Question on setting idle on Keihin PJ carb

[Blown 331]

I finally got my 85 250R jetted correctly and its running great all all speeds but there is still something not quite right. On a PJ the idle is set by turning the choke knob. To get it to run right I have the choke knob unscrewed all the way. And when its like this the choke knob has almost no up and down movement, so when starting I have to hold up on the choke while kicking because it doesnt move enought to click into the open position. I thought scewed out all the way was rich but when I screw it down some the bike cuts out and seems to smoke a lot so I'm not real sure on that. So which way is lean and which was is rich, and why do I need my choke knob screwed all the way out? Would changing the pilot jet affect that? I think I have a 55 in it. The bike has compression, ESR pipe and rad valve. 185 main jet, 110 octane, no air box lid. It idles and runs great at all speeds but I'd like to have normal choke operation. Thanks.

[barnett468]

Hello;

The links below might help until someone can explain it for you. You might need to change your pilot jet to get it is range. If it does not have a flat spot/hesitation right off idle the pilot may be fine.

http://www.carbparts.com/keihin/needles_tuning/jetting_your_carb.htm

http://www.keihincarbs.com/tips/gate.html

[Blown 331]

There is no flat spot or hesitation. It appears those links are for a PWK which does not have the idle adjustment on the choke knob, I have a PJ.

[shortline10]

In my experience its usually a throttle cable issue or small jet issue .
is this a new carb ? is their a little slack in the throttle cable so the slide bottoms out ?
Might be a simple as going up 1 size on the small jet and if its a used carb make sure the small jet is very clean .
If all else is ok lower the needle clip 1 notch so it raises the needle and see if that makes a difference .

[Blown 331]

Used carb. Its probably been on the bike for 10 years and I could never get it to run right at WOT, so just only rode up to about 3/4 throttle or so. lol. Turns out I was always way small on the main jet. Runs great at WOT with a 185 which is much larger than I see other people runnning so I never tried going that big but thats what it takes.
There is slack in the cable. I haven't checked the jet for being clean lately but its always been like this even when I knew for a fact it was clean so I don't think thats it.
Everything you are suggesting is richening the bike. So does that mean having the choke screwed all the way out richens it? So basically I'm maxed out on that adjustment to the rich side. I'll check the needle first, I can't remember where that is at all. Probably 5 years since I've been in there. If nothing else I can always just live with it since it does run great. Just sucks I have to manually hold the choke up to start it like this and it has almost no movement. Thanks.

[El Camexican]

I think (repeat THINK) that you might need to go down one or two sizes on your slow jet in order to get your idle/choke thingamajig (for want of a better term) in the nominal range. That gizmo is enough to drive a man to drink. I gave up on mine and when to a PWK on my Tri-Z, but now that I have my fuel pump issues sorted out I may put the PJ back on and try and to master it.

Good luck and let us know how it works out!

[Blown 331]

Yeah they can be a pain for sure. I've got a PWK on my 01 Banshee. My 90 Banshee has a PJ and its great all around. Both bikes have a single carb. I've also got another PJ, I thought about trying it on my Tecate.

[poohbee1]

I think I would pressure test that motor. 185 main wow that's big

[Blown 331]

That's exactly why I had incorrect jetting for 10 years. lol. This is the first 2 stroke I've jetted so I had problems determining if I was lean or rich at WOT. From what I read, lean would bog and I definitely did not have a bog. It also seemed to be the general consensus that 175 was about as big as a 250R would need so I never tried to go any bigger. I threw in a 175 and it would still cut out at WOT. No bog and I thought 175 would be rich. So I worked my way down and just could not get it. I did not want to do a plug chop for fear of melting it down if I was lean.
And heres how I figured it out. I picked up a built 01 Banshee with a single PWK, it had a 168 main and ran just like my 250R, perhaps worse, it made the most power at about 75% throttle, over that it would loose power. I did some research on that as well an it as general consensus that I was real close on jetting. Then I picked up a 90 Banshee with a single PJ. It was stock other than that, even has stock pipes. This thing was jetted spot on, whoever jetted it knew what they were doing, it runs PERFECT! It had a 180 jet. That was my first clue that the 01 was way lean. I threw in a 185 and it was better. Then I went with a 200, better yet and I had no bigger. I have a 210 on the way and I may even need a 220. As far as I know 230 is as big as they go. That was my clue that my 250R was just crazy lean even though there was no bog. I pulled the 165 and threw in the 185 and it ran perfect. And the power is unreal. By far the fastest untouched stock cylinder I have ever ridden. I'm almost wondering if there isn't some port work I don't know about. Its amazing. Super crisp.
And since all this I did more searching and found other Banshees running up to 230 main and other 250Rs running 200-220 mains (even on this site), so while it seems to be rare, its not just me. I don't run air box lids either, no one seems to mention that and it makes a big difference.

[El Camexican]

Did you check your float height before you started changing jets? If it's off nothing will work right.

[DohcBikes]

This could be a stupid suggestion but no one has talked about fuel mix ratio... could that make a difference? And is 110 octane suggested for a stock 250r... I truly don't know but just throwing that out there.

[barnett468]

Hello Blown 331;

Actually, the links I posted for you are in fact, at least part of Keihins official instructions for tuning a PJ carb as well as others.

I will post detailed directions hopefully within the hour, sorry but I just didn't have time yesterday.

Upm,

[barnett468]

Yeah they can be a pain for sure.LOL.


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

I thought it might help people to "properly" adjust the following section of these lovely PJ series carbs if they understood a little about how the idle speed adjustment/choke knob worked on them. Just skip down to the adjustment section if this doesn't interest you.

The PJ carb is obviously a weird bird. Imo, Keihin tried t do what is nearly impossible, which is to make a carb that would idle properly on all motorcycles it was properly sized for without having an adjustment for the slide. Obviously it didn’t work out completely according to plan.

Many people either give up trying to set the idle or simply live with either no idle or no choke, or use throttle cable tension to increase the slide height so their bike will idle.

The idle speed adjustment/choke pull knob combo, is nothing more than a fuel enrichment circuit.



The way this set up works in principal, is a bit similar to the Keihins on the ATC 250 ES Honda ATC. The main exceptions are, the PJ carb has an adjustable idle air screw and the Keihin on the Honda has a fixed size air jet. The air screw is able to adjust air to the idle circuit from an amount of 0 up to the size of the orifice it controls. The idle speed adjustment knob and choke pull knob on the PJ, performs the same function [works the same way] as the idle fuel screw on the Honda Keihin.

This combo knob has two sections. One section is a needle which is raised or lowered by turning the idle “speed adjustment” knob either in or out. The position [height] of this needle partially controls the amount of fuel supplied to the engine on the idle circuit. The idle circuit provides fuel to the engine during starting also.

The other section is a plunger that when pulled, adds additional fuel to the bike for starting purposes only.



There are a few different ways to explain and approach the idle "speed"/choke adjustment of this lovely design, the following is just one. Some of these items were mentioned above by others.

IDLE "SPEED"/CHOKE ADJUSTMENT

1. Check float level, if it is too low it can cause a lean idle condition, if it is too high, it can cause a rich condition. Set it to the lower end of the recommended height then check idle again before making further adjustments.

2. Make sure there is a little free play in the throttle cable.

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.

4. Warm engine to operating temp then turn the idle "speed" adjustment knob [located on the choke pull knob] until the highest idle is reached.

5. If your idle air screw is more than 1/2 turns out, turn it in until highest idle is achieved but do not adjust it to less than 1/2 turns from fully closed.

6. Turn the idle "speed" adjustment knob until the desired idle speed is reached.

7. If your idle air screw is still more than 1/2 turns out, turn it in until highest idle is achieved again, but do not adjust it to less than 1/2 turns from fully closed.

8. Turn the idle "speed" adjustment knob until the desired idle speed is reached again.

9. Turn bike off and pull choke lever to see if it properly engages. If it does, you are done with this part, if it does not, install a 1 size larger pilot jet. This should hopefully create a high idle and/or rich condition at idle, in which case, you simply turn the idle "speed"adjustment knob in until the desired idle is reached.

If you turn the idle "speed" adjustment knob all the way in and the idle is still too high, simply turn the idle air screw out until the desire idle speed is reached.

If you reach a point where the air screw suddenly has little to no effect on the idle speed, you have reached the maximum range of its effectiveness. Count how many turns out it is from full in and make a note for your records. This point might be around 3 turns out from full in.

If you reach this point and your idle is still too high then the easiest way I know to achieve your desired idle speed is to reinstall the previous pilot jet, raise the slide a hair by putting tension on it by adjusting the throttle cable [this is not recommended because it can possibly create a potentially dangerous situation], return your air screw to its original setting and turn your idle “speed” adjustment knob in until the desired idle speed is reached.

You will need to fiddle with balancing all these things until the idle “speed” adjustment knob is in far enough that it allows the choke to properly function.



Ok, now if you eventually become one of the lucky few that are able to achieve idle nirvana with a PJ carb, but you now have a slight rich burble/hesitation just off idle, lower your needle 1 position at a time by raising the clip on it and PRAY that it goes away and does not affect your idle.



Ok, was that confusing enough?

[barnett468]

Hi Blown 331;

Is your screen name a reference to a stroked small block Ford?

If so, is it the one used in your Avatar photo that appears to have a problem keeping the wheels on the ground?

If so, does it happen to run under 10.5 in the 1/4?

[Mosh]

I would be cautious on taking jetting advice while not listing more specifics. For instance what size is the carb? 34-36 mil?
Any porting done to the engine?

Now assuming it is a stock 34 pj and no porting and a 250 based cylinder you would be way to rich on the pilot. Where is theair mix screw set? I would guess you have that screw backed out at least 3 turns just to get it to run.
The choke circuit on those carbs are confusing. You think they are adding fuel when you screw it out. But you are actually adding air when that knob is pulled up or or screwed out. If that bike does not rev hi when its cold and choked the pilot is to rich.
Either way if the engine is fairly stock other than the pipe and reeds and the carb isa factory 34 with 55 pilot and 185 main I would be working down on jets. Probably around a 42-48 pilot then re check the main.

[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.