Jump to content
Sign in to follow this  

Main jet affecting first 1/8 of throttle?

Recommended Posts

08 kx450f

Since the needle goes into the 'main nozzle' which has fuel passing through it from the main jet, wouldn't the main jet size possibly affect partially open throttle?

Kawasaki 'team green' suggests a mainjet change from 175 to 180.

My issue with lean condition off idle (barely above idle, but not at idle) could be related to the needle taper, as pilot changes do nothing.

So, if I put in the bigger main, might that not make some change to that off-idle condition?

I'm going to try it, then try a needle with a narrower shaft (richer) too.

Share this post


Link to post
Share on other sites
does the needles clip lowered help at all? have you tried going to a smaller leak jet?

I have tried the richer needle setting, but other things were going on at the time and I will try it again.

Leak jet should have no affect. And I went from a 70 to a 55, with no change in my problem area.

Share this post


Link to post
Share on other sites

there is some overlap but for sake of tuning you should consider them seperate circuits.

anything that happens just off idle is the straight diameter of the needle. not main jet and not taper.

Share this post


Link to post
Share on other sites

I think the needle will have the most effect for you. I agree with your main jet theory. Above idle you are not running on the pilot jet anymore and indeed are drawing fuel through the main jet. The limit on main jet size is of course the WOT mixture, if that is right you have no choice but to leave it alone and pursue other options.

There are many other factors involved too from slide cutaway to needle profile and length. I greatly improved the tip-in throttle response by changing needle length not profile. I cheated and washered the stock needles, which pulled them out of the main jet. Helped the midrange too, and I was able to jet down on the mains to the level it really need to be at.

Is the lean condition at slow steady throttle position or are you experiencing a flat spot on acceleration? I suspect slow throttle since you have already experimented with the leak jet settings to no avail.

Carburetors are kind of like the neverending story in my opinion. Just when you think you have them right, the temps change or some new theory pops into your head and there you go again.

Share this post


Link to post
Share on other sites

like Eddie says, with the throttle just cracked your in the straight part of the needle. Clipping or shimming it up will still have you in the straight part of the needle. Is this observation made with the engine good and hot and while riding, or test stand in nuetral cold?

Share this post


Link to post
Share on other sites
anything that happens just off idle is the straight diameter of the needle. not main jet and not taper

Short and sweet, Eddie, and entirely right as usual. I was thinking backwards in my post although deep down I knew better. The needle mixture control happens at the needle jet not the main jet as I alluded to. A larger needle jet, or slimmer needle might be the answer, Although main jet size would influence that setting also to a degree.

Share this post


Link to post
Share on other sites
there is some overlap but for sake of tuning you should consider them seperate circuits.

anything that happens just off idle is the straight diameter of the needle. not main jet and not taper.

Thank you Eddie. I needed that feedback to make sure my limited tuning skills were not running me in circles.

The NCVR needle is stock on the KX450.

Would the next richest needle be a NCVQ?

5TA-14916-VM-00 .NEEDLE (#NCVM)

5TA-14916-VN-00 .NEEDLE (#NCVN)

5TA-14916-VP-00 .NEEDLE (#NCVP)

5TA-14916-V1-00 .NEEDLE (#NCVQ)

5TA-14916-VR-00 .NEEDLE (#NCVR) stock

5TA-14916-VS-00 .NEEDLE (#NCVS)

5TA-14916-VT-00 .NEEDLE (#NCVT)

Share this post


Link to post
Share on other sites

OK, so I went to the Q needle from the R needle on my KX450 carb, and the problem is even more pronounced.

- When riding, in gear, shutting off the throttle drops the engine RPMs down to normal for about .5 sec, then the RPM's rise back up just above idle for about 1.5 seconds. It does not do it on the stand, only when riding.

- Turning the idle screw out, to eliminate the idle, cures the problem

- Adjusting the fuel screw has a minor effect on this specific issue: if you turn the fuel screw out 4 turns, the problem diminishes VERY SLIGHTLY.

Normal fuel screw adjustment setting is 1.5 out.

- Q Needle is in slot 4 from top.

- Pilot is 42. Main is 175 (stock).

- Adjustable leak jet turned all the way out (big leak jet) makes no difference. Before adjustable leak jet, problem was still there.

ONE MORE THING:

- Adjustment of the fuel screw seems to have little to no effect on the way the bike runs at idle. Idle rpms and smoothness only vary very slightly from 0 turns to 4 turns. Decel popping does come back when at 0 turns. The bike does not stall at 0 turns or 4 turns out...

I'M STUMPED:banghead:

Share this post


Link to post
Share on other sites

Something to consider is the clutch basket. I was baffled by a "hanging idle" that is usually caused by a lean condition. But as you were saying it only happend when the bike was moving. I quickly realized that since the clutch basket was notched even tho I had the clutch lever pulled in the clutch was still "dragging" just enough to cause a minor engine braking effect that raised the idle artificially. I determined this on a hunch first then went out and proved it by cruising about 20mph and instead of staying in gear and pulling the clutch, I shifted the bike into neutral and instantly the idle returned to the proper RPM... :thumbsup:

Share this post


Link to post
Share on other sites
Something to consider is the clutch basket. I was baffled by a "hanging idle" that is usually caused by a lean condition. But as you were saying it only happend when the bike was moving. I quickly realized that since the clutch basket was notched even tho I had the clutch lever pulled in the clutch was still "dragging" just enough to cause a minor engine braking effect that raised the idle artificially. I determined this on a hunch first then went out and proved it by cruising about 20mph and instead of staying in gear and pulling the clutch, I shifted the bike into neutral and instantly the idle returned to the proper RPM... :thumbsup:

It's not a hanging idle. Does it ONLY with clutch engaged.

Share this post


Link to post
Share on other sites
unplug the gear position switch.

OK, tried that.

NO change.

The only way I can get it to stop is to turn off the idle.

Then it runs like every other FCR carb bike I've owned.

Accept, there is no idle........

Since turning off the idle makes it go away, would the lowering of the needle position, or the lowering of the slide position, be the issue here?

I am going to try raising the clip all the way to the top to see if that has an effect.

Change the slide cut-a-way??

Share this post


Link to post
Share on other sites

if it idles with the fuel screw closed the pilot circuit is too rich. becuase it is to rich the slide has be higher than it should be for it to stay running.

Share this post


Link to post
Share on other sites

Yeah, I hear you. I had a 42 pilot in before (now a 45) and there was no difference. I will go back to the 42, but I think there is something else going on in there.....

Share this post


Link to post
Share on other sites

possibles-

worn needle jet

junk aluminum fuel screw

tight valves

other worn parts in the top end

cam timing

or similiar

you must get control over the pilot circuit with the fuel screw or it will never be right.

Share this post


Link to post
Share on other sites
possibles-

worn needle jet NEW BIKE

junk aluminum fuel screw BRASS R&D

tight valves HAVEN'T CHECKED, BUT IT STARTS EASY

other worn parts in the top end NEW BIKE

cam timing

or similiar

you must get control over the pilot circuit with the fuel screw or it will never be right.

I will put the a NEW 42 back in, put the clip back on 4 down.

I will also check for any varnishing of the pilot passage way.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×