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Discussion Starter · #1 ·
Hey, has anyone done, or seen done, a big single carb on a 650 brit motor? Not just using the single carb head but like HD uses one big ass carb for both cylinders.
 

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BigDdy31 said:
Hey, has anyone done, or seen done, a big single carb on a 650 brit motor? Not just using the single carb head but like HD uses one big ass carb for both cylinders.
What? One carb for both cylinders is a single carb head. And a Harley carb is way to big for a 650.
 

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You don't really need the BIG carb on a Brit Bike. 30mm on a 650 single carb or 750 single carb is plenty unless you are racing, then 32mm is better, but you gotta REALLY have some hot cams, bigger intakes, high compression pistons to not be over-carburated on a 650 and 750 Brit bike. Most of the time the jetting is changed, not the carb. For example the Bonneville has 2 30mm carbs with around 190 main jets, the TR6 had ONE 30mm carb, 230 or so main jet. Weird, eh? If you gotta have something that looks bigger, get the Mikuni (see my earlier post) 30mm, it looks bigger. Or add the big round pancake air filter to the TR6 and that fills space.

I think a lot of the H-D's you see with Huge-e-o Carbs never see the street. Remember the carb that guy Billy Lane used on the Discovery Channel? Would not work, way too big, choked itself on gas, (kinda like locking yourself in a room with Burritos and nachos for a week.)

BigDdy31 said:
Hey, has anyone done, or seen done, a big single carb on a 650 brit motor? Not just using the single carb head but like HD uses one big ass carb for both cylinders.
 

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Discussion Starter · #4 ·
fouraceswes said:
I think a lot of the H-D's you see with Huge-e-o Carbs never see the street. Remember the carb that guy Billy Lane used on the Discovery Channel? Would not work, way too big, choked itself on gas, (kinda like locking yourself in a room with Burritos and nachos for a week.)
I agree on the common HD carb sitch. It seems there a lot of chromed out 103 inchers that ride the trailer to the show and back again.
 

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Discussion Starter · #5 ·
LOL! Don't worry fellas, I don't want to strap a Super E onto a 650. ;) I'm just looking to explore the possibilities of using the single carb approach for performance applications.

I'm just thinking that one would take a single carb head, increase valve size, take a healthy bore to the cylinder, bump up the camming a bit, maybe increase/smooth the intake, then put on something like Wes suggested (a 30 or 32) slightly bigger and more easily tunable than the carb that came stock on the bike. Maybe go crazy and turn the head around and create some kind of 'ram air' situation.

My initial conclusion would be that one would get easier tunability with single carb, need less room for your intake system after turning the head around, and end up with some smooth performance gains.

Input?
 

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I think that although the tuning of a single carb may be eaiser, you will lose performance over two properly sized carbs. Besides tuning dual carbs is not that hard.
 

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I've been thinking about stuff like that too... It would look fuckin' wicked to "somehow" have an old Linkert or an S&S L mounted to a Trump motor... But it's probably near to impossible... I dunno.. I'm not that much of an engine guru to know about this.

This bike has a Weber on it...

 

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Triumphs, Nortons and BSAs all had single Amal set-ups as stock for years and years. If you want to run one the parts and information are easy to find. IMHO they are better than twin-carb set-ups for most applications, smoother and easier to start.. don't go silly with the cams and pistons. there was a trend in UK in the 60s, for Triumph engines with 11:1 pistons and E3134 ( racing ) cams and all it did was to make them unrideable and blow head gaskets... if anyone is REALLY saying that they need the extra 5% on a rigid frame bike with springers, no front brake and who-knows-what steering geometry, well I suppose everyone is entitled to their opinion...

650cc Triumph Thunderbirds had an SU set-up in the 50s. If you want to know how that worked, the information isn't hard to find. They dropped it because British riders were too conservative... Ariel used an SU on some later versions of the 1000cc Square Four engine

there was a British barb called the B&B before WW2, it was much like a Linkert. Rudge used it quite a lot, but it went out of production in the 1940s. If you want a wierd carb that will work fairly well with a Brit twin, look for one of those ?
 

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650cc Triumph Thunderbirds had an SU set-up in the 50s.

Actually the Thunderbird in the '50s was the conservative 650 of the line. It had lower compression, milder cams, and a battery and coil instead of a magneto. They also continued to use a cast iron head after the other models went to the alloy "delta" head. That particular SU was not a high performance carb.

The problem with using too big a carb on anything is the velocity of the air being drawn through it. Bigger carb, same amount of air flow = slower velocity. Too slow and the fuel won't atomize well. A too small carb will actually run really well, up to the limits of it's flow.
 

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correct.. the T-bird was the 'cooking' model and the Tiger 110 was the 'sporty' one with alloy head and twin carbs. The T110 became the T120 Bonneville after the record attempt at Bonneville salt flats.

I've never seen an original SU outside a parts book, I had a T-bird in the 70s but it had long since been converted to an Amal

the T-bird also had a seriously good-looking primary cover with a 'Thunderbird' logo cast in it!
 

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Vertical twins are 180 degree motors, so only one cylinder is breathing through the carb at a time. Thats why a dual carb 650 and a single carb 650 use the same size carb. A similar dual carb bike will top-end a couple miles an hour faster, but might get pulled a little out of the hole by a well tuned single carb bike. I have a '72 TR6 single carb that uses a 190 main, just like the BSA Lightning twin carb engine on my bobber, same mains. The tune up on the TR6 is a snap, the BSA is a pain, due to cable stretch and idle adjustment procedure.

Bernie
 

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fisherman i just put twin carbs on a 350 norton twin, used x2 stock
carbs with stock jetting.
The "experts" i asked told me i would have to rejet change needles etc.etc.
Started 2 kick & idled, runs no better than before!
kbg...
 

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If you have a Bonnie head use 32mm carbs and 240 main jets with the needle all the way up and #2 1/2 slides. Leave the intakes alone as they and the valves are the correct size even for a 750. If you want more power try 3 angle valve seats, a 750 big bore kit and a T140 inlet cam. You can also fit offset rocker buttons with the stock cam. These give .025 extra lift. Don't for get an oil cooler. Here in Houston you need to keep the head cool.
 
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