Author Topic: PWK carbs  (Read 375 times)

Online RDfella

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PWK carbs
« on: 01.08. 2020 14:57 »
Anyone have any experience with PWK carbs? I've come to the end of my tether trying to tune mikunis on my vee twin project. This project has been on and off for thirty years and I desperately want to get it finished. I spent around £400 on mikunis (including cable choke conversions etc) and I really don't want to spend another £500 on Amals (apart from the fact I may need to modify oil and petrol tanks to fit them). At least not until I know the bike's a good'un. Mikunis have a reputation of being next to impossible to tune (wish I'd known that earlier) and Dellorto  not much easier. So what's the experience with PWK? At least I could buy a pair for £50 so not much to loose money-wise. Just can't afford to waste any more time, as I doubt I'll still be riding in a couple of years. I need to get this carburetion sorted pdq.
'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.

Offline AdrianJ

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Re: PWK carbs
« Reply #1 on: 01.08. 2020 18:47 »
I’ve put one on my plunger flash. Couldn’t get it to start with the original Amal even though reconditioned. Started second kick after a rebuild with the PWK  Specifically these are JRC PWKs. They have a flange added to suit the A10 inlet manifold. Came with 2 pilot and 2 main jets. Starts 2nd kick every time. No air slide but a starting button does the trick.
My only problem is the bike is very reluctant to start when hot. I am worried about vapour lock as the chap who sold it me was absolutely insistent on the use of the heat insulating gasket between the manifold and carb. Any questions about it and I’ll be pleased to answer what I can. Bike has only done 34 slow miles so far so I have only played around with the pilot jet, which is easy enough.
Adrian
'53 Plunger Flash, Steib S500.


Offline Swarfcut

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Re: PWK carbs
« Reply #2 on: 01.08. 2020 19:12 »
RD The more I follow this post, any working carb would solve the short term conundrum of whether you have a runner. A good old fashioned constant vacuum SU from under your scraps pile and a knocked up manifold would do. Or are you more of a perfectionist?

All the best in your endeavours. You deserve a bit of success.

Swarfy.

Online RDfella

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Re: PWK carbs
« Reply #3 on: 01.08. 2020 20:09 »
Swarfy - thanks for the encouragement. It is a runner - done about an hour so far, but that's only five minutes at a time. As I've alluded to in previous posts, these carbs apparently have a habit of running OK but being a pig to start, or starting OK but won't run properly. And mine are no exception. Given the difficulty (virtual impossibility) I have starting it with the kickstart and having to rely on rollers I need better than that for a 'shakedown' before I can finish it off for a road test (whilst messing with the carbs I have to run on a temporary tank, as I can't get to the front carb when it's on). So many loose ends at present it's getting me down, and the carbs are the present hold up. Because of them, I'm no further advanced than I was over a month ago. Which is why I was wondering if I might actually get there quicker with different carbs, bearing in mind the mikunis are unlikely, given other's experience, to ever be right.
'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.

Offline chaterlea25

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Re: PWK carbs
« Reply #4 on: 01.08. 2020 22:50 »
Hi RD,
I have fitted several Mikuni's and sorting the running was not too difficult , maybe I have been lucky ?
My SR has one on it for going on 20 years now, it came with a couple of pilot and main jets to choose from, a 3mm washer can be used to get half positions on the needle adjustment
A C15T that came from Arizona with a Mikuni fitted ran really weak here in mid winter although the owner said it was OK when he rode it here in the Summer, I cannot remember precisely what I changed now but got it sorted
I put another on a Sportster for my sins  *roll*
and some others too
Some time ago there was a long thread on Britbike about getting a Goldie to run correctly on a Mikuni
Consensus was that there were a few different setups that would combine to work equally well ????

Flash Greubon on here has one on his V twin Condor and is really happy with it

John
1961 Super Rocket
1963 RGS (ongoing)

Online RDfella

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Re: PWK carbs
« Reply #5 on: 02.08. 2020 13:25 »
More than luck, I'd say John.
According to online comments from people who have bought them, Mikunis are notoriously difficult to set up. Even those out of the box and supposedly jetted to suit a particular engine are wildly out. The most common complaint (as I'm getting) is that if set up to start well they won't run (or vice-versa). As the pilot jet affects the needle jet and even the main, where do you start? There's 27 different needle jets to choose from and at least 20 needles. Just to help complicate matters, the needles come in single and twin tapers and are measured at 6 points along their length. So one needle compared with the next on the list could be richer at one of those six points, but the same or leaner at the other five. As stated in an earlier post, the only way I can see one could get these carbs right would be to examine exhaust gases at throttle openings of 1/6th intervals. And then remember changing needles or needle jets will upset the pilot system and vice versa. A veritable nightmare. Because my engine is so hard to kick over I can't accept an engine that is not bad throughout the throttle range but needs 20 kicks to get it going. And starting 1st or 2nd kick but won't run anywhere without choke ain't much better.
Oh for the simplicity of an Amal. If I recall correctly they have two needles - petrol or alcohol (the length dependant on carb bore) whilst SU's had 3, lean, std and rich. Only ever had one problem with SU's - a new MG Midget with a huge flat spot (twin SU's). Everything checked out OK, so I ran it with a pencil in the dashpot to determine whereabouts on the needle it was lean, and filed the needle until the flat spot disappeared. Now let's not mention Strombergs ..........


Addendum:
Started her up again this afternoon to try new pilot jets and slide cutaways relieved from 2 to just under 3.

Short history:
Carbs were supplied with 15 pilots. Used to start pretty quickly with choke, but wouldn't run any throttle opening without it.
Then tried 30 pilots. Wouldn't start on choke, responded to throttle OK but a bit rich mid-range.
Today tried 25 pilots and No 3 cutaways. Took an age to get going on the rollers, but ticks OK over and the slow-running screws now have an impact. Sadly, not so good elsewhere. Opens out instantly, so presumably cutaways are about right, but starts farting and banging before mid throttle and won't rev any further.
Conclusion - looks like pilots are now right, but the fact they have such an extensive effect at half and full throttle is astounding. Those on-line comments were spot-on. Guess if I now alter the needle / needle jet (which one to change and what to ???) then the pilots will be wrong again. Arrrrggggggh.

'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.

Offline chaterlea25

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Re: PWK carbs
« Reply #6 on: 02.08. 2020 20:54 »
Hi RD,
Although there are a big number of needles and needle jets listed , I believe only a handful are available?
The prices of Mikuni carbs and parts in the UK is nothing short of scandalous !! Check the difference on Sudco's website, of course they won't send to Europe or UK  *razz*
I got the 36mm VM for my GS project through a friend in USA who then brought it over when he was coming on holiday, huge saving on the UK price
(I found the same issue getting parts for an American made servo drive on my milling machine, I eventually found an independent parts dealer in US who was happy to ship)

For what its worth, here is a listing of the settings of the 30mm VM fitted to my Super Rocket

Slide,           2.5
main jet      180
pilot jet       25
needle       6DH3
needle jet .159 P2

I bought another 30mm with a Norton twin manifold at an autojumble and it has these settings

Slide,             2.5
main jet        210
pilot jet           30
needle        6DH3
needle jet .159 P2

The settings are very similar so the slide /needle / needle jet combinations must be a good combination ??

John

1961 Super Rocket
1963 RGS (ongoing)

Online RDfella

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Re: PWK carbs
« Reply #7 on: 02.08. 2020 21:12 »
Hi John,

At present I'm running (on VM34)
Slide           2.7
Main           230
Pilot            25
Needle        6F9 (clip in lowest groove)
Needle jet   P6

I'm currently ordering a pair of P8 needle jets. As you say, prices are eye-watering.
'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.

Online berger

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Re: PWK carbs
« Reply #8 on: 03.08. 2020 19:59 »
I think the mikuni I have is a vm with 28 mm bore 0.5 air jet ----20 pilot jet  was 25---round slide 2.0----main jet 160  --and needle 5f21. I also bought a 50 and a 47.5 pilot because I was reading the 25 upside down *bash* *bash*  *problem* *problem*

Offline chaterlea25

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Re: PWK carbs
« Reply #9 on: 03.08. 2020 20:13 »
Hi Berger
No wonder you had trouble?  *conf2* *conf2*
Did you buy the carb already (supposedly) jetted?

John
1961 Super Rocket
1963 RGS (ongoing)

Online berger

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Re: PWK carbs
« Reply #10 on: 03.08. 2020 21:18 »
hello john with me not knowing anything about much anyway especially mikunis I did buy it as advertised jetted for a 500 shooting star. so to anyone in the know who knows mikunis like yourself what jets and needle do you think would make it work sort of better, it would not start just on choke nor would it without, it had to be choke and throttle then choke off straight away. once warm it would try to start with no throttle or choke but as soon as I opened the throttle it would stop. so even warmed up it was a case of choke and a handful again. once running it wasn't bad until banked over *dunno*

Online RDfella

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Re: PWK carbs
« Reply #11 on: 03.08. 2020 21:38 »
You're not alone, Keith. Started the project up again this afternoon, this time with the jet needles right up. Starts better with choke and ticks over, but just bangs and farts after just under 1/2 throttle. Won't rev past around 2,000. Tried applying choke, but made no difference apart from running slightly rougher. Am next going to try with the needle clips in the 2nd groove down. If no change I'll have to sling the carbs as I'm not prepared to spend a fortune buying jets only to end up throwing the carbs away a few months later anyway. Having already spent £400 plus on these carbs I'm loathe to double up on that by getting Amals - which will likely require surgery to oil and petrol tanks (meaning refinishing them as well) to get them to fit. Hence my request for comments on PWK carbs.
'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.

Online muskrat

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Re: PWK carbs
« Reply #12 on: 04.08. 2020 22:08 »
G'day RD.
I have a pair of PWK 30's on my Cafe. I had almost as much fun tuning them as a Mikuni. 2 slides, 4 or 5 needles, a few main jets and pilots. Still not perfect but close enough. 1 to 2 kick starter, idle is hit and miss and fuel height is difficult if their used with any degree of downdraft (same as concentrics).
Lost all my data in the fire, I can't remember what my final setting are.
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7

Online Billybream

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Re: PWK carbs
« Reply #13 on: 05.08. 2020 05:11 »
Now my second year using PWK 30 carb, supplied by SRM with correct mounting flange for £50.00
after initial setting and some jet changes, this carb is a joy, no tickling, just fuel on, enricher on and she starts first kick, cold or hot, runs great, so very pleased. Jets and spares are readily available and inexpensive, existing cable fits, just some replumbing to fuel lines and addition of fuel filter and air filter. Can only comment regarding summer use, my bike is garaged in the winter.
Did recent mpg check, and getting 60 mpg on rural run at modest speeds, so pleased alround.
Loads of good information about PWK carbs, I did alot of research before buying.
1960 Super Rocket, owned since 1966, back on the road 2012 after being laid up for 29yrs.

Online RDfella

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Re: PWK carbs
« Reply #14 on: 06.08. 2020 20:34 »
Been looking into this further. Seems PWK were fairly good a few years ago, but today's Chinese ones are rubbish - sometimes salvageable with real keihin jets. I have analysed my Mikuni issue, and figured changing the pilot has caused the needle and main to go ultra-rich. Trouble is, there's hundreds of needle / jet combinations (just for the model / size I have) and at nearly £30 for a pair of jets (including P&P) how long do you go on trying jets? I see the guy quoted below made 200 changes before giving up and going to Amal. His quote is regarding the fact that not only are there something like 40 needle jets available - but they come in two types - Bleed or Primary. Mine are primary.

"Ah, the dreaded Mikuni round slide primary versus bleeder N/J quandary. First, a little history. Few know that the Mik is a rejected Amal design. When Amal was looking to replace the Monobloc, the Mik was one of the candidates. Concentric MK1 won. However, when a N/J mixes the Air Correction air AFTER the fuel is metered - Primary N/J - it will not tilt the fuel/air curve enough for a 4 stroke. It's a good 2S carb, so Amal licensed it to the Japanese - the early YG1 Yamaha carbs still had the Amal logo on them. This lack of F/A tilt means that to properly jet for top end, the mid range - at full throttle - is always too rich, costing you about 15% horsepower, according to Mik. The Bleeder N/J is Mik's attempt to correct this deficiency. The A/C jet becomes VERY critical. My experience has been that the Needle, N/J, M/J and A/C jet become much more of a combo, making jetting from scratch very difficult. To a certain extent, change one, and it messes up the other 3. While experimenting, admittedly under the duress of racing, I made over 200 brass changes and never got it right. It was still better than a Primary N/J, and I won races, but I finally gave up, and bolted on Amal MKII Concentrics, and won the first race, even though it was way too rich everywhere, primarily due to the as-delivered spray bar height adjustment. Throttle response was outstanding, and I run Amals to this day, in spite of their headaches."

So it looks like I'll be heading to Amal before I waste any more time and money . Beware of Mikunis!

 
'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.