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Thunderbolt performance
#818471 08/02/20 10:42 pm
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Hi, I'm building an a65 motor. plan has been do a thunderbolt head for simplicity of tuning/running. I love my lightning but it doesn't really have that much more oomph than my buddy's thunderbolt. Is there a setup that lets you get best of both worlds? Running a mikuni or setting up a single AMAL someway that allows for a more "performance"setting but with the simplicity of a single carb...??
Thanks!

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Re: Thunderbolt performance
drunkenmonkz #818477 08/02/20 11:15 pm
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In real life Thunderbolt gives you more torque in lower revs with it's smaller ports and carb than Lightning so even with it's stock carb in some situations ( town ) can give the same or better performance than Lightning.
My friend has very good experiences with his 69 TR6 where he used a Monoblock built from new parts, so this is the way you could try.

Last edited by Adam M.; 08/02/20 11:17 pm.
Re: Thunderbolt performance
drunkenmonkz #818481 08/02/20 11:45 pm
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If you're comparing discrete models (as opposed to conversions), note that the factory gearing was different for a Tbolt vs. a Lightning (at least in 1966 and '67, which are the only two Factory Spares books I have); 20-tooth gearbox sprocket was standard for the Lightning vs. 19 for the Tbolt. That would also tend to make up for any power disadvantage, if there is one. But you lose in the high end, so if you want to go fast, it would be better to sacrifice a little in acceleration and go for the 20-tooth sprocket.


Mark Z

'65(lower)/'66(upper, wheels, front end, controls)/'67(seat, exhaust, fuel tank, headlamp)/'70(frame) A65 Bitsa.
Re: Thunderbolt performance
drunkenmonkz #818487 08/03/20 1:12 am
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I have a 21 and 22 tooth sprocket to choose from. I'm running a 22 on my lightning and it makes running all day at 70-75mph a bit more manageable. had to do a little custom case filing to get the 22 to fit though

Re: Thunderbolt performance
drunkenmonkz #818501 08/03/20 6:37 am
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My dads oif Thunderbolt had a 22t sprocket, it pulled a decent top speed with triumph silencers fitted, knocked it back to 90 with the oif Megas fitted.

If you want to really change the performance, fit the SRM race cam. It’s still very tractable for road use but not very pillion friendly.


Now let’s all have a beer beerchug

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

Re: Thunderbolt performance
drunkenmonkz #818526 08/03/20 11:47 am
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This has been one of the recurring, entertaining and highly debatable subjects that come up here. First off is that these bikes are not in their best environment cruising down the interstate at 75 mph for extended periods. You can gear them up but it will cost you a bunch in all areas other than high speed touring. Having tried several set ups over the years I still vote for a 20 for the best compromise. a nicely built and tuned T-Bolt with, maybe a 30mm carb can be a joy. There are lots of details to attend to here. Best bang for the buck would b a big bore kit. Cheers, PRT

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Re: Thunderbolt performance
drunkenmonkz #818546 08/03/20 3:19 pm
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Depends on what you want.

If top speed is important, go dual.

If you want a well behaved motor at the throttle opening normally used, the bolt may be better.

Personally, I prefer the DIFFERENT feel I get with single carb.


Rich (last remaining member ThreeMustGetBeers)
"It's not always about going fast. Sometimes it's nice to slow down" (Wendy E.2016)

69 bonney
72 commando
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and a Honda?
Re: Thunderbolt performance
drunkenmonkz #818555 08/03/20 4:02 pm
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I used my 70 Thunderbolt with original head, dual carb small port head and late dual carb big port head, but only with 20t sprocket during my 10 years of ownership. My riding buddy used his Thunderbolt with 21 t sprocket and stock head, during a period when my bike was using small port dual head. In hindsight the most effective combination on the road was his one carb, 21 t sprocket bike, not as fast and hard accelerating as mine, but less vibrations, cheaper on gas and possibility of cruising 70 - 80 mph without making him tired for longer time.

Re: Thunderbolt performance
drunkenmonkz #818629 08/03/20 11:31 pm
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This is a very debated subject and i tend to agree with PRTom.
My own oily frame t'bolt has a 21t g'box sprocket fitted, this makes 1st very tall
for traffic etc use but at 4500rpm the old crate is doing around 75mph so it's
a compromise.
With a pair of +1.5mm i/l valves and some judicious work on the inlet ports
together with work on the piston crowns, retaining the 28mm carb and running
the low end pazon or Boyer at around 33 degs fully advanced allows the old heap to
be ridden at 30mph in top and will pull to 100mph around 6k rpm. It will also
run on 95RON fuel quite happily which is a consideration when living here.
Being a lazy sod , i don't like changing gear more than i have to so flexibility
was the thought behind the head work and retaining the smaller carb. As standard
the old thing would ping if trying to pull from low down and was not happy
at much below 40mph in top.
The silencers/mufflers are the emgo dunsatll replicas lagged a little. The centre hole
is also plugged as they tend to be too free flowing, the a65 likes a little back pressure
at the lower end. Carb jetting as i remember requires the needle to be lifted to centre
and a slightly larger main, 240 or 250.
I also own a 69 t120 and in honesty the difference in performance 30-85mph is negligible
when given some stick. At 75mph the beezer is about the same vibes wise as the trumpet.
What Tom says about the 750 kit is true, but expensive and i'm a mean old git.

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Re: Thunderbolt performance
drunkenmonkz #818696 08/04/20 2:15 pm
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Don't know if I was doing something wrong but while I had 21 on there the fuel mileage seemed to go down a little bit. I thought maybe due to larger throttle opening at highway speeds? Never messed with the ports so maybe i'm not getting everything out of it but revving past 5500 doesn't make it faster at least as far as my butt can tell. I do have a tach on and it seems more accurate than the speedo anyway


69 A65T
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Re: Thunderbolt performance
drunkenmonkz #819105 08/07/20 4:32 pm
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Your OIF Lightning ain't no slouch, Bill. Or maybe I should have kept that to myself?

Re: Thunderbolt performance
drunkenmonkz #819211 08/08/20 2:47 pm
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The secret is out Mike...are you riding to TWO or luxury camping?


69 A65T
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69 B44VS
71 A65FS
95 Trident
Too much moderation is bad for you.


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