Bonneville America Forums Forums Technical Forums Performance Mods backfire on cold start

I have a 2004 speedmaster that i installed short tors. ai removed, one shim on needles and 3.5 turns on air mixture.
on cold start I usually get 1 loud backfire out of the exhaust before bike starts. Does anyone know what would cause this? Thanks.

Make sure your vacuum ports are plugged tight and don't hit the throttle. When you removed the AI did you plug all 4 vacuum ports or use crossover tubes? If not, this could be the issue. You may have California emissions on your bike that need the heave-ho.

Good Luck

I had that problem a couple times but then i'd rejet richer and it would go away. As of now i have a pretty rich needle setup and it doesn't do that at all anymore. the needles have been where all my major gains and changes like the backfires have happened. so i tend to think it could be that a richer midragne by adding another shim might do it.trying different mains or turns on the screws never seemed to help that or change anything for that matter much at all. But of course thats what i experienced so YMMV.

Use a little choke and no throttle. I've found that twisting the throttle when starting the bike when it is cold causes old ladies and kids to drop!

Thanks for all the info. I am waiting for some main jets to arrive and might try one more shim on the needles. I will give that a try.

I get it occasionally too if I give 'er any throttle on start-up. So, I stopped doing that, unless, of course, I want old ladies and kids to drop; then its kinda fun. And I have 160 mains and 45 pilots with 3.5 turns.

Try leonard's advice before you try mine. As i recall that was also something that helped me. I no longer twist the throttle either and i think that may have been even more of the cure, but it's been a while so i can't say for sure. but try that if you do twist the throttle normally on startup. and it's funny that we even do that because it starts right away every time when i don't touch the throttle.

I was even getting a BF without touching the throttle. So now before I engauge the choke, I turn her over a couple seconds before engauging the choke. Since doing this not a single BF. Vegas

I've heard a lot of people over on triemphrat saying that throttle play during starting is what caused the infamous exploding airbox. Leave it be!

thanks for all the replys. went for a ride yesterday. 55 degrees in nj. No throttle 1/2 choke still backfired.
Will try no choke next time i start it.

If you changed the pilots to the 45s, you will never need the choke again.

Waiting for 45 pilots and 125 mains from newbonneville.com.
I guess i should turn the pilot screw back in to 1.5 turns after I install the 45 pilots.

This is an interesting old thread I just stumbled upon. I get backfiring when it is colder and drier. I figured I was just running a little lean in the cold air since I also chug a little at idle with 3 turns on the screws. Was going to go back out .5 turns on the screws this weekend to see if this helped.

FYI - start it up in the garage for maximum effect on the neighborhood.

Ditto for me. I have 45s and always use the choke on cold start, but it's off in less than a minute.

Mine does that (backfires on cold start) occasionally, but seems like it only happens in the winter. I'm thinking maybe a combination of stale gas from the last tank in the fall and cold air.

Ryan,
Lately Vera was farting almost every start. So, I'll spin her then while she is spinning I'll pull out her choke a bit. Fires right up with no more backfires. Was using 3/4 choke, but occasionally she would backfire. Then no choke. Then no choke with a pull out. That did the trick. Fires right up.

I put my bike up on my new Sears jack today, and removed the seats with the intention of pulling the snorkel. Well, I found I'm the proud owner of an exploded air box! I had a pretty severe backfire on a cold start over a year ago (before I found this site). I figure that's what did it. The plan was to install one of the two uni's I just got from Brent, and maybe drill out the bottom of the box. Now, I'm not sure what to do. Should I get a replacement box and go with plan A, or just bite the bullet and get the freak? I'm not sure if the bike is still under warrenty. 03 TBA w/ 2000 miles. I put on BUBS slash cuts, and had my dealer do the ai removal, and re-jet w/(I think) 128 or 130 mains, and 45 pilots, w/1 shim. The bike runs strong IMO, way better than stock. I guess I'll start by calling the dealer tomorrow. Any thoughts would be appreciated. Charlie

unclecharlie,
I posted this in another thread.... Read your owners warranty manual and you should see the part where emissions stuff (includes airboxes) are warranted for 5 years and 18k miles.

Thanks Grump. But, do the mods void the warranty? Charlie

That would depend on your dealer... they're the one's who make the call.

Charlie, I think if you bring it to Alex at Branchvile you woill be OK with the waranty stuff. I would call him in advance to check, though. You don't need to mention anything about the jet changes.
Matt

Branchville did the ai removal and put the jets in, so that shouldn't be a problem. I tried to call Alex today, but they must be closed on mondays. I will post my progress (or lack thereof). Charlie

And it was DEFINATLY the throtle twist prior to start that caused mine! wont do that again...good thin I had draggin jeans on as I was sitting on the seat when it went

My dealer told me the warranty is only as good as the dealer,as he's the one who will present the problem to Triumph.It is up to him to overlook any mods to the bike or not.I thought I saw on this site that by U.S. law they(Triumph or dealer?) have to prove that the specific mod was directly responsible for the problem if they want void the warranty because of that mod.
Good Luck,
Mike

I finally got around to calling my dealer today,Alex @ Branchville Motors, regarding my exploded air box. He said he'd have to talk to Triumph to see if they would cover it, and call me back. He called me back, and said they went for it, even though the bike was out of warranty. He also said they would be doing an ignition upgrade(?).I was at work and didn't have time to ask him about this. I'm guessing that they are aknowledging the ignitor problem ???????? Charlie