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To expand on what Dave said I would add this. What you have right now is a motorcycle that is idling too fast on 1, 2, and 3 cylinders. If it were possible to turn in on number 4 you could bring #4 down to the other 3 cylinders. (vacuum goes down when you turn in on a carb screw. Also mercury or numbers on gauge go down when you turn in) Problem is you can't adjust #4 to match the other 3 cylinders as there is no adjuster on #4.
What you have to do is bring the vacuum up on the other 3 to match #4. When the other 3 cylinders go up #4 will come down.You can do as Dave said or you can do it this way.Really no difference from what Dave said. Just my preference. Turn the other 3 screws out exactly 1 turn at each of the 3 carbs.. Now when you start the bike it probably will not idle or will idle too slow so you will have to turn the main idle screw in to get it to idle.
Check the gauges. Are all the gauges higher than #4 now? That is the 3 cylinders have higher vacuum. If not do the same procedure. Turn all 3 cylinders out 1 more turn. Again start the engine. It might idle low or not idle so you might need to adjust the main idle screw again. Eventually you will get in a position where number 4 is the lowest vacuum. You have inverted the situation you were in originally. Now the #4 cylinder is working harder. (confusing because low numbers means working harder) The other 3 are sluffing off. So.....what you need to do is adjust the lazy 3 cylinders to match the #4 cylinder.
I know this sounds confusing but just keep in mind when you see a number or mercury travel up that cylinder is working less.An example is when you come down a highway ramp. Vacuum numbers are sky high because the engine is doing nothing. When you go wide open throttle you will see zero vacuum almost. As your bike sits right now #4 is high because that cylinder is not doing it's share. I hope this didn't confuse you more and it helps you get going. :)
 

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He said it was synced and running well, then went bad during a ride. That makes me think something had to come loose or break. That wouldn't affect 3 carbs and not the 4th. So it seems plausible that something is amiss with the #4 intake system.
 

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He said it was synced and running well, then went bad during a ride. That makes me think something had to come loose or break. That wouldn't affect 3 carbs and not the 4th. So it seems plausible that something is amiss with the #4 intake system.
That might be a good thought. Wasn't there a member who had a similar issue that turned out the arm was broken or slipping on the throttle shaft. The arm would move but the throttle plate did not advance with it???
 

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Dave is dead nuts right on this. Everyone says #4 isn't adjustable. That's kind of a misnomer, because when you sync the carbs, you're adjusting the throttle plate opening. The screw that adjusts the #4 carb throttle plate is the base idle screw and all others are sync to it. Opening or closing #4 will affect the deflection in the manometer/sync tool and because they are all linked mechanically, affects the throttle opening on the other three carbs. You can pull the intake runners and visually inspect the throttle plate opening between #2 and #4. See if one is out and check the plate and screws at the same time.
 

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Dave is dead nuts right on this. Everyone says #4 isn't adjustable. That's kind of a misnomer, because when you sync the carbs, you're adjusting the throttle plate opening. The screw that adjusts the #4 carb throttle plate is the base idle screw and all others are sync to it. Opening or closing #4 will affect the deflection in the manometer/sync tool and because they are all linked mechanically, affects the throttle opening on the other three carbs. You can pull the intake runners and visually inspect the throttle plate opening between #2 and #4. See if one is out and check the plate and screws at the same time.
Yup...how about #4 is the only carb that is not individually adjustable.:) You bring up another thought to me. Wasn't there also some one that had issues of some sort with the carb adjusting screw not adjusting when he turned it, or something like that? Wouldn't fit these symptoms any way though.
 

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Not the intake tube. There is a hose that comes off the base of that intake tube, and if that's missing I suspect there would be a real loss of vacuum on that cylinder.
I was assuming high vacuum from his first post, at least that's what a pegged gauge means to me.
 
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Discussion Starter #27
Thanks for the posts. Before reading them, I got to thinking what happened with the bike.



The bike ran fine until last week when it began a rough idle. Yesterday, I adjusted the idle mix screws and found a bad spark plug terminal on #3 (which was probably the reason for the bad idle...should have stopped fixing things right there). But since the mix screws were re-adjusted, it seemed easy enough to re-balance the carbs again.



Well, the first CarbTune reading had two pairs of carbs in sync but the pairs were not in sync so I began adjusting them. While adjusting, I thought to myself "this isn't right" because I kept backing out the base idle screw. Even to the point where I thought it might back all the way out. So I stopped. That's when I found that the idle return spring on #3 had detached itself.



Because the spring wasn't engaged, it allowed the two adjuster screws on that side (right side) to move ever so slightly on the throttle shaft when applying pressure with the screwdriver to make adjustments. That little bit of movement threw everything out of whack - three adjuster screws and base idle screw. Each (false) adjustment compounded the problem until things are as they are now.


I'm confident that Dave, RedWing, etc. hit the nail on the head, now that I've had time away from the bike to collect my thoughts and your thoughts. I've not gotten out to the bike yet today...will get there later today...but I'm sure that's what's going on. Thanks for all the help, including how to fix it (and the long explanation Redwing...that brings it into focus for me). Slow steps and minor adjustments should bring #4 and the others inline with each other. I greatly appreciate it. I'll report back after I get to the garage and spend a bit of time with it!
 

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Discussion Starter #28
GREAT LEARNING EXPERIENCE for me. Thanks folks.


I never thought about how the main throttle stop/idle screw (indirectly, I guess) controls balance through carb 4. It was a process but worked like it was explained. I fiddled with the screws but wasn't getting the feedback I wanted. So I screwed everything in and did half turns until the bike started. After warming up, I fiddled in small increments and made notes (cuz the old memory is bad) as I went along. More fiddling and it's good to go again! Thanks again for sharing the knowledge. If I can get back out there today, I'll take and post a pic of the CarbTune readings.
 
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