Black Sparkplug and Unburnt Fuel

lukecanna

Member
Joined
Sep 25, 2017
Messages
79
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

Active Member
Joined
May 20, 2017
Messages
709
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.
 

Stoneman

Active Member
Joined
May 20, 2017
Messages
709
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

Well-Known Member
Joined
Sep 23, 2013
Messages
3,674
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
 

crassius

Well-Known Member
Joined
Jul 23, 2012
Messages
4,575
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
 
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