Dropping Cylinder? Misfire?

[jak444]

Hi all,


I've tried lots of things listed in these great forums related to this problem I'm having... but still some issues :bang head:



The bike runs perfect for the first 5 mins of the day... then when it's warmed up it starts backfiring/misfiring. The exhaust note also changes - sounds like it's dropping a cylinder.


Carbs were overhauled a couple of months ago.


I've pulled the plugs and the number 2 cylinder was wet with fuel... I'm guessing it isn't getting good enough spark.


I've tested the pickup coil, HT coil for 2nd cylinder, HT lead, Stator and regulator/rectifier. Pickup coil, HT coil and HT lead were within specs.


Stator had slightly high resistance - about 0.6ohms on all readings - workshop manual says 0.48 max. Pretty close but a little high. No short to earth though which is good.


R/R seems to be faulty - getting infinite resistance across all diodes?
Bike also is charging at the battery at only 12.9v at all rev ranges. Obviously this is too low.


So..... my questions.... if the R/R is faulty as it seems, could this cause low charge at the battery? I've never had any problem with overcharging, blown fuses/globes etc.


Also... if the R/R does indeed cause my low charge problem, could this be the cause of my problems with backfire/misfiring/dropping a cylinder?
I'm confused because I don't see how this would only happen when hot, and only seem to affect no. 2 cylinder?


Maybe I am missing something.


Expert advice appreciated!


It is a 1992 model by the way.



Sorry for the long post

 

[42Below]

Im sorry I cant give expert advice but I can share an experience.
Im an ex car mechanic. My mate is a very experienced bike (ex Yamaha) mechanic.
We both raced KTM super motos. He was a KTM tech by this stage.
My bike started losing power like a wsitch going on and off. I couldnt find why. He tested every darn thing he could think of.
He had another KTM in parts by coincidence in his home garage ('the surgery') and put the stator in just to try it. It fixed the issue. My one had tested within spec.
A new stator fixed my issue, never misfired again.

I realise my story probably does no more than undermine your faith testing, sorry!
Damn machines.........

 

[jak444]

Thank you... and I concur... damn machines!! haha so frustrating when they start playing up. :bang head:

Fun when they behave though! unk:

Interesting that your stator was in spec, but was still the problem. I've read that on a few other threads too.

I guess I will wait for a few more responses and then see!

 

[sdt354]

A cold engine runs fine when too rich. You appear to possibly have more than one issue. Do a stator A/C output test. Diode test is best done by switching polarity of your test leads, continuity in one direction only. 12.9 is not enough charging. Battery voltage, after full charging , then sitting overnight without the charger on, is?

 

[jak444]

A cold engine runs fine when too rich. You appear to possibly have more than one issue. Do a stator A/C output test. Diode test is best done by switching polarity of your test leads, continuity in one direction only. 12.9 is not enough charging. Battery voltage, after full charging , then sitting overnight without the charger on, is?

Thanks,

The stator is putting out about 18v AC.

Battery is brand new.

So it may be too rich on one cylinder as well? Didn't think of that, thank you.

 

[Bill Seward]

18v at idle, or a higher RPM? A good stator should put out 45 volts AC, tested across all combinations of the 3 wires. (1-2, 1-3, 2-3) This should be tested at 2000 to 2500 rpm.

If you're only seeing 18v at higher rpm, somethings wrong. I'd bet a stator, but it could possibly be a problem with the flywheel magnets that generate the AC in the stator coils. I'd think that would be pretty unlikely, though..

 

[jak444]

It was approx.18v across all three combinations - at idle.

I thought it seemed low, but since all combinations were the same and all other tests were good I dismissed it.

So looks like there's an issue there then.

Question is - could that cause the symptoms I am describing?

I think the bike has been undercharging for years, it's had the hot start issue for at least 7 years.

This misfiring when hot is new though.

Thanks for all the help - you guys are awesome :biglaugh:

 

[caseyjones955]

Obviously a charging issue should be corrected, some ign systems dont appriciate low voltage. You said you had the carbs done, I know there are a few guys out there doing carbs that may not have the skills that our own Dannymax does and ruling out carbs done by an unknown may be premature. Hopefully getting system voltage back cures the issue.

 

[Traumahawk]

The hotstart issue can be corrected with a more powerful battery, upgrading to a 4 pole starter (which is what yamaha did in later years) or upgrading/cleaning your grounds including those on the starter itself.

If your R/R isnt where it should be with putting out enough charging voltage, a misfire can start when the bike gets warmed up, and the fan comes on....because it sucks up available voltage). Also check out the wires....grounds....etc, for corrosion. Voltage might wanna flow thru something thats cold (even with corrosion on it) but doesnt doesnt wanna flow thru something hot.

Even the Vmax spark plug boots can be taken apart and cleaned.