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Syncronizing Carbs

1549 Views 3 Replies 3 Participants Last post by  truckel
I am reposting some stuff from the MSN group here so we don't loose it forever.

Kerry is the man....

From: MSN NicknameEvilKenieval (Original Message) Sent: 6/25/2008 5:09 PM
My RH carb has too high of an idle. The idle set screw has lost contact with the plate and it still won't slow down. Both carbs are properly synchronized for throttle response. How do I correct?

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Recommend Message 2 of 12 in Discussion
From: Kerry Sent: 6/26/2008 1:15 AM
Depends on what's wrong. did you set the cables with the throttle stop screws out? Are the carbs mechanically balanced? Be sure both butterflies are closed all the way when you set the cables. Check that the throttle butterfly actually closes? I don't know if it'll have the effect with these carbs; some carbs axial thrust on the throttle shaft will stop the butterfly from closing all the way. Lift and drop the venturi slide? Check timing and advance with a strobe? Weak springs will result in an early advance, which will speed up the motor. High float level (idles better cold than hot)? Air leaks? Put a little petroleum jelly around the throttle shaft where it enters the carb body - that leaks air if it gets worn. Sticking throttle cable (push down on the throttle arm, feel the cable to make sure it has a little play) ?


Recommend Message 3 of 12 in Discussion
From: MSN NicknameEvilKenieval Sent: 6/26/2008 7:18 PM
Thanks, Kerry. That list should give me a start. I think about now I owe you a bottle of Crown Royale. Except for cleaning the carbs out and resetting the float, I've done nothing to them. I figured I should get the timing and valves set right first and then go to the carbs. That's now done and I set the jet screws perfect yesterday and ended up with a 2.3K idle and with the idle stop screw on the RH carb not even touching the plate! It's sitting 1/4" off the plate while in the same position (same number of screw turns) as the LH carb's idle stop screw which is in solid contact with its plate. Does this carb have an adjustment for length of throttle linkage and the cable is somehow set 1/4" too short?

I don't fix 'em. I just ride 'em - but this bike sure wants me to change that way of thinking.


Recommend Message 4 of 12 in Discussion
From: Kerry Sent: 6/26/2008 7:31 PM
The throttle is part way open. That's why you have high RPM.

For starters, you should check/adjust the timing before playing with the carburetors, as timing will affect how the motor will respond to carb adjustments. you'll have to back off the cable adjusters on the back side of the carbs.

You can mechanically balance the carburetors. Screw the idle stop screws out far enough that they won’t touch the stops and check that each throttle butterfly closes all the way. Barring a mechanical problem with the carbs, that’s simply a matter of pushing down lightly on the throttle control arm on the side of each carb and checking to see that the throttle cable isn’t holding it open – usually there’s just a slight looseness at the twist grip. If pushing down on either throttle control arm pulls the twist grip tight, you have to loosen the cable on the inner side of the carbs.

Adjust those cables to the point that you can feel the throttle control arms move just as you start to twist the grip, and make sure they move together. I hold one closed, twist the grip ‘til I feel pressure on it, hold the grip in that position and feel the other, putting a little twist pressure on the grip to see if it feels the same. The method of adjusting them is pretty obvious, but you might want to take a cheap 10mm spanner and grind the sides down to use it on the locking nut. This becomes what we've humerously come to call an SST, or Special Service Tool – a term from some now forgotten 1950s service manual.

That having been done, turn the throttle stop screws down ‘til they touch the stops without moving the arms, then give each one turn down. When you start the motor, turn each screw the same amount ‘til you get a reasonable motor speed and re-adjust to about 1200 rpm when the motor warms up, again turning them identical amounts. Adjust the mixture screws to produce the highest rpm, and re-set the throttle stops. (I favour about 1400 – keeps the oil going to the top end and – in slow traffic riding – keeps the motor from stalling as your battery gradually goes dead. The battery does not charge at this low RPM)

At this point your exhaust pulses should be identical. If they are not, you should look for the reason – like lower compression in one cylinder, valve adjustments, carbs not performing optimally, etc.


Recommend Message 5 of 12 in Discussion
From: MSN NicknameEvilKenieval Sent: 6/27/2008 1:32 AM
Thanks a bunch! I'm pretty sure timing is not the problem when both idle stop screws, set in the identical position, have one a 1/4" off the stop plate. That cable adjustment has to be at fault.

For lurkers, you might wonder why I don't just back off the left hand carb idle screw until I get 1.4K at idle? If I did that the RH cylinder would be trying to run at 2.3K while the LH cylinder would have to be running at .5K to get a 1.4K idle. That's massive cylinder opposition and is evidenced by blurred rear view mirrors at idle.


Recommend Message 6 of 12 in Discussion
From: Kerry Sent: 6/27/2008 8:25 AM
Don't be "pretty sure" timing is not the problem. Check the timing. If you set the carbs and then adjust the timing, you'll have to set the carbs again. It does sound exactly like the cable on one carb is too tight or that the cable is binding somewhere, and setting the cables so they open both butterflies at the same time is a mechanical setting - idle stop setting will be affected by timing, but mechanically balancing the carbs isn't really "tuning" - it's a preliminary step, not affected by anything else.


Recommend Message 7 of 12 in Discussion
From: MSN Nicknamedinosaurthatroars Sent: 6/27/2008 9:29 AM
throttle cable routing can also pull one cable to tight has to be very free (often a problem with clip ons and drop bars ) paul


Recommend Message 8 of 12 in Discussion
From: MSN NicknameEvilKenieval Sent: 6/27/2008 3:33 PM
The bike was timed a month ago at full advance. I will look for a bind in the RH throttle cable. Where is the adjustment for the throttle cable length? Is it the standard type two nut cable adjuster? Is there a picture in one of the site links?

Thanks, guys!


Recommend Message 9 of 12 in Discussion
From: MSN NicknameEvilKenieval Sent: 6/27/2008 8:55 PM
Success! I found the cable adjustment via the LINKS section of this site and it was two nuts as I suspected. Because my idle stop screws are so far off, I only gave the cable adjuster 1.5 turns then locked it. This did drop the idle on the RH carb without effecting throttle cable synchronization.

Next, I balanced the cylinders up with it running. The manual wants you are to feel for "even exhaust" but I'm much more precise and measure vibration. When vibration is eliminated, they are in perfect synch. I could probably balance a dime on that engine with it running right now.

Still, it's not very receptive to setting at a specific idle. I was looking for 1200-1400 RPM. When I was done synchronizing the idle, I found myself at 1,000 RPM. Any attempt to increase the idle on the RH carb resulted in it climbing too high. Using a hairpin turn of the RH carb set screw, and then reynchronizing, I'm now at 1,100 RPM. That's still low.

What is the sudden climb in the engine RPM a sign of? Do I need to back off the cable another half turn? I still get some exhaust pop out of the RH carb for an incorrect float setting (It's very sensitive one way or the other). Would that cause the RPM jump on the RH carb?


Recommend Message 10 of 12 in Discussion
From: Kerry Sent: 6/27/2008 11:58 PM
Start with adjusting the throttle cables so that both throtles start to open simultaneously. Turn the throttle stop screws 'til they just touch the stops, then turn them down the same amount. One turn, then back them off the same amount to get 1200 RPM. Turne the mixture screws (in is lean; out is rich) until you get 1200 RPM with the lowest throttle stop setting. If you're too rich, the bike tends to idle more slowly hot than it does after the initial warmup. A high float can do this, too, and make for an irregular idle. Assuming both cylinders are individually timed correctly, if the motor doesn't produce the same power on both cylinders, adjustment to the carbs is just correcting for a problem, like low compression, intake air leak, malfunctioning carb, a worn valve follower in one cylinder, valves out of adjustment - that exists somewhere else in the motor.
Air leaks in the intake passage, weak advance springs, different amounts of throttle on the individual cylinders - these are a few things that can make the idle very hard to set.
The small phillips head screw in the right front of the headlight bezel adjusts lateral position. Tilting the whole headlight adjusts the vertical position. Basic adjustment is high beam centre is the same height ahead of you and low beam height is always lower than the headligh centre ahead of the bike. Just a little lower would be nice, but that would make the high beam too high - and peripheral coverage on the CB headlights is not really great. Make the adjustment with the normal amount of load on the saddle/carrier, and keep in mind that the beam gets a little higher at high speed since the front of the bike will lift a little, especially when you're in the 80 mph plus range - which you definitely should not do at night. I just read about a chap colliding with a bear at about 45 mph - the sort of stuff that doesn't belong outside of nightmares and Russian novels.
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what is the "throttle butterfly" please.
thank you
The disc in the carb throat nearest to the intake valve.....the other one is the choke plate (or choke butterfly)...
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