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Sudden change: backfire and low idle

Old 07-10-2011, 11:45 AM
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Sudden change: backfire and low idle

Took the bike out yesterday, fairly warm so I put the choke back in after just a mile or so. The idle went down to about 500 but didn't die on me. So I pulled the choke back out to get about 1000 RPMs. After the temp needle started going up, I put the choke back in but I was still only getting about 600 RPMs. The bike was also backfiring something fierce on deceleration. I usually have some burbling but these were sharp loud explosions out the muffler. I recently did 8541Hawk's carb mods (filled the slide holes from the Dynajet kit and richened the needle on the rear carb) after which I did a carb sync. I also installed an air filter for a car in place of the K&N. The bike ran better than it ever has. I tried richening the pilots jets, but that didn't do anything productive. Any thoughts of what to look for when I pull things apart? Acceleration is good though I haven't wound it up past 5500 or so.

Thanks,
Greg
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Old 07-10-2011, 01:57 PM
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I'd say that you have richened the mixture plenty, based on the decel backfire. Maybe the pilots need more adjusting to get the idle right.
You shouldn't need the choke for a mile this time of year
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Old 07-10-2011, 04:06 PM
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if it was running good and then did this, check everything over because something came off, loose, etc. Set your TPS to 500 if you haven't and re-sync the carbs. These things are finicky and a little adjustment can raise holy hell. Also, i assume you set the fuel screws at the carb bottoms.
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Old 07-10-2011, 05:05 PM
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I assume you cleaned the carbs while working on them, including removing and soaking the pilot jets.

About a year and a half ago, my bike dropped a cylinder 1/2 way through a ride. Limping almost 20 miles to get back home, I was shooting ducks out the exhaust... sounded like a shotgun, must have scared the crap out of a few motorists.

It turned out that the pilot jet in the rear carb was completely plugged. I had to soak it in Berryman's B12 carb spray for 1/2 hour to dissolve the crap in there.

I since changed to 48 pilots.
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Old 07-10-2011, 07:47 PM
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I kind of have the same story, recently did 48s and the slide hole fill and some screw adjustment, mine has more exhaust popping like the OP, probably because its rich. but why would I still have carb farts when I run into colder weather?
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Old 07-10-2011, 09:19 PM
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Sure you didn't get some crappy gas?
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Old 07-10-2011, 09:30 PM
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Originally Posted by xeris View Post
I'd say that you have richened the mixture plenty, based on the decel backfire.
I've forgotten what mains I have off the top of my head. They are either 170/175 or 175/180 from Dynajet. They are smaller than what most people are running with aftermarket exhaust though. I have #48 pilots installed, but the initial settings were 1.5 turns out front and 1.75 turns on the back, so fairly lean. I opened them up a half turn each.
Originally Posted by xeris View Post
Maybe the pilots need more adjusting to get the idle right.
You shouldn't need the choke for a mile this time of year
My garage stays quiet cool and hot here is 80 degrees. I know, I'm a wimp.
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Old 07-10-2011, 09:35 PM
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Originally Posted by nath981 View Post
if it was running good and then did this, check everything over because something came off, loose, etc.
I'm usually fairly methodical except when I've bolted the tank down only to find the velocity stacks still on the bench.
Originally Posted by nath981 View Post
Set your TPS to 500 if you haven't and re-sync the carbs. These things are finicky and a little adjustment can raise holy hell.
TPS was set a while ago, but I'll take a reading again. Thanks.
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Old 07-10-2011, 09:45 PM
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Originally Posted by VTRsurfer View Post
I assume you cleaned the carbs while working on them, including removing and soaking the pilot jets.
I only messed with the slides and the diaphragm seemed to seat nicely when I put them back together.

Originally Posted by VTRsurfer View Post
About a year and a half ago, my bike dropped a cylinder 1/2 way through a ride. Limping almost 20 miles to get back home, I was shooting ducks out the exhaust... sounded like a shotgun, must have scared the crap out of a few motorists.
My bike idles smoothly, even at 600 RPM and acceleration is good, I just get that shotgun effect when I back the throttle all the way off.

Originally Posted by VTRsurfer View Post
It turned out that the pilot jet in the rear carb was completely plugged. I had to soak it in Berryman's B12 carb spray for 1/2 hour to dissolve the crap in there.
I since changed to 48 pilots.[/QUOTE]
I'll take the jets out and give them a cleaning. One thing I remembered today was the last time I took the carbs off, there were small black specks in the gas that was in the fuel line. I think I'll order some new fuel lines, they might be disintegrating on the inside.

Thanks, I'll let everyone know what I find over the next few days or so.
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Old 07-10-2011, 09:46 PM
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Originally Posted by Thumper View Post
Sure you didn't get some crappy gas?
I did get gas that day, but I think the problem started before that, not 100% sure though.
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Old 07-10-2011, 10:29 PM
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+1 on replacing the fuel lines, and check the tank for contamination as well.

I suspect I got some bad gas on a trip my wife and I made up to Santa Barbara. We rode the 33 east from Ojai, and I had to gas up at the Pistachio Farm, which has an above ground tank. My bike started misfiring, only very intermittently, on the way back home the next day. That was the start of my pilots clogging up, I suspect.

Just a heads up, if you ever ride up that way. With our limited range, the Pistachio Farm is the only option for gas that far out on the 33.
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Old 07-11-2011, 07:56 PM
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I got the airbox off and noticed something out of the corner of my eye as I was getting ready to pull the carbs. As the saying goes "Well there's yer problem" At least I think that is the problem. When I did the carb sync the other day, the old vacuum line plugs were starting to crack so I replaced them with some that I had in the garage. The rubber on those was getting a little hard so one just slipped off. Bought some fresh plugs and spring clamps today and everything is back together. Tomorrow, I'll get her warmed up and readjust the idle and see where I stand.
Attached Thumbnails Sudden change: backfire and low idle-vacuumplug.jpg  
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Old 07-11-2011, 08:08 PM
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ha ha! like you said, "fairly methodical", the key word being "fairly". Easy to do, huh.
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Old 07-11-2011, 08:52 PM
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Originally Posted by nath981 View Post
ha ha! like you said, "fairly methodical", the key word being "fairly". Easy to do, huh.
Those things are always the worst to diagnose, cause everyone's trying to figure out internal problems or broken parts. I was just messing with my sparkplugs and didn't put one of the coils back on... took me a little bit to figure out why the damn thing wouldn't idle...
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Old 07-12-2011, 08:22 AM
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I had a similar problem the solution was here
06-25-11 A High Carb Disaster! Naw Just a Geek On a Bike
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