Can someone explain to me PWK then PJ

[atctim]

I recently upgraded my 1986 Flat Tracker 250R from the stock-ish 250R motor to a Hetrick bulit ESR 330 cylinder TRX motor. It came with a PJ carb (which happens to have an unused electrical connection on it). It is a 39mm bore. I received a 39mm PWK carb from a friend and put it on (from the get go). After several failed attempts at tuning it I swapped over to the PJ. It ran perfect with the PJ - I mean really really really good. FYI - both carbs had the needle clip at the middle position, both had 185 main jets. The PWK had a 50 pilot jet, and the PJ has a 52 pilot. Why such different results from such similar carbs? The PWK idles fine, and runs fine until just over 1/2 throttle (which tells me - wrong main jet size), then breaks up the rest of the way. Both carbs are also 2 turns out on the air/fuel screw, and running a pod filter (same filter). I found it so very odd that they run so much different - the PJ runs awesome idle through WOT. Ideas? Theories? Comments are appreciated - I want to learn.

[El Camexican]

I have 2 identical 38mm PWK AS carbs. One new, one well used. All else being the same the older carb requires a little more opening on the slide and a little leaner setting on the air screw to idle the same as the newer carb. It also seems to fluctuate at idle more at high altitude, but seems a lot less finicky about where the needle is set when running at 6,000+ elevation.

So what could it be? Well in your case I would look at the cut of the slide as it may be different between the two carbs and the fact that they are flat out different, but in my case I think it has to do with the slide being a little worn on the older carb.

The fact is conventional carbs are amazing if you consider how old the technology is, but archaic when you consider what a closed loop EFI is capable of. Look at it this way, every .0005” of tolerance and wear in your carb will change the way it works compared to another, so set each up to run best and forget about it.

[ATC-Eric]

Different needles?

[82 250r]

1/2 throttle is where the needle clip position ends and the needle taper begins. Main jet not a factor around 1/2 throttle.
Try to tune in one or both of those making only one change at a time.

[El Camexican]

Different needles?

Sorry Mr. Eric, not sure if your asking or telling. I used the same needle in both cards (it's a JD), or are you telling me that yours are different? That would make a difference. I mentioned the cutaway on the slide which is totally different (not being rude, you likely knew that), the I think the 38 PWK's come with a #7, but if you buy a KTM carb they come with a #6.5. Too expensive for me to play with different cutaways.

[Daddio]

Here's my 2cents. I've spent many a day helping Hartley get his 300r dialed in. I've noticed that the more high strung the engine the longer it takes to get it dialed in really crisp. You say the PJ came with the engine. It's possible that someone already took the time to dial that carb in for that engine. In helping Hartley I noticed that most generally, every carb will need to be jetted a little differently, even if it's the same carb, model and size. I also know that if a carb has a good deal of wear, it will perform a lot differently than a new one, just like El Camexican said. From the description you gave about the way yours runs with the PWK, without being able to see or hear it run, my guess is that it's too rich on the main. And like Eric said, it could also be the wrong taper on the needle making it too rich too soon.
With Hartley's 300r, we would have to keep notes so we knew how it reacted to each change we made. It was so temperamental that even changes in the temperature would throw off the jetting. It was a pain in the ass to deal with until we got it figured out. But when we did figure it out, it was very hard to beat. It was an animal!!! He tried several carbs, but the one he liked best on his was a 39mm PWK. He said that PJ, stood for, piece of junk. If I remember correctly he ended up running a main jet in the 120s, like maybe a 128. It seemed like it would be scary lean but it never had any sort of engine failure. Of course his was a drag race only machine. WOT all the way.
So the best advice I can give you is to keep trying and eventually you will get it right.

[C.J]

Weeelllll,,,,,

PJ's don't have any electrical connections on them.

Difference between a PJ and PWK is the PJ doesn't have an idle adjustment screw that changes the slide height, but rather you turn the choke knob to raise and lower the choke slide to Chang you idle. They also have a regular flat slide where as the PWK has the D shaped slide with the little air foil on it.

Both are SUPER finicky carbs to deal with,, the PJ being worse so than the PWK. The best version of the PJ is the quad vent. They aren't as bad as a regular PJ.

But that being said,,,

The biggest difference is going to be the slides. That will change up how both carbs run with the same jetting since the PWK will atomize the fuel/air a little better but IMO, the PJ is a harder running carb. Just my .02¢ though so take it with a grain of salt.

Take the time to dual both carbs in and see what you come up with. I would also suggest a different needle for the PWK,,,


Here's a thought: Did the PWK come as a 2-stroke carb? 2-strokes and 4-strokes have different emulsion tubes (the little bit that sticks up that the needle slides down into). That piece can also wear out or it can have a larger diameter for a different needle.

One of my PJ's for reference

[El Camexican]

we would have to keep notes so we knew how it reacted to each change we made. It was so temperamental that even changes in the temperature would throw off the jetting.

That right there is huge. Most every serious drag racer has a mini weather station in their trailer to calculate the corrected altitude. It is amazing to see corrected elevation at 0 feet at 9:00am, 4,000 feet at 2:00pm and then negative 500 feet as the sun sets. What I found was that being rich had very little effect on performance, but as soon as I’d get lean my engine would fall flat on its face.

[Louis Mielke]

PJ's are junk, at least all old ones are. Are you sure the one with a throttle positioning sensor is a PJ Tim?

[ATC-Eric]

Sorry Mr. Eric, not sure if your asking or telling. I used the same needle in both cards (it's a JD), or are you telling me that yours are different? That would make a difference. I mentioned the cutaway on the slide which is totally different (not being rude, you likely knew that), the I think the 38 PWK's come with a #7, but if you buy a KTM carb they come with a #6.5. Too expensive for me to play with different cutaways.

I was talking to Tim. Saying that if they appear equal, they may have different size needles in them.

[Blown 331]

I have read that a PWK may require a larger main jet that a PJ for whatever reason. I have bikes with both and both run good. But I don't have any experience swapping from one to the other on the same bike.