New Stock 39mm pwk jet block solves over rich issue

Jetting and intake information
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marknrom
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New Stock 39mm pwk jet block solves over rich issue

Post by marknrom » Mon Dec 13, 2021 8:19 pm

My oem stock 39mm on my 91 500 needle jet wore oblong. Tried to punch it out and replace from the jet block, didn't work out so well. Ended up crushing the chamber ruining the jet block.

Went craigs list shopping and found a like kind 39mm short body with the understanding it was off a 03 ktm 300. Appeared to be a mirror copy of the oem version.

Inserted proper jet sizes, needle. The needle was sticking in needle jet opening. Won't try to swap it out, i learned. So i got to drill it to size.

Replacement carb seller thereafter informed me the carbs off a 125 not 300. Likely explains the problem, hopefully no other sizing issues arise.

I need an accurate known size of the needle jet. Can't trust my worn out one in the prior carb, i damaged it further in failed removal.

Anyone?

Idea to determine: Insert needle in existing oem 39mm jet block / center hole, mark where it stops going into needle jet (top) then mic that spot. Does that seam valid? It isn't tapered I understand.

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Sandblaster
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Re: Needle jet size, modify to stock 39mm pwk

Post by Sandblaster » Tue Dec 14, 2021 1:17 am

So much will depend on your altitude, fuel type, and engine set up.

Former Kawasaki Factory Engineer Doug Haas recommends the following:

Main Jet: 170
Pilot Jet: 55 or 58 if hard starting
Needle: N82M
Clip: 2
Slide: Modified to be 7.0.
Spark plug: B9EVX
Air Screw: 2.0 turns out
Race this setting from just about sea level to about 1,200 feet
If bikes are for kids I'll never grow up.

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marknrom
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Re: Needle jet size, modify to stock 39mm pwk

Post by marknrom » Tue Dec 14, 2021 1:56 am

Sandblaster wrote:
Tue Dec 14, 2021 1:17 am
So much will depend on your altitude, fuel type, and engine set up.

Former Kawasaki Factory Engineer Doug Haas recommends the following:

Main Jet: 170
Pilot Jet: 55 or 58 if hard starting
Needle: N82M
Clip: 2
Slide: Modified to be 7.0.
Spark plug: B9EVX
Air Screw: 2.0 turns out
Race this setting from just about sea level to about 1,200 feet
Issue is the needle jet, not the needle and not jetting and settings you address. Easily misunderstood as the needle jet is very seldom discussed.
Please see orig post again and you'll recognize the oversight. Can you measure an oem 39mm not out of round / worn out needle jet opening (measure idea included in post)??? Thanks Sandblaster!

PS: since you introduced needle model, i was going with the N82P, leaner than stock? Correct?

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marknrom
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Location: Reno Nevada

Re: Needle jet size, modify to stock 39mm pwk

Post by marknrom » Wed Dec 22, 2021 7:25 am

More from author of post:

PWK NEEDLE JET WEARING OUT DISCUSSION- FIX


If the needle IS RUBBING the needle jet you may have a bent needle or needle clip. If there is a flat spot the damage is done. It's an old carb and yes carbs wear out.

KTM OWNER (they also use the pwk carbs) COMMENTS:
Try a leaner diameter needle such as Sudco L or M. Also the older carbs tend to "like" leaner L1 needles such as DGK. My experience is the 300 likes the C series needles, CEK for example but if a worn needle jet, leaner diameter, CEL, CEM but again leaner L1 such as CGL, etc.

RESPONSE TO OFFSET WORN NEEDLE JET VIA JETTING:
You’re NOT alone but why do folks always mess with the mj when the issue is low off idle part throttle? Your way to lean with the 160 in your 38 MM carb. DO NOT go by the jetting of the newer bikes using the smaller 36 MM carbs. Your mj should be in the 172 - 175 area. Also seriously consider a leaner 7 slide. Yours is probably a 6 or maybe 6.5.

DIRECT EVENT (KX500)
It happened on my kx500 carb. The hole should be .115" mine was worn to .118 to .123 oval shape. Even with a needle 4 sizes larger it still didn't run right. So it does happen but most people will never figure it out. They'll just keep messing with the Jets and needles and never have it work right.

AND ANOTHER (One solution)
I just fixed my needle jet on a PWK 38. "Egg shaped" on the top of the needle jet, which is in the metering block.

Sputtering off idle, would smoke and sponge excessively.

The conventional wisdom is to replace the carb.

However, I was able to drill out the damaged area on top and tight press in a very small nylon bushing which the needle fit perfectly.

The needle now fits correctly with no leak by, no binding and quick response. Problem solved; starts easy, idles and limited smoke.

Saved $200 on a new carb. Even if the nylon wears away every season, still easy to fix. Under 1 hour, and the bushing was a whopping .48 cents.

“The size is ultra critical, on the stic, going from a 114 to a 115 is a big jump”. Implies .114 is stock.

.114 is the stock needle jet opening size, Forum comment states .115 is stock. ????
jet block needle jet drill insert.png
jet block needle jet drill insert.png (99.64 KiB) Viewed 1124 times


The needle jet is located in the removable jet block as seen above. The jet block is upgradeable w/ aftermarket performance jet blocks; “the stic" references an aftermarket product name of one such seller of performance jet blocks. Some sell the block preinstalled in new carbs, leaving the new OEM jet block on the table. That’s were you look to buy a replacement.

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Sandblaster
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Re: Needle jet size, modify to stock 39mm pwk

Post by Sandblaster » Thu Dec 23, 2021 4:25 am

Interesting
Thanks for posting :ugeek:
If bikes are for kids I'll never grow up.

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marknrom
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Re: Needle jet size, modify to stock 39mm pwk

Post by marknrom » Tue Dec 28, 2021 8:13 am

Post Author Update:

Problem:
Oem kx500 carb (Keihin pwk 39mm) running rich, having to reduce jet sizes to compensate.

Cause:
The cause maybe a worn out needle jet.

Determination:
You know you have a worn out needle jet if your carb noticeably runs rich. Reducing the jet sizes may not cure the richness for long especially if the needle is not centered, instead pressing on the side of the needle jet wearing the opening out further.

Not knowing better I chased the problem over time by reducing both jets; my idle jet was in the high 40’s, main was in the mid 150’s, still ran rich.

The needle jet is not a replaceable item and the jet is incorporated in the jet/metering block, not a replaceable item, so I was told.

Determining if the needle jet is worn out, the block that it resides in has to be removed.

Removing the jet/metering block
The jet is mounted in the removable block located on the underside of the carb, above the bowl, above the floats and is held in place with 2 screws. The screws are security bit type and somewhat common. If you don’t have the proper bit you may need to hack on them, drill out center tip or what have ya to get your tool or bit to engage.

Once removed you can view and measure the needle jet. If oblong shaped no need to measure, its toast. No round needle with compensate in an out of round hole (jet opening).

The jet should be .014 - .015). You can use the depth of your needle to establish the size of the opening by marking the point it stops in the jet, then mic the needle at that mark. Probably have to remove the main jet to allow needle to drop.

Found new blocks for sale
After months and months searching I found a source who sells new jet/metering blocks for the pwk.

Cost was reasonable; I paid $41 plus shipping. A lot cheaper than a new carb in the $300 range.

It came with 2 replacement security bit screws, the bit, the entire gasket with the o-rings attached, preinstalled on block. Make sure all the ports are clear including the singe one on the opposite side of the dual port openings. The hole is very very small.

I slightly greased the top of the side o-rings to avoid any potential dislodging or tearing when reinstalling.

The company markets their high performance replacement jet/metering block upgraded unit - Stic Super Torque.

They also offer new carbs with the performance block.

They do not publish the availability of Keihin OEM jet/metering blocks for purchase. You have to ask for an OEM unit if you don't want the upgraded unit they offer. The company's web page represents the upgraded performance unit only.

Contact info Phone: 715-479-7822 Web page: http://www.sticsupertorque.com/ Email: Vortex@sticfuel.us

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Sandblaster
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Re: Needle jet size, modify to stock 39mm pwk

Post by Sandblaster » Tue Dec 28, 2021 5:11 pm

Wow!
That is great news.
I had no idea that they offered the stock metering blocks.
I have two of the upgraded units and never thought to ask them.
Thanks for your post and going through the effort to track this down.
If bikes are for kids I'll never grow up.

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Sandblaster
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Re: New Stock 39mm pwk jet block solves over rich issue

Post by Sandblaster » Wed Dec 29, 2021 6:12 pm

I posted links to this thread on Facebook.
All to very positive reviews.
Thanks again! :ugeek:
If bikes are for kids I'll never grow up.

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