Black Sparkplug and Unburnt Fuel

[lukecanna]

Hi forums,
I am running 27:1 fuel mixture so 37ml of oil to 1 litre fuel. I use a short billet intake and I am experiencing alot of Unburnt fuel in my exhaust and I mean alot! As well as a black spark plug which I suppose the problem is the short intake. How do I solve this problem? Do I change the position of my slide?
No other mods on my bike and 44t sprocket

 

[Stoneman]

I’ve found the engines we have put together run a little rich out of the box.
May need to fill the jet with solder and resize it smaller.

 

[lukecanna]

I’ve found the engines we have put together run a little rich out of the box.
May need to fill the jet with solder and resize it smaller.

Ive been thinking of doing this what size do you recommend

 

[Stoneman]

I wouldn’t know what size you need but a size or 2 smaller.
Is it running ok? Have you checked the float level and needle settings?

Stock set is normally pretty good for most

 

[FurryOnTheInside]

I don't think it's the length of the intake manifold, but perhaps the angle? The carb should be pretty close to level.
It's also common to raise the clip on the needle by a notch or two to lean out the a/f mix at 0 to 3/4 throttle.
Another cause of unburned fuel being dumped into the exhaust is the poor design of the transfer ports. For effective Schnuerle scavenging they should direct the flow of fresh charge towards the intake side, away from the exhaust port.

https://en.m.wikipedia.org/wiki/Schnuerle_porting

http://www.dragonfly75.com/motorbike/transfers.html

 

[lukecanna]

I am going to try changing the clip and yes my carby is on a slight angle

 

[crassius]

when tuning, it is best to consider how motor is performing rather than looking at plug and muffler - too many short trips can make a black mess like that