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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 11-05-2012, 10:41 PM   #1 (permalink)
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Popping on overrun

Here's the thing, I've disabled the AI, turned my mixture screws out 2.5 turns, rejetted to 130 mains with one shim under the needle and this is my setup:

-Snorkel removed
-Unifilter air filter
-Norman Hyde togas

Why do I still get popping on overrun (deceleration?). Is it just something I have to live with with the Hydes or am I still running lean on my pilot jet?

Going from the stock 1.5 turns to the 2.5 turns on the idle mixture screws did help soften up the popping, I think. We'll see, because the popping is worse when the engine is fully warmed up than when it warming up.
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Old 11-05-2012, 11:12 PM   #2 (permalink)
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what about the other jet?
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Old 11-06-2012, 12:48 AM   #3 (permalink)
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The pilot jet? I didn't change the pilot jet.
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Old 11-06-2012, 06:26 AM   #4 (permalink)
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Changing the pilot jets to 42 should reduce the popping. You won't need as many turns out on the mixture screws, 1 turn should be close.
Note that many aftermarket open exhausts pop a bit on the over run.
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Old 11-06-2012, 06:36 AM   #5 (permalink)
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BTW. The stock baseline setting is 2.5 turns out on the mixture screws, not 1.5 as you say.
So before you go changing the pilots try them at 2.75.
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Old 11-06-2012, 09:23 AM   #6 (permalink)
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IMO, I'd bump it down to 125 maybe 120, and from there. start at 1.5 turns out and work your way up.

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Old 11-06-2012, 10:11 AM   #7 (permalink)
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My mixture screws were 1.5 turns out on '01 Bonnie with the plugs covering them. I had to drill them out and turn them out to 2.5 turns.

This is my problem. The bike is difficult to warm-up from cold, and stumbles and has trouble idling when cold. The power when accelerating with quarter throttle within the first few miles of start-up is weak and stumbles sometimes. When things are warmed up the bike idles fine and power delivery is instant from closed to open throttle.

All these things suggest to me a lean condition. However, my air box mods are very modest with just the unifilter and snorkel removed. I do live exactly at sea level so I do have more oxygen, however, the bike shouldn't be running this poorly when cold. I did the WD40 intake manifold check and to see if there were any vacuum cap leaks or leaks in the rubbers and found none. Is there any other area of leakage I should be testing?

I don't see how going to 125 or 120 mains will solve my problem, however.

Last edited by luckyduck; 11-06-2012 at 11:21 AM.
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Old 11-06-2012, 10:45 AM   #8 (permalink)
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130 mains are probably a bit rich with the airbox baffle in place. 120 - 125 will lean out the mid to top end but not the lower end of the rev range and won't improve cold starting.
If the screws were 1.5 turns out it seems to me someone's been in there before because the factory setting is 2.5 or thereabouts (see Haynes manual).
You need to richen up the low rev range by turning the screws out in 1/4 increments until it runs better and it it needs more than 3 they're likely to drop out so you need to go up to 42 pilots with less turns.
Have you balanced the carbs ?
Note - These bikes aren't fully warmed up until they've been ridden for 20 miles.

The only way you're going to sort the A/F ratio out properly is on a dyno.
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Old 11-06-2012, 10:49 AM   #9 (permalink)
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Mine has always idled badly or not at all when cold. When I take off on a ride, once I push in the choke, I have to keep the throttle goosed until it's thoroughly warmed up, about 15-20 minutes. It doesn't bother me, because it runs very nicely when it warms up, and I don't want to mess with it.
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Old 11-06-2012, 11:13 AM   #10 (permalink)
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Lucky, You still haven't told us what size pilots you're running. I'm assuming you're running stock 40s. I'd try going to 42 pilot jets and see what that does for you.

I'm also right down here at sea level and have had to go to 45s on the pilot jet with my idle mixture screw out 3.0 turns. I was running 42s and really didn't notice how much my bike still popped and surged at times when the bike was cold. Perhaps this is also why my valves were tight at it's 12,000 mile service and needed adjustment. I don't know, only speculating. Since going to 45s, it's really quieted down on the overrun and no more surging in the lower RPM range.
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