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Another fcr jetting thread! with lots of questions!

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Hi there.

 

I had better start by saying that i am in the UK.

 

I have just fitted a new fcr mx 39mm (hot start, choke and tps) to my 2001 drz400s (now sm), it has E cams, a straight through fmf exhaust and a very small looking 3x3 mod, but it is 3X3!!!

 

I left the carb as it came with what i assume is the EMR needle, 160 main and 45 pilot, i set the screw at around 1.5 turns out.

 

The bike started first try and ran with a smooth (if a bit fast) tick over and rides absolutely fine with no bogs of any kind, but if i remove the airbox cover the bike feels a lot faster, still runs smooth as before but with a slight noise/note increase from the exhaust.

 

Turning the fuel screw down also seems to make it run a bit better but doesn't alter the tick over speed, after reading a lot of threads on jetting it is obviously rich, i have ordered a size 40 air jet and have an EMN needle, 45 & 48 air jets and 150 ,155 and 160 main jets to play with, i ordered the needle and jets at the same time as the carb but tbh when they turned up i kind of chickened out of messing with the jetting  :blush: and dismantling my new carb, so i decided to try it as it came and to go from there.

 

I also used the airboot adapter that came with the carb, its a black plastic adapter that looks like it is made for the drz as it fits perfectly, although i think this carb is originally made for the yzf425, does any one have any idea what this adapter is made for? i have never seen one mentioned before and it pushes the carb towards the engine actually making it hard to get the standard S rubber over the manifold because it is so tight, as opposed to the rubber seeming to be too short.

 

adapter.JPG

fcr 39mm.JPG

 

This adapter fit into the airboot perfectly but it is 65/75mm long (half of the end is tapered) meaning that the carb is a very tight fit and is a pain to work on, it will probably be easier for me to take it off to re jet than try to battle with it whilst on the bike  :cry:.

 

Also do the ali type fuel screws work and fit ok on these? I don't want to spend £40 on the R&G one, it should hopefully be a one time setting, i have no need to adjust it after it is jetted correctly.

 

Thanks

John.

Edited by Bassit

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i'd put 155 and retry the bike with and without air box cover.

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What does that gold brass connector next to the choke do? Does it need a an overflow hose at all?

Actually next to the airflow screw

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i'd put 155 and retry the bike with and without air box cover.

Ill give it a try at the weekend, Thanks..

What does that gold brass connector next to the choke do? Does it need a an overflow hose at all? Actually next to the airflow screw

Thats for the fuel line, the angle of the photo doesnt show it properly though.

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Give it all the air it wants...don't restrict it and subtract fuel. If it runs better with the airbox cover off, open up up the airbox hole until it runsvabout the same with your current jetting.

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Give it all the air it wants...don't restrict it and subtract fuel. If it runs better with the airbox cover off, open up up the airbox hole until it runsvabout the same with your current jetting.

Sorry, but I don't like that recommendation at all. Lots of research has been done the past years on these carbs. The consistent thing I've read is that the 3x3 is the optimal opening.  Beyond that and you're on your own for jetting. 

Edited by BlueHeart

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Opening the air box more than needed can be detrimental to performance. Giant holes everywhere can cause turbulence and loss of air velocity, both bad for carb'ed engines. Keep the opening a reasonable size (3"x3" is recommended because it is the size of the factory opening on models that came with the 39mm FCR from the factory) and you'll be fine. Stray from 3"x3" and you'll have to put more time in experimenting to find the proper jetting.

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Give it all the air it wants...don't restrict it and subtract fuel. If it runs better with the airbox cover off, open up up the airbox hole until it runsvabout the same with your current jetting.

Thanks for the suggestion. :thumbsup:

 

This is probably the cheapest way to getting the carb somewhere near tuned as it doesn't require any new jets or needles?!.

 

But, it has a couple of very major flaws in that.

 

You wouldn't know how much more of the air box to cut out and could easily cut out more than is needed.

&

You would then have trouble fine tuning it with just the air/fuel screw, assuming you did cut the right amount, you would need to get lucky to get it close to perfect.

 

The end result of both scenarios would be that you would (probably) end up having to buy new jets and needle(s) after all.

 

Interestingly though William1 ended up cutting more out his air box to get the optimal tuning, albeit with a "wide band commander" hooked up and lots of testing and feedback before cutting.

 

The thread with William1's testing is below and it is the best jetting thread i have read, its well worth a read, all 10 pages :) .

http://www.thumpertalk.com/topic/436377-wide-band-commander-qs/?hl=wideband

 

Thanks for all the replies guys. :thumbsup:

Edited by Bassit

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Sorry, but I don't like that recommendation at all. Lots of research has been done the past years on these carbs. The consistent thing I've read is that the 3x3 is the optimal opening.  Beyond that and you're on your own for jetting. 

3x3 is what everyone does because the associated jetting is somewhat predictable (unless you use a DJ kit...then it's all fawked up).  An engine is an air pump.  The more air it pumps, the more power it will make....period. 

 

Yes, There are things like trying to tune runner lengths for the second order harmonic, which most certainly is not being done with the DRZ, and other optimizations that can have huge gains, but at the end of the day the goal is to let the engine eat as much air as it wants, and then introduce enough fuel to support that.  The carb should be the bottleneck in the system, not the airfilter, and most certainly not the airbox.  This is basic engine stuff...not specific to motorcycles. 

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Thanks for the suggestion. :thumbsup:

 

This is probably the cheapest way to getting the carb somewhere near tuned as it doesn't require any new jets or needles?!.

 

But, it has a couple of very major flaws in that.

 

You wouldn't know how much more of the air box to cut out and could easily cut out more than is needed.

&

You would then have trouble fine tuning it with just the air/fuel screw, assuming you did cut the right amount, you would need to get lucky to get it close to perfect.

 

The end result of both scenarios would be that you would (probably) end up having to buy new jets and needle(s) after all.

 

Interestingly though William1 ended up cutting more out his air box to get the optimal tuning, albeit with a "wide band commander" hooked up and lots of testing and feedback before cutting.

 

The thread with William1's testing is below and it is the best jetting thread i have read, its well worth a read, all 10 pages :) .

http://www.thumpertalk.com/topic/436377-wide-band-commander-qs/?hl=wideband

 

Thanks for all the replies guys. :thumbsup:

No, it would certainly require new jets/needles/and lots of fine tuning, because you will be moving away from the known jetting range associated with the 3x3.  That's why the 3x3 is easy, because it's better than stock and the jetting to satisfy the known amount of additional air has been established...very little or no guess work/dyno time required to make it run right with the 3x3.

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3x3 is what everyone does because the associated jetting is somewhat predictable (unless you use a DJ kit...then it's all fawked up).  An engine is an air pump.  The more air it pumps, the more power it will make....period. 

 

Yes, There are things like trying to tune runner lengths for the second order harmonic, which most certainly is not being done with the DRZ, and other optimizations that can have huge gains, but at the end of the day the goal is to let the engine eat as much air as it wants, and then introduce enough fuel to support that.  The carb should be the bottleneck in the system, not the airfilter, and most certainly not the airbox.  This is basic engine stuff...not specific to motorcycles. 

I agree with what you're saying, but I think I should have been clearer. I don't think it's a good recommendation unless you have the ability to investigate and tune. If you're like most it seems, we just want to ride.  :ride:

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If you're like most it seems, we just want to ride. :ride:

Exactly, which is why I did the 3x3 and called it a day

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Ok, so i managed to make time to take the carb off and investigate.

 

The jetting was not standard for an fcr as i wrongly assumed :facepalm: .

 

OBDVR needle 5th clip.

190 main jet.

38 pilot jet.

fuel screw around 1.5 turns out.

 

It ran great at this setting albeit with a high idle and ran better with the air filter door off.

 

I changed the settings to:

 

OBDVR needle on 3rd clip.

190 main jet.

42 pilot jet.

R&D flexi screw @ 2 turns out.

 

Again it runs great (no stumbles, stutters or flat spots), the idle is now fine but it still runs better (more power, much beefier sounding exhaust note) with the air filter door off, so must still be rich ?? right ??

 

The temp here is now around 6.5c (and wet), i'm not sure if that makes much difference to the tuning but it was lots of fun when trying out the new settings. :ride:

 

And just in case anyone is interested in the air boot adapter that came with the carb, i bought the aluminum adapter that is always recommended and it made things awkward when fitting the carb, so i ended up using the one that came with the carb as it pushes the carb further towards the engine because it is longer than the aluminum one.

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190 is way too large. dvr at 4th or 5th clip + 165-170 could  be  a starting point

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190 is way too large. dvr at 4th or 5th clip + 165-170 could  be  a starting point

 

Thanks, i kinda knew 190 was too large :(, the next largest main i have is a 160, i also have a 40 pilot if that combo may work, i have the emn needle too but i am pretty sure that needle will be too rich all over.

 

Can i ask what is the best method to check the mixture, without any tuning equipment.

I'm just removing the air filter door and going for a short ride to check if it runs any better, this cant be the best way to check if rich or not.

 

When i think i have the jetting about right i intend on taking it to get fine tuned on a dyno.

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mikuni test for main jet:

get the warm engine rpm high enough that it is on the cam and in its power band. This may need to be as high as 4000 rpm with some cam choices. Apply full throttle slowly. Let the engine accelerate for a couple of seconds until it has settled in and is pulling hard. Quickly roll the throttle off to about the 7/8ths position. When you do this, the mixture richens slightly for a second or so.
If the engine gains power as you roll the throttle off, then the main jet is too small and you need to fit a larger one.
If the engine staggers slightly or has a hard hesitation, then the main jet is too large and you need to fit a smaller one.

 

pilot jet:

warm engine, set idle at 1800rpm , slowly close fuel screw untill idle goes down 3-400rpm or engine dies. from that position open the screw 1.25 turn

 

imho to get a good starting point you need to buy something: a larger main jet for the drv neddle or a ncyr neddle for the jets  you already own.

i'd go to the dyno run with 160 and NCYR at 2nd-3rd clip, a dual taper neddle more refined than dvr or emn. pilot jet according the method posted above, checked with new needle already installed.

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Im running an fcr with full fmf exhaust

If i ride hard its fine but if iam putting through trails and idiling alot the header starts to glow red right at the head

How do i fix this?

 check the pilot jet as suggested above.  glowing while the bike is moving is unusual

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