"Reading" spark plugs for carb jetting?

Paul LeClair

Paul LeClair

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Location
Calgary, Alberta, Canada
so, I am back working on the RGS, 1200 barrels, 80.4 mm pistons, 36 mm inlets and exhaust ports matched to 38 mm i.d. headers. Mikuni RS 36 carbs on Red's angled intake manifolds, WITT ignition, TEK coils.

the spark plugs are blackened, almost sooty.

as there are several circuits in a carb, how the heck do you "read" the plugs to diagnose which carb circuit is causing the rich condition?

thinking about it, the bike probably spends most of its time at part throttle, ranging from barely opened throttle to say half throttle, so the rich condition is probably mostly the needle and needle jet. I'll have to check the needle number, but from memory I have the richer of the two possibilities in the carbs. I have the carbs off the bike, I'll pull them apart tomorrow.

who else here is running RS 36's on an RGS?

Paul LeClair
 
Paul, I think since the move to the wondrous unleaded shite the art of spark plug reading is nigh on impossible.
 
That is right Grant, but most true for fine tuning etc. however if they are black and sooty it is well and truly rich.
Paul, you?ll no doubt get masses of advice from this forum. I found a brilliant step by step guide for setting up my Keihin FCRs on the net, no doubt the same can be found for Mikunis. When you get close you?d be best with one of these O2 sensors.
 
You have a solution in a first time. Try the "colortune"
http://www.carbtune.com/ (clic on "colortune" on the left menu)

See here ...

[youtube]http://www.youtube.com/watch?v=fL8llgibKGA&feature=related[/youtube]

[youtube]http://www.youtube.com/watch?v=6hFUvQ4gaPc[/youtube]

[youtube]http://www.youtube.com/watch?v=zrBcCZCRfms&feature=related[/youtube]

Xavier
 
Tend to agree with Grant- modern fuels seem to leave much less crap on the plugs.  Best thing would be to get the bike on a dyno.
 
Reggie3cl said:
Best thing would be to get the bike on a dyno.
Click to expand...

only if you have a braked dyno, not with an accelerator type of dyno, please. Friend of mine got his bike set up on an accelerator type dyno where the AF-ratio looked very good. On the race-track, he burned a hole into one piston and constantly had 140?C oil-temperature...

Color tune is good for up to a maximum of 3.000 Revs, after that the heat can cause the transparent part to melt. Good to see if accelerator pumps work, good to see idle mixture setting etc., but for midrange, I fear it won't tell enough, especially as it can not simulate what happens under load.
 
Hi all

Bob (RustySpokes) sent me an excellent link to a 1977 Gordon Jennings article on reading spark plugs.... Thanks, Bob!

http://www.strappe.com/plugs.html

when I was reading about hotter and colder plugs, I realized I have still been running the colder NGK B9Es plugs that I stuck in for my last trip to the dyno, and they are simply too cold for ordinary street use, so that likely explains the blackened plugs, they are not maintaining a high enough temp in street use to burn the crap off...

I am also going to have another look at my ignition timing, maybe retard the advance a few degrees just for fun and see what happens.

while I have the bodywork off, the carbs off, the exhaust off, and the valve cover off, I have a few projects to complete, and I am going to swap in a leaner set of needles into the Mikuni RS 36's to see how it runs with the leaner needles...

Paul LeClair
 
Paul
Thought it was always best to change one thing at a time when setting up manually, or you are not sure what improved things or made it worse.
Moved over to using an Innovate LM2 and now I can change lots and get almost instant feedback.
Also record the runs and see where the bike is rick or weak.

Steven
 
Hey Paul

If you fit a Nitrous Oxide sysytem that will lean out your mixture for you ........  :D ....... Maybe only at WOT so may not help ........  :D

Sorry could not resist knowing your ''whilst I'm at it tendancies ''

                                Cheers
                                        Matt

 
Laverdalothar said:
please don't trust the color index mentioned in that article. It is true for leaded fuel only!
Click to expand...

Gordon wrote the article in 1977..catalytic converters were on virtually all US made cars in 1975 and "lead" began to be phased out of US gasoline starting in 1973. Lead in the form of tetra-ethyl lead was once a common additive, but since the 1970s, its use in most of the industrialized world has been restricted. For many tuners..reading spark plugs with unleaded fuel was the challenge.

He wrote this with unleaded fuel in mind..as the spark plugs will not look the same as when read with "lead" in the fuel. The classic slightly brown or tan spark plug insulator disappeared with lead...tetra-ethyl lead was not in the fuel to color the plug by 1977 in most US gas stations, especially in California, where Gordon lived.

Typical "octane booster" gasoline additives include ethanol, MTBE, ETBE, isooctane and toluene. Most modern pump fuel "octane boosters" burn very clean off the insulator of the spark plug (with the exception of toluene, not in ordinary US pump fuel..not very common). The spark plug porcelain will look new..even after many miles.

His notes and illustrations are very relevant to modern tuning....look for the mixture ring at the base of the porcelain insulator..after the spark plug heat range and ignition timing are trued up. It works.
 
it is the "mixture ring" at the base of the porcelain insulator that intrigues me, my current plugs are too dirty/dark/black from being too "cold' but I am looking forward to having a read of a set of hotter fresh plugs when I get the bike back together...

as for the comment about changing only one thing at a time, yes I will be initially chjanging two things (hotter plugs and leaner carb needles) but I already know how the bike runs with the hotter plugs as I have run them before.

I'll leave fiddling with the ignition timing and the WITT ignition curves until I am comfortable with the carb jetting changes

Paul LeClair
 
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