Spark Plugs?

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jsouthard

jsouthard

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I recently installed a single mikuni 34mm carb. It idles at much lower rpm's and when i'm riding its aggressive but smooth. Big upgrade!

At idle although low its really choppy and won't start without the choke (hot or cold). I recently had a boyer ignition installed.

When i pulled the spark plugs after riding for a minute and they are pitch black.

Any ideas??
 
jsouthard said:
I recently installed a single mikuni 34mm carb. It idles at much lower rpm's and when i'm riding its aggressive but smooth. Big upgrade!

At idle although low its really choppy and won't start without the choke (hot or cold). I recently had a boyer ignition installed.

When i pulled the spark plugs after riding for a minute and they are pitch black.

Any ideas??
Click to expand...

What size jets? Needle?
 
I am running a 240 main jet.

The needle size is the stock that came pre installed on the mikuni.
 
I got the same Mikuni set up on my 850 Mk2. Plugs get sooty, mostly on around town work and yes, the Mikuni must have that choke lever on for 2 miles in summer and 3 in winter (Australia so may be more in snowville)

Big thing is to check that the manifold is tight up on the head with no leak points around the gasket. I find the Mikuni returns 10% better gas mileage and is hell better all round for accelaration and idle than the twin Anals.

The sooty plugs was more evident on around town rides and the consensus from the local British MC crew is- todays fuel is crap. Its part synthetic alco mix, OK for injected Toyotas but 2 valve long stoke air cooled carb motors from the 60's...no.

The cure for this potential sooting up the plugs which can cause hard starting and poor cold running is the NGK Iridium spark plug. Take it from me and several others, these Iridiums go along way make more start less fart.

The standard NGK plug is BP7ES 0.6 gap
Iridium Option BPR7EIX 0.6 gap (this is fixed can't be altered you'll see why)

The Mikuni Carb spec should be -

Model - VM34
Main Jet - 200
Needle Jet - P2
Needle - 6DP1
Idle Jet - 20
Slide - 2 1/2 (OK to go to 3)

It works for me, stock 850Mk2, 150 PSI compression in each cylinder with a Boyer analogue ignition.


Mick.
 
ML said:
I got the same Mikuni set up on my 850 Mk2. Plugs get sooty, mostly on around town work and yes, the Mikuni must have that choke lever on for 2 miles in summer and 3 in winter (Australia so may be more in snowville)

The Mikuni Carb spec should be -

Model - VM34
Main Jet - 200
Needle Jet - P2
Needle - 6DP1
Idle Jet - 20
Slide - 2 1/2 (OK to go to 3)
Click to expand...

Should be no need to run the starting circuit for that long. I use mine for about 20-30 seconds to start (maybe slightly longer in cold weather) and then turn it off and raise the idle temporarily. Running with the starting circuit on this long is presumably what is causing your sooty plugs. Seems like your idle jet is far too small. Specs I have call for a 40 idle jet in the 34 mm and a 45 in the 36 mm. I am running a 40 idle jet on a 36 mm Mikuni and it seems slightly too small (air screw all the way in) but works fine. Your main may also be slightly small. Specs I have call for a 240 main in a 34 mm Mikuni on an 850. I am running a 280 in a 36mm at about 1000 metres elevation. Good luck!
 
I find the modern fuel needs a hotter grade plug such as NGK BP6ES or a Champion N8Y to keep the plugs clean and soot free.

I'm sure the NGK BPR7EIX is a resistive plug and if surpressor caps are used that'll weaken the spark. You should be OK removing the caps with the old Boyer Black Box, not so sure about the Micro and Power boxes.

Cash
 
cash said:
I find the modern fuel needs a hotter grade plug such as NGK BP6ES or a Champion N8Y to keep the plugs clean and soot free.

I'm sure the NGK BPR7EIX is a resistive plug and if surpressor caps are used that'll weaken the spark. You should be OK removing the caps with the old Boyer Black Box, not so sure about the Micro and Power boxes.

Cash
Click to expand...

Aren't you worried that the hotter plug may hole a piston?

The use of resistor plugs, caps and wires also depends on your charging system. It took me a while to figure out why the battery on my Matchless G12 was not charging. I had installed a 3-phase SPARX alternator and solid-state regulator and was running resistor wires AND caps. Not sure why. Battery wouldn't charge. As soon as I got rid of the resistor wire, all was OK. I guess the extra resistance played havoc with the regulator. Also seems to depend on spatial location (ie. how close the regulator is to the spak plug wires). Fun stuff!
 
This is interesting stuff, about which I've posted questions and my own addled observations in the past.

I find Iridium fine-wire spark plugs BY FAR the best in all my motorcycle applications, including my 73 850 Commando, an 84 BMW airhead, 97 2V Ducati 900, and 06 EFI Harley Deuce, plus my wife's 07 Triumph Bonneville and Harley Nightster.

I have a Sparx 3-phase alternator and a Boyer analog EI on my Norton. I run resistor plugs and resistor (but not 5K) caps. All seems well. I was getting a lot of soot on the plugs a while ago but it was carb related. I think I have that dialed in now....sort of!

But any and all information is good....hope this helps you, and thank you for yours!
 
Thanks for all the individual tips, Its suggested I'm running too small an idle jet, and I'm not happy with the duration of choke on before I get proper idle and acceleration, otherwise it spits and pops.

When the motor is warm enough it pulls really well, but the off idle performance when cold to warming up is where things are suspect. Its possibly the Needle Jet and Needle itself?

I guess the advice on the main if fair enough too...I;m new to Mikunis and can't translate it to Amal experience.

Anyone got a recommendation for a Needle Jet and Needle to suit a stock 850Mk2? (Australia is 90% sea level)

Thanks, this forum is great, much appreciated.

Mick
 
I run a grade hotter plugs in my 850 that has over a tad 9:1 compression, 3S cam, twin 34s etc and is used at times hard. my mate uses N9Y in his bog standard Mk3, never had a problem. My plugs are always clean with no signs at all of heat stress.
I'm sure it's not hot plugs that melt pistons but advanced ignition.

Cash
 
My completely stock with points Mk I 850 blackens the plugs with my 34 mm Makuni that has a 220 main jet, 35 idle, 6DH2 needle and 2.5 slide. Only in cold (for California) weather does it start with the choke. Any other time I only kick it over once with the choke, then take the choke off and it over where it starts with no throttle and idles pretty smoothly at 1000 rpm. Idling in a confined space, the exhaust smells a little sooty or gassy to me.
 
ML said:
Anyone got a recommendation for a Needle Jet and Needle to suit a stock 850Mk2? (Australia is 90% sea level)
Click to expand...

needle jet 159 p2, needle 6DH3 for 34 mm Mikuni on 850
 
My experience with buying carbs, even if for a specific application, is they come WAAAYYYY rich. Makes sense - beats the alternative! - but it's up to the owner/operator to dial 'em in.

My experience in this regard with the FCRs is the subject of a different string. Not directly relevant, but perhaps in terms of showing the orders of magnitude of adjustments needed, perhaps of some value.

Good luck! - BrianK
 
I put a 34 MM Mikuni on my Mark 3 last summer. I bought it from Rocky Point Cycles. The owner, Stan Smith asked me my elevation so he could set it up for me before shipping it (I am in coastal Southern Calif, so at sea level).

The carb works perfectly and the plugs stay clean.

On warm days and when the engine has already warmed up, I do not need to use the choke at all. On those days when i use it, it is only for 20 or 30 seconds. Bike idles just fine after that.

Maybe you could check with Stan at Rocky Point for the specs he uses on the Mikunis he sells.

Good luck

Keith Kelly
 
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