Weak rear cylinder. Backfire, surging - Star Motorcycle Forums: Star Raider, V-Max, V-Star, Road-Star Forum
 
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post #1 of 10 (permalink) Old 07-28-2016, 01:13 AM Thread Starter
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Weak rear cylinder. Backfire, surging

I could use a little help diagnosing an on going problem with my 1100 V Star.
This entire riding season I've been having issues with my bike backfiring and surging at a constant speed. Runs good when I'm on the gas hard, but if I'm just trying to cruise at a steady speed it runs like crap.
I also used to start it when cold by pulling the "choke" about 3/4 way out and it would fire up. Now I have to keep cranking it until it catches and slowly add some choke while cranking. It also doesn't idle as high as it used to with the choke out.
Here's what I've done in my hunt for an answer.
Changed plugs. (old ones looked good but were old)
Adjusted valves
Changed Fuel filter
Ran several tanks with Seafoam
Synced carbs several times.
Yesterday i removed the carbs and cleaned them. They weren't really bad inside at all.
Checked plugs and double checked for correct gap. Plugs are good and so is gap.
Synced carbs again.
Tonight on the way home it's surging and backfiring.
When I got home I felt the pressure from each pipe.
Front cylinder is a strong steady pulse.
Rear cylinder is weak and is the one that backfires.

I have a bike trip planned for next week and I don't trust my bike right now!
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post #2 of 10 (permalink) Old 07-28-2016, 07:13 AM
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what's the compression difference between the cylinders? i would start there, all the tinkering it the would won't make it run better if cylinder compression is low. if it's low bad rings or valves leaking or adjusted too tight., also vacuum leaks will cause the exact problem your having

dumb bikers don't get to be old bikers
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post #3 of 10 (permalink) Old 07-28-2016, 11:01 AM
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Also check for cracks in the manifolds and air leaks.
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post #4 of 10 (permalink) Old 07-28-2016, 04:52 PM
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if the jets arent dirty check for air leaks
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post #5 of 10 (permalink) Old 09-02-2016, 08:46 PM
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Did you ever figure this out?
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post #6 of 10 (permalink) Old 09-02-2016, 09:46 PM Thread Starter
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Pauli466 pointed me in the right direction. Compression test told the tale.
Turns out the rear cylinder exhaust valve was too tight as he suggested. Thanks Pauli466!!

I followed the directions from the service manual and adjusted it.
I looked online and found some other instructions that said once adjusted within spec , rotate the engine and check again. Sure enough it was too tight again. I rotated and adjusted 3 times and triple checked it.
Bike fired up pretty good after that.
Gas mileage is back to normal. It's not backfiring and bucking.
It still runs a little rough and there is some popping that wasn't there last year, but it's actually rideable now.

Did a great 2 up trip with the wife all around the Pacific Northwest. Anacortes and Whidbey Island, and the Olympic peninsula and back into Victoria BC. The bike never let me down once.
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post #7 of 10 (permalink) Old 09-03-2016, 05:35 AM
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i found the cap over the carb sync vacuum port had deteriorated creating a very small leak, so i used a piece of hose with bolt stuck in the end and the exhaust stopped popping. i also resealed all the exhaust joints,at the heads and mufflers with ultra copper.

dumb bikers don't get to be old bikers
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post #8 of 10 (permalink) Old 09-03-2016, 11:11 AM
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I will do a wet dry compression test once I find someone with the compression tools. Where exactly should I look on sync vac port?
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post #9 of 10 (permalink) Old 09-04-2016, 12:48 PM
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Ditto on Pauli466. Check your vacuum-check nipple covers to ensure they're fully sealed. Had one of those go bad on mine and it produced exactly what you describe.

2007 V-Star 1300

-Cobra Slip-ons
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post #10 of 10 (permalink) Old 09-04-2016, 04:51 PM
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Make sure that whoever does the compression test understands how to do it on a V twin....as in make sure you hold the throttle open....

Sick Mick* 2006 V Star 1100 Custom

--- Striping, lettering and paint by Sick Mick 6* Tree - 2" extended forks 21" front wheel - HK Sideburners - Progressive rear shock Pods Sick Mick solenoid delete - Spring seat - and other stuff
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