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Discussion Starter · #1 ·
i pulled in a gas station tonite and as put my foot down my pant leg caught the little knob on the kickstand and dragged it down. because i was still in gear it killed the engine. no big deal. when i started it back up (after about 5 mins) it backfired once thru the carbs just as it started running and i felt the side covers pop against my legs. it's never done that before and it spooked me. i had the performance package from british customs (exhaust, airbox removal and a rejet) put on last year but it's been running fine since. i've been reading that backfiring thru the carbs is an intake valve being open while igniting due to bad timing. could the kickstand kill switch have caused the timing to be messed up or is that just coincidence? should i be concerned about the intake/exhaust mods? also, i can't seem to find a definitive answer to the question: can a backfire thru the carbs damage anything? any info is good info, unless it's bad.

it's an 06 thrux.
 

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Hi... that backfire was caused by unburned fuel in your cylinders, causing a very rich condition on restart.

When your side-stand went down, it cut off the ignition while the pistons were still flopping around, sucking in gas and air. On restart, the raw gas ignited with a 'POP'.

When I was in High School (1969-73) sometimes I would drive a my mom's '65 Dodge Dart GT with a 225 cubic-inch 'slant-six' engine. When I got near the school...on a long down-hill run, I would downshift the TorqueFlite to first gear, bringing the revs up, and then switch off the ignition...

After a second or two I would turn it back on, and 'BANG' there was huge POP from the muffler!! Loads of fun when you are 16 with a car-full of your pimply faced buddies, but...hmmm, not so much now...although, I still like a good 'BANG' once in a while!

Cheers,
BLIGHT
 

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Discussion Starter · #3 ·
i understand how that situation could cause the unburned fuel to build and pop out of the exhaust on startup due to it sucking in fuel without burning it then blowing it into the pipes, but i guess i can't wrap my head around how it got out of the intake side. i would think that the valve should be closed during ignition....or doesn't it matter?
 

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i understand how that situation could cause the unburned fuel to build and pop out of the exhaust on startup due to it sucking in fuel without burning it then blowing it into the pipes, but i guess i can't wrap my head around how it got out of the intake side. i would think that the valve should be closed during ignition....or doesn't it matter?
Hmmm, that's a good question! My SuperHawk 996 V-Twin with dual 48mm flat slides (!) used to backfire through the airbox, usually when idling. From the SH user group feedback it is common on these big twins.

The only thing I can think is that your idle mixture is too lean, causing an occasional pre-ignition (no spark required) that occurs before the intake valve has fully closed. But that doesn't make any sense to me either...you wouldn't be on the compression stroke if the intake valve was still open!

Perplexing, but I've experienced the same thing... even a couple of times on the Triumph.

Cheers,
BLIGHT
 

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Backfiring throught he carb is one of the symptoms of a lean mixture. As you say you have rejetted to allow for those mods, and assuming the carb hasn't been touched since, the lean condition could be caused by an air leak.
Check any vacuum hoses or manifold vacuum blanking plugs for soundness.
 

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+1 on the lean mixture, BUT, if it just did it that one time and hasn't done it since, I wouldn't worry too much. She could be just chastising you for dropping the kickstand with the bike in gear.
 

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i understand how that situation could cause the unburned fuel to build and pop out of the exhaust on startup due to it sucking in fuel without burning it then blowing it into the pipes, but i guess i can't wrap my head around how it got out of the intake side. i would think that the valve should be closed during ignition....or doesn't it matter?
The Bonnie uses a wasted spark system, that is both plugs are fired as the pistons approach top dead centre, the cylinder that receives the wasted spark is just completing the exhaust stroke and about to start the induction stroke when the spark occurs so the exhaust valve is closing and the intake valve is opening (valve overlap). If there happens to be unburnt fuel / air mix in that cylinder, due to the last induction charge in that cylinder not being fired, then it is obvious that a "spit back" through the carb will occur when the "wasted" spark fires that remaining fuel / air mix because the intake valve is opening as the mix is burning.
 

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I'm resurrecting this thread because I'm having carb backfire problems after airbox removal and new jets. Hell, I've always had this problem, even stock.

My setup right now: airbox removed, no AI, 150/42 jets, Emgo Reverse cone mufflers. Idle mixture screws turned 2.5 - 2.75 turns out.

I have balanced the carbs and installed new Irridium IX plugs. I've tested both the coil and spark plug wires to make sure that they're functioning correctly.

Also, the bike runs like crap with choke pulled out and stalls cold with choke in. Even with choke pulled it drops into lower RPMs.

Maybe I'm running too rich? The jets that were supplied to me came from Newbonneville and he said that that was fine. I have not yet dyno tested.

I need some suggestions. I can order some 145's if need be.
 

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My setup right now: airbox removed, no AI, 150/42 jets, Emgo Reverse cone mufflers. Idle mixture screws turned 2.5 - 2.75 turns out.
Considering the standard jetting is 110/40 and 2.5 turns out of the idle screw, I feel you're running too rich throughout the range. For example if you increase the size of your idle jet you expect to have to reduce the screw opening a little, yet you have it opened even further than stock.
For the mods you have I would have increased the main jet by about 15% to something like 125. A 150 seems too big. It's something like 36% larger than stock.
 

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It's running with 150 main jets fine other than cold. Thing is, I won't know until I'm on a dyno and they're expensive and I'm broke!
 

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Idle mixture

If you're backshooting through the carbs, it sounds to me as though you're too lean at idle. Try adjusting your mixture again and richen it up. You didn't say what type of air filters you have, but I'm running 150 mains on mine with UNI pods. They breathe a good bit better than K&Ns. Your Emgo mufflers are probably pretty free-flowing as well, so the 150 mains may not be too far out there.

Try adjusting the mix and let us know how that works out.
 

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I have a 2007 Bonneville and had some scarry loud backfire after filling up. I also have a BC upgrade. Could not figure it out for about two days. Considered mixtures and bad gas, but it all boiled down to one of the rubber nipple covers came off of the right intake manifold and was running lean because it was sucking in air at this carb. I don't know how I didn't notice this for two days. I had extras that came with the upgrade package. good luck.
 

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i pulled in a gas station tonite and as put my foot down my pant leg caught the little knob on the kickstand and dragged it down. because i was still in gear it killed the engine. no big deal. when i started it back up (after about 5 mins) it backfired once thru the carbs just as it started running and i felt the side covers pop against my legs. it's never done that before and it spooked me. i had the performance package from british customs (exhaust, airbox removal and a rejet) put on last year but it's been running fine since. i've been reading that backfiring thru the carbs is an intake valve being open while igniting due to bad timing. could the kickstand kill switch have caused the timing to be messed up or is that just coincidence? should i be concerned about the intake/exhaust mods? also, i can't seem to find a definitive answer to the question: can a backfire thru the carbs damage anything? any info is good info, unless it's bad.
I have the same problem here man. 2006 Bonnie and getting the backfires and pops. I took it to a friend who told me that it shouldn't hurt my bike at all but that i needed to get my carbs checked out. I'd like an answer on this too.
 

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Considering the standard jetting is 110/40 and 2.5 turns out of the idle screw, I feel you're running too rich throughout the range. For example if you increase the size of your idle jet you expect to have to reduce the screw opening a little, yet you have it opened even further than stock.
For the mods you have I would have increased the main jet by about 15% to something like 125. A 150 seems too big. It's something like 36% larger than stock.

I agree Earl's too rich low down but 125's with pods will be way too lean, minimum of 138-140. Try going back to a 40 pilot at 2 turns out and also possibly experiment with 1 shim.

Don't worry too much about an occasional backfire through the carbs, my jetting is spot on and I still get it once in a while.
 

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Bang!

Last year I ran out of fuel just as I pulled into my garage. I wanted to see how many miles I could get till I had to use reserve. I switched it over in the morning and tried to start it. It must have been lean and it blew the top of the air box off. It is not easy to change so I plastic welded the thing back together that was 5000 miles ago all is well.
 

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Sounds similar

Hello all,
I have been searching and reading trying to find a cause to my problem and this sounds similar to what I am getting on my 05 Bonnie.

The bike has been sitting since last September. I ran the bike out of gas, or the carbs that is before parking it for the cold season. It has been sitting on a trickle charger ever since. I know I should have gone out and started it a few times, but as things go, life happened and I neglected my Bonnie. Now she is giving me grief.

Yesterday the thing took forever to start up and idle. Once it did start idling I have been hearing and seeing some sort of spitting, sputtering from the carbs. Specifically from the rubber hoses connecting the carbs to the engine. I say seeing because there is a lite residue where it is spitting out. I am also hearing a lot of popping from the exhaust. The part that really worries me the most is when I try to apply any throttle, the engine bogs down and wants to die. Even after letting the engine warm up. I ran into this behavior once before while riding and like an idiot I did not know that was a sign to switch over to my reserve. Gas was low. Not used to watching my mileage as a gas gauge. What could be causing this? Hopefully I can fix it before the riding season gets here fully.

Thanks for any suggestion.
 

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Hello all,
I have been searching and reading trying to find a cause to my problem and this sounds similar to what I am getting on my 05 Bonnie.

The bike has been sitting since last September. I ran the bike out of gas, or the carbs that is before parking it for the cold season. It has been sitting on a trickle charger ever since. I know I should have gone out and started it a few times, but as things go, life happened and I neglected my Bonnie. Now she is giving me grief.

Yesterday the thing took forever to start up and idle. Once it did start idling I have been hearing and seeing some sort of spitting, sputtering from the carbs. Specifically from the rubber hoses connecting the carbs to the engine. I say seeing because there is a lite residue where it is spitting out. I am also hearing a lot of popping from the exhaust. The part that really worries me the most is when I try to apply any throttle, the engine bogs down and wants to die. Even after letting the engine warm up. I ran into this behavior once before while riding and like an idiot I did not know that was a sign to switch over to my reserve. Gas was low. Not used to watching my mileage as a gas gauge. What could be causing this? Hopefully I can fix it before the riding season gets here fully.

Thanks for any suggestion.
Have you checked for vacuum leaks? If it's been sitting awhile the rubber caps or hoses on the vacuum fittings might be cracked and leaking.
 

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this is EXACTLY what my bike has been doing since i pulled the airbox a long time ago. with the stock carbs i'd get a frequent backfire through the carbs, experiemented with jetting, idle mixtures, etc to no avail. i thought the problem would go away when i installed FCR's but still nothing. i even had the head rebuilt (not for this problem in particular, but when it was sent out for porting and polishing), so with FCR's and a rebuilt & flowed head i STILL get the backfiring through the carbs! i tuned the bike with an A/F gauge and basically it's pretty darn close to 13:1 though the range, and had it dyno'd to confirm that, making great HP, but my idle mixture has always been in the high 13's low 14:1. i am guessing there may just me a magic sweet spot that the mixture screws need to be at, but have not found that spot yet, and am kind of bummed. i know it's not hurting the bike, but I will admit, it is INCREDIBLY EMBARASSING if you are at a light or stop at an intersection where there are a ton of other bikes around, and poeple are admiring your bike, and then it all of a sudden backfires/throws up on itself. just isnt good form in my opinion and i would do anything to make the backfiring go away! the rest of the RPM range the bike is solid and totally rips though. i've even adjusted mixture screws that allow me to just reach down and adjust on the fly while i am riding. no d-tool/screwdriver needed. anyu suggestions would be greatly appreciated! i have no vassuum leaks, rotted vac caps, exhaust leaks, etc.
 

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Chade22,

Have you put clean new gas in your tank, since September ?
Old crappy gas will cause those problems and the gas we have now breaks down real quickly, don't ask how I know.....
 
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