Problems with RPMs droping

[t6nis]

Hi guys.

I have a problem with my B4. It starts ok and runs ok, but it does not idle, and the RPMs do not drop some times when i rev it standing. It like stucks around 4k and then drops really-really slowly, or doesnt at all. When i turn the idle screw, it starts to drop and then the rpm drops so low that it dies.
It cant be excess air, because i have sprayd brake cleaner to the carb manifolds and the rpms did not rise. The spark plugs are all nice and brownish. Could it be the diaphragms?

[Squishy]

Hello.

If it's not too much air (either by leak or mixture) then it's either stuck needles/diaphragm or out of sync carbs.
You already said you checked air. However my question is how did you change the mixture screws properly while it was running? I know it can be done but they are very hard to reach and it's impossible to turn them all at once.

As for the diaphragm, you can easily check this by removing the airbox and running the bike. Just check whether the needles go back once you let go of the gas.

If it's not this I would suggest syncing the carbs. If it's off RPM will hang.

[t6nis]

I turned the idle screw that is on the side, not the mixture screws. Those are 1.5 turns out, like the factory default is.
I am going to try and sync the carbs and see what happens. Im also going to clean the needle chamber. I saw a  video of a b4 carb cleaning where the dude pushed the needle chamber( the place where air and fuel are mixed. little brass tube with small holes in it and  with the pilot screw at the one end?) out. I hope that would make a difference.

[Squishy]

Oh sorry, I thought you meant idle mixture screw.
Turning the regular idle screw does nothing except increase/decrease throttle valve position with the throttle closed.

You could try setting the mixture screws richer but unless you severely changed the setup (like open air filters) this should be necessary.

Cleaning the carbs is always a good idea but I think you should do this last. The brass tubes are called emulsion tubes. They are not really prone to getting clogged, unless you used really bad fuel or let it sit for very long time. They are more prone to wearing causing overly rich setup.

Anyway I'd just remove the airbox and check the function of the needles/slider if that's ok sync the carbs.
If valve clearance check is coming up you might as well do that since after that you usually gotta resync the carbs anyway.

[ventYl]

it sounds like huuge de-sync of carbs to me. the engine should run rough also. if you ask if it is possible that they de-synced so quickly, yes it is possible, i saw it once on B6. check if some of synchronization screws is not loose.

[t6nis]

okay, can someone tell me what would be the optimal PSI or bar for one cylinder? I mean for syncing, not for cylinder pressure.
 I know that they all should be in the same range, but what would be optimal for a normal B4.

[Squishy]

okay, can someone tell me what would be the optimal PSI or bar for one cylinder? I mean for syncing, not for cylinder pressure.
 I know that they all should be in the same range, but what would be optimal for a normal B4.

Doesn't really matter as long as they are equal

[ventYl]

I did it today and mine had somewhere around 0,7bar at ~2000 RPM (around 0,8bar at ~1400 RPM). But probably there's nothing like optimal pressure because it will differ from altitude and some other factors. Set idle RPM, then ballance them, fix the RPM and don't care about absolute values.

[t6nis]

Okay, ran into another problem today. Was riding home from work and suddenly i felt a power drop, the sound changed and the bike wanted to die. Pulled into a parking-lot and figured out that the 1&4 cylinder were not firing.

Got it into a garage and tried another coil, that did nothing, no spark whatsoever. i fiddeled around a bit and pulled the signal plug from the CDI and then reconnected it. This brought the spark back and i rode it home.

Can someone please explain this problem to me, why did it happen and how can i avoid it? Or is it a sign of a CDI thinking of wanting to go bad? :D
I have dealt with bad caps and burned down CDIs on B4 before, but nothing like this.
Have any of you guys ever seen this?

[ventYl]

any signs of rust or any other kind of non-conducting contact?

dying CDI can be observed mostly only using oscilloscope by measuring the drop of the voltage at the time of "fire". tbh I don't know the right numbers as I've never seen original B4 CDI in mint condition. One of mines is repaired by me (I should be able to provide you numbers from mine if you want) another one is replacement 3rd party CDI.

[t6nis]

nope, i didnt see any rust whatsoever.

All the CDIs that were on the verge of dieing showed trouble at starting or didn't start at all.
I hope this problem will not occure

[ventYl]

well i can't be helpful in this case as I never seen CDI dying

[greg737]

Your description of yesterday's ride home from work might be indicative of a malfunction that is partially a heat-related thing. 

Whatever is causing the misfire on 1 & 4 could be a situation that gets progressively worse as the weak component heats up, going from just barely adequate to run the engine when cool or warm to full misfire when it gets hot.

The time you spent working on the bike after the complete 1 & 4 misfire caused you to stop might have allowed the problem component time to cool off again, making the bike nominally rideable again for long enough to get you home.

[t6nis]

yeah, but i was working on it for like 10-15 minutes.
I waited for transport to garage about a half an hour or so, and after that it still ran on two cylinders. Right before unplugging and plugging the connector back in i tried it with a different coil, and it still ran on two cylinders. Only after unplugging and replugging the problem disappeared.

[interfuse]

If you jiggled the low voltage wires going to the coil and you are now running on 4 cylinders again you've got an intermittent connection. You can test this by running the bike on 4 cylinders and jiggling the wires to see if starts running on 2 again.

The wires going to the coils are small, old and exposed to wind. They tend to start breaking near the spade connector. I'd cut off the connectors, strip back the wire a little and crimp on new spade connectors.

If you don't fix this you will lose the connection again and probably at the most inopportune time.