Any ideas what could be causing my plug fouling, dry, sooty black, no oily stuff.
Doesn't happen during road use only with high speed track runs. Clean up plugs and it's fine again until the next hoon.
Nothing obviously wrong, carbs I'm pretty sure are jetted and operating correctly, couldn't be something as simple as an incorrect gap could
it, thinking the extra load of high rpm might just be enough to stop an over gapped plug firing correctly, maybe crap leads, cheap plugs?. Worst
fouling on cylinder 4, then 3, 2 is very light and 1 never seems to get it.
Thoughts?
[Edited on 4/8/09 by norfolkluego]
What carbs? assuming bike or DCOE's, with a common air box, not enough air getting to the rear carb inlets during hard use??
i would clean the air filter
r1_pete is bang on the money
You need to look at the design of your airbox/filter --- "standoff" fuel vapour is getting blown back from the mouth of the front carb to
towards the rear most carb.
It you have an airbox may be you need to use a rear facing air intake.
After sorting that out you may have to revise the jetting.
You know I hadn't thought of that.
Carbs are DHLA 40s with trumpet socks poking through the bonnet side into lovely cool fresh air.
Thing is with the shape of the Luego bonnet No 1 cylinder trumpet poke just out of bonnet side, No 2 is about level with it, No 3 is inside, No 4 is
well inside, equates exactly to the amount of fouling.
We did the Borough 19 sprint at Hethel on Sunday and it killed my first timed run, cleaned up plugs and left trumpet socks off and it was OK after
that. Any suggestions to a permanent fix to get plent of air to 3 and 4. These are double trumpet socks and the one to cylinders 1&2 sort of
shields the one to 3&4. Think you might be right, think it just being starved of air.
Should also add that I haven't got an airbox as such just the double trumpet socks and the trumpets are quite short, with the socks on the one for cylinders 1/2 stands about 1 to 1.5 inches proud of the one for cylinders effectively masking it from the fresh air flow add in fuel vapour being blown out through the sides of the first sock and I think that could well explain the fouling. Intrigued by the rear facing air intake, wouldn't that be in a low pressure area effectively reducing the air flow to all the cylinders albeit solving the 'standoff' vapour problem.
Anyone got any nice quick fix for this, running without the trumpet socks seems to cure the problem but I'm worried something unpleasant is going disappear inside the carb.
I think you need either an airbox, or some sort of ducting. Being twin 40's, there should be a choice out there.
I'd look for some sort of airbox given the "half in, half out" situation and trim the bonnet to suit. Have a look at
these onse for inspiration..
In theory a front facing intake gives pressure gain but if you do the math the pressure gains only start to become worth while at F1 speeds --- ie
getting towards 200mph.
And to get the pressure gain the airbox has to be big enough to slow the air right down before the air goes into the carb intake trumpets.
Air boxes rear ward facing intakes give a much less turbulent airflow in to the carbs which can be very benefical.
[Edited on 6/8/09 by britishtrident]
The math for the maxiumum possible dynamic pressure is here ( http://en.wikipedia.org/wiki/Dynamic_pressure ) just plug in the metric units and get
the answer in pascals
Which seems fine until you take into consideration 1 psi = approx 6900 pascal.
100 mph = 44 m/s
Air density at 15 c = 1.3 kg/ m^3
Do the math and it works out the pressure gain from a forward pitot style intake is always going to be less than 0.2 psi even at 100mph
Because the airbox can't slow the air down to zero velocity air will always be moving inside the airbox a more realistic estimate would be about
0.05 psi.
At 60mph you might gain 0.005 psi of free supercharge.
[Edited on 6/8/09 by britishtrident]
Guys, as ever, thanks for your help (and in taking the time to explain how the problem comes about not just how to fix it)