Hello,
Ive been having trouble syncing my carbs. The left cylinder pulls way more vacuum than the other, so the manometer won’t level at all. The carbs were just rebuilt by Bing. I did a compression test and it was strange. I saw online to remove the carbs first before testing, so I did both:
with carbs on, no filter: L=65 R=120
without carbs on: L=130 R=125
what on earth is going on??
thanks!
One other clue; the plug on the left cylinder is black and smells of fuel. The right plug looks perfect. I have verified that the valve clearance is right.
Here's the procedure I use....
1) First... Adjust your valves. Airhead valves tighten during use. The number one cause of low compression in Airheads is tight valves.
2) I agree with removing both carbs from the rubber intake manifolds. Opening the throttle is not the same since the slide is positioned by air flow and not the twist grip position. (Use some WD-40 to lubricate the carb when pushing the carb back into the rubber hose.)
3) Remove both spark plugs. Insert the plug tops back into the plug caps, and then lay the plugs on the cyl or cyl heads so that they make contact and can spark. If the plug is not allowed to spark, then you may burn up a coil !!
4) Screw a known good compression tester into one of the plug holes.
5) Kick or use the electric starter to turn the motor over FAST ! Kick or push the starter button a LONG TIME until the maximum compression reading is obtained.
6) Record the maximum reading. Install the compression tester into the other cylinder and repeat Step 5.
7) Both readings should be over 100psi and within 10% of each other.
* Airheads don't typically have compression issues. Not anything near the rate of other air-cooled Jap, English, or Italian bikes.
* Blacken plugs can be caused by low compression. But it is far more common for blackened plugs to be caused by the wrong heat range spark plugs OR poor adjustment of the carbs. Go buy yourself a set of NGK BP7ES plugs and start there.
* Best compression readings will not be available unless the engine has been run 100 miles within the last week. That is to say, don't expect wonderful readings if you just pulled the bike out of a barn. A better use of your time is to change the oil, adjust the valves, and then go ride the bike.
Hope this helps.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
My opinion - except for the wet/black plug, to me the left is ok. The compression test with the carb installed and throttle closed would be scary low numbers (been there). With carb removed it pops right up there like it should. The right side has a problem. 120 with the carb installed and throttle closed should not be possible. That side is sucking in a bunch of air somewhere! Is the throttle butterfly plate closing properly? Hard to believe the rubber connector could leak that much, but check that. Make sure the steel spigot is securely screwed into the head. Most folks seal those threads with loctite. The Bing folks have anything to say?
Now, when you compression test with the carbs installed, are you twisting the throttles wide open? If you are, that brings other factors into play. The intake vacuum will work to lift the slides. They should bounce around, and they definitely prevent getting a true reading - but maybe the right slide is slamming open and sticking there? Just throwing out crazy ideas. If that slide were sticking, balancing the carbs would be obviously impossible.
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