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by Mal Marsden - 06/16/22 7:00 pm
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Allan G, davidhernandez, gavin eisler, MarksterTT
Total Likes: 13
Original Post (Thread Starter)
#833231 12/14/2020 8:53 AM
by Mark Parker
Mark Parker
Has anyone used the JRC PWK carbs? I have two 34mm OKO PWK carbs on my Firebird and after about 1/3 throttle if plays up, before that it's excellent. These are not JRC carbs, I am just wondering what spec needle they use.
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#834786 Dec 29th a 09:38 AM
by Mark Parker
Mark Parker
These 34s are now working. It was rich on the needle, lean on the mains frown

The biggest jets I had I put in, 140s, bigger number than in the 38 TMs on the 883. I thought it was getting too much fuel in the top half throttle, like lower down, it would cut out. The 140s may be right, they are close 135s are not good. So I'll try finding some 145-150 etc. Great bottom end and midrange surge. Poor thing, now I'll be able to check the mains.

When I get it right I'll post exactly what I found best. Just knowing I needed around 140 mains would have been useful.
2 members like this
#835480 Jan 4th a 05:18 PM
by DMadigan
Here are a couple of Koso Oko 34mm carbs on a T140 parallel port head.
[Linked Image from]
They just fit but nearly impossible to get at the idle screw on the right carb. Getting the fuel line on would also be a problem. It would have to be installed before mounting the carb.
The rubber coupler they supply is odd, one side fits the carb but the other is smaller, like for a 30mm port.
Probably the easiest mounting is to replace the intake stubs with flanged ones so a slightly curved manifold or even a single carb manifold.
I do not remember seeing left/right hand carbs.
1 member likes this
#835537 Jan 4th a 11:23 PM
by Mark Parker
Mark Parker
They are close on the Thunderbolt head, they look similar width, though I can get a short screw driver tip in and adjust the right hand carb, it has a hex for going in a driver so easy to turn. Just need to not burn yourself on the case or motor. I have no balance tube so it's easy enough to set the left, take it off and set the right the same number of turns out.

I have the power jets connected, my problem was thinking it was going too rich, when the mains were lean. 135s it was cutting out the motor the 140s are better. The needles are short and sharp so I thought they were over rich to run up high. But really it depends on the design of the carb. So with 140s they work. I'll try 150s and see what they do plus slightly leaner needles when and if the arrive.

Markster, the BSA head is fantastic in not being crowded with the head bolts. The standard 1970 68-701 port is very narrow at the guide It can be made 38mm or a little more without breaking through on that Lightning casting. The Thunderbolt breaks into the pushrod tunnel, but you can fix that. The R3/Trident is also crowded by the studs. And so cannot have the same fun with it I think.

It would not surprise me if the ports actually put more air into the cyl at lower rpm with kinetic energy. Like a blower. Not particularly on high boost but making a difference, because something does. Even though it's 9-1 it doesn't seem like it. Which may be why Matt's 750 outfit kicked butt beating 1200cc rigs using just 6,600rpm and taking 4.4seconds out of the lap record. Though his has hi comp pistons. I need some numbers to compare.

So this is 1st and 2nd opened a fair bit, 3rd and 4th less and less. I think 3rd and 4th would go pretty well. Need to turn gps on on the camera which was flat. Hopefully will have rev counter to wire in soon.

1 member likes this
#836757 Jan 14th a 10:35 PM
by Mark Parker
Mark Parker
See if images work?

[Linked Image from]
1 member likes this
#836873 Jan 15th a 11:00 PM
by Mark Parker
Mark Parker
There are different versions. These are KOSO 34 with power jets. They are nicely made and work very well. But do not jet the same as Amals. Things effect each other quite a bit. Needles are different, to other 34 needles, but std pointy ones work with the sufficiently large mains. Me thinking rich when actually massively lean on the main jet. Seeing kit's do not quite include big enough sizes.

The manifolds are bolted on and are oval going into the head. The manifold spacing by chance is pretty exact to line up with Nick's Weber manifold. This head flows marginally more than Nick's and has slightly oversize exhausts as well, if that makes a difference. It lets the valve sit higher on a used seat.

If I do them a spare head and the manifolds are oval and at the exact angle it should be a bit better. I'd be tempted to use a std spec bottom end strengthened for rpm, even good std old pistons. Just to see what a 654 engine can do.

First picture is std 27mm port right with partly done oval on the left, it extended to the wall on the left was welded on the upper right for a mounting bolt and lower bolt hole moved down.

[Linked Image from]

With one manifold 34mm with oval end squashed in a vice because we are high tech.

[Linked Image from]

My vacuum cleaners are not precision and I'm not confident figures are exact, but the photo shows the modified little port from the other side. What it actually does by the feel of it on a stock engine would probably have upset BSA.

[Linked Image from]
1 member likes this
#837345 Jan 20th a 03:25 AM
by Mark Parker
Mark Parker
The beauty and the ease and the smoothness of a 90degree 750.

Setting up the rev-counter but the battery is dead. I fitted a modern rec/regulator wired it -earth in the hope it was not the old alternator but no. Pulled the cover and it stank.

Anyway this video is me trying to get home on a half dead battery with no charging. I put it into 2nd early so it would have something to pull against and you can hear when it goes onto the main jet. It sounds so busy. Not on the main long enough to see much but the noise changes and it gets light at the front.

1 member likes this
#838917 Feb 3rd a 10:16 AM
by Mark Parker
Mark Parker
Seems happy enough to rev to 7,300, and pulls well. I can probably try 8,000 or more. Though it pulls fine from down low and it seems pointless in a way.

An interesting test may be a 1/4 mile with someone who can get off the line (and have the throttle on the stop). A '71 Lightning did a 14.3 and they seem to be changing at 7,000. This feels fairly strong in that range, it likely revs higher but I don't think it needs to.

1 member likes this
#835424 Jan 4th a 04:07 AM
by Mark Parker
Mark Parker
It's great when things work the way they should. The 140mains work with the std needles in the leanest clip, and it's brilliant. I need bigger mains to test. and leaner needles to also test. However it now works and 60-70mph is barely off idle with just me. Up hills is great.

These carbs are inexpensive and very nice. They look nicer than Mikuni carbs being smaller like an Amal. They should last like a Mikuni with chrome slide, throttle action is light. 34mm is a good size with a head like this.

Oil pressure when hot was dropping and around 30-40psi going slow. So I cleaned up and fitted the piston type relief valve without a gause, so pressure is 75psi and 50 or more going slow.

What I think is happening, just changing to this head and carbs... maybe: Stock heads are around 109cfm. It's not the cam, nor bore and stroke stopping them breath and restricting power, it's the inlet port drilled at the factory.

The Mk IVs claimed 56.5HP because the heads were done better, even with lower 9-1 compression. A '71 head I have seems similar but the casting number was removed before casting? Usually they are 71-2202 rather than the more common earlier 68-701. And it's only 30mm. It may be why BSA claimed 54hp for the 71 Firebird.

Which raises an interesting point regarding performance, highlighting what breathing does.

If the port flows better by a large margin and stay relatively small it seems a game changer, better than you might think.

Though bigger means more flow, more power, if it's proportional 14% bigger =14% more flow. Then 34mm = 124cfm, which isn't guaranteed. Or 32mm = 116.6cfm, also not guaranteed (because the port shape is not good.) It moves power up the range a bit and slows intake speed till higher rpm, as it needs the same vacuum pulling through the bigger port to get more air.

But what happens if the 34mm port gives say 150cfm? 25.8cfm more than the previous 34mm port. If it even had 124.
So now you have a 14% increase in size with a 38% increase in flow. This means a 24% increase in gas speed when you reach that vacuum. But the thing is, and I'll try to measure it some time, at less vacuum, speed and cfm are... 'up'. What does that mean? Why would anyone need or like it like that?

Hot cams like the 473 work when gas speed reaches a certain point and the timing dynamics force or carry more air in past the valve with high kinetic energy and can in fact put more air in than the cylinder actually displaces. What blowers do. The sooner the air speed approaches that speed the sooner any reversion is overcome and nice power begins. So what's increased is 'response' all the way through to a higher top end, depending on exhaust limitation I guess.

It's not particularly a bottomless pit of power, but it is nice and very willing.

My son heard it from a distance and thought it was a modern bike, till he realized. The draw back is a bit of a wall of vibes when it revs, something being 90degree could fix. I have a rev counter/speedo ordered, when ever that gets here. And will try getting a GPS Go-pro acceleration graph some time soon, but as Ben says wait till after double demerit points.
1 member likes this
#835487 Jan 4th a 05:52 PM
by Hillbilly bike
Hillbilly bike
I used readily available TSS slightly angled rubber connectors to mount 32mm Mikuni TS flatslides on my T140D with parallel ports using aftermarket air filters...
Attached Images
1 member likes this
#836013 Jan 8th a 06:20 PM
by DMadigan
With a 360 crank there is no need to go for a larger carb (at least not 40mm) because the intake strokes do not overlap and the low through the carb will be more consistant. You will not get the resonant effect of a single cylinder so tract length is not as important.
With the offset crank you would have to look at the peak combined flow and compare it to the peak flow of a single cylinder.
1 member likes this
#836027 Jan 8th a 10:51 PM
by Mark Parker
Mark Parker
Ben's 750 is 90 as is brother Pauls 750. The Firebird is stock crank configuration.
1 member likes this
#836515 Jan 12th a 10:38 PM
by Mark Parker
Mark Parker
Yep that's interesting. The carbs they show differ a little. But an interesting way to get mixture worked out. Getting idle and early settings first.

I just tested the 150 mains. It's really a joke. 1st is too low like neutral except you move, 2nd revs up really fast and harsh like it's gone nuts, the bars seem to get bigger. It's very distracting and no rev counter or light to tell where rpm is going. It needs rubber mounting, balancing for rpm or 90 degrees.

Midrange is stronger. If Matt builds a spare engine maybe just use a strong balanced for higher rpm stock bottom end.

I probably need to try bigger jets. Carburation, ignoring getting any plug readings, seems close, just one little glitch when turning it around 1/2 way from something. I could try lifting the needles a clip. But idle is sensitive. I'll get Ben to have a go make sure it's not just me.

Even when the rev counter is fitted as insulated as I can get it it may get hard to read. Rubbers need to be exact because sometimes they make it worse.
1 member likes this
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