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Mark, what are you using to measure port flow?

Also what are those PWKs off of? Or are you buying them new? I see a handful of 80s/90s dirt bikes had PWKs on them.


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They are made under license in China. But the pairs have different internals. They are meant to fit 2 and 4strokes but the differences look like the one's Ben has a 4stroke. It basically runs straight up. And seems much closer to being correct. But we need to test it a bit and I'll let you know and see if I can identify which is which. How to get the best most suitable ones.

They are marketed by sellers who probably do not know much about them. There is a company that sells them in Oz, I'll ask them after we test a bit more. They are high quality and not expensive.


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From what I have read, the PWKs made in Taiwan are better than the mainland China versions. I do not know if it is a quality problem or some slight differences.

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I can answer a couple of questions. One is I use 4 vacuum cleaners hooked up to poly pipe to suck through some old cylinders bolted and sealed in my bench. I use a plastic box over the head and cyls with a seal on the bottom and two air flow meters in the top. They tell you how much air is drawn through the box. There is a manometer hooked up just below each cyl depending which side is being tested. The other side being blocked off. I have a plate with a 162cfm hole and measure the readings on the meters at 28"w, add them together and divide into 162. Whatever the numbers on the meters through the head I add together and multiply by that number. I can use one meter but it's really pulling rpm. There are a few sets of numbers based on the rpm of the meters, it doesn't matter which is used as long as it's the same one.

Without the box, I also run the vacs flat out and note how many inches the manometer pulls. I can turn vacs off individually and I also have a tap that adjusts test pressure I use 28"w. You can do it with less vac but You need 10"w.

With 28"w I have a graph for gas speed. With a second manometer with a probe to put in the port and I can watch how many inches that pulls and have an idea of speed.

I took Ben's 734 for a brief run late this afternoon. It idles at 14.7-1 I turned the idle screws a bit and ended up putting them back. It goes a bit rich driving off but didn't really watch after that, it pulls fantastic and you need to watch where you are going. There is no flat spot once moving. It's really lovely, just surges forward when you turn the throttle. As it revs more power comes in. I didn't have it on the mains but Ben said it's really good, better than before. It makes you laugh and you are thinking that's ridiculous. And all the time no vibration. Though I didn't use 7,000.

There is no hint of reversion with the big headers and system. It's not nothing and then top end, which is so annoying to ride on the road.

So at 62mph there is zero vibration in the bars, it's like the engine is turned off, same at 70. I did not notice any at all. The gearing is 20-38, so the C/r 1st is fairly tall as are the rest.

[Linked Image from i.postimg.cc]

Pwk 34 carbs. These are different to mine on the Firebird and have a different emulsion tube and a longer hex jet. Mine comes with extra short round jets with a slot. Hard to get exactly right. These type with the long hex jet work straight up on the 734. And seem they are already pretty right, or will dial in easy. They say they are for 2 strokes but I doubt it. I'll ask them.

https://www.ebay.com.au/itm/1536977...prpgw8r-8J8w6xMY4dWln_-KP8vd3Clj7zHlmunE

Last edited by Mark Parker; 04/26/22 9:17 am.

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I ordered two of these carbs from China and will see how they go on the Firebird. I think these are a 4 stroke version of it. $72.25 the pair with tax and shipping, so $36.13 each and Ben has a bucket of jets. They look the same as Ben's from the same place. They are sold as a pair.


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I see you bought the more expensive ones or did you just get them through Ebay? Mine have the hex main jet, about 8.8mm from the top of the hex to the bottom, marked K140. Manufactured by Oko Motorist Accessory, made in Taiwan. Power jet on these goes through.
I bypass Ebay when getting parts sourced from China whenever possible.

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OKO is said to be a good quality PWK type.


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Ebay, but Oz dollars are about 78c US. Ebay is the most reliable here. I've had bits other ways, but sometimes not frown I'll get photos of the jet set up. But I'm not pulling them off a running bike. May have to fit different mains on Ben's so will wait.

OKOs are on the Firebird I think, quality wise they look the same. Wish I had them in the 1970s.


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I was watching a youtube video from Eric Weingartner a few days back and he was talking about dimpling intake ports, and his tests show that dimpling everywhere had no gains for him, but dimpling where he would have filled in with epoxy had reasonable gains for him. Have you played with dimpling intake ports?

Last edited by MaaseyRacer; 05/28/22 2:26 am.

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I haven't tried that but maybe it caused local disturbance, and blocked that area a bit.


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G'day Mark
"Wish I had them in the 1970s.", have you ever heard of the UK made carb called 'Gardner' (made by Ron Gardner).
Ron was making his carbs back in the late sixties/early seventies. They are a flat slide (for air metering), taper needle with a flat (for fuel metering) device, needle is centre choke located and is actively fed by fuel.
The Lectron carbs from the US are a copy of Ron's carb (he told me that himself), Ron has used his carbs on modern formula 2 outfits here and obtained the same horsepower figures as the original motor's fuel injection (which is unsurprising due to the way fuel is presented at the needle on the carb).

Sorry if this is old news for you.
Cheers
Phil

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I've not actually seen them, but read about them. I ran Lectrons for a while, I bought some s/h for a $100 or so the floats deteriorated and they would not stop flooding in the end, probably should have bought some bits, as they are very good. Super expensive to buy now. The best economy I ever got on the BSA around 80-90mpg (Imperial G)because I could set up the lower end perfectly.

And probably because it was over advanced. An ignition with throttle position sensor could maybe have that advance for cruising and back it off when you wanted power. It would have to be backed off for kick starting as well.


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Originally Posted by Twin Pot Phil
G'day Mark
"Wish I had them in the 1970s.", have you ever heard of the UK made carb called 'Gardner' (made by Ron Gardner).
Ron was making his carbs back in the late sixties/early seventies. They are a flat slide (for air metering), taper needle with a flat (for fuel metering) device, needle is centre choke located and is actively fed by fuel.
The Lectron carbs from the US are a copy of Ron's carb (he told me that himself), Ron has used his carbs on modern formula 2 outfits here and obtained the same horsepower figures as the original motor's fuel injection (which is unsurprising due to the way fuel is presented at the needle on the carb).

Sorry if this is old news for you.
Cheers
Phil
Lectron UK has a slightly different story
https://www.lectron.co.uk/buying/buying-guides/the-history-of-lectron-carburettors/


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Think I'll drive to find a place, to be surly"
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Hillbilly,
Well they wouldn't admit to copying would they.
Ron's carb does pre-date the Lectron by some time, if you could see an early version of Ron's carb you can see how the Lectron came to be.

FWIW People have the same 'hassle' with Lectrons as they do with Gardners, because when setting up they 'think' AMAL instead of Gardner.
The Gardners are a great carb and fuel feed the engine with a much better mixture than most others.

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I would love a set of Lectrons, but they are just so expensive. Can get Chinese weber 40DOCE for around $300au. That would be nice to try on a T/bolt head. On an RGV framed thing, it has space.

I could get slow running easy on the Lectrons but I don't know about the rest. Ben has a fuel/air gauge on his thing which could make the process easy.

The pwks are cheap as are VM copies. pwk is smaller and looks unobtrusive, people have to know when they look at the twin pwks on the Firebirds T/bolt head. And after a while I get what are those carbs? Not like VMs you can see from half a mile.

Actually whatever the carb is, it's how good the head is that makes them shine.


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I see quite a few 2 stroke dirt bike riders like the Lectrons...I see stories from 4 stroke racers that they have off idle issues but make good power...Drag racers like them...Anything different or claiming impressive power gains like Lectron is going to be controversial...I do land speed racing and don't see many Lectron users...And my Mikuni TM carbed Triumph is as fast as any similar class Lectrons.. So...if Lectrons were 200 bucks each I would for sure try them as you will never know unless you try something..


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i was going to experiment with lectrons but put on keihin FCRs instead on the advice of leo goff

the FCRs work very well. the accellerator pumps are addictive.


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Gardner carbs have a seperate float (Ron did design a 'Concentric' version for Norton, but it never went into production as AMAL just massively reduced their price to keep the business!), Ron's floats do not froth or flood.
The beauty (and the downfall to the 'Amalisti') of the Gardner (and lectron) is the single jet, fuel metering is done by the taper on the needle, the flat on the back of the taper acts as the fuel siphon to the venturi (looks great and very informative when strobed), custom tuning is done by modifying the taper at the appropriate point.
The early versions suffered from a delay on an accelleration, but this was cured with the change to PTFE slide (slide sticktion). They present a very clean presence to the venturi.
Granted, they ain't cheap.

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So called "smoothbore carbs" can have stumbles at part opening due to turbulence under the hollow slide.
Flat slide carbs work well and are very responsive
The vertical flat slide carb on my race bike has no needle. The slide is about 2mm thick anodised alloy with a taper on one side, there is a pivoted arm with a roller on one end that rests on the taper, the other end moves a plunger which partially covers the spray tube.
When the throttle is opened the tapered slide moves the arm, opening more of the spray tube
The carb is made by Michael Blixt Racing and is available in 34mm and 38mm


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The Blixt carb looks interesting ....Does it have an idle function?


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Think I'll drive to find a place, to be surly"
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It has no pilot circuit or throttle stop but will idle if the cable is adjusted. It does have a reliable cold start.
I don't know how it would perform on petrol as it is designed for methanol. It uses Dellorto main jets


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Would like to try an E85 pump fuel engine..The ethanol content vares from about 60-85 percent but I have been told the alcohol is more tolerate of rich air fuel mixture so just jet accordingly...Most E85 users are modified supercharged electronic injection car engines but some are carbureted...


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Another advantage of high alcohol content is cooling, the fuel cools the engine from the inside


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Moving on to a big valve 34mm head. I'd been working on this before, and I was looking at it in light of other ports I'd done and it was pretty obvious the bottom turn was too sharp. I think what happens is the air leaves the wall and will not follow it. I cannot actually see this but I can see the flow increase. It's around 4cfm better.
So this port is 34mm going through a 44.5mm valve. I'll try to show it so people can see it better. Incase they are interested to improve one. I don't know which is better really. The 44.5mm valve or the 42mm. The big valve is around 10cfm better but it may not make much difference on a 744. Because the 42 is already about 60% better than std. This is 70-75% better, but it may not use it.
Anyway two photos down the port one with the valve. You can see the valve seat and the exhaust valve in the chamber beyond.
[Linked Image from i.postimg.cc]

The Exhaust valve is way too big, a std size exhaust valve is plenty big enough and the bigger one makes no difference to power.
It shows how deep the bowl now is, the port goes up either side the valve guide and is very wide. The speed of the air forces it against that wall and it follows the wall down onto the back of the valve. The valve stem and guide obstruct the air and force it out wide and the greater width allows it past unobstructed, it then follows the wall back in onto the valve.

[Linked Image from i.postimg.cc]
A std port narrows there, you might think it makes it venturi, but it restricts it and slows it down. If the roof of the port goes down and then backup the air will not stay on the roof. BSA did not have the port right. The bigger and rounder it gets the worse that can be, why this is oval and coming in low to go high in the bowl and down in one smooth sweep. So it can have large capacity without going high near the entry, as it goes wide instead.
The light shinning in the bowl on one makes a shadow. The floor is raised with a smooth turn to feed the air onto the back of the valve as smoothly as possible. With the valve removed it flows less.
There is dark fill in the top of the port because it used to be bigger, 38mm though it flowed much less. The valve seat is only just big enough and luckily fairly central.

[Linked Image from i.postimg.cc]


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PM's over the limit Mark.

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