In my 2 stroke racing days I was used to doing a "Plug chop" to check mixture:

Get a decent length of run at full throttle.
Clutch in, cut motor, stop.
Pull spark plug/s.
Check colour.
white = too lean.
light tan = perfect
grey = too rich.
black = way too rich.

I seem to recall reading that this technique is not valid with today's Unleaded petrol, as the plugs do not colour as described above? Any experts care to comment?.

Any yes I know today we use rolling roads, laptops and sensors, but in the absence of these?

Cheers

Fred W B

I can tell you one thing. They still go black when its way too rich.

I still tune my bikes this way

just remember that lean kills engines, rich kills plugs, I know which is the cheapest to replace

Yep, still works I understand

Proper mechanicing

And they go when you're running horribly lean...!
Link to previous thread

Ed.

The Haynes manual for the ST1100 still talk about the chop being used to tune....

A plug cut would tell you the average fueling, and could be a help.
If you looked at a plotted dyno sheet, if the mixture is a little on the lean or rich side at a certian RPM, you would see a slight fall in the BHP and torque. You would never see this on a plug cut and would never know.
We have seen 5 BHP on a 100BHP car at lower RPM, we corrected this and the car sounded no different than it did before.

Yep that how I take a look at the mixture on my lads bikes, still works for me.

Cheers

Rich

quote:

---

Originally posted by matt_gsxr
I can tell you one thing. They still go black when its way too rich.

---

Thanks for the speedy responses guys.

Trick with the two strokes always was to get as close to lean as you dare, as the engine made best power then.

There was a fashion in the kart racing at a stage to run the top end mixture so lean it would just about sieze at the end of the longest straight, this was prevented by momenterialy choking the air intake with your hand (to pull in extra fuel) just before the critical point each lap.

Cheers

Fred W B

[Edited on 19/5/10 by Fred W B]

quote:

---

Originally posted by matt_gsxr
I can tell you one thing. They still go black when its way too rich.

---

this has been the method that I have set my twin 40's on a 1.8 zetec along with balancing the carbs and I have done 7000 trouble free miles. My old many swore by this method, he was a mechanic for 50 years