Tuning Trumpie carbies

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Joined: Mon Nov 28, 2005 6:17 pm
Location: sydney

Tuning Trumpie carbies

Post by chopperal » Sat Sep 29, 2007 6:01 pm

Tuning Triumph carbies

(Can't remember where I got it from)

(Note: when trying to sinchronise the slides, I use two short lengths of something like screw driver thick metal bar or something similar.
I twist the throttle enough to place the metal bars (or whatever) under the slides. Then I release the throttle so that the slides are resting on the bars.
Next, I very slowly twist the throttle, while at the same time watching to see if either of the bars dips before the other. If one does I either adjust that one to dip later or adjust the other to dip at the same time as the earlier one. Once they dip together you're ready to move on)

I've never done the ride test for the needle position. On my 73 Bonnie it's the middle slot, and as far as main jets are concerned, I just use the one recommended in the manual.

Tuning carburetors is taken in steps. You absolutely must perform each step in sequence. Each step affects the next one in sometimes mysterious ways. Follow these steps, and your Amal Monobloc, Type 6, Concentric, IRZ, Bing, Mikuni, Tillotson, Linkert, or other motorcycle carb should be ready to deliver its utmost.

In these steps I assume you have two or three carburetors. If you have only one, you may ignore the references to the plural.

Step 1. The Mechanics.
1.Remove all air filter assembly to get access to the intake mouth.

2.Take all slack out of the cables, and back off the idle speed adjuster. On slide carbs, the idle speed adjuster pushes on
the bottom of the slide to raise it. The slides should now be resting at the absolute bottom of their travel.

3.Adjust each cable until there is about 1/16” slack. Make sure the slack remains when the bars are turned to full-lock.

4.You now want to make sure that the slides rise at the exact same time. On a British triple, refer to the factory manual.
On other bikes where the carbs are close enough together, place one finger of the same hand on each slide lightly. Twist the throttle slowly. You should feel exactly when the slide begins to lift with each fingertip. If one slide lifts before the other, give it more slack in the cable. Don’t exceed 3/32” slack in any cable. If you get this condition, you need new cables or adjusters. On some twins like the Triumph Bonneville, the carbs are too far apart for one hand to reach each slide. In this case use either your eyeball, or your ears. In a quiet shop, you can listen and hear when the slack is taken up and the slide begins to lift. You want to hear one simultaneous “clink”, not two distinct ones. If you are good with a steady hand, you can see just when a slide just begins to lift, and then peek at the other carb and twist a bit more to see if the second slide is a little later, right on, or lifted too soon. I have also used a mirror to watch one carb while I used the naked eyeball on the other. Pay special attention and lots of time to this process! Twin carbs must be synchronized perfectly if they are to work properly!

5.This is crucial. Now you must screw each idle speed screw in so that it just touches the slide, or butterfly. Note that I said just touches, not raises. You are just taking all the slack out of each carbs screw just like you have taken all the slack out of the cables, and everything remains evenly balanced. Double check to make sure that each screw is just touching the slide, but not lifting it.

Step 2. The Idle, or Tickover.
1.Find the idle air screw, and screw it gently all the way in on each carburetor.
2.Back off each air screw 1 ½ turns.
3.Start the motorcycle. The slack in the cables will likely not permit the bike to idle at this time. Even if it does idle fine right now, you will need to make sure it is set correctly.
4.Let the engine oil warm to about running temperature. You should not be tuning a cold engine.

5.Keep the engine running with the throttle, and on a twin remove the plug wire from the second cylinder. On a British triple, refer to the factory manual.

6.While running on one cylinder, screw in the idle speed adjuster until the engine runs with your hand off the throttle. IMPORTANT: Note how many turns you gave the idle screw!

7.Turning the mixture screw in or out by ½ turn increments, listening for the engine speed to increase. If the engine slows, back off until you reach peak idle speed. If the idle speed becomes excessive, back off on the idle speed screw and re-adjust the mixture screw until peak idle speed is set. You want the screw to be screwed in as far as possible while maintaining the highest engine speed. That is, if the engine runs just as fast at 2 turns out as it does at 1 ½ turns, leave the mixture screw at 1 ½ turns.

8.Now connect the second cylinder’s spark plug cap. The idle should increase dramatically.

9.Remove the spark plug cap from the cylinder you just adjusted, and adjust the idle speed and mixture as you did for the first cylinder, as in steps 6 and 7.

10.Now when you replace the plug cap, the idle should be far too fast. Lower the idle beginning with ¼ turn on each carburetor’s idle speed screw. Count the turns, and do not back off more than you turned it in! You will ruin the balance by turning one screw excessively.

11.You may find that throttle blips affect the idle speed until everything beds in. Make adjustments in the idle speed in ¼ turn increments only, and evenly from side to side.

Step 3. The Slide Cutaway.
1. If you’ve got a new carb on a new engine, you want to judge the slide cutaway. The slide cutaway is indicated by a number such as #2, or 3 ½ marked on the slide. Throttle performance from 1/8th to ¼ throttle is controlled by the slide cutaway. Hold the throttle at 1/8 open and if you get lumpy running and some black smoke, the cutaway is too small. If the performance is improved by closing the choke slightly, or placing a finger in the carb throat, the cutaway is too large.

2.Make sure each carb has a matching slide on twins!

Step 4. The Main Jet and Needle.
1.Replace the air filter assembly from here on. All testing will be done on the road, as a loaded engine is best suited to tell its story.

2.From about ¼ throttle to ¾, the needle’s position relative to the main jet comes into play. Take the bike out and on a level or slightly uphill road, hold ¼ to ¾ throttle. If closing the throttle slightly improves performance, you are lean and will need to raise the needle’s height. Closing the choke and improving performance will confirm that the needle must be raised. If the exhaust is black, and performance is sluggish, and pickup slow, the needle must be lowered as the mixture is rich. Cutting the ignition with the throttle in the suspect position, and removing the spark plug can verify a rich mixture. A sooty black plug will confirm a rich running diagnosis.
3.The size of the main jet itself comes into play at about ¾ throttle to WFO. The signs of leanness and richness are the same as for the needle position. Generally a larger main jet number indicates a larger main jet, increasing richness.

4.If a main jet is changed, obviously you must re-check the needle position. Conversely, if the needle cannot be dialed in a larger or smaller main jet may be in order.

5.Performing an ignition chop and plug reading at any stage will result in a sooty plug for a rich condition, whitish for lean, and milk chocolate for a good mixture.

6.Pre-mix ratios, fuel condition and octane, humidity and altitude all affect carb tuning. Tune for the day and conditions you will ride in.

cheers, al :)

Posts: 1065
Joined: Wed Aug 22, 2007 8:51 am

Post by Chucky » Thu Nov 05, 2009 9:43 am

Two 3/16" drill bits work GREAT for visualising your slide movement! and getting it perfect.


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