Spark plug color

c.cito · Oct 6, 2010

My plugs have always been a source of annoyance. Here's a photo of the head with plugs in:

My bike setup is:

75 MK 3
Single Mikuni VM34
Needle 6DH3 at 3rd notch
Slide 3.0
Main Jet 230
Pilot Jet 35
Air Screw 1.5 turns
Cheap pancake air filter
Pazon ignition (31 deg before TDC)
Iridium plugs gapped at about .27 (if I remember correctly)

Ignore the R and L I wrote on the head, that is to help me remember what push rods go where. The lean-side header is getting pretty discolored. What would cause one side to be lean and the other to be rich with a single carb? The timing seems to be fine, but when I time it the mark does jump around some. The bike is a hard starter and has a tendency to die easily. Any ideas? Thanks in advance.

-paul

kommando · Oct 6, 2010

A single carb will always bias one side, on A10's there was a gasket available that allowed this to be altered, but a large bias is more likely to be an air leak.

highdesert · Oct 6, 2010

Your say your Mikuni slide is a 3.0 ? That is pretty lean. I am at 6000 feet, and tried a 3.0 because I thought my altitude warranted it. Mistake, went back to 2.5 all all was well.

The standards slide is a 2.5. Your's is too lean, especially for an 850, may account for your motor's tendency to die, and also effect starting and idling somewhat although that is mostly the pilot jet size and your 35 is right size. Order a 2.5 in, unless you are over 7000 feet or higher alititude.
All your other Mikuni settings look perfect.

Can't help with why one plug may be a different color than the other. My single Mikuni luckily did not give that symptom, both plugs always looked identical.
Would try get the 2.5 slide, and then take the carb to head manifold off and reinstall with new gaskets coated with a light high temp gasket sealant from an auto store.
Possible one side is letting in air.

c.cito · Oct 6, 2010

Forgot to mention that the manifold is a MAP, and I just read that some folks feel they are not as good as others... could this cause a cylinder bias?

I will check out a 2.5 slide too... I am really grasping at straws here.
-paul

highdesert · Oct 6, 2010

I had the exact same MAP with my 34 Mikuni for 15 years, no problems of any kind with the MAP. get the 2.5, you are too lean

c.cito · Oct 6, 2010

Just ordered a 2.5 from MikesXS... $20 + postage. Will reassemble sometime in the next week or so and see if that helps. Now if I can chase down that annoying oil leak from the head I will be good to go (until I manufacture another issue I MUST address!).
-paul

MexicoMike · Oct 6, 2010

It seems clear from the pic that the cylinder on the left in the pic is ingesting oil. So valve seal/guide is the likely culprit. It could be the rings but it is less likely. It might be a fuel issue but with a single carb that's pretty much impossible.

xbacksideslider · Oct 6, 2010

What Mike said, there's an oil control issue.

swooshdave · Oct 6, 2010

There must be an easier way to check the sparkplugs. Pulling the head seems like a lot of work...

:mrgreen:

BrianK · Oct 6, 2010

A few datapoints.

When I reringed my engine, and it needed it, both heads looked pretty much like your left ("richer") head.

Your right intake valve is surprisingly carbon free. Almost like its running as hot, or nearly so, as your right exhaust valve. Is valve lash to spec? I wonder if it might not be a bit tighter on the right, particularly as to the intake valve. Certainly, with a single carb the potential for different valve lash would be the first thing I've checked - and you may have already, in which case, never mind....

T'would be useful to see closeups of your plugs, although with modern gas, plug reading's not the art it once was, and in any event without a proper plug "chop," the results are generally not all that instructive.

Best of luck.

splatt · Oct 7, 2010

The light side is spot on , it is not to lean !,I would be looking at why the other side has signs of oil washing, it has washed the carbon of the head, inlet valve looks wet. A #3 slide works well

stockie1 · Oct 7, 2010

To ad to the other comments, it looks like mechanical faults are combining with your weaker slide to make idling and low running marginal. From my Norton experiences runing twin amals, I have found crappy low running from poor valve sealing. Not so noticeabe at cruising RPM. I would check the valve seatings whilst the head is off. Latter on a good tool to have is a leak down testr to keep an eye on the valve and or piston leak down.
Obviously the shitty looking intake has a uide or some such issue!

Best of luck Richard

c.cito · Oct 7, 2010

How else do you see the business-end of the plugs! You think that's bad, you should see me do an oil change

Here's a few shots of the intake valves thru the ports:
First the L valve (as per the original photo)...

And the R valve...

These look GREAT! Right?? RIGHT?!? No? Huh.... Well at least it's a cheap fix! Right? RIGHT?!?!?

-paul

MexicoMike · Oct 7, 2010

Bad guide or seal or valve stem or a combination of any of the three. You have the head off - easy to check/replace whatever is needed. At a minimum I would do the same thing to both intakes, whatever the "bad" one needs.

diy570 · Aug 31, 2011

rather than start a new thread, i figured i'd post here to see if i could get an opinion on this plug. does it look like its close to being where it should be? the bike was running really rich when i got it and i moved the clip on the needle up (single mikuni 36mm). now i just want to be sure that i'm not running too lean. i've got about 300 miles on these particular plugs. thanks for any advice.

comnoz · Aug 31, 2011

diy570 said:
rather than start a new thread, i figured i'd post here to see if i could get an opinion on this plug. does it look like its close to being where it should be? the bike was running really rich when i got it and i moved the clip on the needle up (single mikuni 36mm). now i just want to be sure that i'm not running too lean. i've got about 300 miles on these particular plugs. thanks for any advice.

I would say it is pretty close. I would suggest one step colder on the plug as the side electrode looks on the hot side and yet the edge of the plug looks plenty cool. You probably have room to go slightly leaner with a plug change. Jim

Josh Cox · Aug 31, 2011

There must be an easier way to check the sparkplugs. Pulling the head seems like a lot of work...

That is very funny

.

The induction bias problem is quite common on single carb twin cylinder engines, remove your tufnol spacer on the carb flange and sand at an angle across the face, so it points the carby towards the lean cylinder, or go back to twin carbys.

The bikes do go faster have have much more sex appeal with two carbs.

comnoz · Aug 31, 2011

Josh is right, mixture bias is common with a single carb. Manifold differences or compression and cam timing can make the problem worse. Check the compression and leakdown first and then cam lift. Jim

Hobot · Sep 1, 2011

To un-infomred hobot that plug looks pretty darn spot on mixture wise, with soot on rim and hint of gray on the ceramic. We are in the New World Order now so lean burn gas will not color plugs as the golden leaded days. About only thing I can see off is hint of thermal hook staining too close to the base so might bump up a heat range and/or tweak timing ahead a tad. Thermal band should land about mid way up hook. Might open gap a bit too. A V8 box of plugs sure comes in handy for this level tuning game. I've not yet made it to indexing the gap facing mixture swirl.

http://www.4secondsflat.com/Spark_plug_reading.html

diy570 · Sep 1, 2011

hobot said:
We are in the New World Order now

i still like to pretend that we aren't :lol:

hobot said:
Might open gap a bit too.

i have them set at .026 now, they came out of the box at .030, maybe i'll set them at 28. i still have a mild stumble/hesitation if i roll the throttle on somewhat quick from an idle. if/when i get rid of that, i will be content.

Spark plug color

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