News:

Mat Oxley 1986 IOM TT


click above to read more about Mat.

Main Menu

Needle Jet Question

Started by Czakky, October 04, 2020, 09:27:25 PM

Previous topic - Next topic

0 Members and 1 Guest are viewing this topic.

Alain2

Does anyone find that a bigger needle jet makes the main jet start engaging earlier in the throttle?

I was reading on dirt bike forums and a guy mentioned that claim. Not sure what type of needle jet he was using though.


1973 RD350, 1977 RD400, 1979 RD400, 1980 RD400, 1985 MJ50, Goped Zenoah 30cc.

Czakky

I felt that with a bigger main that the powerband was earlier and the flat spot before the powerband was for sure more pronounced.

What I find strange with changing needle jets is my flat spot was not effected by a main jet change even on full throttle. I tried going up and down on mains and the only way to start eliminating the flat spot was with needle jets.

Czakky

I just can't leave well enough alone.... Every time I think I've got this flat spot before the powerband solved my second test ride proves otherwise.
I went back down to P0s and after multiple test rides it seems pretty well gone :celebrate: for real this time. Now I've got this bad off idle stumble that improves when I crank in the idle air screw but by the time it's gone (the off idle stumble) it won't idle.
This is at 32.5 pilots 1.25 turns in or more. Even at 1-.75 turns in it still isn't perfect.
So I think I've finally gotten to a point where I can feel what's happening with this thing.
Here's my jetting followed by my hypothesis:
32.5p
240m
P0
Leanest clip on needle
If I got to the second clip position, off idle (1/8 or less throttle actually) is fine but my flat spot is there. I can get a decent idle too. With my current settings no flat spot hard to idle and no flat spot.
If I go to yet another leaner needle jet (emulsion tube) I'll be able to raise the needle and likely go to a richer pilot.
How's my logic?
Who wants some needle jets?

edgefinder

#18
Quote from: Czakky on June 13, 2021, 04:56:40 PM
Now I've got this bad off idle stumble that improves when I crank in the idle air screw but by the time it's gone (the off idle stumble) it won't idle.
I have been in this spot a couple times. Pretty sure you need a needle that the taper starts earlier. Start of taper is closer to bottom groove of needle. What needle is in it? Your trying to richen with the pilot and mixture screw.

Quote from: Czakky on June 13, 2021, 04:56:40 PM
So I think I've finally gotten to a point where I can feel what's happening with this thing.
It sounds like your much closer where adjustments make a difference. When you started it was so rich small adjustments wern't noticeable.

Quote from: Czakky on June 13, 2021, 04:56:40 PM
Here's my jetting followed by my hypothesis:
32.5p
240m
P0
Leanest clip on needle
If I got to the second clip position, off idle (1/8 or less throttle actually) is fine but my flat spot is there. I can get a decent idle too. With my current settings no flat spot hard to idle and no flat spot.
If I go to yet another leaner needle jet (emulsion tube) I'll be able to raise the needle and likely go to a richer pilot.
How's my logic?
If the air bleeds are 1.8 you should try 1.5 and 1.3 to lean it a little. They are cheap and do make a difference in how it accelerates from a stop. Sometimes enough that you can get the needle towards the middle clip.

m in sc

shim the needle. put it between the slots.

Czakky

I'll try both and report back. The difference between the two clip positions is pretty dramatic.

teazer

At what revs and/or throttle position are you seeing/feeling that difference?

In theory, the needle and needle jet are there to moderate the main jet. Normally the pilot jet and pilot air jet/adjustment control idle and just off idle.  As the throttle is cracked open, the cutaway becomes the main control system and from there up to say 3/4 it's all about the needle and needle jet.  Main jet does impact all the way down but main jet has to be set of full throttle operation and the needle jet spray bar height and main air jet control the fuel slope.

What the heck is a fuel slope?  Well we tend to conflate revs and throttle opening and that's understandable, but we usually get things wrong because of that.  At full throttle the air:fuel ratio changes as revs rise.  Two strokes typically require a richer mixture at full throttle & full revs than a 4 stroke, so the air jet or spray bar are used to change how much richer or leaner it gets aas revs rise.  That's what they mean by changing the fuel slope.

That's also why people say to get idle right first and then main jet at full revs.  After that the needle and needle jet are used to tweak that fuel slope at lower throttle openings.

In this case, it sounds like a wrong cutaway or wrong needle jet taper or the point at which that taper starts.

Which is why I asked where is the issue in terms of throttle opening.

Czakky

The problem ive been trying to tune out originally is around 5500-6k rpm in higher gear mainly when I'm at 1/4-3/4 throttle. Full throttle it'll zing right past the flat spot. I previously had it pretty well cleaned up with P2s when temps were cooler. Now it warmed up it is pretty bad again. Went to P0s on 2nd clip, minimal change now I'm at 1st clip (leanest) and flat spot gone, but now it seems to lost any signal from the needle jet at closed throttle. So now I will either wind in the air screw and it won't idle for longer than a few seconds or if I go the other way on the air screw it wont take throttle until it's like 1/8-1/4 open.

teazer

That indicates that it was marginally rich with cold dense air and has become too rich with warmer less dense air.  Try O-8 needle jets to clean that up.

The idle is now too lean, so try going up two sizes in the pilot jet.

Needle jet size doesn't usually have that much impact on idle but circuits do overlap.  Try that and see what changes.

m in sc

needle should have zero effect on idle if the slides are set correctly. if they are too high on the idle screws it it will affect it.  :twocents:

Czakky

I will check to see how far my slides are at idle. That was definitely on my radar during tuning and idle has always been okay with these pilots with other needle jets. Now it's just super picky with the air screw where at fatter njs it didn't have much effect and small throttle openings were never an issue.

Czakky

I measured slides at fully closed and I was able to get a regular aerosol straw in there with some resistance. I don't think my slides are set too high. I threw fresh plugs in and went for a very quick ride with the needle spacers (.5mm). Checked plugs, thought they looked a bit lean, but maybe just not enough mileage. I intentionally threw them in warm and didn't use choke to start it.

Czakky

BTW the bogging is back with these spacers, although not as bad and idle is very touchy. I'm able to get her to warm up and pull from small throttle opening but only idle for like 10 seconds before dying. :bang:

1976RD400C

#28
Can you make a video and take us for a ride?
'76 RD400 green  '76 RD400 red   '84 RZ350

Czakky

I'd have to tape my phone to my chest...

I believe it was edgefinder who said that if my needle jets were too big I'd get more signal from them than the pilots. I have to believe this to be the case as when I get down to near correct on the needle jet the pilot is too big. I played with what needles I have and i was only able to get her to idle at 3+ turns out..
Second issue I have is my bogging thing. This seems completely dependent on needle position. I tried raising the needle (richer) two positions and my small throttle position 0-1/8) lean condition is gone. I measured my needle and it makes sense as the taper is beginning right where that slide position is. Looks like I've got to go leaner on the needle jets again... Who ever said jets are cheap can go to hell!