Carb imbalance weirdness - Star Motorcycle Forums: Star Raider, V-Max, V-Star, Road-Star Forum
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post #1 of 14 (permalink) Old 02-17-2016, 04:42 PM Thread Starter
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Carb imbalance weirdness

Just finished my 650's first big winter overhaul, which included:
1) carb removal, cleaning and reassembly.
2) GAK kit + 127.5 jets + 3.5 on the idle screws + 2 #4 washers as shims on each needle.
3) valve clearance check
4) AIS removal

Already had a set of Cobra drag pipes with lollipops when I bought it. Last step was a carb sync after I made sure it was running okay.

Good news: Bike starts and runs okay.
Bad news: I cannot sync the carbs now.

Facts: The front carb is drawing something on the order of 15 lbs of vacuum and the rear's only drawing 5. Fiddling with the sync screw produces changes in the tone and quality of the idle, but no substantive change in how much vacuum either one's drawing. They simply never get close to each other. Loosening the screw (counter-clockwise) should improve the rear's vacuum and lower the front's, but it simply doesn't; I've gotten the screw to the point where it's almost falling out of the linkage, and it doesn't help. Tightening the screw (clockwise) does little more than roughen up the idle until it eventually dies and won't start until loosened a bit.

I already checked for air leaks around the filters (tightly mounted to the carb) and the boots/flange where they met the cylinders; no change in RPM.

I'm already resigned that I'll have to get shoulder-deep in the carb(s) again, probably starting with a "bench sync", but I'd really appreciate having an idea of what I should be looking for. Everything seemed A-ok when I put it back together...

Last edited by Arnold_Kupferkopf; 02-17-2016 at 04:49 PM.
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post #2 of 14 (permalink) Old 02-17-2016, 05:33 PM
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better check the valve adjustment again, if one is too tight you'll not only lose vacuum the compression will be off too. do a compression check before you start tearing things down. even overtightening by .001 past 0 lash will cause serious problems. definitely don't rev the engine before checking clearances again. when i set clearances i manually hold the rocker up when inserting the feeler gauge because rockers can be sticky when cold. messing with carb jets will not affect vacuum the position of the butterflies will and there's no way they got that far out of position on their own. even the choke won't affect vacuum because it's a bystarter type. also at idle you should be pulling almost a perfect vacuum at least 18-20inhg at idle 30inhg is a perfect vacuum but i'v never seen it on engines if the ais is connected it might read a bit lower. low vacuum= leaks and the leak can be a valve. the sync screw only affects the rear carb butterfly the front butterfly is controlled with the idle knob only. my guess would be the rear exhaust valve is tight.

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Last edited by pauli466; 02-17-2016 at 06:18 PM.
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post #3 of 14 (permalink) Old 02-17-2016, 07:47 PM Thread Starter
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Did another vacuum + compression check tonight, VERY eye opening

Rear cylinder: 95 lbs of compression, but only 5 lbs of vacuum.
Front cylinder: 15 lbs of vacuum, but only 35 lbs of compression.

The 35 compression reading was an absolute shocker, especially given how much vacuum it was producing. This bike ran fine and mostly runs fine now, I would have thought I'd see or hear major symptoms for something that serious.

Clearly now I'm looking in the direction of my valves being tight; do you still think it's the rear valve?

Last edited by Arnold_Kupferkopf; 02-17-2016 at 07:49 PM.
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post #4 of 14 (permalink) Old 02-17-2016, 10:06 PM
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I hope you held the throttle open when you did the compression test...if not....your readings are not close to being accurate.

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post #5 of 14 (permalink) Old 02-18-2016, 05:47 AM
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Quote:
Originally Posted by Arnold_Kupferkopf View Post
Did another vacuum + compression check tonight, VERY eye opening

Rear cylinder: 95 lbs of compression, but only 5 lbs of vacuum.
Front cylinder: 15 lbs of vacuum, but only 35 lbs of compression.

The 35 compression reading was an absolute shocker, especially given how much vacuum it was producing. This bike ran fine and mostly runs fine now, I would have thought I'd see or hear major symptoms for something that serious.

Clearly now I'm looking in the direction of my valves being tight; do you still think it's the rear valve?
yes i believe the exhaust valve it to tight on rear cylinder. i would start with that one first then move on to the rest. you might be able to remove the rear exhaust valve cover with the head cover on. try 1/4 drive with a swivel. when the rocker is on the cam heel push up on the rocker with your finger to be sure it's against the cam heel this will also squeeze the oil between the rocker and cam out for accurate measuring. forget lining timing marks, just visually watch the rocker for top out then roll the engine 1/4 turn 12 o'clock to 3'oclock then adjust. your compression test wasn't done right or your not reading the gauge right because a cylinder can't fire at those readings. minimum pressure of 90 is needed and at 90 psi it would run like a model T recheck compression after adjusting like this http://www.dansmc.com/compression_test.htm

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Last edited by pauli466; 02-18-2016 at 05:59 AM.
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post #6 of 14 (permalink) Old 02-18-2016, 10:50 AM
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On your other thread you said the valves were not making any noise. Bad sign. No noise means no valve lash (clearance) and that means the valves may not be seating. If the exhaust valve isn't seating it will burn quickly because A) fire blasts through the gap every time the cylinder fires and B) the valve is cooled by conduction from it through the valve seat when it is closed. If you want an engine with no valve noise you are going to have to look for something with hydraulic lifters.
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post #7 of 14 (permalink) Old 02-18-2016, 04:33 PM Thread Starter
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D'oh

D'OH! You're all right. Forgot to crank the throttle while I did my compression test. Not sure why that slipped my mind, probably because I was so focused on the carbs and the valves. Either way, it didn't help the rear carb which is still stuck at 5 pounds of vacuum even after loosening the valves up.

I've got the carb out on the bench now, so I'll redo the test properly when I think I'm ready to reinstall. Still no idea what (if not overly tight valves) could be causing it to produce so little vacuum.

In the meantime, as a gesture of my thanks for your assistance, please enjoy this camera footage of what my last week in the shop has looked like.

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post #8 of 14 (permalink) Old 02-18-2016, 04:50 PM
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Now that you have the carburetor(s) off, run the compression test again. That will completely eliminate them from the picture and give you an unobstructed intake, which is the idea behind holding the throttle open.

Last edited by Charon; 02-18-2016 at 04:52 PM. Reason: correction
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post #9 of 14 (permalink) Old 02-18-2016, 05:26 PM
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the compression readings you posted can not be correct at 95 psi the air/fuel mix will just barely ignite. the front reading at 35 psi would be impossible and the engine would be firing on the rear only with the front one popping/ coughing or nothing at all. i'm at sea level and with over 50,000 miles the compression is 160lbs front 155lbs rear for every 1,000 feet of altitude engine looses around 1-2 lbs of compression and 1inhg in vacuum. with a few drops of oil in the cylinder i can get 165lbs on each ( wet test ) my valves are set .003 in and .005 ex in inches not mm and there's just a hint of tappet noise which is absolutely necessary no noise= too tight on clearances

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post #10 of 14 (permalink) Old 02-19-2016, 04:17 PM Thread Starter
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You're quite right, I did the test wrong. I redid it and came up with 140 lbs on the front and 120 lbs on the rear.

I removed the carbs, completely disassembled and re-cleaned and re-checked everything, then reinstalled. My mission was to painstakingly double check that I'd covered all of my bases internally. I even went so far as to swap the slide/needle/diaphragm assemblies from the front to the rear, just to make sure that wasn't the issue.

Warm it up, hook up the testers, and... same exact situation as before. 5 lbs of vacuum on the front, 15 lbs on the rear, no detectable external vacuum leaks, and turning the carb sync screw still results in zero (I say again, zero) change in the pressure from either carb. I actually accidentally unscrewed the sync screw so far counterclockwise it fell out while the bike was running; zero change in vacuum pressure, zero change in idle. Turning it too far to the right rapidly slows the idle and eventually kills the bike, but the vacuum never changes.

Last edited by Arnold_Kupferkopf; 02-19-2016 at 04:20 PM.
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