I just cannot work this out.
Bike info: Open K&N filter, and short muffler with less recistance, simliar to a Jardine muffler. (sealevel)

1. I installed 55/155 jets and 3 washers on the needle.
Result was that the bike ran fine and felt good throu the hole range and alomst no backfire. BUT, I could start it cold without choke and also turning the mixture screw inwards didnt affect the idle at all. Outwards I had to turn something like 4 turns and it started to stall. Therefore I asumed that it was a bit rich.

2. Rejetted down to 50/150.
Result, bike was ok in the low and midrange, but started to backfire and bogging when adding full throtel. It got a litle better when the bike warmed up.

3. Rejetted ones again to 50/155.
Result, bike is still backfiring on hard acceleration but just occasioanly, and mostly on high revs and especialy on second gear. Third and fourth gear works better almost no backfire. Runs fine on low and midrange. Max idle is somewere 1,5-2,0 turns out.

Could it be something else? Recently did valve adjustment after fixing the cam chain tensioner and plug leak, so the valves should be ok.
Should I stay on this setting 50/155 or go back to 55/155? I havent had the time to check the color of the plug yet (by the way the plug is new) Exhuast leak? vacum leak? Could it be normal to get some backfiring on max revs and full throtle?

If the backfire occurs when you let off the throttle or shift gears....ignore that for a bit while you concentrate on what makes it run best.

For the main jet size..... keep going up in size until you notice the bike becomes slower and less responsive - then back down a size.  There is a roll off test that can help to determine if the main jet is too rich - while under full throttle slowly roll the throttle closed a bit....and if the engine speeds up or runs smoother as you close the throttle a bit...it indicates you may be too rich.

For the Pilot, Mikuni claims that the best pilot jet size is the one that allows you to get the best idle mix at 1.5 turns out.  If it takes more than 2 turns to get the best idle mix....go up a size on the Pilot Jet.

Then ride the bike and see what happens as you open the throttle slowly from idle and accelerate slowly.  If the bike runs smooth and accelerates well you probably don't need to fuss with the needle.  If the bike surges and doesn't accelerate smoothly - you need to raise the needle by removing another washer.  If you raise the needle it could richen up the mix in the upper rpms and you may be able to drop the main jet size.

Backfire/afterfires - As Versalgen indicated, less restrictive pipes make the backfire louder and more noticeable.  The backfires occur on a bike that is jetted to run properly.....and making the engne run richer does help to reduce backfires at the expense of performance and mileage.  The backfires occur because when you close the throttle you are dropping the tapered needle into the needle jet and closing off the fuel supplly.  The carb then is only providing fuel through the idle circuit, and the engine goes so lean that the spark plug cannot ignite the mixture.  As you continue to coast the unburned fuel builds up in the exhaust and is either ignited by the hot exhaust - or is ignited by the flame the next time the spark plug does ignite the mixture.  You can minimize this by raising your idle speed a bit which allows more fuel flow, or by rolling off the throttle slowly when you shift so that the slide does not drop all the way back to idle between shifts.   An engine that backfires on deceleration is more a product of a free flowing exhaust system than it is of one that is jetted wrong. Backfiring on acceleration is an indication that something is wrong.

To lesson the backfires between shifts you can increase the pilot jet size to richen the mixture - but doing so will make the engine run a bit rich while riding normally and will hurt the fuel mileage.      

Henrik wrote on 04/17/14 at 14:40:12:

From wiki answers...
Quote:


There are some clues in this answer, so questions to clarify...
Is it true backfire (out the intake)?
or afterfire (out the back end)?
About what rpm does it backfire? you say after a hard accel, but is it medium, high, or really high rpm?
and you're trying to go faster in the same gear?

and it could be... dum dum dummmmm... bad petcock!

I have changed to a raptor petcock so it shouldnt be an issue.
I think the backfire comes thru the exhaust. It is during acceleration, but happens in the end on high revs.

Nope, no fuelfilter

I have never heard of these bikes having a timing issue.  The ignition system works.....or fails.....not much in between.

Tested the bike today. Had some backfire but it got better as the bike got varmed up. Going to order 52,5/152,5 jets to se if it gets better.
Another question, I noticed that I have a small exhaust leak were the muffler mets the header. Could feel pulsations with my hand. Possibly this can contribute to the issue, but from what i have read this should give deccelration backfire?

WD wrote on 04/18/14 at 07:32:55:


I have converted my bike to a VM36 Mikuni Round Slide.  It is not inexpensive, and most likely not for eveyone, and it does not make the rumbling popping afterfire noise go way completey - but it does make it easier to control and the bike is faster to react to throttle changes.  If you want to ride nice and quiet you just roll off the throttle a tad while you shift - and when slowing down you don't let the throttle close completely. It takes just a little extra brake as the engine is not doing as much (or any) compression braking when the throttle is not completely closed.  If you want to make a lot of noise then you go ahead and let the throtte slam shut when you shift (Pow), and when slowing down you close the throttle completely (Bang, putt putt putt, Bang, putt putt).  It all deponds on your right wrist how much noise you want to make.

The same is true with the CV carb when jetted correctly and you can reduce the amount of noise you make....but the VM carb is a little more responsive about what you choose to do with the throttle.