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#891800 09/26/22 1:27 pm
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PEH Offline OP
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Has anyone Flow bench tested a Thunderbolt Head Or TR6/7 as i have both engines, with the single carb manifold still on? Trying to decide whether to go single or twin carb on my next Flattracker.

Last edited by PEH; 09/26/22 1:30 pm.
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Mark Parker has done extensive tests on the t'bolt head.
I don't think he's ever done one with a single carb manifold on it
but he's done a couple of modified ones for us and is getting
ludicrous figures from them.
From memory with no manifold around 105 cfm at 400 thou at the
valve is about right for standard.

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PEH Offline OP
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Thanks Nick, I have been following Marks work, quite amazing what he can get from from an A65.

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yer getcher smoother bottom end ( of idle ) with a single carb .But it gets all confused past 2/3 revs ,
n'aint got the ' Tuned Pulse ' enhancement of proper ' one on one ' intake & pipe ' set ups .

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Originally Posted by Pre Unit
yer getcher smoother bottom end ( of idle ) with a single carb .But it gets all confused past 2/3 revs ,
n'aint got the ' Tuned Pulse ' enhancement of proper ' one on one ' intake & pipe ' set ups .

Im a firm beleiver that the single carb models on the later bikes only have those atributes because they are using smaller ports and smaller carbs. Any one thats switched to the early small port head will atribute to better bottom end, even better top speed. - this is comparing an unmodified head of course.

The firebird uses 1 3/8" downpipes, a 70 firebird with the same head that was used on the lightning has a good spread of power, showing that the bigger pipe isn't always better. This has also been a feature with the dunstall down pipes and was noted when Ariel sent their version of the A10 to find out why their 650 with 1 3/4 pipes didn't make as much power as the A10, after fitting smaller down pipes the bike gave the same results. (an edited/shorted version from the Roland Pyke memoirs)

Unless your wanting higher RPM's the single carb in my opionion would be better. with or without inproved flow. try having some downpipes made of the smaller size and that will yield some good results.

On a similar note:
On my Siamese pipes I welded up a "ramp" at the bottom of the port, about 1/5th in height from memory. It made jetting a little tricky and need a #4 slide in the end with a bigger pilot jet and low positioned 5 ring needle with 105 needle jet. It sounded odd for a twin at tick-over, but it went really really well! with dunstall silencer it pulled the fastest that I have been comfortable on it. I thought I was loosing something and ground the weld away. It sounded better at tickover but lost a LOT of power, though still had more poke than the standard pipe setup.


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (now rebuilt)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

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" Trying to decide whether to go single or twin carb on my next Flattracker. "

A bit of vaugely relevant stuff here : https://www.accessnorton.com/NortonCommando/bsa-a65-stroker.20192/

Weber on a A 65 with a few other bits , 2nd last .
If your Aiming to be at the front , youll be coming off the line at / past half revs , on the cam , at least . A Race Cam ? .say 3500
unless its a rolling start .

Though of course ( nowadays ) some people ARNT refering to ' Flat Tracks ' , when they say ' flat tracker ' . ?

https://i.pinimg.com/736x/25/df/b8/...6--vintage-bikes-vintage-motorcycles.jpg

If its ' Just ' a Road Bike , no doubt your not aiming for 8.000 rpm , so ' we 're being a bit irrelevant .

https://i.pinimg.com/originals/3f/b5/7e/3fb57e68be6b224d46899d6b72bed7e0.png

https://i.pinimg.com/564x/fb/fe/2c/fbfe2c8ba1fee23999f5136b3981bbd9--track-equipment.jpg

Some pretty pictures . Note the Oil Cooler . If its NOT being run W.F.O. the optimum output becomes less of a consideration .

Last edited by Pre Unit; 10/01/22 1:31 am.
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It will be used for racing, The Lighting based one i built last year goes well!

20221001_103739.jpg 306793534_5597509903640639_301591988610476525_n.jpg
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Velly Nice .

Ancient ' Classic Bike ' mag. ( Early 90s ) Had some stuff on Chris Vincents Sidecar Champs. motor .

Basically ( Like a Vincent ) once you hogged out the ports & fitted 1 1/4 + carbs , it stated to breath .

Though Rod & crank life start to ' appear ' once your full wellie . Generally 60 hours before considered
unreliable . See the Triumphs were supposed to run 2.000 race miles before major maintanance .

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https://www.accessnorton.com/Norton...-and-after-comparisons-2014.17385/page-5

heres more stuff on a ponced up Dual 34 mm t Bolt ed .

Scroll down too .
---------------------------------------------------------------------------

https://www.bsaownersclub.co.uk/bsaoc_forum/index.php?topic=6525.0

" I still have the Spitfire and an A50 with a Chris Vincent top end on it but have never got round to trying it. "

Or REALLY , Do tell .
Might be worth getting in touch & twisting is arm . Pictures , Virnneers , and suchlike .
silastic copies of PORTs Even ! . In the Flesh . 2019 post , so likely still extant .

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PEH, is it a big bore motor in the Lightning? It looks like it pulls well. The filters will restrict it a bit I imagine but maximum hp would convert to around 53hp at the engine. Aldana's bikes look like they had a lot. The BSA factory tested a 650 with 66hp and a 760 with 79hp. Aldana's had air filters but I'd think he had most of that hp. With a twin carb head I think you could tap into that punch. It doesn't have to be all in the top end and using that may be dicey, but it could have great progressive midrange controlled with the throttle, I don't know if you could get that kick with a single carb.

This bike at the start is my stock Firebird with the ported Thunderbolt head and twin carbs. I imagine it would wheelspin quite easily on the dirt. What would it be like with 11-1 rather than 9-1 compression? Would you look for a bigger cam? I might fit a .016" head gasket some time to get 9.6-1 but without dynoing it would be hard to compare. As it is I imagine in a lighter better set up frame like the one above it would be a lot of fun. But a splayed head would clear the frame, plus you could use a tuned length intake.



mark
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Just found a DEVIMEDE M.C.M. Article , https://www.bsaownersclub.co.uk/bsaoc_forum/index.php?topic=4269.0

Some ikle prity pictures of the componentry , from in the day , at this link . Ive even printed them for me .

As a RACE Engine , below half rpm is virtually irrelivant . For a Dirt track motor , the Throttling / carburation , on & off
are a prime consideration .
For making passengers hair stand on end , the WEBER gives prime incremental engine breaking modulation and transition ,
into tight bends theyve been known to grab the window frame with both arm and prepare for exit . this isnt acievable on
' average ' carburators . or many, even . Even if this is with four wheels on a supple responsive platform .

If its aiming to be at the front subtle incremental instantaeneous accuraccy / feedback , and dependability are paramount .
As is a ability to focus on the Task at Hand .

For these reasons , id think a D C O E would be rather difficult to better . Also the cable hook up and lightness are a decided advantage .

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The DCOE 40 is a great bit of kit but too bulky to fit in the standard frames on a solo.
You can get away with them on a featherbed frame though it's a bit tight, it depends on the engine.

Plenty of sidecar outfits used them over the years with good results on beezers, Triumphs, Nortons, Weslakes etc.
Once you have the body mounted it can have between 27-36mm chokes so is very adaptable.

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seen in N Z in the early 80's two differant triumph 500 units ( Daytona - whatever ) with 40 DCOE's on em ,
Could be a bit awkward with the air cleaners , But worth a try ? Tho the BSA frame tubes not virtical like the T100 ,
youd think thered be as much room ? .

Parently , its 118 m.m. front to rear - face . without stacks . 90 mm bore spacing . ( centers ) Used to be 30 bucks 50 years ago . New .
You could buy them in town . !

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Originally Posted by Mark Parker
PEH, is it a big bore motor in the Lightning? It looks like it pulls well. The filters will restrict it a bit I imagine but maximum hp would convert to around 53hp at the engine. Aldana's bikes look like they had a lot. The BSA factory tested a 650 with 66hp and a 760 with 79hp. Aldana's had air filters but I'd think he had most of that hp. With a twin carb head I think you could tap into that punch. It doesn't have to be all in the top end and using that may be dicey, but it could have great progressive midrange controlled with the throttle, I don't know if you could get that kick with a single carb.

This bike at the start is my stock Firebird with the ported Thunderbolt head and twin carbs. I imagine it would wheelspin quite easily on the dirt. What would it be like with 11-1 rather than 9-1 compression? Would you look for a bigger cam? I might fit a .016" head gasket some time to get 9.6-1 but without dynoing it would be hard to compare. As it is I imagine in a lighter better set up frame like the one above it would be a lot of fun. But a splayed head would clear the frame, plus you could use a tuned length intake.


The firebird with stock exhaust sounds much fiercer than the 734!!


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (now rebuilt)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

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Mark, It is a John Hill 750, I will be playing with the tuned inlet lengths on the Red one when it next comes in for a check up. Its a shame i cannot seem to find any info at all on single carb tuned engines, yet they use them all the time in classic scrambles, very odd. Just for education i might just play around with a single and a twin carb head to see what we can get, and more importantly what we can actually use on these short and loose tracks, im thinking 50ish would be enough unless the rider is very skilled. Aldana's bikes were mostly used on 1/2 mile and mile tracks so the big horse power was needed. I will contact him and see if i can get any info on the subject. Everything on the Red bike is light weight when possible and safe. It has transformed the average owner/rider into a trophy winner! He cannot believe it was possible. The firebird sounds great,

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I think unless you fabricate a decent inlet manifold the single carb setup will
not be usable. The snag is then the manifold will be too long to fit.
This is only my opinion but using 2 x 28-30mm flat slide carbs would be the
better way to go on the t'bolt head. You can buy flat slide carbs cheaply
and easily now.

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I put some quietening things in the 734 mufflers, plus it's geared so much higher and I short shift. It also has power on for much less time and it's not wide open because the front is trying to come up. Everything it does it does so much easier. The Firebird is surprisingly fierce. I'd love to get quarter mile times on both, they have amazing acceleration. The 734 makes you not just smile but laugh. If you are ever out here have a go on them. I didn't understand before it was possible to make them do this. The Firebird is turning into a very old school hotrod.


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Nick, i have just bought some 30mm flatslides so i will give them a go,

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Would defo like to try them, My daughter is in Sydney at the moment, so who no's i may make it over there.


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