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OUR 2 CENTS WORTH
Ever wonder why some bikes run better than others with the same components?
Why does my buddy’s bike have this jetting and mine doesn’t like that jetting?
For conversation sake, lets assume (Never assume.) that both bikes are in the same condition, same electrics, tuned up, altitude, etc…. (Mileage unknown.)
Why do some bikes require different jetting?
Story:
We had a customer’s motorcycle that began to run rich. All the normal tune-up stuff was ok. He is the original owner and no changes were made. (Exhaust, fancy air filters, etc.)
Question: So
why does the bike run richer? Failing ignition? Dirty
air filter? Float malfunctioning? Bad
fuel?
Answer: Worn carburetor needle jet and needle.
Just like pistons, carburetors are working hard. Slides are moving up & down, being pulled & pushed with intake pulses constantly wearing the inside of the needle jet.
The needle jets and needles are very precise and wear is immeasurable. The detection of this problem is sometimes difficult. (Unless you spend a $100.00 per hour on a Dyno.)
When you drive
a bike at normal speeds, you are generally in the pilot circuit, slide cutaway,
needle jet and in the needle a bit. These circuits work together delivering fuel
to your motor. A little wear in the needle jet and needles is why some bikes
with open exhaust, open air filters SOMETIMES run better with almost stock
jetting. We wrote a tech article on exhaust systems and tuning and basically
said: Select an exhaust system you like and jet accordingly with an open mind.
Same with this needle jet, needle topic: Do not go into jetting a bike with a
preconceived idea about jetting. Keep an open mind.
We do not know if needle jets & needles are still available or if you really need them.
This was just a story and maybe this will help someday.
SLIDE INTO PERFORMANCE
A lot of literature and chat room posts are helpful in diagnosing ill running motorcycles.
We wrote a tech article on ageing and worn needles and needle jets recently and talked about how aging components can mess up the diagnostic process. We prefer to write about running issues that are not in print or rarely talked about. Generally, these things have baffled us in our service department and are experienced real life fixes.
When it comes to carburetors and jetting, the diagnostic problem must be cut in half.
Is it rich or lean? At ¼,½, ¾, full throttle is it rich or lean? Many publications and discussions can help with this.
Here is a real life story and an experience that has worked for us from time to time.
When cruising down the road your bike is normally running about 2 ½ to 3 ½ thousand rpms. Most of the jetting conversations talk about pilots and needle positions. We feel that is a wide range. Why? There is a sweet spot that is overlooked when transitioning from pilots to needle position. ”The slide cutaway.” Sometimes with pod filters, drag pipes and other custom products you must have a determination to get the job done. Rather than, “I’ll live with its attitude.”
First, pilots, cutaways, needle jets, needles and main jets work together. They cross over, overlap, and need to be delivering fuel and air as needed.
Oh yea!
The cutaway!
The sweet spot!
Here is what we do in many high performance or aggressive situations. We will disassemble the carb bank and remove the carburetor slides. Then we will remove 8 thousands at a time from the bottom of the slide (On a lathe.) this will richen up the sweet spot (Between the pilot and needle position.), which is normally overlooked. Of coarse the carbs will need to be synchronized again for absolute determination of progress. Sometimes after doing this modification, we have gone back to stock pilot jetting. Here is the problem. This tip takes a lot of work and hours, but no money.
Sounds simple?
Some of these tips have cost us many hours in our service department and we would like to support people having annoying carburetor issues. Hope this helps someday.
** Not used with a 25% braking load to simulate heavier bikes that might ride with a passenger now and then. So, if you weigh 130lbs and you tune your bike perfectly then pick up a chick that weighs 250lbs your bike will need more fuel. So, i