Mikuni Carbs (a little carbon)

[joecnc2006]

I have a little over 4.5 hrs test stand the 1/2 VW, 92mm x 86mm with 28mm carbs. the inside of the carbs had a little carbon and a little on the bottom of the slides, cleaned them up but just curious what may cause this if someone else has noticed this also.

The heads came from Scott @ Hummel Engines he made them for me with valves installed and ready to install. I rechecked head torque and they did not move at 23fp for the 10mm studs so they did not loosen, and rocker gap seems fine. I called Scott and he was a little puzzled and did not know what would cause it.

But since I cleaned the carbs with B12 I moved the needles to the second notch, I did notice one was in the middle and the other was in the second notch originally, I guess from factory.

[leshoman]

I have the same problem.

[Dan_]

I don't know if this is the answer for sure but,  I would say the answer has to do with a short intake combined with the valve overlap...  Possibly a short exhaust path could be involved as well.  Inquiring minds want to know.


Scroll down to valve overlap here:  https://en.wikipedia.org/wiki/Valve_timing

[leshoman]

it does not seem to change or affect anything so i just watch it.

[stevejahr]

I second valve overlap.  The exhaust remains open for some duration past TDC and the intake opens some duration before TDC.  Thus both valves are open together and allow higher pressure from the cylinder to enter the lower pressure intake.  Since the intake is very short with the typical Mikuni setup you can get carbon back at the carb.

Given a longer intake runner and separation of the carb from the valve this would likely not be a problem.

It would also be helped with proper exhaust tuning wherein a negative pressure pulse is reflected back into the exhaust port around this time to help suck that last bit of spent junk from the cylinder.

It can be worse at lower RPM as the amount of time for overlap increases.