Question on setting idle on Keihin PJ carb

[Blown 331]

Alright, lots to answer here.
I haven't actually measured the float level but it does not run over or starve.
A 250R head gasket is a piece of steel with a thin layer of copper riveted to each side. I'm running just one layer of copper as my head gasket, no second layer, no steel center. So it should have some decent compression, I have ran it on 93 but that was with improper jetting.
Yes My Mustang in my avatar has a supercharged 331. I only took it to the track one time and it has some bugs to work out of it but first pass was 11.1. Lots more left though. Its full weight with leather and AC as well.
Maybe I'm misunderstanding but I think I'm getting conflicting info on how the choke knob works. I don't really see how raising the knob would let in more air, it would never start cold like that.
As far as I know it has no porting, its a stock cylinder, I had it bored and a fresh Wiseco. Its a 1985 so it had a round slide carb stock. My PJ is a 36. When I got this carb it had a 75 pilot!?? And was supposedly on a 250R. I had a 55 laying around so I used that. I'm going to order a smaller pilot and see what happens. I can't say if it revs high when the choke is pulled or not because the way I have it adjusted the choke really has no movement.

[Mosh]

I know it is completely confusing and I am probably not helping with the theory of operation, but let me try to explain how the choke knob works.
That Knob is a plunger attached to an air mix circuit that goes from the intake (filter bell side) down to the bowl, then draws fuel up through the idle circuit. So if your pilot is too big, the more air you bleed past the choke plunger, the more fuel it pulls into the engine at idle. So whenever you screw that choke all the way loose, you are giving it all the air the circuit is capable of, trying to lean out the over rich pilot jet. Hence you get no idle RPM raise. Think of it this way. What happens when a 250R starts running out of gas? They rev up cuz they are running lean correct? So what you have is too rich of pilot, so you are trying to mix as much air as possible through the choke knob just to get it to run. That is why when you turn the choke knob down it starts blowing thick black smoke. You are taking air away from the idle circuit causing it to go rich.

Or how about this if it makes you feel better.

My wife's engine is 20 over no porting, CR head gasket, 50-50 mix of 110 and 93, with Boyeson reeds and ESR pipe with no lid on the box. Pretty much identical to your engine in theory. She runs a PJ 36 mil off a CR dirt bike. She is at a 45 pilot and the choke knob is about 2 turns from the maxed out position.

My stock 86 TRX with a pipe and V force reeds, ran a PJ 34 carb. 42 pilot 165 main. And the choke knob was about the middle adjusted. That is where you want it.

My ported 250R race engine at 250cc with a PWK 38 runs a 45 pilot...Yes no choke idle adjustment on that model...

My other ported engine 250CC with pipe and reeds runs a 42 pilot on a PWK 38 quad vent..Yes no choke idle adjustment on that model also...

But when you choke any of those carbs cold, the idle real high when started.

Are you sensing the pattern there? None of my machines run a huge pilot jet and they all run really well. 2 of my friends 250R's I have tuned all run 42 pilots. The only time I have had to dump a huge 50+ pilot on a Keihn carb was if it was a big bore, or on a modded 250cc Tri-Z where that thing wanted a 50 pilot just to even idle. But Z's like to be rich. And that had a pod filter.

And the only time I have seen a huge pilot come out of a Keihn that "barely ran" was when the reeds had a 1/8th inch gap in them and the piston was rocking back and forth in a worn out jug about 1/16th of an inch. Someone had a 60 pilot jammed in that to make up for the wasted engine. It ran but that is about all it did if you held the throttle 1/2 open.
I read guys posting all the time how they are running a 50+ pilots in a mild modded stock based 250R and they claim it runs great. I can't for the life of me figure out how they even run at all, when in my experience any pilot jet that big in a 34-36 even some 38 mil carbs, makes my eyes water from the raw fuel pouring out the pipe just before it stalls and fouls the plug.

I am sure some need a 50 pilot here and there, but the one thing I have in my jet box is an abundance of 50 and larger pilots, but I can't keep 42-48's in stock cause every bike I try to jet I pull a huge pilot out and drop in a somewhere in the 40's.

[barnett468]

Alright, lots to answer here. I haven't actually measured the float level but it does not run over or starve.

Maybe I'm misunderstanding but I think I'm getting conflicting info on how the choke knob works.

My PJ is a 36. When I got this carb it had a 75 pilot!?? And was supposedly on a 250R. I had a 55 laying around so I used that. I'm going to order a smaller pilot and see what happens.

I can't say if it revs high when the choke is pulled or not because the way I have it adjusted the choke really has no movement.

I already admitted I might be wrong about part of my tuning/function description [see disclaimer below, lol], however, once the knob on the choke is turned fully in or out, the small brass air screw on the left side of the carb is the final determining factor as to whether you need a different size pilot jet and whether it needs to be smaller or bigger than the current one.

From post 13.

The following is not from Keihin or the internet. If any of it is incorrect I apologize and encourage corrections. It is also incomplete.

3. Turn the air screw all the way in until it just barely stops and count the amount of turns. The average might be 1, the max might be 3. If it is less than 1/2, and your idle screw knob is at the max limit, your pilot jet is too small. I would install 1 size larger.

From one of the Keihin links I gave you. When a "mixture screw" is located on the air cleaner side if the carb slide it is an air screw and turning it in increases the ratio of gas to air.

“Turning IDLE MIXTURE SCREW (CR, PWK, PJ, PE) controls the amount of air to the IDLE and SLOW CIRCUIT. This SCREW is located on the air cleaner side of the throttle slide and turning the SCREW out will lean the mixture and turning the SCREW in (clockwise) will richen the mixture.”

If the air screw Is all the way In you need a bigger pilot.

With a bigger pilot, try setting your air screw to 1 or 1-1/2 turns out. Start the bike up, then adjust the choke knob/Idle adjuster until It will Idle decent on its own. Then fine tune with the air screw.

FLOAT LEVEL - Kehin says the spec is 16 mm on all PJ carbs. Since you don't know for sure what it is I would check it anyway. Even though it runs well you still can't get the idle/chokre to work together and an incorrect level can throw your adjustments off especially on that model.


You can also check with Duncan racing on how the different circuits work and how to jet it etc.

619-258-6306

Yes My Mustang in my avatar has a supercharged 331. I only took it to the track one time and it has some bugs to work out of it but first pass was 11.1. Lots more left though. Its full weight with leather and AC as well.

Very cool. Novi or Vortec? How much boost at max rpm?

[Blown 331]

Ok. I'm convinced my 55 pilot is too big and the source of my problems.
And yes Mosh, other than the head gasket difference, your wife's bike sounds pretty much exactly the same as mine. That thing must run good because mine sure does! lol.
But just to clarify this choke knob operation. The idle circuit basically has 2 feeds, a fuel feed (pilot) and an air feed (turning choke knob). Pulling the choke knob does add air (which is opposite of normal choke operation), but this extra air pulls more fuel through the pilot which actually richens the mixture. So that is why pulling the choke raises idle RPM, because more air, and in turn more fuel is added. And the choke knob only affects idle operation, once its off idle the pilot takes over by itself (only limited by size) and the choke knob no longer has an affect on the pilot. Is that right?
On thing that kind of makes me not want to change the pilot is that this thing is super crisp. Throttle response off idle is instant. I'll give it a try though. Before I order a new one I'm going to check my other PJ and see if it might have a smaller pilot. Thanks for everyone help.

And the Mustang has a Vortech S-trim, 14psi.

[barnett468]

Ok. I'm convinced my 55 pilot is too big and the source of my problems.

But just to clarify this choke knob operation. The idle circuit basically has 2 feeds, a fuel feed (pilot) and an air feed (turning choke knob). Pulling the choke knob does add air (which is opposite of normal choke operation),

Ok, good to know, thanks for the info and correction, my comments were based on fading memory, I apologize for any confusion they caused.

Just curious, how did you determine for certain it it regulates air and not fuel?

How far out from 0 is your brass air screw?

As far as the idle circuit having only two “feeds”, why does Keihin and Jason Hall and many others say the brass air screw affects idle, and its setting aids in determining pilot jet size?

The air screw does something and I don't remember ever seeing one used on a main jet circuit.

And the choke knob only affects idle operation, once its off idle the pilot takes over by itself (only limited by size) and the choke knob no longer has an affect on the pilot. Is that right?

Not exactly, off is off and on is on. If you do not turn the choke off it will still affect the idle circuit only, however, the idle circuit contributes fuel to the engine throughout all throttle positions.

As far as the idle circuit goes, it always supplies fuel to the engine throughout the entire rpm range, however, the “percentage” of fuel it contributes changes depending on throttle position etc.

The “idle speed”/pilot circuit fine tuning air adjustment knob [did I get that right, lol], has a continuous affect on the amount of fuel the pilot flows throughout the pilots entire operating [effective]range.

Changing the knob position, changes the amount of fuel the idle circuit contributes.

The more the throttle is opened, the lower the percentage of fuel it contributes compared to the main jet. You can see this in the Keihin chart I gave you the link to. The chart shows the idle circuit falling off to 0 at the 3/8 throttle range, however, this is incorrect. They show it this way because its effect/contribution has become so minimal at this throttle position that carb companies don’t consider it to be note worthy.

You might be able to see this if you have a carb [not efi] and an AFR meter on your Mustang, by taking it for a drive on a flat road, and checking the afr ratio at steady throttle positions at 1500, 2500 and 3500 rpm, then afterwards, turn your idle mix screws in all the way and check it again. Since the effect will be small at higher rpm's, your Supercharger might make the results inconsistent.

And the Mustang has a Vortech S-trim, 14psi.

Cool, I had a screamin Vortec on my 396 Camaro at around 10 lbs boost, and a whisper quiet Paxton/Novi on my 302 Mustang at 14 lbs. Love superchargers when they’re cranked up high. I call them “stupidchargers” because the amount of HP they make with high boost is so much its stupid imo, lol.

[Blown 331]

The brass scres is idle mixture so you can fine tune the pilot. I'm 2 turns out on mine but honestly it doesn't seem to do a whole lot. Turning the choke knob is supposed to be the idle speed adjustment.
I wasn't trying to state facts on the choke knob operation, I was trying to put its operation in my own words and asking if I understand it correctly. As far as the choke knob adding air, I'm going off Moshs explanation.
I do have a wide band on the Mustang but its fuel injected. A buddy of mine has a ATV / motorcycle dyno with wide band. He lives about 50 miles from me. I to haul a trailer load of bikes down there, should be interesting.

[Mosh]

Ok. I'm convinced my 55 pilot is too big and the source of my problems.
And yes Mosh, other than the head gasket difference, your wife's bike sounds pretty much exactly the same as mine. That thing must run good because mine sure does! lol.
But just to clarify this choke knob operation. The idle circuit basically has 2 feeds, a fuel feed (pilot) and an air feed (turning choke knob). Pulling the choke knob does add air (which is opposite of normal choke operation), but this extra air pulls more fuel through the pilot which actually richens the mixture. So that is why pulling the choke raises idle RPM, because more air, and in turn more fuel is added. And the choke knob only affects idle operation, once its off idle the pilot takes over by itself (only limited by size) and the choke knob no longer has an affect on the pilot. Is that right?
On thing that kind of makes me not want to change the pilot is that this thing is super crisp. Throttle response off idle is instant. I'll give it a try though. Before I order a new one I'm going to check my other PJ and see if it might have a smaller pilot. Thanks for everyone help.

And the Mustang has a Vortech S-trim, 14psi.

That is sort of what I am trying to convey. But as I stated before I tend to think of things backwards as it helps me remember better or understand better for myself but probably confuses others. Just let us know what happens when you go leaner on the pilot.