Keihin 38 PWK AS

dogger315 · Post by **dogger315** » October 22nd, 2011, 10:16 am

I know most of you run the larger PWKs, but I recently swapped my
39.5 for a 38. My bike is intended exclusively for MX so I'm trying
different things to improve throttle reponse and low to mid range
power.

The 38mm PJ was the OE carb on later model 500s, so it should be a
straight forward swap and work fine. I jetted the 38 with a 172 main
a 42 pilot and the same "DGN" needle/3rd clip that was in the 39.5.
2 out on the airscrew, VP C12 and Honda HP2 @ 32:1. Mid 60s, 40%
humidity and 500 feet - just about perfect "standard" conditions.

What a mess this created. Now the bike is very hard to start, it is
so lean that it runs better with the choke on. The 172 main is probably
fine but I will need to bump the pilot up to a 48 or 50 to duplicate the
choked setting. I'm surprised that a 1.5mm change in carb size caused
such a difference in jetting requirements. Both carburetors are new with
new jets.

Is there anybody else running this size carb and if so, did you run into
similar problems? Any helpful thoughts or input is appreciated.

dogger

Kuma · Post by **Kuma** » October 22nd, 2011, 10:58 am

I'm probably not the one to give carb advise but,
I had the air striker on mine, it started great in the morning cold with the choke but once warmed up it was a bitch, that is until I learned that there's only 2 ways to get it started. One is have Bob kick it, he can wing it over like it has an electric starter. The other way I found was that it likes to have the throttle held wide open, then it would start within 2 kicks.
I have since pulled it off and put the PJ back on. Planning to put a 39.5 on it soon.
I know, not much help.

cmotodad · Post by **cmotodad** » October 22nd, 2011, 1:54 pm

I run a 38mm AS on my 500 and it runs good, clean on the bottom thru the mid up to the normal sign off rpm on top. Not sure of jetting but I have a richer needle tham a dgn. If you like , I can do a check.

Tharrell · Post by **Tharrell** » October 22nd, 2011, 2:40 pm

Somebody needs to dig up that vid of Brit starting his bike.
I know this is a jetting question but, I liked that vid.

scooter5002 · Post by **scooter5002** » October 22nd, 2011, 5:12 pm

Perhaps greater minds than mine have the answer (Roosty?) but would not the smaller carb mean a higher air velocity due to the smaller bore, thus creating the lean condition? I could be totally my wrong. Something my ex reminded me off constantly.

cmotodad · Post by **cmotodad** » October 22nd, 2011, 9:46 pm

The high velocity seems to draw more fuel as it has more fuel quicker.

iggys-amsoil · Post by **iggys-amsoil** » October 23rd, 2011, 6:33 am

I think you'll have to go bigger still on the pilot, like a 55.

dogger315 · Post by **dogger315** » October 23rd, 2011, 8:43 am

Thanks for the help guys.

If you like , I can do a check.

I would appreciate that - especially the pilot and needle.

I think you'll have to go bigger still on the pilot, like a 55.

You may be right, but I think I'll start with a 48 and work my way up as
needed. I don't know what the choke adds in terms of comparable jet
size, I'm just guessing an increase of 6-8, but it could be much more.
Also, while the bike did run better with the choke on, it was rich because
it was making spooge and too much smoke - that's why I want to start
with the 48.

dogger

bigpower · Post by **bigpower** » October 24th, 2011, 7:44 pm

I've had some recent experience with the 38 AS, but the one I used was from a 97 KX250. I found the DGN needle not suitable for that particular carb. It did come with the proprietary Kaw triple taper needle, and I have yet to go back to testing it again. I have found that because of my riding style, I can't jet my PWKs as lean as a lot of these guys do here. My Gorr ported 98 500 with the 39.5 prefers a 48 pilot and the DGN on the 4th. I still only get about 1-3/4 turn range on the airscrew, and if the temps are below 50f, I gotta go to the 50. But ya gotta remember, my trails require lots of part throttle op's, so a little fat is better than a little lean, or you risk making an ashtray that couldn't be used in a pinch, if need be.
If I remember correctly, the aftermarket 38AS comes jetted with 55/170 and the DDJ, so if thats the carb you have, I'd start there. Like I stated, the KX carb has a triple taper needle, the main taper being 1'34, the diameter is 2.705( I had to mic the bitch as I couldn't find too much along the lines of numbers), and I read that if changin from that type of needle to the 48N style, you need to go up 3 sizes on the main. That would have meant I needed to go to a 178 to try the DGN. Seems strange that the 2 carbs would have a different size orfice, but there must be a difference in the specs between the OEM and the aftermarket versions.

I was running the AS with a 50 pilot, 170 main when i tried the DGN on the 4th clip and this is exactly what happened...idle was fine, but once we got into the throttle settings where the needle starts to take over, flat on its ass and dying from starvation. Up top, she was good, too. I pulled it out, put a stock CGJ in, ping city unless I was running 98 octane fuel or better. I've got the stock Kaw needle back in it and ran it on the 5th clip, rich as all get out, so I think the next time I try this carb I'll be running a 55 pilot, 175 main and the KX needle on the 4th and go from there. Dont need to make another ashtray.

Good luck, and hope you can get it sorted out

lewisclan · Post by **lewisclan** » October 25th, 2011, 6:34 am

Tharrell wrote:Somebody needs to dig up that vid of Brit starting his bike.
I know this is a jetting question but, I liked that vid.

is this the bike in question?

dogger315 · Post by **dogger315** » October 25th, 2011, 7:59 am

the aftermarket 38AS comes jetted with 55/170 and the DDJ, so if thats the carb you have, I'd start there.

This is the OE carb from a 2000 CR250. It comes pre-jetted with a 180
main, 48 pilot and a Honda "boutique" needle. The same carb also came
on the 99 CR250 with a 185 main, 42 pilot and a 1370D needle. Neither
of the carbs were anywhere close to decent jetting for a 250.

I tried a 172 main, 48 pilot and the DGN with a 3rd and 4th clip and it
was still way lean. Now, I have a 175 main, 52 pilot, 1368 needle/3rd
clip. I'm going to try these new settings today and see if I'm moving in
the right direction. I also rechecked the plug gap, it was at .018. I
reset it to .022

Thanks for the info.

dogger

dogger315 · Post by **dogger315** » October 25th, 2011, 3:23 pm

The new jetting is good to go! Bike starts easily and pulls clean and strong
through every gear. Did some plug chops and got a nice light tan color.
Good response to airscrew changes so I'm in the middle of the "ballpark".

The new carb is also an improvement over the 39.5. Throttle response
is improved and the bike is super strong in the low to mid range with no
appreciable loss on top, which is exactly what I was looking for.

My original jetting was way to lean on both carbs. It's good that I had
to run with the choke on or I would probably have a hole in the piston
by now. I'll probably do a tear down a little sooner than normal to be
on the safe side.

I also spent some time modifying a Works Connection skid plate to fit.
I had to cut notches out of both sides and trim some material off the
back to clear the modified cradle. I also need to cut the "hooks" off of
the over-the-frame mounts. There isn't enough room between the
engine and frame for them, so I will need to weld the modified mounts
to the frame instead.

dogger

Gmbond · Post by **Gmbond** » October 25th, 2011, 4:35 pm

So what jetting specs did you land on??

I'm not in search of top max power as I'm a single track and tighter trail rider/racer. I want smooth bottom thru mid with decent top end for when needed. I know the 41 and 41.5 aren't for me.

lewisclan · Post by **lewisclan** » October 26th, 2011, 7:12 am

Ive got at least 2 bikes with the AS 38 diled in I never had a big problem getting one on, and I like the AS for trail riding . if you dont get it figured out by the weekend I can open 1 up and see what I put in there and let you know

dogger315 · Post by **dogger315** » October 26th, 2011, 9:58 am

So what jetting specs did you land on??

!75 main, 52 pilot, 1368N needle/3rd clip, 1 1/2 airscrew,
5.0 slide. VP C12, Honda HP2 32:1

WX conditions are very close to standard.

Always interested in what works for others, Lewisclan - Next time
you have it apart.

dogger

lewisclan · Post by **lewisclan** » October 27th, 2011, 6:50 am

will do

AMA714 · Post by **AMA714** » November 2nd, 2011, 7:20 pm

I have been trying to get the plug color off of coffee black since I put my AF together. I think the 06-09 250F airbox is too small for the big lung on the 500. I always use C-12 and 927 at 32:1

I started with a PJ with 170 main 48 pilot 2 turns out on air screw. 6.0 slide 1369 needle 2nd from top. Bike started and idled great, was a little chuggy down low, mid not bad, fat up top. Not great, but got around a track fine. It was just kinda fat. Never fouled a plug, but never really pulled that clean either.

Decided to get a PWK (39 Air Striker off a KTM) Thought I would set it up with the base that I had on PJ since it was close. Was a bitch to start, no idle at all, and still fat and burbly everywhere. dropped to a 42 pilot, airscrew was past 3 to make a difference and not much of a change.
Ended at a 38 pilot 2 out, 1369 top clip and a 160 main... started coming alive, was easier to start, idled fine on a 70F morning and pulled real nice and clean. By noon it was 85 and bike started to be fat again... Gotta love TX. Im afraid to go any leaner... but the plug color tells me I need too.

Anyway, through it all, I have learned that the 500 is tough, and even jetted shitty its fast. I also think that the CRF airbox is limiting. I ported it with 6 new holes, and that made a big difference. I wish I had another 500 AF here in San Antonio to try and compare.

2strokeforever · Post by **2strokeforever** » November 2nd, 2011, 7:35 pm

i guarantee you theres at least 3 better needles for your bike

iggys-amsoil · Post by **iggys-amsoil** » November 3rd, 2011, 7:29 am

X2

I've noticed over the years of reading these threads that no matter if a PJ PWK and what ever porting is done they like the 14.. or a D.. needle. which is for the 3/4 throttle opening.

However mine and a couple of J's bikes do need the E.. ticket needle.

2strokeforever · Post by **2strokeforever** » November 3rd, 2011, 11:27 am

mines happy with the CJL right now

dogger315 · Post by **dogger315** » November 4th, 2011, 10:48 am

I also think that the CRF airbox is limiting

Back in 1997, Team Honda started "venting" the airboxes on the CR250s,
the same year they switched to the Aluminum frames that reduced the
available space for an airbox.

My 500 uses the flange, filter and airboot from a 2001 CR250 and the
airbox is about the same size as a gen 3 CR250 except, by leaving the
heat guard off, the bottom is opened up to allow more airflow with a
baffle in place to prevent too much crap from getting to the filter.
Switching to the 38mm carb, and using what is essentially an opened
up CR250 intake and filter system (which was/is very similar to the OE
system on the CR500), I don't think the airbox size is a problem for my
application.

I'm real happy with the jetting I landed on. That's not to say I won't
be trying different combinations to make it even better. It's been a
bunch of years since I last raced 500s, and I'm using different carbs,
reeds and ignitions now. I'll need a little time to get a handle on
everything. For the jetting, I'm glad I've amassed a couple of pounds
of Keihin brass over the years - looks like it will come in handy now.

dogger

britincali · Post by **britincali** » November 4th, 2011, 12:45 pm

Bobs AF had massive airbox restriction when on its maiden voyage, so bad it would bog bigtime at WOT. 10 mins with a knife and vice grips fixed it

100hp honda · Post by **100hp honda** » November 4th, 2011, 8:13 pm

britincali wrote: 10 mins with a knife and vice grips fixed it

sweet. got any before/after pics ?

BannedCR500Riders.com

Keihin 38 PWK AS

Keihin 38 PWK AS