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Vintage Technical Tips & Tricks Technical and maintenance tips and links. DO NOT POST YOUR QUESTIONS HERE!! Please post questions to the general forum.

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Old 02-28-2009, 09:55 PM   #1 (permalink)
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Maximizing horsepower from a '76 T140

I am interested in maximizing the performance of my T140 bobber and would appreciate anyone's thoughts on do's and dont's.

Thanks
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Old 03-01-2009, 07:35 AM   #2 (permalink)
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Have it dyno tuned first, than explore gearing....Cheers, Don

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Old 03-01-2009, 08:47 AM   #3 (permalink)
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well how deep are your pockets?

there are big bore kits out for the 750

just by making it a bobber and removing weight(not making more HP)but you power to weight ratio is going up making bike feel and take off faster.
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Old 03-01-2009, 10:06 AM   #4 (permalink)
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Absolute max performance for a Bonneville without ungodly stroking or phase change on the crank, water cooling on the head or anything that makes it look like something other than a Bonnie, and assuming an unlimited budget:

If you don't do all the machinework youself, MAKE SURE THAT ONLY ONE PERSON OR SHOP DOES IT ALL (including balancing). You NEVER want multiple machinists pointing fingers at each other when you show them your blown up mess of an engine.

Split the cases, clean out the sludge trap and dynamically balance the crank. Two camps - heavy vs. light flywheels, I'd go with heavy.

Big-bore cylinders with plain iron liners. There are several good manufacturers. MAP big bore kit is tried and true for reliability. Carefull crosshatch hone finish to exact tolerance recommended by piston mfr. If your big-bore kit doesn't come with a head gasket, use a composite one with metallic flame ring; seal with copper-kote.

Rigid, lightweight, shot-peened rods. Carillo makes some, there are others. High compression forged pistons with top of the line rings for plain iron lined cylinders (I would NOT drill the skirts for oiling, not necessary). Balance all of these reciprocating parts, balance the rods on both ends.

Lumpy cams from Megacycle, JOMOs if you can find 'em, others are out there (commonly known cam grinders).

All new high quality bushings and bearings in the engine & tranny.

High strength pushrods, mushroom head valve adjusters in carefully cleaned and lightened rockers. Properly radiused cam followers.

Ported head, bench flowed to prove the porting scheme. Use a reputable firm that is known for Triumph work, or follow a proven scheme if you are doing your own porting; take lots of photos and have a pro review them. It's too easy to ruin a head.

WATCH YOUR HEAD TORQUE. proper sequence, proper torque, your big-bore kit may specify values. Loctite everywhere.

High end valves in bronze guides, with springs as recommended by the cam mfr/grinder, titanium retaining hardware. 3-angle valve face grinding.

Double plug mod to the head, Irridium plugs indexed to aim the open spark gap at the center of the combustion chamber when bottomed out. Run two double output coils (Dyna or other).

Magneto-electronic ignition such as the ARD mini-mag. No points, no battery input required, low mechanical drag.

If you are going to ride on the street, you'll need an alternator at least, to run the lights. Use shims to align the rotor with just less than half of the rotor surface aligned with the stator, to reduce mechanical drag. Otherwise, yank it off.

Belt drive clutch (one that includes it's own sealed bearing), with sureflex plates.

4-speed gear cluster with all new buchings & bearings, run in 80wt gear oil.

Large-bore exhaust with nominal flow-through mufflers or tuned megaphones.

Kehin FCR flat-slide carbs with carefully configured airbox and single, large free-flow filter. You can run open velocity stacks, but you'll be spending lots of money on new pistons & rebore very soon.

High output oil pump.

Additional crankcase venting through the timing chest and rockerbox tops.

Lightened timing gears (either lightweight gears, or drilled OEM bits).

I prefer "easy" run-in for the rings, but the majority of the race crowd likes "hard & fast". STRAIGHT PETRO LUBE, 40W, for break-in. Highest octane gas you can get; make friends with guys that race, they can keep you supplied in 110.

I'm not getting into what oil to run after break-in. I would run castrol GTX 20/50.

Probably not absiolutely needed, but it can't hurt to drill and saftety-wire all oil-related and adjustable stuff.

I'm probably overlooking a few things, but this should get you close.
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Old 03-01-2009, 10:24 AM   #5 (permalink)
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like GP said there are a lot of things to do but $$$ is king and street-ability goes out the window.


i love going fast but hate to say it Triumphs have a limit to HP and any engine is only as strong as the weakest link. change one part have to beef up another.

when i get a speed itch i go get my buddies Suzuki GSX-R1000 ( i know not a Triumph) but power in 4th gear doing 80mph just touch throttle and up goes the wheel and doing way over 120mph in seconds.
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Old 03-01-2009, 09:33 PM   #6 (permalink)
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Maxiumizing the performance is a multifaceted question. Are you talking about mpg? Perhaps you are speaking of top speed. If top speed in the 1/8 mile, 1/4 mile, Bonneville Salt Flats? I doubt you are speaking of AHRMA Racing. For what its worth the weakest on a 76/77 Triumph 750 are the spokes. If they couldnt stand up to a stock bike the will not stand up to a Hot Rod
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Old 03-02-2009, 01:07 AM   #7 (permalink)
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My first question is cruel.

What is your weight(lbs) to height(in) ratio?

Told you!

BTW, mine is 2.5

OK, why do you want more power? Braggin' rights or are you going to put it the track? Prolly not since it's bobber.

More HP, lots more, will cost you $$$$$, reliability and streetability. GPZ listed almost all the things you will need to do.

A little top end work, Mik TM or Kehin FCR 32's, a good ignition system and less restrictive exhausts will net ~5HP. More will add cost$ with splitting the case, crank work, rods, pistons, cams and followers which will add maybe another 8HP.

After that, the costs rise exponentially and reliability decreases by the square root.

My '73 750 Bonnie dyno'd out at 64HP by paying attention to detail and slight mods.

Can I notice it? Not really after riding my S4 for the past 6 months.
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Old 03-08-2009, 04:31 PM   #8 (permalink)
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T140 horsepower

There are lots of tips and tricks in previous posts here but in my experience one of the biggest tune ups you can do is to balance everything and i do mean everything. The Triumph valve train lends itself to quite a bit of lightening but be careful. Many companies offer pre-made dural rocker nuts etc and these are worth getting but weigh them before fitting. With respect to the rocker arms the ends can be smoothed off to good effect but the arms should only be gentle ground on their faces nothing off the height so in section it will resemble a rugby ball or US football depending on what side of the pond.
Another thing to check is that everything is actually where it should be. Often referred to as blueprinting making sure the crank bearings are all in line, that the piston bores are square to the crank from both the front and the side. Any good engineering workshop should be able to check this if you don't have the equipment yourself.
Head porting is good but don't be fooled by shiny surfaces. It is the shape you are after and don't think bigger is better. Triumph valves are big anyway so more port shape than size. If in doubt seek a specialist. Once you have all things corrctly aligned I usually dowel the halves together so if they come apart again I can get them back exactly. Steer clear of wild cams, on the T140 it's the exhaust you want to change, the inlet is already pretty good just mellowed by the exhaust (Norman Hyde & Nourish make good cams). I'd also avoid lumpy pistons.
There is an old saying for Triumphs, let them breath. Either run open carbs or K & N's and free flow pipes. A nice short reverse mega phone gives real good grunt once you've established carb settings. I've followed this procedure on a few bikes over the years and whilst you can go so much further it gave me a vast improvemnet over stock performance without sacraficing reliabilty. No good going a zillion miles an hour if you have to entertain a rebuild every 1000 miles!!.........don't forget your brakes either!!

Last edited by GrandPaulZ; 03-26-2009 at 10:42 AM. Reason: fixed cam info
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Old 04-19-2009, 01:14 PM   #9 (permalink)
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Thanks to all for your replies and detailed comments. Per your questions here is a little more color about what I am looking to do.
  • I plan to ride on the street - no serious racing - but also ride the local curves (100 mile radius).
  • I am not capable of doing any of the work myself.
  • I'm looking for as much bragging rights as I can get without losing reliability or requiring rebuilds every 1k miles.
  • My budget is not endless but I can afford to do it right.

I want to strike a balance between improving reliability and performance (i.e. impressive ability to get off the line and go fast).

Hopefully this will give you a closer idea of what I am looking for and refine the to-do list.

Thanks again.
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Old 04-19-2009, 02:30 PM   #10 (permalink)
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Maximizing horsepower from a '76

In order:

1 - Basic engine mods will give you a little bit of HP and great reliability. Ride the curves eh? All the HP in the world is useless if you can't hook it up. Spend the money on suspension and tires. But, if I recall your bike is a bobber and handling will not be a strong suite. I know, I'll hear about that one. Basically, if bobbers handled, that's what race bikes would look like, vintage and new.

2 - Oh oh! You will find lots and lots of snake oil salesmen out there. I don't know where you live but a shop that specializes in the other brands will not be good for you, unless they have a BI tech who knows the old stuff. Interview them/him and get references. Check out the references. Have a friend also check out the references.

3 - Funny thing about bragging rights, aka posuers, it's all show and no go. They end up dead or severely wounded right away. Be careful what you wish for.

4 - Expect to pay ~$7K-$10K to do it right with lots of cool stuff to brag about that doesn't get in your way. CNC's can be fun. I suggest learning about basic maintenance for your steed, it will pay off in the long run. get the owner's manual, parts manual and service manual.

You have great bike that handles well in stock form. If you have the need for real speed and handling and want to stay with a Triumph, I recommend a TT600 or Speed 4.

* I plan to ride on the street - no serious racing - but also ride the local curves (100 mile radius).
* I am not capable of doing any of the work myself.
* I'm looking for as much bragging rights as I can get without losing reliability or requiring rebuilds every 1k miles.
* My budget is not endless but I can afford to do it right.
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