hate, its putting them back in *L* oh well

[KEITH KRAUSE]

Sometimes the plug lead swap is not definite in helping you troubleshoot.
With a suspect bad plug lead placed on a good looking plug, and a known good plug lead placed on a previously fouled plug (even if you tried to clean it), the bike may still only be firing well on 3 cylinders. This will make you think the test proved nothing. Actually, just moving a possibly bad plug lead can make it work better for awhile. Also, re-using a carbon-stained sparkplug is questionable. If the carbon is glazed on, cleaning it just won't work well. The spark will want to track down the carbon layer instead of jumping the gap. Hope this makes sense.
If it was my bike, I would take off the plug caps and inspect the ends for any corrosion. Clean up the ends with a wire brush or cut new ends if you have enough cable. Twist them back on tight and check their spark with a new plug. Then replace the plug that was fouling with the new plug, correctly gapped. Then I would make sure the ignition timing is correct. If you're timing is correct and the spark is good, I would re-test. If it starts fouling the plug again, THEN I would look at the carbs.
The first thing I would do is make sure the mixture screw to that problem carb is adjusted at least similar to the other ones. Have you compared how far out it's adjusted compared to the other ones? It does'nt have to be exact, in fact, it's common to find all 4 screws set at different points. But they should all be within 1/2 turn of each other. How is it set?
The next thing I would do (if you can) is synch the carbs. If one carb is pulling more vacuum than the others, that carb will run richer than the others. With a mercury vacuum tool, the levels should be set so the difference between the "highest" and "lowest" level is no more than 1 cm.
Let us know how it goes.

[Anonymous]

hrmmm, the idle mixture screws are related to syncing carbs? i've never owned a multi cylinder bike before so although I know about syncing carbs I have never actually done it yet. I did set the throttle plates using a drill bit to get them similar and the mixture screws are all the same. cant remember exactly what there at around 3 though, when I cleaned the carbs they were all equal so I just returned em to the same setting they were at before I disested the carbs. they did not have any caps covering them though and they are flush with the top of where they thread in.

if it was a bad plug lead though it would have fouled the clean plug I put in the cylinder or the clean plug in the cylinder I moved the lead to(leads 1 and 4 can swap and leads 2 and 3 so the lead went from cylinder 3 to cylinder 4) when I put the clean plug in cylinder 3 it turned black and power miss was still there underload even though I went and road it soon as the new plug was in(engine was already hot). when I moved the lead to cylinder 2 the plug in the cylinder stayed clean, leading me to realize its gotta be the carb on that cylinder.


and its electronic ignition..... as far as im aware I cant change the timing.

[KEITH KRAUSE]

I was only saying that maybe the mixture screw in that carb was adjusted too far out. I was not relating the screw to the carbs being synched. As for the synch, you can't be sure how well the carbs are synched without using a vacuum tool.
Regarding the lead swapping test, you lost me a bit. You can't put lead #3 on #4 and run the bike. Anyway, just moving a bad electrical connection can cause it to work temporarily. I would have checked all the leads so I know they are good. The coils/leads/caps are usually quicker/easier to check than carb inspection. If you feel it's the carb, and it may well be, then check what can cause a carb to run rich. By the way, 3 turns out on the mixture screws sounds too far out for a stock motor.( Based on other members post's here)
I've already mentioned the carbs may need a tool synch.
You say the carbs are clean and the floats adjusted. Are the floats moving freely and no chance of sticking?
I would assume the jets are tight and the correct size.
Do you smell any gas or get any fuel (even a few drops collecting in the tube) spilling out the overflow tube for #3? A failing float needle valve would cause this or a bad o-ring for the valve's seat.
It can't be the petcock, it goes to carb #2.
Does the choke plunger look like it's seating down completely?
Aside from carbs, the valve clearances need to be set correctly for good combustion.
It should'nt be the timing because you only have a problem on one plug.
I always check the timing before any carb work. If your modules/pick ups/sensors are mounted on a movable plate, the timing is adjustable.

[Anonymous]

fixed..... pulled the carbs today and the main jet for #3 was on the bottum of the floatbowl. I must have neglected to tighten it, sure didn't take long to vibrate out though.


and I swapped the leads between 2 and 3 as well as 1 and 4. those are the ones that can swap on my bike. I had to pull the tank to figure that out as the wires have factory labeling that says 1-3 on the wires that were on 1 and 3 and the wires that were on 2 and 4 have labbeling that says 2-4 so when I first did the lead swapping it didn't run then I figured it out.

runs great now though, my intake boot vacuum adapters arrived this week at the bike shop so i'll get them synced in fairly soon.



still hard to believe it was something so stupid *L* oh well live an learn

[KEITH KRAUSE]

I mentioned a loose jet twice in my replies. It's happened before.
I was the only one who stuck with you on this. Where's my atta-boy?

[Anonymous]

your also the only one that replied. I do thank you for the help.

at least I got a bunch of other testing done that I wanted to do anyway

[KEITH KRAUSE]