 Carbs backfiring and hesitation
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Joined: Jul 2012
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QUESTION:
What are the different reasons that a carburetor would be back firing. I do not mean the popping on decel. I can hear the carbs popping through the air filters and there is hesitation. fuel mixture might not be right. It is very difficult to start with or without the choke. I have to open the throttle to get it to start. and it won't idle very well.
Carb popping causes:
1. vacuum leak
2. carbs need to be synched
3. carbs need to be cleaned (AGAIN)
4. fuel mixture is not right
These carbs were recently rebuilt at a local shop, and sadly, I don't trust the work from most any shop.
I have not rebuilt Triumph carbs, but I have rebuilt Honda Goldwing carbs, and Honda 750 carbs. So, I don't think I would have a problem doing this if necessary. But where is the best place to get a carb kit and a how to?
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Re: Carbs backfiring and hesitation
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Joined: Feb 2008
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Loquacious
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Loquacious
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Carb farts are generally a sign of overly lean mixture, either caused by a vacuum leak or improper tuning.
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Re: Carbs backfiring and hesitation
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Joined: May 2007
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Worn Saddle
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Worn Saddle
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I'd replace the four 'squirrel condoms' first.
A positive attitude may not solve all your problems, but it will annoy enough people to make it worth the effort. Herm Albright (1876 - 1944)
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Re: Carbs backfiring and hesitation
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Joined: Jul 2012
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oldroadie,
that was the very first thing I did.
And with having to open the throttle to start, combined with carb farts?
lean mixture?
I think we are getting somewhere.
Last edited by goregalore; 07/31/2012 8:43 AM.
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Re: Carbs backfiring and hesitation
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Joined: Jul 2006
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3/4 Throttle
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3/4 Throttle
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I all ways have to start the bike with the choke out and the bike backfired through the carbs during warm up....I just changed out the pilot jet to a 42 and put in Truxton needles...I don't need the choke to start,,didn't notice any carb backfire
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Re: Carbs backfiring and hesitation
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Joined: May 2007
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Worn Saddle
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Worn Saddle
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Quote:
oldroadie,
that was the very first thing I did.
And with having to open the throttle to start, combined with carb farts?
lean mixture?
I think we are getting somewhere.
Good about the vacuum caps. Since the carb is a CV and doesn't have an accelerator pump opening the throttle really doesn't do anything to start the motor. Have you opened the mix screws yet? Either that or one pilot size bigger is what I'm thinking. Go for the screws first, simple and cheap is always better.
A positive attitude may not solve all your problems, but it will annoy enough people to make it worth the effort. Herm Albright (1876 - 1944)
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Re: Carbs backfiring and hesitation
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I have NOT made any adjustments on the carbs yet. I am reading my owners manual etc... I honestly do not know much about this bike.
Like standard settings etc.
I want to know the basics before I start screwing with it, so I can atleast get it back to where it was before I started tinkering.
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Re: Carbs backfiring and hesitation
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Joined: Jul 2006
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3/4 Throttle
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3/4 Throttle
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I think what would help is what mod's have been made to the bike...so for example I have a 2006 speedmaster with British Custom cams, no air box K&N pods only and British Custom Cocktail Shaker pipes and I live at sea level.......all of this effects the jetting of the bike.
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Re: Carbs backfiring and hesitation
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Joined: Jul 2011
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Oil Expert
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Oil Expert
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Quote:
I think what would help is what mod's have been made to the bike...so for example I have a 2006 speedmaster with British Custom cams, no air box K&N pods only and British Custom Cocktail Shaker pipes and I live at sea level.......all of this effects the jetting of the bike.
exactly.
2007 Speedmaster and miss it!
2013 T-Bird Storm and Luvin it!
Catching a yellow jacket in your shirt at 70 mph can double your vocabulary
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Re: Carbs backfiring and hesitation
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Joined: Apr 2007
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Loquacious
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Loquacious
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When you said they were rebuilt, could you tell us what the dealer installed in terms of main and pilot jets along with the type of needles and if they were shimmed. You may have to pull off the float bowls to see what is in your carbs.
Because the bike won't start even with the choke pulled I'd say something is messed up or you have a big vacuum leak as already been stated. The choke is really an enricher that allows extra fuel to be drawn into the engine through another series of feed holes. You shouldn't have to open the throttle (unless you are really really rich?).
The next thing is to know what modifications have been done to the exhaust and intake. That will dictate what you should have in place.
I would agree with Ed and richen the two idle/pilot mixture screws, one under each carb. You might want to pull the plugs to see if you really are too lean. White = lean, Black = rich.
12 Rocket Roadster
03 Bonneville America
69 BSA Firebird Scrambler
73 Yamaha TX 750
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Re: Carbs backfiring and hesitation
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There were 2 mods.
1. removal of airbox and replacement with K&H pods
2. Sceptre exhaust from team triumph.
So those 2 combined will probably need new primary, and main jets
I am trying to find out from the previous owner if he replaced the jets when he had the carbs rebuilt.
Last edited by goregalore; 08/01/2012 7:38 PM.
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Re: Carbs backfiring and hesitation
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Joined: Jul 2006
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3/4 Throttle
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3/4 Throttle
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I am running 42 pilot, 145 main and truxton needles...like I said after I increased the pilot seemed to take out the carb backfire and need to use the choke on start up.
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Re: Carbs backfiring and hesitation
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Joined: May 2007
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Worn Saddle
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Worn Saddle
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Ah, modified at both ends. That's a horse of a different color; you'll need to open a carb up see what's in there before you can proceed. The main jet and pilot (slow) jet will need to be larger than stock with that much air flow.
A positive attitude may not solve all your problems, but it will annoy enough people to make it worth the effort. Herm Albright (1876 - 1944)
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Re: Carbs backfiring and hesitation
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Those mods are almost 10 years old. He said he changed out the jets, and will try to figure out what he changed them too.
The other symptom, I was riding and backed off just a little on the throttle and it sputtered and wanted to die.
But on my way home I got it to 90 without any issue at all. I could have easily gotten more out of it. But I had the wrong helmet and glasses on to handle that speed.
And I will pull the plugs to see if they are lean or rich.
Last edited by goregalore; 07/31/2012 8:42 PM.
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Re: Carbs backfiring and hesitation
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Joined: Feb 2007
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Fe Butt
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Fe Butt
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Carb to intake rubbers on the right way? (It does matter) Sticky valve after sitting? ( you did say it sat a while) Quote:
But on my way home I got it to 90 without any issue at all. I could have easily gotten more out of it. But I had the wrong helmet and glasses on to handle that speed.
I run an indicated 120mph on my way to work with no helmet and little round (John Lennon type) sunglasses.NOTGATT 
I learned all I need to know about life by killing smart people and eating their brains.
Eat right ,Exercise ,Stay fit, Die Anyway!
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 Re: Carbs backfiring and hesitation
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I will check. Sorry, Big Dog, I am not as much of a man as you are 
Last edited by goregalore; 07/31/2012 10:00 PM.
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Re: Carbs backfiring and hesitation
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Joined: Jan 2005
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Should be Riding
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Should be Riding
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Back in 2003 I recall when Robert had to reverse the direction of the manifold to carburetor rubbers. This was taken just after that exercise.  Click in this image for the post. 
Blowing gravel off rural roads
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Re: Carbs backfiring and hesitation
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Re: Carbs backfiring and hesitation
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New Tires
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New Tires
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New jets came with the Septres, so I would guess if someone was paying close to $600 for mufflers they would have changed to the Team Triumph recommended jets. What does your bike idle at and what are your AF screws turned out to?
Also, I may be wrong, but I thought opening the throttle was not advised when starting.
Once you've got this sorted out, balance the carbs, you'll be pleasantly shocked.
Every normal man must be tempted, at times, to spit on his hands, hoist the black flag, and begin slitting throats.
H. L. Mencken
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Re: Carbs backfiring and hesitation
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Thanks Macmc,
I called the septre rep and he knows Robert. the previous owner.
He said the jets they send are 45 pilot and 122 main.
But robert also installed the K&N filters.
I have a tach on the bike but it is not hooked up yet. So, I don't know what it is idling at, yet...
But I am sure now that the best thing I can do is just pull the carbs and clean them again. check the jets. and adjust the pilot screws.
Now where is the best place to get a new carb kit or atleast the gaskets.
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Re: Carbs backfiring and hesitation
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Joined: Feb 2007
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Fe Butt
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Fe Butt
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Not sure what altitude you live at but I bet you should have about a 150 to 155 main unless you have baffles in those Sceptres.
I learned all I need to know about life by killing smart people and eating their brains.
Eat right ,Exercise ,Stay fit, Die Anyway!
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Re: Carbs backfiring and hesitation
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Joined: Jan 2005
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Should be Riding
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Should be Riding
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Quote:
Thanks Macmc,
I called the septre rep and he knows Robert. the previous owner.
He said the jets they send are 45 pilot and 122 main.
But robert also installed the K&N filters.
I have a tach on the bike but it is not hooked up yet. So, I don't know what it is idling at, yet...
But I am sure now that the best thing I can do is just pull the carbs and clean them again. check the jets. and adjust the pilot screws.
Now where is the best place to get a new carb kit or at least the gaskets.
You are NOT running 45/122s with the freak. No way. And you have at least one shim under each main jet needle. You shouldn't need a gasket kit or anything else for the cleaning. Without reviewing your post, have you run through several tanks of fuel mixed with seafoam? When you clean the carbys, pay attention when you drop the jets and air mixture screws as the part order is crucial and the itty bitty o-rings and washers are hard on the eyes. Hum, never mind as you have been there before. 
Blowing gravel off rural roads
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Re: Carbs backfiring and hesitation
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From the research I have done and using the calculator. It should be 155 main and a 42 pilot with the pods and the exhaust, plus a shim.
But that sounds like a good plan. Run a couple tanks of seafoam might really help. The backfiring isn't that bad after it warms up.
Oh, and no bluing on the exhaust yet.
Oh man, please ALL the help I can get is great.
Last edited by goregalore; 08/02/2012 1:56 PM.
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Re: Carbs backfiring and hesitation
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Joined: Jul 2006
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3/4 Throttle
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3/4 Throttle
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Your carb should be easy to pull, with pods only...take the tank off, and un-screw the carb...they should slide out on the kick stand side..there is a allen head screw on the side of the carb bowl to drain the fuel...un-due the screw on the bottom of the bowl...main jet has a nut and screw on it the pilot is in a "hole" and you need a skinny screw driver to get it out...the jet size is marked on the side of the jet...should take a'bout 1 hour to do it....
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Re: Carbs backfiring and hesitation
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Joined: Dec 2006
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New Tires
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New Tires
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Quote:
Not sure what altitude you live at but I bet you should have about a 150 to 155 main unless you have baffles in those Sceptres.
I'm not sure the original Septres have baffles, mine don't.
Every normal man must be tempted, at times, to spit on his hands, hoist the black flag, and begin slitting throats.
H. L. Mencken
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Re: Carbs backfiring and hesitation
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Here is a shot of one of the plugs. Bike does appear to be running lean. -I did put seafoam in the tank. -I also adjusted the pilot screws. Right now at 3.75 turns out. Plan on riding to dinner tonight so should be able to run some fuel through the carbs. I live in Southern Indiana. SO elevation is not really an issue. Also checked the exhaust. No baffles. 
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Re: Carbs backfiring and hesitation
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Fe Butt
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Fe Butt
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If that is the correct pilot setting you probably need a larger pilot jet.
I learned all I need to know about life by killing smart people and eating their brains.
Eat right ,Exercise ,Stay fit, Die Anyway!
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Re: Carbs backfiring and hesitation
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Joined: Feb 2007
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Fe Butt
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Fe Butt
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I just know they made baffles for the Sceptres, I haven't seen anyone running them yet but know some do.42 pilot jet is gonna be too small with open pipes and pods.
I learned all I need to know about life by killing smart people and eating their brains.
Eat right ,Exercise ,Stay fit, Die Anyway!
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Re: Carbs backfiring and hesitation
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Joined: Dec 2006
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New Tires
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New Tires
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Dogg the original Septres didn't have baffles available, but I think they became available in later years. Isn't there a jet calculator somewhere, to help him? 
Every normal man must be tempted, at times, to spit on his hands, hoist the black flag, and begin slitting throats.
H. L. Mencken
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Re: Carbs backfiring and hesitation
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Seriously thank you gentleman,
Took an hour ride last night.
And it ran good but periodically had a carb pop. accel or decel, it didn't matter.
Then we got caught in a storm, riding a motorcycle with seeing that much lightening flashing across the sky right in front of you is awe inspiring. Luckily it wasn't raining that hard. But boy it hurt at times. I still loved it!
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Re: Carbs backfiring and hesitation
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Okay,
pulled the carbs.
155 main 42 pilot, and no shims.
Needle designation NBAD
And again we know the bike is running lean since the the plugs are white.
SO, I will closely check the carb to manifold rubbers to make sure they are good. And if so then I should order shims and a 45 pilot.
Last edited by goregalore; 08/03/2012 6:59 PM.
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Re: Carbs backfiring and hesitation
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I think we found the problem. This one is worse than the other. but both were cracked. Shall we say "VACUUM LEAK" 
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Re: Carbs backfiring and hesitation
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Worn Saddle
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Worn Saddle
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Ooh, that'll take the starch out of the throttle.
A positive attitude may not solve all your problems, but it will annoy enough people to make it worth the effort. Herm Albright (1876 - 1944)
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Re: Carbs backfiring and hesitation
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Ordered the head to carb rubbers from madss.com. 9.99 each RUBBER,HT, CARB TO HEAD ENG NO 55616 AND HIGHER US CAL ONLY bike bandit part #1388148 #8 on diagram carb schematic And ordered a 45 pilot and shims from fast eddys
Last edited by goregalore; 08/04/2012 8:14 PM.
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Re: Carbs backfiring and hesitation
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Saddle Sore
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Saddle Sore
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I would throw on those rubbers and run it for a while to see how she does before doing any additional mods.
We have to assume the carb set up is what Robert was running and I don't recall him having issues.
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Re: Carbs backfiring and hesitation
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Fe Butt
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Fe Butt
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Always best to make 1 change at a time to determine weather it is good or bad. If you make more than 1 change at a time you won't know what was good and what was bad if you have an adverse result.
I learned all I need to know about life by killing smart people and eating their brains.
Eat right ,Exercise ,Stay fit, Die Anyway!
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Re: Carbs backfiring and hesitation
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One thing at a time is the way to go. If I were you I would replace the rubber and see how it runs. Next I'd readjust the A/F screw and test that out. If all is well I'd sync the carbs before changing the jetting.
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Re: Carbs backfiring and hesitation
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That is what I was thinking I should do. I was just getting prepared.
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Re: Carbs backfiring and hesitation
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Should be Riding
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Should be Riding
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Quote:
I would throw on those rubbers and run it for a while to see how she does before doing any additional mods.
We have to assume the carb set up is what Robert was running and I don't recall him having issues.
+1.
Blowing gravel off rural roads
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Re: Carbs backfiring and hesitation
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Should be Riding
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Should be Riding
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Kevin, doesn't that take the cake? Really. "Please de-varnish my carburetors mr stealership" Okay oldcustomerwhomweusedtolovenowwewillripyouoff. Big4Cycle, Way to go. Brilliant attention to detail. Marvelous work. Exceptional diligence. R U KIDDING? Pull the carbs and not see the rubbers as bad? 
Blowing gravel off rural roads
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