PWK Carb Adjustment on a 1000SP

[monkeyodeath]

Just got a '79 1000SP in nice shape. Wasn't running at first but managed to get her going nicely with a little carb cleanout/float adjustment. Was hoping to get some advice from the sage folks on this board.

The PO of this bike fitted it with Keihin PWK carbs after experiencing some issues with the Dell'ortos and wanting to reduce throttle pull effort.

I think these are clone carbs but they seem to run fine and I didn't see anything that seemed too bad upon disassembly to clean them.

My question is -- what's the best way to balance these things? I've got a carbmate and am able to get the idle nice and stable using the directions here: https://www.thisoldtractor.com/moto_guzzi_loopframe_carburetor_-_setting_them_up.html and here: https://www.thisoldtractor.com/moto_guzzi_loopframe_carburetor_balancing_synchronizing.html, but:

- The idle mixture screw, especially on the left cylinder, seems to do almost nothing.
- As soon as I rev a little off idle, the right side shows much higher vacuum.

I realize that the throttle cable setting might be the reason for the cylinder imbalance on throttle, but I also feel like I'm just shooting in the dark as far as adjusting these idle mixture screws.

Just curious if anyone else has pointers or experience running these carbs on these engines. The PO seemed to find a jetting that he liked so I'm hoping I don't have to mess with that, but I'm trying to figure out how to best get the cylinders balanced, both in the speed and mixture screws.

[guzzisteve]

Idle mixture I use a Colortune made by Gunson. You see the spark flame in a glass bottom spark plug. Spot on stuff. Also works on FI, push buttons and see it go from blue to orange. You get actual not after the fact w/exhaust sniffer.

https://www.gunsonshop.com/contents/en-us/p209_G4074-G4171-G4172-Gunson-Colortune-See-Through-Spark-Plug-Kit.html

Amazon has it for $150

[nc43bsa]

If adjusting the idle mixture screws has next to no effect, chances are the idle circuit is plugged, either in the carb body or the idle jet.

[monkeyodeath]

Well, turns out something was going on...

Was riding it today on the freeway and starting to feel a bit dismayed at the lack of power (this is my first time even riding a Guzzi, so was not sure what to expect)...

All of a sudden, the bike stumbled a bit, backfired, and rocketed forward with what felt like twice as much power.

Not sure what was happening before, and what cleared to all of a sudden give the bike some life. Stuck ring? Clog in the carbs I didn't get (I used an ultrasonic cleaner!)? Fuel supply issue? Either way something cleared and it feels much more like a proper large-displacement bike.

Tomorrow I'll re-adjust everything, now that the idle has gone from 1100, where I set it initially, to 3000 RPM.

[Frenchfrog]

possibly a blockage clearing

[Ratso88]

The PWK is a 2 stroke carb.
It can be made to work on a 4 stroke but will never be optimal.
It’s just not designed for it.

The FCR would be a much better choice.

It is what you have and if you’re satisfied that’s what’s important.

[Antietam Classic Cycle]

The PWK is a 2 stroke carb.
It can be made to work on a 4 stroke but will never be optimal.
It’s just not designed for it.

The FCR would be a much better choice.

It is what you have and if you’re satisfied that’s what’s important.

Steadfast Cycles sells a kit with PWK carbs to replace the Amals on Triumphs and Nortons:
https://www.ebay.com/itm/204487499347

The late Mike Tiberio was among the first to adapt these to a Guzzi using manifolds from an Amal equipped Eldorado. Mike being a racer was a stickler for proper jetting and running. His setups worked perfectly well - better power than the VHB, and equal in fuel economy. Sounds like they were "optimal" to me. I have  the same setup to use on an upcoming project.

Here's Mike's write-up on the conversion:

I have been posting about this on the loop list, but I should really spread the
news. I have been looking for a replacement for the old square slide VHBs for a
while. I found a chinese made Keihin carb knockoff modified to replace british
Amals. Mating a pair of those with a set of the Guzzi Amal manifolds (yes small
valve roundfins came with Amals for a while in 1974 I believe) is possible and
practical. Other than buying the manifolds, the carbs, and a pair of off the
shelf throttle cables, the whole conversion required no machine work or other
special fitting or cuttin (other than fuel line)...

I have never installed a totally different carbs on a bike and had them work as
perfectly as these carbs did tonight on my Eldovert. It all jelled when I got
some throttle cables in from MG tonight. I asked the guys at MG to measure the
freeplay of their throttle cables, and asked them to send me a pair of whichever
ones had the most freeplay. It ended up being their "no name throttle to VHB"
cables. I knew the Keihins ate up more cable than the VHBs so I
thought I'd start with these cables. As I was doing some measuring tonight
trying to figure out how much I was going to modify the cables, I realized they
would work with my Thom 2C throttle and the Keihins as delivered. Then I
realized I had everything I needed, and by 8:30 I had them installed. I went to
start the bike and it didn't even cough. Oops, I left the spark plug wires off
(I removed them to get them out of the way). Once I had spark, choke, the bike
started right up and settled into a decent idle. I rode off down the street, and
by the time I went 3 blocks, it was idling high, oops I left the chokes on.
Chokes off and it idled perfectly, and ran great in my residential neighborhood.
Quick to get home, finish buttoning up the tank bolts and put the saddle on, get
my jeans on and head out for a top speed run. Bike pulled cleanly to 100, and I
ran out of road. I'll have to watch the gas mileage, and do some plug chops to
get these carbs any better set up they are that close. I uploaded some pix so
you can see the sano install with the Guzzi Amal manifolds...



They have carb mounted chokes, but a cable kit is available, I might spring for
that. They take 1/4" fuel line, but since I was set up for filters that take
1/4" line, that issue was no sweat. I was using dual spigot banjos on my VHBs
for a crossover, I'll either have to "T" each line to do the same or use both
petcocks. I do have a set of Tonti petcock adapters, so that will give me a
reserve back if I don't go for a crossover. The inlets are about 2" dia, so I'll
probably pick up a pair of unifilter foam socks to handle the air cleaning
chores. There are no clearance issues between the float bowls and the bell
housing. In other words, a bolt on perfect fit as long as you have the Amal
manifolds... Oh yea did I mention the throttle pull is oh so much lighter...

More later...

The carbs are made by a Brit Bike specialist called JRC Engineering
(http://www.jrceng.com/). They are "Keihin PWK" oval bore (28/30) semi flat
slide carbs. They are big with the moto cross crowd. JRC doesn't do retail, and
has dealers. I used Specialty Spares 253/853-1420. They were $100 a pop plus $15
shipping. The manifolds were made by Moto Guzzi to allow mounting of Amal carbs
to Eldos back in the early to mid 70's (either Dell'Orto couldn't supply enough
carbs to Guzzi, or Guzzi didn't pay their bill!?!). No one ever liked the amals
on guzzis, so the manifolds are easy to find used. While a lighter throttle pull
is one reason to convert, I was hoping for more precise metering and
adjustability, together with a sense of lets do this because we can...


Dec. 5th, 2011

Still on the bike. All I did to jetting is go one smaller on idle to 35, and lean out the ims to 2.25 turns off seated. 37 to 39 mpg with my convert.

still occasionally pops, but then so did the vhb's. Bike does have 155000 miles, so I don't expect perfection. Throttle spring is soooo much lighter. Like FI. I recco this mod to anyone that has 29 or 30 mm vhb carbs...

[monkeyodeath]

Having better luck with the carbs, but I'm noticing a "gurgling" sound coming from the left cylinder area when I shut the engine off. Does anyone know what this might be?

[Ratso88]

It’s interesting that someone has picked up on the PWK knock offs for use on a four stroke.
I guess they can be adapted and made to work and do the job, better than a 50 year old Dellorto.
It’s definitely worth a try. Especially at the price it’s being offered by Steadfast.

I do prefer the Keihin over the Mikuni.
I run the PWK Air Striker on my race bikes, all two stroke 250 and 500.
I’ve run the FCR on a DR650 build and it made a huge difference  over the stock CV.

I hope you can get your SP sorted out, seems promising.

[steven c]

 I have an Eldo with AMALS  but with a convert motor in it, I was thinking about trying these but it has been years since I read about them. Having said that I have had no issues with the Amals, bike has always run okay. I just thought maybe they would give a little more zing to it.

[Ratso88]

Having better luck with the carbs, but I'm noticing a "gurgling" sound coming from the left cylinder area when I shut the engine off. Does anyone know what this might be?

Could be fuel in the bowl boiling off. ???

Just a wild ass guess, really.

[someguy]

The PWK is a 2 stroke carb.

While this is true we used to run the "OEM" ones (not the clones) on our small bore 4 stroke race bikes with very very good success. But they were small 26mm IIRC.
IMHO getting the throat size right for the displacement is the key.

[SemperVee]

   Might just need a complete and thorough cleaning..

[monkeyodeath]

Just wanted to update this in case it helps anyone else...

As is so often the case, the answer was to clean the carbs AGAIN and then go ride the bike hard.

I think there might have been a stuck ring issue or some shmutz in the carb or fuel line, but it's cleared up now.

Managed to get the carbs pretty well-balanced with a Carbtune manometer.

This is my first Guzzi, so I can't really comment on these carbs and how they compare to the Dell'ortos...but they seem to run really nice all through the rev range. Throttle pull is very reasonable and choke is barely needed to get 'em going. A little popping on decel and I might try Mike's recommended idle jet size to see how that gets on, but otherwise, so far so good.

The PWKs do seem like a pretty common carb on all kinds of applications, and appear to be a drop-in for Amals, so fairly popular for British bikes. They are available from Keihin, but there are also tons of copies out there from various companies, in sizes that Keihin never even made. Fit on the 1000SP is pretty great, super-easy to remove (stock airbox was removed by PO, so just running pod filters). Mine are the 30mm size. Confusingly, I'm not sure if Keihin themselves actually made a 30mm PWK, but I've found that most of the parts are interchangeable between all of them if, like me, you're clinging to your old Sudco catalog to reference parts when trying to find them elsewhere (RIP Sudco!)

The carbs on my bike are almost certainly copies -- the Keihin logo area is visible but ground off -- but they seem to be of decent quality aside from a little rough casting in the float area around the overflow drain -- no big deal. The idle mixture screw on one of them was a bit loose, so I bought a genuine Keihin rebuild kit and the included mixture screw and spring worked much better.

Adjusting the idle mixture screw on the right-side carb when it's on the bike is a little tough for tuning while running, so I just set mine at Mike's recommended turns and since that seemed fine, I left it alone. You can buy idle mixture screws with attached knobs that would make it much easier to adjust them if you wanted.

Chokes are currently on each carb, which requires a little hand-switching to turn them both off, so I might someday consider a remote choke kit.

So, despite nominally being a 2-stroke carb, seems like the PWK works just fine on a 4-stroke. As far as the aforementioned FCRs... I have those carbs on a Ducati 900SS and they are pretty sweet, but I do wonder how much, if any, I'd gain by going with them, especially considering how much they are and that the 1000SP heads might not have much more to give without some serious reworking.

At first I was put off by the fact that these PWKs were knock-offs, as that's not usually a route I go, but as I said, castings looked alright and I can't see anything noticeably crappy about 'em so, it might not be as bad as it sounds. Probably better than some knackered ebay carbs.

At the very least you can still buy genuine Keihin seals, jets, floats, and float needles, so if the casting is good it's possible to upgrade everything else to quality parts.

Thanks again for the advice for this new Guzzi owner!