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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 06-03-2008, 05:44 AM   #1 (permalink)
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Carb Tuning: Pilot jet size and mixture screw settings

This may be old hat to many of you, but it was news to me and would have helped a lot with my carb tuning problems in the past weeks were I to have found it on the forum. So, for those who don't know:

What I discovered from my own carb tuning research was a little-discussed property of pilot jets. That is, that the pilot jet supplies 75% of the air-fuel (a/f) mixture at up to 1/4 throttle cruising, and only 25% of the mixture at idle.

With 42 pilots (with the mixture screws set for highest rpm i.e. correct a/f ratio for idle) I had roughness problems 2-3000 rpm (on the dyno it showed up as lean at 14+:1 up to 1/4 throttle) and turning out the pilot mixture screws didn't make significant difference to this roughness at cruise, but did make the idle way too rich. I managed to get my 1/4 throttle cruise set perfectly at 13:1 a/f ratio by going up to 45 pilots and turning the screws right in for 13:1 a/f at idle, verified on the dyno. Now both idle and steady cruise are 13:1.

Problem for me was not knowing what exactly adjusting the pilot screws was doing to cruise fuelling (without having to check that on a dyno), because being set right at idle didn't mean the pilots were set right for cruise.

So, the relationship we need to consider for pilot jets, seems to be pilot jet size should be chosen for the low throttle cruise fuelling (checked on a dyno). Pilot screw setting for idle fuelling.

(If anyone can say more on this I'd be interested as there doesn't seem to be much info available)
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Old 06-03-2008, 12:03 PM   #2 (permalink)
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Stock needles?

If so....I ran stupid lean in that range with stock needles....regardless of shims....that's using a 42 pilot jet.

I tried Thruxton needles and Dynojets....and settled on the DJs.
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Old 06-03-2008, 12:25 PM   #3 (permalink)
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Thrux needles.
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Old 06-03-2008, 11:45 PM   #4 (permalink)
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Good info, Frank. As a general rule I have found that the best way to go is to adjust in the following order:
  1. Float heights
  2. Main jets
  3. Pilot jets
  4. Low-speed screws
  5. Balance
Minor a/f funkiness between the pilots & screws can sometimes be compensated for by fine-tuning float heights, if necessary.
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Old 06-04-2008, 06:23 AM   #5 (permalink)
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Another thing...

Just to throw another variable into the mix, and that is the rate at which the slide raises the needle, which is tuned either by changing the spring rate OR the airhole in the slide itself. The Dynojet kits includes changes to one or the other of these, and Jenks guide also mentions it, so there must be something in it.

I would think that if the slide moves quicker, this will bring the taper on the needle into play quicker. I also think shims may go some way to replicating this effect, but then the taper is not ideal through the rest of the range, which may explain some other problems.

These carbs are complex beasts!

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Old 06-04-2008, 12:46 PM   #6 (permalink)
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Thanks for your input guys. I agree, float height, slide and needle are all important (I haven't discovered what float height does to fuelling throughout the rev range/ throttle opening range though, so comments welcome). What I found interesting was the differential between pilot mixture screw adjustment and pilot jet size.

On this forum, it's common to hear everyone talk about 'how many turns out on the pilots screws' etc, then others talking about setting the screws for best idle. What I found was that setting the screws for best idle doesn't necessarily mean you're setting for best low-throttle cruise fuelling. You can't really tell (unless you're very experienced) what the fuelling is doing at cruising speeds after you alter the mixture screws and/or pilot jets, unless you have an a/f gauge or a dyno measure at that throttle opening.

We go to a lot of trouble and expense having our bike's dyno'd for main jet size, maximum power and at wide open throttle. However, where we do most of our riding isn't at WOT or even 50% throttle, it's down the lower end of the throttle opening, with intermittent, short bursts into 50 and 100% throttle (acceleration and overtaking etc) for normal road riding. So it follows, in my thinking at least, that also paying close attention to fuelling at up to 25% throttle would be pretty crucial for overall best performance, since you could be running very lean for most of your riding (as I was) when your 100% throttle dyno curve says otherwise.

Others who are more experienced may disagree, of course, and if I'm wrong it'd be good to hear other views.

All the best.
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