High speed misfire question

[guyfawkes]

Yes, I did search first but couldn't find this one.

1980 SG special imported from the USA to UK many years ago, not had it more'n 9 months or so and putting right the neglect it suffered before (20k miles when I got it) and lack of servicing... mods are basically limited to aftermaket (louder silencers) and a different (better) fuse holder... engine is mechanically beautiful and I say that because it has been partially stripped and what needed doing was done.....

Problem is this, on long motorway runs, it will run all day and all night at 5500 rpm, but take it up to 6000 / 6250 and I get a sudden and bad misfire, like losing two cylinders, the only instant cure for this is not backing off or even slowing down, but blipping the kill switch to off momentarily cures it, until you speed up again...

parked and inspecting it I'm damned if I can see the problem or recreate it ..... *something* that the momentary kill switch effectstemporarily cures it, where 30 seconds a 4500 revs doesn't, so not fuel starvation (tried prime and checked for kinked pipes) but kill switch shuts down electrical system and will create a high manifold vacuum no???

I'm thinking is there a relay somewhere (aks BMW style relay for everything wiring looms) that gets weak and could cause this? or any other suggestions.....

one thing, mebbe not significant, but headlight comes on with ignition, NOT when motor fires..... other thing is must investigate battery charging, which seems ever so ever so slightly weak, but this could of course be mr yamaha electrics / joins and the onset of winter.

any tips / advice gratefully rec'd

[Dragonrider]

Well...lots to check electrical...at high rpm ignitions use a lot of power...surprising amounts...

Make sure you have a solid battery...should be 12.7 volts or so with the bike off, and not drop much with the key turned on...

Check connections...particularly connectors to the ignition box, and wire condition and connectors to the ballast resistors and coils.

Check the condition of the fuse box (many replace these with the new spade fuse connectors for much improvement.

Check for good ground cable connection to battery and frame.

If at high rpm, a bad connection is dropping voltage to the ignition box it could be loosing its mind.

ALSO...check the pick-up wires...suspect them (tip on this site, just search).

Would not sweat the headlight coming on with the key...the latching relay is triggered by a feed from the alternator and that line has a diaode in the line somewhere in the harness. Common failure and many people bypass the relay. You might make sure that the bypass was done safely and will hold together.

That should get you started!

Good luck and keep us posted!

[guyfawkes]

I have a "gut feeling" that this problem is in some way temperature related, and by temperature I include coefficients of expansion, because when the motor is lightly used it will rev its balls off with no sign of any misfire, after 100 miles of sustained 6000 rpm oops there she is, yet at 5500 it doesn't appear.

the pick up wires one you mentioned I did read about, and I must check as soon as we get a dry day, but it doesnt seem to match the symptoms well enough as yet.

[scalded dog]

I had the same symptoms, I thought it was the little black box, turned out to be the pic-up wires, I fixed that and don't have any problems now. Later 'Dog

[ARRON]

i had a high misfire ,i found that the mechanical advance was siezed and wouldn't advance the timing.its the one behind the vacuum advance.i took mine off and cleaned and lubercated the posts ,it now works awesome!!

[John]

Misfire

Sounds like you have the very common problem experienced with most other XSsives, myself included. Remove the cover from the left side of the engine, locate the two wires leading from the pickup coils. Gently try to stretch them. The most common problem is the wire strands will break INSIDE the insulation, and will create an 'open' at different speeds, preventing the related coil from firing. If you discover any stretch, then use an ohmeter and probe the wire near the pickups, and probe downstream as far as you can, then do the stretching. If broken wires are the problem, they will show up in this test. Mine was the wire for the pick up to the 1-4 ignition coil. Drove me nuts for a couple of hours. That was before I discovered this site, and its' many knowledgeable folks. There is a lot of experience here that can be tapped into. Good luck!

[scoobes]

hi guyfawkes,make sure that the vacum pipe is in the correct place(inlet rubber)try sucking the pipe to make sure that it is clear.if you rev the bike up when you are not riding does it still misfire,or is the problem only when the bike is under load.are you in the uk.

[skids]

I'll bet it has something to do with lean conditions at the higher rpms because of those after market mufflers.

[Bikeboy]

misfire

Hey i have the same problem. Before i cleaned my carbs this past summer my xs ran pretty good, no misfire at all. But every since i cleaned them my bike misfires at high rpm. When i get down on it, and run through the gears, it's at about third gear at 4000 or so rpm's is when it start's. But only does it when i get down on it. But doesen't seem to do it if i go through the gears slowly. Any answers guys.

[mitch]

Not Carbs

Sounds like u have a sticky vac advance or a bad pick up wire go up top and search both . My bet is a pick up wire, not to hard to fix, lots of info on that here .......MITCH

[guyfawkes]

haven't had a chance to check the primary wires yet, still raining, but I can absolutely discount any changes due to possible different backpressure or vacuum hose being in the wrong place.......

the motor will rev its tits off for a passtime, ___UNLESS___ it has just done 100 miles at 5500 rpm, then it doesnt like anything over 6000

[MartyA]

Did I read somewhere about coils heating up after a while, creating increased resistance? Bear with me, I am NOT an electrician. Wonder also if there is a plug wire kinked or broken inside, or a worn spot on a plug wire. When I had the head off my XS this summer, I found one of the plug wires had been rubbing on the cam cover. It had not worn through or caused any problem, but there is an indent in the rubber. I wrapped it in electrical tape as a precaution.

[skids]

That is true Marty. Also, plug caps can show near-normal resistance until they get hot. I always goe to the cap and wire pairs to test resistance across from cap to cap and if I see problems there, the caps themselves are tested next. 'Course I am not an x-pert!

Originally posted by MartyA
Did I read somewhere about coils heating up after a while, creating increased resistance?

[guyfawkes]

hot?

figure in rain and 85 mph wind chill on ambient temperatures maybe just into the double figures celcius and all that **** runs cooler than it will at tickover under a canopy...

no, I like the "feel" of the pick up wires fault, especially as momentarily flicking the kill switch cures the problem until you build the revs up again, as that moment of kill will make a big difference in the vacuum, and I can see elevated temperatures _inside_ a motor casing occurring at 6200 rpm as opposed to 5500, and especially after an hour plus running at >5000 rpm, especially is electrical joints, I can "see" differing coefficients of thermal expansion causing my sort of problem.

Ironically because it is still a relatvely slight problem it wasn't so easy to figure out what might be the cause until I came here....

hope fully it will be dry enough tomorrow so I can pop that cover and test it.

[scalded dog]

I am having similar problems, I fixed the pick up wires, bike runs smooth, until I get on it, if I stay down on it, it will finaaly push me back on the seat like a turbo boost or something, if i keep revs up over 6500 the bike runs like it has a power band, but I don't like to do that. any one else had this problem. I also can cruise at normal speeds shifting normal and the bike runs smooth. Later 'Dog