Engine misfires above 4K rpm but only under load

[Fentible]

Just finished recommissioning a 1980 UK model xs400 custom which has stood idle since 1995. Took it up for its MOT test this morning and which it passed no problems.

On the way back from the tester I opened it up a little only to be rewarded with a misfire over 4000rpm up to about 7, which is as far as I revved it. Bike revs cleanly on the bench so its only under load.

Points gap is spot on as is the timing, new plugs fitted although the dealer only had resistor plugs and I'm not sure about the wisdom of running these with suppressor caps. Owner suggests that a similar problem existed on last road use and new condensers were fitted at the time to try and resolve it. Its unclear if it eliminated the problem as it coincided with the bike going into storage.

Anything else I should be looking at. Carbs were stripped and cleaned with new jets and float valves. I'm also running a small in line fuel filter to avoid any residual dirt blocking the new float valves.


Cheers

Fent

 

[willem]

so as you said in the other thread, it's running rich on one side and lean on the other. That causes the misfires

there's a lot on getting your bike running well in here as well

 

[Fentible]

Thanks Willem, that's a really useful link

 

[Yamaguy55]

Check to be sure both side's ignitions are up to par. If one side isn't sparking fully, you'll that for a symptom.

I'd try some Iridium plugs, they fire with less voltage. they're better anyway. But misfire under load is more often ignition than fueling, although both do assist each other in your misery.

 

[Fentible]

Thanks Yamaguy, I do have a nice healthy spark to both plugs but I guess that doesn't mean they aren't breaking down under load. I'll run a coil test tomorrow.

 

[Yamaguy55]

Hose the points down with WD40 to be sure there is no crud partially draining juice away. Sometimes the lube from the cam contact picks up enough stuff to conduct. If the point wire or HT lead fom the coil are questionable, repair or replace. It doesn't take much to siphon away high voltage. if the points are pitted, replace. Kill switches and side stand switches cause this too. I doubt you have a side stand switch, but you do have the kill switch.

 

[xschris]

Have you synced the carbs yet?

 

[Fentible]

Cheers Yamguy will check out. Forgot to mention that there was a bit of pop as the revs came back down, put this down to the lean side?? No intake leaks discovered. Might replace the points, there was no obvious pitting but according to the owner they've never been replaced so are 1980 18K originals.


XSchris - yup carbs sync'd, although Im slightly confused about the float height, manual suggested 32mm but I seem to recall that was a misprint??

 

[Sesty]

its in that carb guide posted in your other thread.

brass floats should be 27mm +/- 1mm and newer plastic floats 22mm +/- 1mm

 

[Fentible]

Sorry I will rad it thoroughly...............honest

 

[Fentible]

............................'read'

 

[Scorpio1963]

Test will be tomorrow make sure you study

 

[Fentible]

Test will be tomorrow make sure you study

Aaagghh..................I hate tests