Topic: 78 550 coils

[uksparky]

I have miss fire at idle it comes and goes bucking at low throttle 30 mph.. problem is #3 cylinder but full throttle all is ok......  i used timing light on plug wire at idle and on off strobe light....swapped plug wires same #3 cylinder  PO replaced plug wires at some point......cleaned coil connections the same problem...........should i get the same coils i have the green Dyna coils 3 ohms production date 6/92... i also have electronic ignition    best place to get coils that fit with no problems

[Yamahawk]

Hi uksparky,
    As our coils feed two cylinders with each coil, if the coil is bad, both cylinders usually will exhibit the same problem. I see your troubleshooting has  involved changing plug wires, but have you replaced the plug in the #3 cylinder? It seems like it may be something other than the coil, so you may want to replace the plug. If that isn't it, I would check the #3 carburetor for a blocked or dirty pilot jet or passage. The clue here, is that it goes away with full throttle. I believe it is one of the two, so good luck in your quest!
Charlie

[uksparky]

Plugs were new only 2k miles on them ....could be the cheapest way to go























I was thinking the same thing...i cleaned the pilot jets again but no blockage....did this some months back......so the coil if it goes  bad the both cylinders will exhibit the same problem.......ok i have the strobe light on #3 at idle its a pulsating light ..it stops for a few seconds engine rough then back to normal..........coil




 

[Deltarider]

The easiest check first. All CB500/550s are prone to misfires due to (often intermittent) arcing between HTleads and/or caps to the engine (= GND). This is best observed at idle in the dark. Since the last work done was on the wires, I'd start there. Some spray a mist of water to be better able to detect it. What plugs do you run, with or without "R"? And what resistance plugcaps do you measure?

[dave500]

as per delta check the plug cap resistance,if they are the old factory ones and one is a dud replace em all.

coils that fit.

[Yamahawk]

As Delta and Dave said, check to see if your plugs are resistor plugs, and check the HT lead going into the coil, and the cap. If they are old wires, I would replace them for good measure. I know you swapped plug wires, but try that again. And, it isn't uncommon for a plug to foul. Pull the #3 plug and look at it.
Charlie

[dave500]

if you have solid wires they'll be fine,its the caps resistance that ages over time,i never use resistor plugs as the resistor can fail before the actual electrodes,i just don't trust resistors in spark plugs in such a hot environment,the caps don't get so hot,plugs will only foul if somethings up,like poor electrics or wrong fueling.

[uksparky]

Thanks for all the info guys ..just want to clarify one thing ..the previous owner replaced the plug wires, i just swapped/ pulled 3 and 4....same idle miss fire #3...two thousand miles ago i did a carb clean and new plugs NGK D7EA..........all ran fine until this week. I will pull 3 and 4 plugs swap em over

Just pulled 3 and 4 plugs...look at the electrode it's worn down, I will replace them all

[Yamahawk]

I take it the plug with barely any electrode was #3? Plugs are cheap, I try to replace them every spring, whether they need it or not. Good deal on finding that out!
Charlie

[TwoTired]

The plug with the worn electrode is running too hot.  That, along with the high spark current from 3 ohm coils is what is eroding the plug's electrode.

If you don't address the lean mixture for that cylinder, expect to replace spark plugs frequently.  (maybe even valves, too)


Cheers,

[uksparky]

Thanks Two Tired ...yes your right too hot....but plug tip not white.. all mixture screws 1.5 turns out.... turning out riches  mixture right...float heights all the same.. idle jet 42  main 90 but i never checked the main jet needle groove settings
would you recommend staying with NGK or another brand like BOSCH 7701.... not helping the situation i have the high comp pistons and KH DE camshaft longer duration lobes

[TwoTired]

Thanks Two Tired ...yes your right too hot....but plug tip not white..

Note there is a difference between a white center electrode insulator (with no deposits) and an actual deposit build up that is white or white-ish.  The chart I posted is old when there was still lead in the fuel.  Today's unleaded gas formulation will alter actual plug deposit appearance somewhat, because the deposited chemical residue has a different composition.  The key factors are the electrode material loss coupled with the deposited material, possibly from oil.

if you have been running the engine with a hot cylinder for a long time, that cylinder and piston will have different expansion states than the others.  This makes them wear differently leading to dissimilar wear-in clearances.  Have you checked for even head pipe temps?

all mixture screws 1.5 turns out.... turning out riches  mixture right...float heights all the same.. idle jet 42  main 90 but i never checked the main jet needle groove settings

Yes, for the PD carbs turning them outwards enriches the idle mixture.
You have 90 mains?  Still have the stock 4 to 4 exhaust?
have you checked the carb vacuum balance?

would you recommend staying with NGK or another brand like BOSCH 7701.... not helping the situation i have the high comp pistons and KH DE camshaft longer duration lobes

I don't have a reason to use plugs other than NGK.   However, with High comp pistons and cam change,  Probably better off going to a colder heat range spark plug.  D8 or possibly even D9.  Just expect a longer warm up cycle for a responsive motor.

Cheers,

[uksparky]

Yes stock exhaust ...carb balance done...   i will try 2 turns out on #3

[Deltarider]

That left plug centre electrode in your pic is abnormal. I find it hard to believe that it is caused by erosion alone. Somebody must have used a file to... to what?
Dave, more folks don't like "R" plugs. I myself am not that comfortable with them. The "R" set I used (for less than 10.000 km), shows a much increased resistance on all 4 plugs. My advice: stick to resistor caps.

[uksparky]

No file down Deltarider all brand new when i installed them 2k ago #3 looks like its done 60k in a auto engine lol....closest town/parts store is 15 miles away i picked up a set of Autolites 4164 cross over to NGKs..out of stock on the NGKs ...will install Saturday

[uksparky]

The spark plugs fixed it, I decided to do a carb sync after turning mixture screws 2 turns out..minor adjustment
Thanks again guys