XJ900f suddenly idling issue.

My first 4 cylinder bike (and yes, it still intimidates me a little)
1988 XJ900f.

Valve play is okay.
Carbs recently overhauled and synchronised.
New spark plugs and 0-Ohm caps.
Starts very well, runs very good.
Fuel filter is mounted.
Battery charging is okay.

But since a few days, when engine is warm and when i have to halt a little longer at a traffic light, the idling is a little uneven.
As if 1 cylinder misfires every 5-10 secs.

If the coils were to blame, it wouldn't start so well and perform good when accelerating?

 

Two coils four spark plugs, left coil looking towards forks sparks 1 &4 right coil 2 & 3. Check Cylinder 1 plug cap remove it check wire is free from corrosion spray with WD40 replace cap and try the bike. Rev the engine above 2000rpm to see if there is any difference as alternator doesn't charge below 2000 rpm. Its not your coils if it was cylinder 4 would be misfiring too.

 

No corrosion, i recently installed the new 0-ohm caps and clipped the plug cables with 5mm to ensure 'fresh' contact.
I do not understand why revving over 2000 rpm..
It happens ONLY with idling.

 

I tested things, had it idling with warm engine. After a minute or so it starts stuttering. at that moment battery voltage is 12,30V.
When i turn off the light, the stuttering seems to (..) halt.

Hmm, 12,3V should be enough to feed the ignition?

 

Interesting, my 650 maxim does something similar, never tried turning the light on/off though. My feeling is a momentary hiccough/hiccup is indicative of a weak misfire (ie an air leak)..

 

I cannot imagine the plug caps are theproblem. Does the TCI ignition need the 5 K Ohm resistance??

 

When the bike is warm and acting up, spray some starter fluid, not carb cleaner, on the side of the carbs. If the RPM's go up, your throttle shaft seals might be bad. Or you have a leak in the boots in that area.

 

Thanks for the tip! I will make a test drive today with new 5 K-Ohm plug caps.
Are these shaft seals difficult to replace?

 

Armed with a can of brake cleaner i did a test drive.
No signal for an airleak along the shaft.
The problem still exists.
But, when i turn of the light, it idles much better?

Battery voltage when idling with lights on 12,4V
Without lights 14,3V

All contacts and plugs are cleaned last year.

 

The 14.3V is a good sign, the 12.4 with lights on is not. Wonder if you have voltage reg issue? I would guess that the stator is okay because it is putting out charging AC, but the reg is maybe not doing a good job once the lights are turned on. Still you are above the 11.5V threshold for spark that the brain box needs. Or so I have read.

 

Generator has new coils, but the xj900f genny does not do a lot when idling..would the regulator be a problem?
But, i went measuring..
12,4 Volts when idling lights on, was 10,7V at the fuse box??
All connectors are cleaned and okay but 2 years ago i replaced the ignition switch- i suspected the old one- for a JMPD-specimen.. could it be..?

 

No point asking on here, get your voltmeter out and start measuring volts across things - one end on battery +ve, the other following along the circuit till you get to the regulator (I forget what's in the circuit or I'd list each item one by one).

 

I do not find the point where the 2 volts are 'taken'.
Does it happen more often that the ignition switch is the troublemaker?

 

The ignition switch can be an issue, and when the output (12V switched) gets too much drop it results in overcharging the battery as that is the sense voltage for the Rect/Regulator.

I am just guessing, but I want to say at least a volt of drop is typical from the main fuse output to the fuse box input. Not sure if you posted the charge voltage for above 2500 RPM or so but it should be 14.5V +/- .3VDC

You need to consider the battery also. If it is weak from not fully charged or just defective the amount of power available from the AC Generator at idle may not be enough to overcome the total system drain and the weak battery

Is the headlight stock wattage?

 

Max charge voltage is okay. I will measure/check the ignition switch.. so that can be a cause?

But.. The big main fuse is only for the starter motor and-relay? Am i mistaken?

Headlight is a normal H4. 55/65Watt.
The battery is a 5 year old Motobatt, seems fine, resting voltage is 12,8V.

Thanks for thinking with me!

 

I know this is a completely different suggestion than anyone else had, but I once dealt with a rough idle on my 650 Maxim. It was similar to your description, where the bike ran fine cold but would misfire when warmed up. The result wound up being an overly rich mixture on one of the cylinders. The fuel enrichment plunger on cylinder 2 wasn’t bottoming out properly with the choke off.

Have you checked your mixture settings with a color tune spark plug? Or, have you verified that your fuel enrichment circuit isn’t allowing extra fuel into one of the carburetors (assuming that’s a thing on the 900F)?

 

It sounds strange, especial after running thouygh all connectors but.. it does not seem to have a main fuse??

All spark plugs have a sounds colour...

 

It may not I suppose. We mainly deal with the early 80's XJ here, and documentation for the 1988 seems a bit limited. Do you have a service manual that would identify fuse and location, as well as any AC Generator tests that would be beneficial.

When I check Yambits it shows 3 glass fuses for the 1988, a 10, 20, and 30 amp. Do you have the glass fuses?

 

Yes a circuit in the manual, but not the location.. The fuse box is a more modern one with 'normal'fuses.. i do not know the English word for it (plastic with 2 'legs')

 

Red wire ign. switch 11,9V
Brown wire 11,6V
Battery 12,4 (a bit empty due to all the tests i think)
so both sieds have a bit loss.. Hmm.

 

Also with battery charger connected, when i test the ign. switch connector Voltage drops within seconds from 13,4 to 12,4 Volts (So you measure back and forth to get a reliable result... about 0,4V loss it seems ..

When idling, the genny does'nt do much.. should the battery have the capacity to overcome this?
I.O.W., is the 5 year old Motobatt not sooo fresh anymore?

 

If this is similar to the XJ750 then at idle with a fully charged battery the voltage should be very close to 12.6 after a very brief recovery from spinning the starter

Yes an older battery that cranks may not be able to keep up, or put another way the available current from the AC Generator at idle is spent charging the flat battery that is slightly low on voltage. This is why Yamaha stipulates that when checking the charging system a fully charged battery is step number 1.

Commonly called a blade fuse

 

Hmm the battery is 'stealing'the current from thje ignition...
B.V. at idle after a minute or so is 12,4 V.
Idling rpm is prescibed at 1100 rpm because of the low charging at idle on these bikes..

 

In my experience a 5 yr old motorbatt will be scrap, but yours seems to be holding up well.

I repeat my earlier advice. Use a voltmeter to chase down a volt drop, worst case scenario is the loom is corroded and that's where the drop is -ie everywhere. The question of load has a bearing as well - engine running (not idling) and generator output normal, you should, as you state have that voltage at the battery, at the lights, and coil +ve terminals. If not, first check would be earths, ie between battery earth and frame, then locate the loom earth points and check these. If all ok then measure between battery +ve and generator output, then follow this along until you find the loss - any part of the loom, switch, relay (relay bases can be worse), plugs and sockets, until you get to the last item in the line. Use the workshop manual electrical diagram.
You can measure from batt earth, still looking for a lower voltage than the generator output.