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#811694 06/06/20 7:18 pm
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cant get the bike to smoothly transition from past 1/4 throttle.
Running 30 pilot at 1 1/4 turns out
idles strong and from idle to 1/3 throttle is smooth
needle position in fourth clip from bottom. if i slowly let on throttle it makes power but slightly stumbles especially at 1/2 throttle
but if i give a handful of throttle it stumbles like crazy

plugs are sooty!

if i lean the needle one position the bike tends to run lean especially off idle and will flat line and then pick up rpm with a handful of throttle

if i richen the position to the 3rd the stumble gets worse after 1/4 throttle and lacks power.

I cant find a happy medium and doing everything one step at a time
it seems the idle mixture and pilot are correct

the main jet is a 130, idk if the main is too rich or not and effecting the needle position.
also tried a leaner needle jet and no noticable change

These jrc pwk carbs are starting to wear on me

Any suggestion and does anyone know what needle size this is (JJK? idk)

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Apart from asking why you fitted the PWK in the first place, there is a sequence to tuning a carb. The first one is the main jet at Wide open throttle (preferably about 3rd gear up an incline). Following this needle jet and needle position with the pwk I believe there are different needles too.
Then slide number, then idle mixture although you may need to do this periodically just to keep the bike running.

Setting a carb up from scratch is a chore and certain components like slides begin to become expensive.


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what kind of motorcycle is it on?

it sounds like its rich in the midrange.

will it run steadily at wide open throttle or not?


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tommy #811720 06/06/20 11:22 pm
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I had a similar problem with a PWK 28 on my B44, which I eventually fixed by fitting a thicker needle (JJN if I remember correctly) and also my modifying the slide to have a bigger cutaway (I used a sanding drum in a dremel to remove approx 1mm). Slides are expensive so I chose this approach instead of buying a new one which seemed to work.

My problem seemed to be caused by too much fuel bleeding past the needle jet straight section needle jet holder, so a thicker needle jet helped. This area comes in to play at around initial starting to 1/4 throttle. Additionally I felt that the slide cutaway was insufficient and needed to be enlarged to allow more air in.

I think your pilot jet 30 is OK but maybe increase the main to 160, note that the main jet doesn't have much effect unless your are over 3/4 throttle or so. I would put the needle jet in the middle slot.

Last edited by gunner; 06/06/20 11:23 pm.

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tommy #811748 06/07/20 11:16 am
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As with any slide carb up to about 1/4 throttle is controlled by the difference in diameter between the parallel section of the needle and needle jet.
Above 1/4 throttle the taper comes into effect, the bigger the angle the more fuel for a given throttle opening.
Needles with a longer parallel section transition to the taper at a greater throttle opening.
Moving the clip towards the top of the needle has the same effect as a longer parallel section
Get the main jet correct first, once it will hold WOT under load and give a nice light brown plug colour, then look at other settings
Richness or weakness below 1/4 throttle is mainly controlled by the needle jet size, don't try to compromise by adjusting idle settings, get this right
If it starts stumbling at 1/3 throttle and gets worse the further the throttle is opened with sooty plugs, then get a needle with a smaller angle of taper (thicker at the tip)
If it starts 8 stroking and backfiring at part throttle with white plugs, get a needle with with a larger angle of taper (thinner at the tip)
Once the needle jet and needle are correct adjust the pilot air screw to get a smooth even idle, if the screw is not between 1 and 2 1/2 turns out then a different size pilot jet is required. Less than 1 turn a bigger pilot jet, more than 2 1/2 turns a smaller pilot jet.
Finally the slide cut away controls the mixture up to about 1/8 throttle, bigger cutaway is leaner


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kevin #811798 06/07/20 11:55 pm
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at wide open throttle it stalls then pick up steam, which to me is a lean setting, i bumped the main jet till it pulled decent at wot throttle.

but back to the mid range i continue getting a rich mixture and what confuses me is that when the needle is at its leanest setting the stutters doesnt appear now till 1/2 throttle but my idle to 1/4 range is flat and seems lean.

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By moving the clip in the needle you are just moving the point where the taper starts to take effect. To make the mixture leaner in the range from 1/4 to 3/4 throttle you need a needle with less angle / shallower taper / larger diameter at the point. I believe these carbs use Keihin jets and needles


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Originally Posted by Andy Higham
As with any slide carb up to about 1/4 throttle is controlled by the difference in diameter between the parallel section of the needle and needle jet.
Above 1/4 throttle the taper comes into effect, the bigger the angle the more fuel for a given throttle opening.
Needles with a longer parallel section transition to the taper at a greater throttle opening.
Moving the clip towards the top of the needle has the same effect as a longer parallel section
Get the main jet correct first, once it will hold WOT under load and give a nice light brown plug colour, then look at other settings
Richness or weakness below 1/4 throttle is mainly controlled by the needle jet size, don't try to compromise by adjusting idle settings, get this right
If it starts stumbling at 1/3 throttle and gets worse the further the throttle is opened with sooty plugs, then get a needle with a smaller angle of taper (thicker at the tip)
If it starts 8 stroking and backfiring at part throttle with white plugs, get a needle with with a larger angle of taper (thinner at the tip)
Once the needle jet and needle are correct adjust the pilot air screw to get a smooth even idle, if the screw is not between 1 and 2 1/2 turns out then a different size pilot jet is required. Less than 1 turn a bigger pilot jet, more than 2 1/2 turns a smaller pilot jet.
Finally the slide cut away controls the mixture up to about 1/8 throttle, bigger cutaway is leaner
I have no experience with the PWK, but a lot of experience with flat slide TM Mikunis....On the Mikuni , changing the needle position or needle type usually has a noticable effect on wide open main jet performance...might be factor on the PWK....or not...


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tommy #811853 06/08/20 12:52 pm
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Originally Posted by tommy
at wide open throttle it stalls then pick up steam, which to me is a lean setting, i bumped the main jet till it pulled decent at wot throttle.

but back to the mid range i continue getting a rich mixture and what confuses me is that when the needle is at its leanest setting the stutters doesnt appear now till 1/2 throttle but my idle to 1/4 range is flat and seems lean.

tommy, if your motorcycle is a triumph 650, and if it has two carburetors, then JRC recommends starting with a 137 main jet with 35 pilots. if it is a 650 with one carburetor then JRC recommends 135 and 35. those are close enough to be the same. you say you bumped up the main until it ran well at open throttle. what jets did this carburetor come with? where did it come from? is it new?

if its some other type of machine they recommend starting with other jetting.

if your machine runs well at open throttle, your main jet is probably close enough for the moment, and if your pilot is 30, then it is probably not too rich.

i suggest that you put in a 35 pilot jet and see whether that cures the off-idle stumble. you may be using a too-rich needle position to compensate for a lean pilot and then suffering from a rich needle setting after the pilot fades away.

if you richen the pilot and reset the idle mixture, then leaning the needle position again may clean up the midrange. use the original needle jet to start.

some of this may be helpful:

https://cdn.shopify.com/s/files/1/0...or-tuning-guide-pwk-26-30-32-34.pdf?6408

https://cdn.shopify.com/s/files/1/0...atside-carbs-installation-guide.pdf?6408


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tommy #811930 06/08/20 10:57 pm
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ive tried the 35 pilot and it would idle highest at 3 1/2 turns out, with a 135 main jet. Needle in the 4th position and same symtoms at the mid range. leaning out the needle only made it worse before 1/4 throttle

i did some measuring of the stock needle and it seems to be an LFP, using a micrometer and taper and angle calculator.

I have a jjm needle which i can try

Also i took the plugs out and i noticed the right cylinder is richer than the left

Last edited by tommy; 06/09/20 1:10 am.
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the 3.5 turns out with a 35 pilot appears to indicate that the 30 pilot is the correct diameter. most carburetors of this type arrange the correct idle mixture to arrive at 1.5 to 2.5 turns out. if you need 3.5 turns out to get enough air with a 35, then 30 is probably correct.

leaning the needle should have no effect before 1/4 throttle. that is strictly pilot jet and throttle cutaway, at least on round slide carbs.

i can't help you with needles because i don't know anything about keihins except the FCRs. mikunis are cheap and easy, and the recommendations are well-known.

i would suggest flogging the PWK carbs on eBay, and replacing them with 30mm mikunis that have accessible literature on tuning. the TM flat slide mikunis are better than the VM round slide instruments, by all accounts. personally, i use VMs because i have six pounds of jets, needles, and slides, and the exprience to use with them. but the flat slides are supposed to be better.

i have ridden hillbillys T140 with TMs and it was awesome.

Last edited by kevin; 06/09/20 2:44 am.

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Hello Tommy,

Sorry to hear your having issues tuning our carbs, we have all our tuning literature listed on our website
PWK Tuning PDF

PWK Info

If you have any further questions please contact me [email protected] or (951) 940-5411


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Check your float height. It’s specified as 19mm, mine were 25 and 23, fixing it cleaned up my transition when opening the throttle. You don’t mention what motorcycle and any modifications (stock pistons and cams, stock air cleaners, stock mufflers or?) since those affect things.

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I agree with Kevin on this problem which echoes my experience with tuning the PWK. I tried using a 35 pilot jet which simply resulted in turning the pilot air jet out by 3 1/2 turns instead of the expected 1 1/2 turns to get the idle mix right.

We need to know what bike this is on but hopefully, the diagram below will help.

As can be seen from the diagram below idle mix screw has a most effect at up to 1/8 throttle. This is then closely followed by the following:-
- air screw slow jet, diminishing to zero just below 1/2 throttle
- throttle valve cutaway increases in effectiveness from about 16th throttle the diminishes from before 1/4 throttle
- straight diameter works from idle, increasing to1/8 and diminishes after 1/4 throttle to 1/2 throttle
- clip position starts working from 1/8 throttle up to around 1/4 then diminishes until 3/4 throttle
- the main jet barely starts working until 1/2 throttle and only really comes into play near WOT.

Try doing plug chops at 1/2, 3/4 and wide open and check the plug colours, then report back with some photos.

[Linked Image from pjmotorsports.com]


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tommy #812042 06/09/20 11:52 pm
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^^^this is a mikuni diagram.

[Linked Image from jba.bc.ca]


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Ive tuned carbs before and this one is leaving me a bit helpless lol. Perseverance or madness!

i checked float height and they where at 19mm.

I can recall the only time i was able to ride the bike thru out the rpm range was when i removed the filters, ran a 35 pilot with 135 main and it was at the fourth clip position i believe. It was still rich but rideable. Great feeling!

i will try again with recommended settings and methodically approach it once more.

Just wondering would polarity on the ignition coil for dual output coil effect anything? maybe i need to check that

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maybe you need to tell people what sort of bike youre trying to fix.


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Originally Posted by kevin
maybe you need to tell people what sort of bike youre trying to fix.
As well as any modifications to the intake, air filter, and exhaust system. Even the folks who made the carbs can only make recommendations for a known, stock engine and exhaust system.
Trust me, I tried doing the same thing with Mikunis with pretty much the same results.


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its a chopper running points .

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Originally Posted by tommy
I can recall the only time i was able to ride the bike thru out the rpm range was when i removed the filters, ran a 35 pilot with 135 main and it was at the fourth clip position i believe. It was still rich but rideable. Great feeling!

Filters shouldn’t have any noticeable effect through most of the range, BSA Used to quote a larger main jet if using a bell mouth but nothing more. So it’s possible your filters need looking at. What composition are they?

Paper - have they been subject to moisture? If so replace them.

Cotton dry type - have they been washed with spirits and dried?

Cotton oiled type - as above but have they oiled? Have they been given two much oil?

The likes of K&N filters always look nice and red when they are new (if pre oiled) but if you clean them and try and get them as red then they will be over oiled (done this a couple of times) you want just enough so they are pink but no more than that.


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sorry 68 t120, yes it is a chopper running points with uni filters pods and cocktail shaker mufflers with a cycle shack style baffle

rebuild motor, compression is 140psi cold in both cylinders

Timing is was dynamically checked and points look good

Last edited by tommy; 06/11/20 12:52 am.
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with 32 pilot 21/2 turns out, might go back to a 30 pilot,

a 138 main jet and needle clip in fourth position

no filters i can run it through the power band, tad bit rich but finally some results and a good reference point.

makes good power with no hesitation at wot

I installed a velocity stack with a mesh filter and the stumble is back from 1/4 to 1/2 range

changing the needle position did nothing to cure that stumble

i ordered some needles and will give it a try

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its been awhile since i posted and wanted to give an update. I tossed the pwk's and but AMAL 930 premiers. Bike runs better but i keep fouling plugs.
The bike only has about 300 miles on it so im sure it still needs to be broken in. Everything seems in check with the bike. Installed a pazon electronic ignition and went over timing and the valves.

Bike idles best with 1 1/8 turn out on both carbs so im thinking the pilots are right. I tend to cruise around with throttle position at about 1/8 to a 1/4. At 1/2 throttle and full throttle it pulls nicely. Got me thinking of maybe installing 3.5 slides since it seems to foul the plugs around that range.

Anybody run 3.5 slides on a dual carb head?

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Hi

Yes, 3.5 slides will be a good move.

John

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Just noticed that at the start of this thread you had sooty plugs also. With the AMAL’s the jetting should be fine as out of the box unless your running different than a stock setup bike.

Exhaust will change matters (the 1970 firebird for example with the same carbs and head needed a richer main jet and a leaner slide than the Lightning)

Using an Air filter or not will change matters, like the firebird jetting I had to use a 3.5 slide on my Lightning when I started using velocity stacks.

Then if you using different cams etc. If you have t done already, pop some new plugs in and go for a run. It might also be worth checking the fuel level in the bowl.


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68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (now rebuilt)
67’ D10 sportsman (undergoing restoration)
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