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Twins Technical Talk Technical Talk for Hinckley Triumph Twins: Bonneville, T100, Speedmaster, America, Thruxton, and Scrambler.

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Old 11-07-2012, 03:48 PM   #1 (permalink)
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Low torque at the low end

I posted a thread earlier about modding the intake and exhaust to get more power, then trying to get the tuning right. I'm on the right path now, smoother idle, no bogging, and a significant boost in power, but only when I'm in 2'nd or 3'rd gear and in the high rpm ranges.

What can I do to get more power in 1st gear/low rpms? I don't really want to lift the front wheel off the ground like some, but it feels flat in places and I want to fix that. Is this a jetting issue, or something else? I haven't synched the carbs yet after the mods, waiting till I get everything just right.
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Carbs: 135 main, 40 pilot, stock needle, 2 shims, 3.25 turns
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Old 11-07-2012, 04:14 PM   #2 (permalink)
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A smaller gearbox sprocket will lower your gearing and make things feel more lively at all revs. I believe your 790 came with a 17 tooth sprocket as standard, you could try a 16 tooth available at places like this:

http://www.newbonneville.com/html/18..._sprocket.html
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Old 11-07-2012, 04:26 PM   #3 (permalink)
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Enlarge the air intake cover to the same size as the inside of the filter. (drill and dremel)
3.25 turns on the mixtures is too much as they're almost at the point of falling out, try 42 pilots, thruxton needles (no shims) and 1 turn out on the mixtures as a baseline.
You could drop to a 16 tooth countershaft sprocket if you're after more bottom end grunt. If you do you'll be feeling for a higher gear that won't be there when you hit 50 mph.
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Old 11-07-2012, 04:53 PM   #4 (permalink)
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sometimes a little backpressure on the exhaust will give you more low end torque.
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Old 11-07-2012, 07:20 PM   #5 (permalink)
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You are running really big main jets for still having the stock air box (even though you have removed the snorkel and restrictor plate). I'm running 125's right now, and am thinking of going smaller first before I go larger. just curious, what is your MPG looking like?

Mind you, I'm completely inexperienced (well, gaining experience as we go), but I'd love to hear what you're doing and the results you're getting.
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Old 11-07-2012, 07:29 PM   #6 (permalink)
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the best bang for the buck on your 01 bike is to get rid of the air box and then tune the bike right.I dont know how low of rpms you want the power to be .There is not alot you can do at real low rpms (under 3000) with your 790 01 bike unless you go to a big bore and or stroker.A good set of fcr carbs will help some but they are not cheap.What are you looking to get it to do?
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Old 11-07-2012, 07:33 PM   #7 (permalink)
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Jag..mine does 53mpg (imperial) with the below mods.
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Old 11-07-2012, 07:40 PM   #8 (permalink)
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making more cylinder pressure early will boost early torque, cams for better lo end, raising compression ratio just under denotation. optomizing with a good tune that advances timing as well as good air fuel ratios using premium fuel, going into the engine gets $$$$, more cubes works for sure depends on your wants and willingness to $$$$$$$$$
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Old 11-07-2012, 08:01 PM   #9 (permalink)
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Thanks for all the responses. So I'm definitely not interested in the 16 sprocket, as I do a lot of long distance touring. I thought about enlarging the air filter hole with a dremel like Denbow said, but this makes me think I'll have to use even bigger jets to compensate, and I'd hate to lose a ton of MPG.

I started with 120's/40's/2.75 turns before the mods, then opened everything up and tried it without changing anything. Poor idle, bogging and popping when pulling the throttle, headers even started glowing (obviously very lean). Tried 130's and a slight improvement but still major bogging. Next tried 135's. Much better but still a bog. Added a shim, slight improvement. Added a second, even better. Adjusted to 3.25 turns and now a nice quick response to snapping the throttle. If I go back to 2.75 turns or less, there's the bog again. So The jets and screws seem to fit the config according to this chain of events.

What effect does adding the Thruxton needles and/or a 42 pilot have? I've heard people losing MPG pretty bad after adding 42's. Thanks again!
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2001 Bonneville - Red & Silver
Mods: AI gone, restrictor plate gone, snorkel gone, Emgo open mufflers (BCE-037) with JC Whitney baffle insert, K/N filter, crankcase breather filter.
Carbs: 135 main, 40 pilot, stock needle, 2 shims, 3.25 turns
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Old 11-07-2012, 08:09 PM   #10 (permalink)
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Quote:
Originally Posted by rodhotter View Post
making more cylinder pressure early will boost early torque, cams for better lo end, raising compression ratio just under denotation. optomizing with a good tune that advances timing as well as good air fuel ratios using premium fuel, going into the engine gets $$$$, more cubes works for sure depends on your wants and willingness to $$$$$$$$$
higher comp will help some alittle more timing will help some and it has to be tuned right.You wont get it with cams though on these 4valve heads with out killing the high end .stock they have to much valve for real low rpm power and not enough flow for high rpm power.That is why they come geared to use alot of rpm going down the high way un like a hd or a car motor that spend most of there life at lower rpms.
people are getting pretty good hp numbers from 865s with hc piston kits cams and the works but the torque does not go up as much.a 904 with all the same mods will allways make more torque and the bigger you go the more torque you get but as you said its not cheap you got to want it to do it.The stroker motors rock when it comes to low end torque.
BUT they allso cost more to build.
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