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Is it possible to botch a carb synch this badly?

  • Thread starter Thread starter Guest
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G

Guest

Guest
Well, considering I was the mechanic, the answer would have to be yes.

1978 GS1000, VM26 carburetors, warm engine, valves within spec, timing spot-on. As dusk descends and St. Pauli Girl beckons from the refrigerator, I get the Morgan carbtune out, put the gas tank on a nearby trash can, turn on a floor fan, hook up the vacuum hoses and light up the engine. Boost the idle to 1750 RPM and have a look at the steel bars. Not too far off. A tweak here, a tweak there and soon I'll be downing a cold one and enjoying the satisfaction of a job well done. But the steel rods aren't cooperating. More tweaking. The engine begins racing. I try to bring three down to the lowest level . . .one of the set screws seems to have bottomed out . . . one of the rods leaps to its limit . . . the engine begins coughing and I shut it down.

In looking back, it appears that in rushing the last part of the tune-up, I neglected to remember that vacuum adjustment screws are sensitive and only require small adjustments. One overadjustment led to another and now I'm figuring I'll have to pull the rack, bench synch and go at it again.

Unless someone has a better idea . . .
 
Did you replace the intake boot O-rings? Sounds like you have a vacuum leak or something. If everything is sealed up properly adjusting VM's is pretty easy.
 
Go back to a baseline. Make sure the #3 carb is closed with the idle screw backed out. Then do the bench sync
 
Exactly the same thing happened to me when i set the jet needles too low (lean).
 
Did you have the vacuum port blocked off? (Or were you still using it connected to the petcock?)
 
On my 78 750 the pilot adjustment screw is one turn out, idle screws out one and a half turns. The top I adjusted by eye then sync'd them with a motion pro carb sync. Took only 15mins. I have exhaust and k&n pods.
 
Trying to synch my '78 GS1000 carbs using a 5 yr old Morgan Carb Tune and nothing
but problems. My kibitzing neighbor asked if I had calibrated the Carb Tune and I
explained the expensive Morgan was precisely engineered and needed no
calibration adjustment capability.

He returned after a few minutes with a 4 into 1 vacuum 'T', and hooked all 4 Carb Tune
hoses to one vacuum port. No 2 columns were a close match! Dust, age, corrosion?
Useless. Disassembled, blown out, wiped parts with microfiber towel, reassembled
and still showed 4 different column heights hooked to the same vacuum port.

15 minutes with a borrowed 'mercury carb stix' manometer the following weekend
and all is well.
 
Salty, I capped the vacuum port and ran a clear 5/16 fuel line to the tee, petcock set to prime.

The engine was purring nicely when I hooked up the Carbtune. I am now fairly certain that it was a sequence of overcorrections on my part that led to the current state of imbalance, not unlike when a driver swerves to miss a squirrel, realizes he's gone into the oncoming lane, then overcorrects and flips his Ford Explorer into a ditch.

I'd be curious to know if anyone else finds it awkward to use that long Motion Pro carb tuning tool, the combination screwdriver-box wrench. After reading of people synching their carbs with one hand on the throttle I'm wondering if it might be a better method to loosen all the locknuts first with a box wrench, adjust the set screws to achieve balance, then tighten the nuts while holding the screw in place. My problem with the Motion Pro tool is you can't see the set screw as you're tightening the locknut; it seems to me if you could see it you might notice any small rotation as you're tightening the nut.
 
jskellington said:
I'm wondering if it might be a better method to loosen all the locknuts first with a box wrench, adjust the set screws to achieve balance, then tighten the nuts while holding the screw in place. My problem with the Motion Pro tool is you can't see the set screw as you're tightening the locknut; it seems to me if you could see it you might notice any small rotation as you're tightening the nut.
Click to expand...

That's what I do, get them all even, shut the engine off, gently snug the locknuts up, start the engine and recheck the balance, then tighten them properly, and start up and recheck again. Each time snugging or tightening, hold a light twist on the screwdriver so you can feel if it moves while the nut is being turned.

I can usually get them all on the first try.
 
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