fouled spark plugs

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I have just tried to start her again today after riding yesterday.
Bike tries to fire up with no choke but wont run.
When i depress the choke and try to kick her , there is completely nothing. Doesn't attempt to fire up.
Just ruined another set of plugs.
Whats my problem.
 
cjatwell said:
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 .
Click to expand...
I know about the scientific method of changing one variable at a time and all that, but I like the blunderbuss method for problems like this. In the end it's usually quicker and you personally acquire a certain amount of knowledge in the particular issue. Buy a rebuild kit for the carb, take it off and go through it. It could be; the air enrichment plunger isn't sealing off completely when you turn it off, the air jet in the pilot circuit is plugged, the float valve isn't set right or isn't sealing completely, the floats are filled with gas and don't float like they're supposed to. There has to be lots of other reasons too, but you'll come across it all on dissection. Or it's as simple as the air screw that's screwed in all the way, (base setting is one and a half turns out from lightly seated and turning it out makes idle and low throttle settings leaner.)

If you don't feel like going through the rigmarole, you can save yourself some dough by taking the carb off yourself and then sending it off for a rebuild. Speaking of scientific method, when all else fails I've found parts that have probably had a pox put on them and my juju just wasn't strong enough and the part had to be removed and sequestered from other parts to keep it from spreading. :D It's a widely accepted phenomenon in science.
 
You tried to start it up with NO choke?
40 degrees here, just took the commando for a ride. Depressed choke lever and one good firm kick to get it lit up, then immediately raised
the choke and kept the idle going with the throttle. You NEED the choke lever down with NO throttle and a good kick to start it,
the choke is NOT for warmup, but starting only. Have you verified Mikuni settings?
 
L.A.B said:
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?
Click to expand...

The voltage required to jump a spark gap is generally described by Paschen's Law: v = f(p*d) which states that voltage is a (somewhat nonlinear) function of gas pressure times gap distance. Higher compression and larger gap need more voltage.

Increasing the gap puts extra stress on the coils and ignition modules and is not recommended.

The black sooty plugs sounds like a rich carb to me.
 
just bought some brand new plugs and she fired up no problem .
Went for a 10 mile ride and she is sweet.
I will let bike sit overnight with no plugs in and replace them tomorrow
and see if she starts any better.
 
I know about the scientific method of changing one variable at a time and all that, but I like the blunderbuss method for problems like this. In the end it's usually quicker and you personally acquire a certain amount of knowledge in the particular issue. Buy a rebuild kit for the carb, take it off and go through it. It could be; the air enrichment plunger isn't sealing off completely when you turn it off, the air jet in the pilot circuit is plugged, the float valve isn't set right or isn't sealing completely, the floats are filled with gas and don't float like they're supposed to. There has to be lots of other reasons too, but you'll come across it all on dissection. Or it's as simple as the air screw that's screwed in all the way, (base setting is one and a half turns out from lightly seated and turning it out makes idle and low throttle settings leaner.)

I assume that turning the airscrew clockwise turns it in all the way to seated .
What are the settings for sea level
 
turn the air screw on your mikuni about one and a quarter turns out from seated for normal, should be close enough
 
Sea level -3000feet won't make fuck all difference 'what you described before sounds like your carb is flooding, unless you ride around at idle playing with the air screw wont help ,mikuni's seem to like choke to start cold, none when hot and when the are not fully hot they seem to flood very easily if you use the choke and the amount of kicking is usually multiplied by the number of people watching.
purchase a rebuild kit :?: :?: ,you should be able to pull the carb apart and check the needle and seat, float height ect, without needing any parts.
 
I run a Boyer with a CNW Crane coil conversion with a .042" plug gap and she runs beautifully. I wouldn't run a big gap with standard coils but these coils are made to run these sort of gaps. I think the coil is the determining factor here. My man who fitted the new head put new plugs in and set the gap to .026". I go home, pull the plugs, check the gap, sigh, open the gap to .042' and, after playing with gaps before, I think it runs better with the larger gap. Nothing I can specifically put my finger on, it just feels better.

I run a 34 VM Mikuni without an air filter and now with the new head I've been fine tuning the carburetion. I found that it was lean in the top end and very rich in the low end settings. I went from a 220 to a 240 main jet and from a 37.5 pilot to a 30 and it cleaned up it's act no end. I used to go through a set of plugs every few thousand miles with dark plugs and deposits on the electrodes. The plugs ( BPR6ES) are whitish with a hint of tan on the centre electrode and no deposits whatsoever. The bike is running much stronger with clean pickup everywhere. The odd thing is that the air jet is a #2 which is the leanest you can get. Most recommend a #1 which is ten sizes smaller. ??
I was using some oil before the rebore and I think the oil contamination was affecting any tuning by plug colour/condition. It made me think that the bike was running rich everywhere. Anyway mate, don't go thinking that there is a blanket set of settings which will make your bike run well. Like women, they are all different and need to be tuned in isolation to keep them happy. I'm going to have mine dynoed soon so that I can get the tune perfected. Basically, while on the dyno, the operator puts a sensor up your exhaust pipes and runs your engine through the rev range and at different throttle openings. The sensors will tell him how rich or lean the bike is running at the various throttle openings. You then strip the carb and make the appropriate chenges to jetting/needle/needle position etc so that you have the ideal fuel/air ratio throughout the rev range and at all throttle openings. Back on the dyno for fine tuning and away you go.
 
Fullauto, curious what altitude you are at?
Your pilot jet is a 37.5? I thought this effected ONLY idle mixture, and really only when the Mikuni air screw has to turned out more than about 2 full turns
or so, does the need to change to a different pilot jet. The main jet is of course from what, 3/4 to full throttle only?
So, almost all our riding is done off the slide number, typically 2.5, and needle position (middle standard) .


Regarding plug gap. I run a high out put single Andrews Coil with dual plug leads, purchased years ago from Stan at rockypointcycle.
Stan said at the time to open the plug gap to 35 thou, and that is what I have been running for 12 years now.

This plug gap got me wanting to experiment, so I just opened them up to 42th or so, went for a ride, and "thought' I could just feel the
slightest occassional firing hesitation. Over 40thou may be around the limit. I went back down to 35, all is well again.
 
highdesert said:
You tried to start it up with NO choke?
40 degrees here, just took the commando for a ride. Depressed choke lever and one good firm kick to get it lit up, then immediately raised
the choke and kept the idle going with the throttle. You NEED the choke lever down with NO throttle and a good kick to start it,
the choke is NOT for warmup, but starting only. Have you verified Mikuni settings?
Click to expand...

I would second that switch off the choke on the Mikuni ASAP after startup.
 
cjatwell said:
I have just tried to start her again today after riding yesterday.
Bike tries to fire up with no choke but wont run.
When i depress the choke and try to kick her , there is completely nothing. Doesn't attempt to fire up.
Just ruined another set of plugs.
Whats my problem.
Click to expand...

When you mean ruined plugs what do you mean you can clean up a set of plugs that have fouled pretty easily.. :D
 
Just a slight change of subject, a post earlier was on o2 sensors and got me intrested, I found you can buy a simple kit, gauge, sensor and leads for the price of 3-4 dyno runs, you need to make a bracket to attach to exhuast[ can be purchashed but i thought it was dear compared to the rest of the kit] but the bracket is nothing technical.
highdesert said:
Fullauto, curious what altitude you are at?
Click to expand...

do a google search looks like sea level to me, WA stands for Wait Awhile does'nt it :?:
 
Latest update.
Put plugs back in after keeping them out all night . She starts up fine . I am taking the choke off straight away when i get bike going.
Probably something dodgy going on with my carb , or maybe dodgy fuel taps not turning off. Will go for a ride now and leave plugs in all night to see what happens.
 
highdesert, I'm at sea level and the pilot jet affects the mixture at all throttle openings as it still operates with the throttle wide open, adding to the fuel from the main jet. So, it affects the mixture mainly in low throttle openings, not just at idle. I just had to read my Sudco Mikuni book again for clarification. I also think you may be right about the plug gap, as the bike is running beautifully with the odd little hiccup. This may be the plugs. I'll take them out shortly, regap to .035" and try again. The needle clip position is affected by the main jet size so I've been experimenting there as well.
I rode a mate's 850 yesterday with the same setup as mine, 34 Mikuni, standard cam, standard exhaust and 23 tooth gearbox sprocket. The only difference being my new Fullauto Technologies head. The difference was marked. Mine was much stronger from idle up.
 
Re: fouled spark plugs NOW SORTED

Hi All, ive got a 36mm VM Mikuni carb on a 1972 750 & I had the same problem with plugs fouling up after leaving the choke on to warm the bike up.
I now turn the tickover screw in a bit to keep the engine running quicker & turn the choke off straight away, then gradually screw out the tickover screw untill it ticks over at 1000 rpm approx.
My 750 does take a while to warm up but runs well once up to temperature.
My plugs were getting wiskered up after 5 mins of choke even on a new set. Ive also weakened the mixture & it now runs very well without buggering plugs up. The air screw has to be set low to keep the plugs clean.
If that fails get a Suzuki Bandit as I find that good fun to ride although without any character.
At least on a Norton people come up to you & start chatting about yoyur bike.
Cheers all,
Don from UK
 
Ive got the best of both worlds having the Bandit & A commando.
Before the commando I had an Atlas.
I bought my commando in 1973 for £652 & have had it from new.
I know it was a cheeky statement but you have to admit if Norton had made the bandit they might still be in busuiness.
I'm proud of the Norton Heritage but Jap bikes are very good.
If I had to sell one it would be the bandit.
Does this let me off the public folgging ?.
I hope so.
cheers Don
 
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