Question about PJ carb

[Luni]

Im trying to help a buddy get his RM250 jetted after rebuild, and it has a PJ34 carb stock.

Im trying to understand the idle screw as its part of the choke. Its my understanding, screwing out the idle screw actually affects the carb opposite as it would if it was a standard type carb (with its own idle circuit screw) because when you turn the idle out, youre opening the choke slightly and adding more fuel.

So, its my understanding, that if you have to turn that knob out too far, youre richening it quite a bit, and the reason it wont idle is cause the pilot is too lean, and richening it up with the knob is making it idle properly.

However, Ive also read that theres a needle that goes into an air passage, and the choke/idle circuits ARE seperate.

I found royalts thread about PJ jetting, and it helped, but nowhere, nobody ever explains that damn circuit. Everywhere you read, idle should be 1.5 to 2 turns out from closed to idle, if you gotta open it more, you need to raise your pilot, but on a PJ, Ive read its opposite, if you have to open it farther, its cause youre too lean and you need to fatten up the pilot jet.

Ive read about the 2 different types of circuit control on a carb. On one hand, they say if the idle adjusment is toward the engine side of the slide, its a gasoline screw, and its backward from the air screw (out means too lean, where on an air screw, out means too rich). And if the idle is to the intake filter side of the carb, its an air screw.

Someone has to know. Im going crazy trying to understand this damn thing. I keep trying to jet it like my PWK 28 and it isnt working. As it sits, I got the stupid thing to idle, but the knob is turned like 9 full turns out, and the choke rod sits out like almost 3/4 of the way so when you actually choke it, youre not doing much to move it any farther.

Im pretty sure Im right on this, and by being right, means Im going to have to richen up the pilot, and most likely will have to lean out the main since the pilot affects the main, but I just wanna know if anyone really knows how to tune the PJ carb.

And BTW, what happened to Tony (royalt)? I havent seen anything by him on here for awhile. Phragle neither.

 

[sicivicdude]

I have never worked on a PJ but think I've got a firm grasp of the function...

It's my understanding that the pilot air needle works the same way as the one of the PWK and the pilot jet functions the same as well. When properly tuned, the slide should be closed completely and all of the fuel and air supplied by the pilot jet and choke/idle knob. The choke/idle knob has a two stage plunger... One stage controls the air flow through, that's the "screw" part or idle control. The second stage allows fuel past and that's the "choke" part or richening circuit. The two circuits should not intermingle for proper idle control... otherwise the choke stage will bleed fuel into the idle air and it will REALLY richen the mixture at idle and give you WAY false rich readings.

 

[super noober]

whats his current pilot jet? you can see only about a 1/4 inch of the plunger on my 34pj and it idles like a dream. i belive i have a 55 pilot in there

 

[Awk08]

oh yeah i've got a grasp on this pj 34, its like trying to choke a blowjob out of a nun !!!!
they suck to tune, but mine runs good once i got it there
as for that choke/idle circut, i had no luck, i've been everywhere from a 42 - a 65 pilot, it will not idle on that circut alone, period
i used the cable tension to adjust my idle, then i can fine tune it with the choke/bullshit/idle circut, only downfall i see from my using the cable is it will idle slightly higher when i crank my bars to the left, and i actually sometimes use that as a fakey choke, to get it to idle higher when it's semi-cold

the airscrew on mine works the same as all, it adds air, and out means a leaner pilot jet

and btw...."rejet after a rebuild" ?????
if he added no go fast goodies other than rebuilt top end ???
he should be around the stock jetting for that bike, which should be available online somewhere

 

[roadkill]

wow this sounds like a pain in the ass my friend had one on his rm250 and he put a few mods on it so we had to rejet it and we couldnt figure out the idle circuit, then he got a lot of engine work done and went to a bigger pwk and sold his pj

 

[244blackblastard]

My 35 pj is fussy too. Only thing I know to be correct to help is make sure motor is at full operating temperature before adjusting. Seems like I am always adjusting depending on air temp.

 

[Luni]

He did some stuff. Mild porting and milled and mild work to the head. He also didnt have a functionaly powervalve before, and now he does. All these things will make him need to rejet.

Not to mention I really wanna get a grasp on this carb because I like knowing how things work and how to make them work right.

So Civic, if his idle is turned out quite a bit, youre saying its still too rich on the pilot?

I mean like I said, it idles now. It idles quite nicely, I just dont like how far out that thing has to be to get it to idle.

 

[sicivicdude]

He's compensating for too much fuel in some way by needing to add lots of idle air. Either his pilot jet is too large, the air needle isn't allowing bleed air in, or his choke is drooling fuel.

 

[cruelsoul]

mine is idling now since i put new reeds in

 

[Luni]

We actually made headway today. It ended up being lean on the mid as odd as that sounds.

We started with a 48 pilot. Thats what he rode around on last year. But his engine was only working on one ring. The other ring had siezed. So, he was a bit down on power.

Fast forward to lately. We were messing with the jetting, going up, going down, not getting satisfaction with the idle. Finally looked at the damn clymers. The manual says to use the idle circuit on the choke to get the bike to idle. It doesnt state a range of operation for that knob, just to use turn it out until it idles. It says to use the pilot circuit to lean it out to the point where the idle wont kick up any more. If youre more than 2 turns out, youre too lean. If youre under 2 turns, youre too rich. So we had a 35 pilot in there (thats way lean, factory jetting is a 52 I think), and the throttle roll was kinda pingy and not so crisp. I tried adjusting the idle with the cable, and we cant get enough slack taken out of his cable to even hold idle with the cable, so that option is out. So during that test, I had the idle circuit in all the way and I rolled the throttle on. It was horrible. Boggy and hesitant. I shut it off and stopped to think a sec. I decided to test if it was lean or rich by using the choke. I figured if I choked it, and it cleaned up the rolling throttle transition, I was too lean. Sure enough it did. So I was too lean. So I decided to do a test. I put the fattest pilot in I could. I had a 55 in my bag o jets, installed it and kept the idle circuit all the way in. Rolled the throttle and it was CLEAN. So that tells me Im making headway. I then proceeded to adjust per the manual. I used the idle circuit to get it to idle, then I adjusted the pilot screw out 1.5 turns and thats about where the idle got the highest. Sweet! Im in range. Took it up the street at 1/8 throttle. It felt good. Sounded crisp, felt nice. Not too boggy and not too pingy.

Oh I forgot to mention that I also adjusted the float. Royalt (Tony) said in his PJ nightmare thread that they are super sensitive to float adjustments. I learned a nifty trick. If you hold the floats closed and blow into the fuel port, you shouldnt be able to blow anything. If you keep blowing at it, and slowly open the floats the point where your airflow doesnt increase is the starting point for setting the float. I did that, and I was a bit off. Adjusted to 16mm per specification, put it back together, and it didnt seem to run any different, but I was satisfied the float level was adjusted properly.

So, we also found out the guy who helped him rebuild his engine had set his powervalve pretension incorrectly. Youre supposed to take all tension off the spring adjuster, and then rotate the tensioner clockwise 1 full rotation, thats factory spec for PV adjustment. Well it turns out the guy had turned his counterclockwise. So thats gotta be screwing with our powerband too. Cause it felt good, but it didnt feel GREAT when it hit the powerband like its supposed to. And by then we had already had the cops called on us for riding up the street testing too many times, and it was dark, and this is an MX bike so no lights. I believe we are lean on the main. Its sitting at a 160 right now, factory spec is a 170 I believe. It had a 165 in it before and his PV wasnt working and that worked, but he decided to be superman tightening his 165 into the carb and he broke it off and had to extract it. We were too late in getting to a shop to get more jets, so we couldnt finish the jetting process.

So tomorrow he is going to buy 2 more jets. A 165, and a 170 main. We will do the 170, and do a plug chop. If its too rich, we will back it down to the 165. I think its gonna be happy with the 165 or the 170.

Anyway, wanted to share MY experience with this damn carb. The thing to stress is the idle knob DOES increase airflow, but its also increasing fuel, so they work in tandem. Youre choking the carb at the same time youre increasing airflow, but the pilot circuit still works like normal. And even if you have to screw that idle knob out all the way, if you choke it by pulling the knob, it WILL kill the bike when its warm like its supposed to, so the choke is still in tact. So what Im saying is, it doesnt seem like a big deal to have to turn that idle knob all the way out and adjust the pilot screw like a normal carb. It seems just fine and happy running with that knob out all the way. Sometimes it idles a bit high, and you can just brap the throttle and it will come down and be where it needs to be.

Hopefully this helps the next guy out with the PJ carb. I dont HATE It now, I still dont like it as much as a PWK, but at least I kinda understand how its working, and how to work around it to get results.