Bonneville America Forums Forums Technical Forums Performance Mods stuttering at 3/4 to full throttle.

130's 42's and thruxton needles. Uni drop in, AI's removed.
3 1/2 turns out and slight popping on deceleration.
Took her on a run and opened the throttle. At about 3/4 to full she stuttered and seemed to miss. I reduced speed and at 1/2 throttle she was fine.

I've since put 2 shims under the thruxton needles and still had to go out 3 1/2 turns with slight popping!

Is this a science or voodoo magic.
1. whats with the high speed shudder
2. b****y popping!!
3. How would I know if there was an air leak, thus causing the popping.

Jon

maybe your to rich, did it do it before you shimmed the needles? also tr puling your snorkle for a lil more air, might help lean it a bit

Frank

Or maybe you need bigger mains, the spread that you're talking about is leaving the range where the needle affects the mixture, and going strictly to the main jet influence.

Yeah it happened on the way home. Decided to put the shims in when I got home. Thought it may help!
Snorkles have been removed already.
Do you think the mains should be increased?
How would that help? Would it stop the popping!?
I think I'm going popping mad!!

Jon

Increasing your main jet size will increase your fuel to air ratio; if your current mixture is lean, this will help your problem. Basically just like shimming your needle, has the same affect, just in a different throttle range.

If you're running too rich (which is definitely what it sounds like) then you should go down a size on your main jets. The Thruxton needle has a much smaller taper than the stock needle and it allows more fuel to pass as the carb slide opens up. You're just running the Triumph Off Road pipes, right? I couldn't imagine that you'd need anything more than 125 mains for those (with no shims, especially if you're using the Thruxton needles). You should also pull your sparkplugs and check the color. If you're running rich then it will leave a dark, sooty coating on the insulator.

2 shims on Thruxton needles ?!!! That may well be the problem. The needles WILL cause cutting if too rich even at WOT let alone 3/4 throttle. How do i know? Cuz i tried that too. One shim on thruxton needles caused mine not only to stutter, but at wot the engine stuttered and died well before the limiter usually kicks in. Thruxton needles are very rich, and i couldn't run them with any shims at all. Why you are still getting popping musthave to do with the pilots i'd think. Maybe thier size RELITIVE to the neddle setup has something to do with it.

Have to agree with Roundslide, with my relentless pursuit of perfect Air/fuel mixture ratio I discovered one thing. The leaner the mixture the more popping I've encountered. My suggestion would be to take the bike out for a good ride, maybe a half hour or so. Then pull a spark plug and see what color your plug is. I increased my mains (had too with performance pipes) and darn near eliminated all the popping.

Good Luck, Kerby

If I put in the standard needles would that help?
The stuttering happened before I shimmed the Thruxtons. I haven't tried the Thruxtons on 3/4 throttle yet; maybe I won't now!!
I was trying to make the mixture rich to stop the popping. I thought that adding the shims would help eliminate the popping, as it was still present with the Thruxtons.

The popping could be coming from the air/idle mixture screws. Since you have a newer bike you'll probably have D-shaped screws to adjust the idle mix. You should tighten them until they stop turning (don't go gorilla or snug, just until it stops turning) then turn them out 2 full rotations and start from there. If they are still popping give it a half or quarter turn up (to 2.25 or 2.5) and try again. My setup is Epco pipes (48" long, baffles in) 130 mains, 45 pilots, 2.75 turns out. Took me a little while to get it right but I'm happy with it. I'll have to re-adjust after I remove the airbox and do the big bore but that'll be another thread.

Here's a few carb sites that have helped me a bit

CVK primer

More CVK stuff

Begging your pardon friend, but the idle mixture screws are not going to help him at the range he's complaining about. The idle screws will only contribute through about 1/4 throttle opening, while Splodge is complaining about his 3/4 to WOT opening. Since the mains are really singing at the end of the throttle opening, rich or lean, that's where his problems are.

So.....
I'm running to lean... hence the popping. I adjust the idle screws to elliminate this.
At the top end I'm running too rich, so I .......
Oh bu**er... I thought I was getting this.

I replaced the Thruxton with stock needles and 1 shim for the time. Yes, it is still popping. I'll see how it runs at 3/4 to WOT tomorrow.

If I use the Thruxtons, should I be using 125 main jets and 42 pilots. So I should down grade my jets from the 130's?

Jon

Stop trying to fix two issues at the same time. Work on one at a time. Take the uNI out, put the paper back in. Swap out later when you're done tuning.
Get the mains sorted out, then dick around with the medium and idle throttle positions.
Stuttering is probably caused by being too rich, try your stock 110s with the thruxton needles. If it helps you are closer to the ballpark. Go up in main size until it doesn't help, then back down one. No shims with the Thruxton needles either.
Also check your uNI is not opveroiled. That will also cause your problems. Try it with the stock filter until you get it dialed in, then go back to the uNI.

Good point by Pat......when you oil those UNI's, be sure not to use much oil at all. It only takes a tiny bit, just enough to hit all areas quickly. Then knead it into the foam. Too much and you'll be way rich because too much oil IMO could make it more restrictive than the paper filter.

The reason I started this was due to popping on deceleration!!
I had stock 120's 42's and paper filter but TOR's.
I tried to adjust the idle but to no effect.
Removed the snorkles and got loads more power but more popping.
Thought I would change the setup for more power hence the Thruxton needles.
i think I'm almost there. I just need to sacrifice a few more virgins and I think i'll have it.
Thanks for the help,

Jon

Mike,
True, the idle mixture screws will not help the 3/4 throttle problem but like splodge has figured out, he has two sets of problems. The popping on decel should be helped by adjusting the mixture screws. The 3/4 throttle problem sounds like it's too rich. I had the same problem when I put on new pipes and was making adjustments. It was too rich and if I opened up the throttle it would bog down. If I slowly crept up to speed then it would still break up once I hit 60 or so in 5th gear (on a 16 tooth sprocket).

Jon,
Like Pat said, handle the problems one at a time. I'd set the idle screws to 2 turns (as a baseline) and start working on the jets. Once the jet problem is handled, then adjust the mixture screws so that you're not popping anymore.

When I added shims to my Thunderbird Sport needles, I had the same problem. Removed the shims, problem persisted for one ride than disappeared. No clue what happened or why, but I'm leaving well enough alone for now! Only thing I can think of is maybe one of the top gaskets wasn't on properly, although on disassembly, I saw nothing out of place.

Here's my thread and experience:
Carb Weirdness after Shimming

Like Pat said, work on one thing at a time. This is also good advice for any modification. Only do one mod, make sure you have that done correctly, and then move on.

Start over. Work on either the top end (main jets/needles) performance or the popping (pilot jets/mixture screws) and then move onto the other.