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Excuse the dunce’s questions -

Do all thunderbolt heads have the same size inlet ports ?
Do they all have the same size inlet valves ?

It seems to be generally accepted that a single carb gives better torque at low to medium revs . Why is this ? Something to do with the length of the inlet tract , or something else ?

I keep looking at the 1 into 2 inlet branch on my thunderbolt , and thinking what a horrible path it creates for airflow - but it works !

I intend to try mounting twin carbs on a thunderbolt head ( it looks fairly easy to fit two side by side ) , or a single twin choke carb ( Weber / dellorto) . I will buy a second thunderbolt head to mount the new carbs to , to save having my bike off the road while I’m tinkering - hence the questions about whether they all have the same port / valve sizes .

I’m not convinced that a single carb gives more power in any rpm range, all other things being the same.

Remember that in the case of BSA and Triumph 650s, the single carb bike has a different head and ports from the twin carb models.

ive never looked at a dyno sheet.

but i think the single carb head was primarily a convenience and economic alternative to the twin-carb versions in all applications.

i've never seen any assertions that a single carb head was better in any torque or horsepower capacity. they are certainly simpler to tune, but only because they limit you to one-half of the adjustments.

The early ones had smaller valves.
Thunderbolt single carb heads and Lightning/Spitfire etc
heads appeared different from year to year as far as ports
etc go. It depended on who did the machining i think.
The single carb head has smaller ports which give good
speed through the port and althogh they run out of breath
a little earlier, they are often better than twin carbs.
The same applies to triumphs, so many blokes think putting
huge carbs and massive porting on engines is the mutts nuts,
whereas it's often a large step backwards.

The Thunderbolt port is a pretty nice shape compared to a Lightning. They are 27mm with 40.5mm valves on the bigger valve later ones and individually flow very close to a 30mm Lightning, which looks drilled. A US visitor saw ports being drilled at Triumph the way they did it in the '60s. The BSA has shallower valve angles so the port aims more across the valve. The Thunderbolt is better so there seems only 1cfm between ones I measured despite the size difference.

Flow is much better on both when the intake charge is turned onto the valve so it can work properly.

If the Three had been based on a 27mm Thunderbolt port it would have had 10hp more with more grunt, at a guess.

Thank you for that , is there a way to identify the later , larger valve, thunderbolt heads from the casting numbers ?

Yes there is but i don't know it...............

The post-66 heads had all 3/8 bolts/studs whereas the earlier ones
had 5/16 rear bolts. BUT i'm not sure if that is when the valve sizes changed.
The larger valves will go into the earlier heads anyway, just a bit more work.

Hi Mark ,
There seems to be much confusion over a65T casting numbers.
I’m trying to buy a 27mm port , 40.5 mm valve head over the internet.
Can you tell me the casting numbers I should be looking for ?
Thanks

I'll try reading the number on the Firebird when I get a chance.
But why not use a Lightning head? The Thunderbolt is more work, though good with a Weber if you have room. And either welded on manifolds or weld to bolt them on, because the original studs are in the way. The Thunderbolt is also thinner so needs patching if you port it where it needs to go. With care the Lightning doesn't break through to do the same thing.

This is the head on the Firebird, it's 34mm and you can see a dark area where it breaks into the pushrod tunnel. This is patched with an alloy plate and JBweld. Also note the manifold is round where the carb goes on and oval at the head, it goes wide because it cannot go in round because the top and bottom need to be smooth curves so the air will follow them, the bowl is deep to turn the air onto the valve and the floor is raised with a nice curve to do the same thing.



This is with $75au worth of pwk 34carbs. You could use smaller but these are good on this pretty stock 654.



This is partly done, but the bolt holes just need moving out. You could do it smaller. It's good because it can just be 27mm high.



What the port looks like, 34mm Lightning and Thunderbolt.





Pwks are cheap and quality and small and space is limited. You could use a pair of 30s or 32s if you can get some alloy pipe that size.

The 34mm ports above measure roughly 50-55% more cfm than std. And it pulls hard when you turn the throttle and could support a big bore kit pretty well, I would think. Though it hardly needs it.

What year is the bike? if its an OIF then you need a very much different head.

otherwise any single carb head will work, the valve seats can be cut to accomodate the later 40mm valves and the rear stud holes can be gently drilled out to become 3/8 if needed.

Though I have had good success with the early 38mm valves on the early small port lightning heads, after having my current head's valve seats cut to suit the 40mm valves, I wouldn't bother doing it again.... Least not until i have a set of valve seats that need replacing that are 38mm.... then I'll have them cut for 40mm.

Small port A65 twin carb heads have more bottom end grunt than the late ones, they are no worse at top end either - least as far as use on the street is concerned, but that is possibly why you feel the thunderbolt head will give you a better low - mid range. The twin carb will always win at top end as it has a dedicated carb which can sufficiently provide for each cylinder individually and not be over-come by revs.

Allen , the bike is a 69 thunderbolt, not OIF .
I’m looking at putting a Weber twin choke on a thunderbolt head - though if the carb won’t fit in the bike frame , then two carbs on the thunderbolt head.
Do all single carb heads ( pre OIF) have the same 27mm ports ?

The DCOE40 is a good choice but you'll have a job getting it in a road bike frame.
Best to cut the whole rear section of the head off and weld a couple of stubbs on.
Then you can make up a manifold with stubbs at the carb end and use standard
t140 carb rubbers to mount it. I doubt you'll be able to fit it in, the Delorto is a bit
smaller, maybe more chance.
This is with one in a featherned to give you an idea. The featherbed is very long
compared to a small heath frame as the engine had the 'box sited behind it.
You won't need the manifold length but it'll still be tight in there.
https://imgur.com/a/mMNaUml

Thank you Nick ,
Is that a dellorto in the photo ?
I didn’t know the dellorto was smaller .
Do you have dimensions ? Particularly the length of the carb body .
Thanks
What choke , aux venturi , jet sizes etc do you have in that ?
I’m very interested

That is a DCOE40.The carb body plus what air trumpets/filter, there are dozens.


For a standard 650 engine, 30mm main vents 4.5 aux vents, F16 emulsion tubes,
35 Acc pump jets, 125 main jets, 175 air correctors. Will get you started.
The photographed engine has an a10 crank and big valve head etc, it's a race engine.

Can i ask, if you were to run an A65 in a Vintage Flattrack class that run mostly on 1/4 mile loose speedway tracks in the Uk, Would a single carb and a Thunderbolt head be preferable? over the extra expense, time setting up, keeping in tune s twin carb head. And can they be improved over standard factory using a single carb and manifold. I notice that many scrambles bikes run a single carb on there big twins.

Which is more restrictive - the 27mm thunderbolt port , or the 40.3 mm inlet valves ?

That is to ask , is it worth fitting bigger valves ?
I will not be enlarging the inlet, only reshaping it a little, as described by Mark Parker and others .

I am having new valves fitted to a head anyway , so this would be the time to do it .

I will fit a Dellorto DHLA 40 carb . It’s the same size as the Weber DCOE , but a slightly different shape which doesn’t interfere with the cylinder head oil supply pipe .

PEH, I sold a 750 kitted a65 to a guy over here who flat_tracked it.
He had built a frame based on a small heath one but with a spine
top tube/oil tank. He ran a weber on it and loved it, used to rev it
past 8k and it was reliable and smooth.
I suppose the yanks on the site would probably be of more help
as regards flat tracking but the spitfire and the A70 were both twin
carb bikes and they did well in that game.

As for restrictions on ports and valves, the small ported small valve
heads are suitable for tuning and would be adequate for road use.
Mark has done a lot of flow testing with standard and modified ones.
If all you are involved in is 'Traffic Light Grand Prix's' any of the heads
can be made to work very well on a well set up engine.

Thanks for the reply Nick L, Im more getting at how good a single carbed engine can be made to perform, and has anyone got experience of it. I have followed Marks work and very impressed with the gains, Which may be to much for our tracks. I just like the idea and overall simplicity of a single carb, I guess i need to put one on and give it a try.

My own road bike is a t'bolt, up until a year or so ago i also
had a t120 bonny. Mid range say 40-85 mph there was very
little to choose between the two, the bonny would carry on
after 90 whereas the old t'bolt ran out of puff.
The t'bolt has large inlet valves fitted and i have done a little
work on the head but nothing radical.

That is good to know Nick. A direct comparison like that is exactly what im after, seeing as we probably don't get over 50mph on our tracks it would seem to make sense to use the Single carb. I'm also building a Triumph 650/750 Trackmaster which i was considering a single carb head as i have both. This helps make up my mind , many thanks.

Oddly enough, a Thunderbolt that I used to ride (was my fathers) came to us fitted with bonneville silencers, with a 22T front sprocket the bike would happily see 105mph, it had a 250 mains instead of the 230 and went well (I was also about 3 stone lighter back then). As we/he preferred the aesthetics of the OIF megas, it got fitted with a set of campbells silencers and the original 230 main jet. The PO had never changed the needle jet settings which I always thought were on the lean side, but with the OIF megas it ran spot on. The interesting part being that, that bike never went over 90 again after fitting them.

A lot of people forget about exhaust tuning. It makes a big change to how the bike performs and a too restrictive or not restrictive enough can make a bike a good bike with lots of pull or one which is somewhat lacking.

Allan, the bike will do over the ton but it takes a while, the bonny would carry on
at the top end more easily.
Both were/are fitted with dunstall copy type long (similar to oif) meggas with a couple of
exit vents blanked off. I'm not going to spend $800+ on a set of oif pipes for a few mph.
Hacking around at those speeds got my license into bother so i'm happy to keep it
below 90 these days. I'm old now so it's expected eh?

Didn’t mean you don’t know about exhaust tuning Nick, you’ve done a lot more with these crates than I have, was just saying how a different set of silencers changed the bike completely.

It’s nice to get the throttle open full occasionally but these days my job depends on my licence and the family at home depend on me getting back each day. There’s also too many idiots just pulling out of junctions without looking properly these days and that happens when I’m in an SUV, they see me even less on a bike.

PEH, The trackmaster is a lovely frame.

$hit, i miss racing...................................

Nick L. Yes they are. And they work! The A65 750 i built from scratch just over a year ago turned an average placed rider into a front runner in his first outing! Of course he gets the plaudits, i have barely had a mentioned.