Topic: Early spark plug death

[CharlieVT]

1974 CB550
Bike was rebuilt with the help of the the fine folks on this board.
4-1 Exhaust, stock airbox.
I put in new coils that were for a CB750. Everything else in the ignition circuit is stock. Wiring harness refurbished, new points. All tuned up.
I had been fussing the jetting and had been running pretty rich for a while.

The the bike decided not to start. It turned out that the sparkplugs, while they would spark outside the cylinder, didn't have enough spark under compression to get the bike running.

Replaced the plugs with new ones and now all better. Total miles on the plugs that went bad was no more than 1500 miles.

So what caused the premature death of my spark plugs? I am guessing that the CB750 coils and plugwires are the problem. 

I do run the bike hard, and the rpms are often up but I don't redline it.

Thanks in advance for you thoughts.

Charlie in Vermont

[754]

repeated cold starting and  short rides can be the DEATH of plugs..

[750goes]

what style of plug are you running ??

if you are running rich then you may need a hotter plug for a better burn...

but I would try and fix the rich issue first,  then run the right heat range plug...

what sort of plug caps do you have also ??

[CharlieVT]

what style of plug are you running ??

if you are running rich then you may need a hotter plug for a better burn...

but I would try and fix the rich issue first,  then run the right heat range plug...

what sort of plug caps do you have also ??

I have rejetted back down so the too rich problem is probably fixed. I had experimented with larger idle jets but have changed back to the original jets. I had thought with my 4-1 exhaust I might need to run a little more fuel but it turned out not to need it.

I didn't think running rich would hurt the plugs. Wrong?  I know running too lean can cook things too hot, but that wasn't the case.
 
The plug I am using is NGK D7EA, referenced as the equivalent of the NGK plug that is specified in the manual but no longer available.

The plug caps are the original ones that came off the orignial coil's ignition wires.

Thanks.

[Patrick]

Running rich can fuel-foul your plugs. That may have been the issue. Try the new plugs and see how long they last now that you've got the mixture right.

Patrick

[hopterfixer]

Check the resistance of your plug caps, they should be no more than 5,000 ohms.  I had one at 20,000 Ohms when I got the bike. 

[TwoTired]

The 550 uses 10K plug caps, vs the 750 5K.  Perhaps to match the coils, which are slightly different between the models.

Soot from running rich can coat the center electrode insulator, which forms a path to ground, bypassing an arc across the electrodes.

Cheers,

[Deltarider]

The 550 uses 10K plug caps.

Are you sure? As I remember all 500/550 models had 5K plug caps and the 10k were introduced when the Goldwing came along. Maybe in some countries the K3 (and K4)models had 10K. Maybe.
Next spring I'll have D7EA's for the first time. My plugs get fouled nowadays. In the years long gone (no speed limit, proper gas) even Hondadealers would put in D8ES-L's as a routine. They knew these bikes were commonly used for high speed touring. 

Could there be a cloth or something in the airfilter opening?

[TwoTired]

All the 500/550s in my sig have 10K plug caps.  Seems pretty remarkable that all the previous owners would have changed them all out.  In fact, I'm pretty certain that at least some of them are still in as-delivered condition from the dealer in the USA ... with 10K plug caps.

I've been sold D8EA plugs by the Honda dealer several times.  They all ran like crap in the 550 unless the outside temps were in the 90-100F range and I ran on the freeway most of the time.  They also needed a 5-15 minute warm up to be ridable and not stumble at the slightest aberration of the throttle.  Maybe they needed 5K caps? I don't know.  But, with D7EA I can start and go when the engine is cold and the engine is reliable, just choke as needed for engine temps when cold, response is always there.

Doesn't surprise me that D8 plugs are prone to fouling in a 550.  Being a colder plug, you would need hard hot running for them to self clean in the 550.

Cheers,