fouled spark plugs

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cjatwell

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Oct 2, 2009
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Hi all.
I have a '74 MK 11 A 850 Commando. Noticed it getting bit harder to start recently .
Until the other day had no spark at all and wouldnt start.
Pulled out plugs and replaced with new.
Started fine.

The plugs were only in the bike a few months and are now useless.
The electrodes are black in colour and are what i would describe as 'sooty'.
What is the cause of this and how do i prevent it happening .
I run a single mikuni carb .
 
Sounds like you need to lean out your carb, probably in the middle of the range eg 1/2 throttle but it all depends on how you have been using the bike and a full tune is best as changing one setting has a small but significant effect on the neighbouring settings.
 
Give your location altitude and your mikuni needle position and slide number, main jet size, etc.
Are your running a stock dual coil or single high output coil, what are the plugs gapped at?
I have found that, all the above being correct, that depressing the "choke" lever only to get it started and immediately, really a couple of seconds only,
then raising the lever fully up and giving enough throttle so it idles around 1300 or so helps greatly in not loading the plugs with rich soot.
 
highdesert said:
Give your location altitude and your mikuni needle position and slide number, main jet size, etc.
Are your running a stock dual coil or single high output coil, what are the plugs gapped at?
I have found that, all the above being correct, that depressing the "choke" lever only to get it started and immediately, really a couple of seconds only,
then raising the lever fully up and giving enough throttle so it idles around 1300 or so helps greatly in not loading the plugs with rich soot.
Click to expand...


Live in N Ireland UK , sea level . Not sure how to get mikuni needle position and slide number, main jet size, etc.
How do i find this out. ?
Think i run a boyer ignition . Using standard NGK BP7ES plugs.
Was finding i had to leave choke lever on for a minute at least to let her warm up enough , then blip throttle for further minute so she would tickover .
How do i adjust the carb ?
 
If all else fails try NGK BP6ES plugs. They're a little hotter and will self clean better.

Cash
 
When you have time, you can unscrew the top of the mikuni and lift out the spring/slide, then look on the bottom of the slide, there will be a number
like probably 2.5, taking off the four screws bottom float bowl will allow you to unscrew the main jet, this has a number stamped on it, like 240.

Regardless, let's assume your mikuni settings are correct.
In my humble opinion, you are loading up the plugs by allowing the choke lever to be down way too long.
Again, depress it only to kick it over and while giving a little throttle get the lever fully up as soon as possible.
You can let the bike warm up for a short period of time while keeping the idle going with your wrist.
Very short time is warmup, I ride away within a half minute of starting, and have the choke back up within seconds of starting.
Blast and clean your plugs, or replace now with new ones, I think you are leaving the choke on WAY too long.
 
Use alot less choke, start only and switch off straight away , holding the revs up manually until it will idle, electronic ignition :?:if so open the plug gap to about 0.040'',do you ever get above 4000rpm :?:
 
splatt,
you mention to, with electronic ignition, open the plug gaps to 40thou.
Years ago I bought an Andrews high output single orange coil with dual plug wire outputs.
At the time, I was told to open the gap from 28 to 35.
Why would an electronic ignition permit/require more of gap than points?
And would my supposedly high output coil combine with a boyer allow opening the plug gap perhaps even further than 40thou?
interesting
 
Sorry highdesert Im not an electronic scientist and cant tell you why the electronic system will run the bigger plug gaps, all I know is that in good condition it will. Most modern vehicals run at least 0.040'' with a warning that high voltage can kill :shock: so there must be some real output in new systems to help fire the ever leaner mixture
 
While you should certainly correct the root cause, FWIW fine-wire iridium plugs resist fouling a LOT better than normal ones. DAMHIK - new rings this winter though!
 
I'm running champion n7y with a boyer, and two 6v coils... I should gap them at .040?
 
DonOR said:
I'm running champion n7y with a boyer, and two 6v coils... I should gap them at .040?
Click to expand...

I seem to remember Boyer Bransden not recommending increased plug gaps?
 
I seem to remember Boyer Bransden not recommending increased plug gaps?
Click to expand...

LAB you're right, Boyer don't recommend larger gaps.

cjatwell,
For the interim just stick a pair of NGK BP6ES in.

Cash
 
http://www.centuryperformance.com/spark ... pg-26.html


"OPENING THE SPARK PLUG GAP:

For as long as engine builders and racers have been tuning engines there has been talk of enlarged or modified spark plug gaps. The theory is that by opening the gap that you increase the spark kernel, thus improving flame propagation (complete burn across the entire area of the piston), and increasing performance output of the engine. Well, I can say that it does work, but I can also say that it does not work. The explanation for this is that on some applications you will see gains, on others you won't. In some cases you may actually lose performance by opening the plug gap.

On weaker, or stock ignitions, opening the gap puts added strain on the other ignition components in the system. On race ignition systems you are already taking advantage of the output of the ignition box, extended spark duration, and more efficient energy transfer. The only honest way to discuss modifying the plug gap is to experiment on your individual application. You'll either see a benefit, or you won't. Spark plugs gaps are a trial and error type of science when you deviate from the manufacturer recommended settings. If you increase the gap at 0.001" - 0.002" steps and you get to a point where you lose power you will need to close the gap up 0.002"-0.003" to be at the optimum gap. This is what you need to think about:

•The ignition coil may not have enough stored energy to fire, or in the least case, not enough energy by the time it gets to the plug to be able to jump the gap.
•Plug wire leads will break down at a faster rate due to the added resistance as the spark tries to reach ground.
•Rotor, distributor cap, points/condenser (if using an antiquated breaker point distributor), and the center carbon in the distributor cap will show early failures.
•The chance of spark scatter in the distributor cap increases.
All of this occurs because the greater the gap, the higher the voltage requirement necessary to get through the components to jump the gap. The harder these components have to work, the shorter their life span. As a side note, on aftermarket, and high performance rotors the tip is a bit longer to control spark scatter on higher output ignitions."

_____________________________________________

Also: http://www.eng-tips.com/viewthread.cfm? ... 647&page=1
 
The real problem with fouled plugs is that you can't sort the root problem if its carburation because the plugs are fouled. I have box full of fouled plugs, they came straight out of the packet into the engine, fouled and came straight out again.
The best way IMHO to break the cycle and let you get on and sort the real problem is to put platinum or iridium plugs in. They're almost impossible to foul and really do work.
 
L.A.B. said:
I seem to remember Boyer Bransden not recommending increased plug gaps?
Click to expand...
Is that due to the fact standard type coils cant reenergise or store enough current to fire the larger gap :?:
The article isn't really aimed at norton ignitions,aithough it is correct,as most of the dual output coils are for harleys they are designed to run larger gaps ,and do so happily,standard coils may not :?:
 
Hey guys.
Thanks for all the valued input. I am new to ownership so all help appreciated.
I will prob try the easy method first and run NGK BP6ES plugs to see if that helps, combined with less
choke after initial startup.

Does running those hotter plugs do anything bad to my bike ?
Also had a look at the mikuni manual as posted here.

Should i be adjusting the airscrew to make it leaner and if so by how much and what direction. ?
 
splatt said:
Is that due to the fact standard type coils cant reenergise or store enough current to fire the larger gap :?:
The article isn't really aimed at norton ignitions,aithough it is correct,as most of the dual output coils are for harleys they are designed to run larger gaps ,and do so happily,standard coils may not :?:
Click to expand...

As I understand it, the bigger the gap, the greater the voltage has to be to jump a spark across that gap?

And while any standard coil may be able to produce a spark across a .040" or so gap in free air with a new plug, it may not continue to do so under compression,-especially once the plug has become worn or slightly fouled, as I believe more voltage is needed to fire a worn plug than a relatively new one?


I remember reading (many years ago) that "bigger plug gaps were better" and I did try experimenting with larger plug gaps, but I don't think I ever noticed that it had any effect at all?
 
cjatwell said:
Should i be adjusting the airscrew to make it leaner and if so by how much and what direction. ?
Click to expand...


I suggest you follow the procedure described in the VM manual for setting the airscrew position first, by turning the screw to obtain the highest RPM figure with the engine fully warmed up and then readjusting the throttle stop if necessary.

Remember that adjusting the pilot airscrew setting only alters the mixture at idle.
 
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