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Trying to synch GS650 carbs, too much vacuum?

  • Thread starter Thread starter sultan
  • Start date Start date
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sultan

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'83 GS650GLD, 23K miles.
Earlier in the spring I basically rebuilt the engine and carbs. (honed cylinders, replaced piston rings, dipped carbs, replaced o-rings, replaced all gaskets, reset valve clearance, etc.) Once I got the bike back together it actually ran! (somewhat to my surprise) It actually ran pretty well, though it idled a bit high. But I've been able to ride it since then with no bogging, stalling, or other performance issues. However, I know that the carbs still needed synching.

Let me explain the high idle issue. With the idle screw all the way out, the bike still idled around 2K rpm on the tachometer. If I screwed in the idle screw, the idle would increase, but if I screwed it out it would never drop below 2K on the tach. The bike would start with virtually no choke almost instantly. Since I wanted to ride more than I wanted to spend more time in the garage, I left things as-is, and it's been running great all summer apart from the high idle 'problem'.

However, I finally got a manometer:
http://www.z1enterprises.com/detail.aspx?ID=1689
and decided to tackle the idle issue and get the carbs synched. I figured that compared to everything else, this would be an easy job. No such luck.

I went to calibrate the manometer, and hooked all 4 lines into cylinder 4. No problem. Used the screws on the manometer to balance the levels (they were about mid-range on the site glass at this point) and then hooked each hose into its proper cylinder. Upon starting the bike, at least 2 of the columns in the manometer rose to the top of the site glass. (I was ready for this and killed the engine before anything got sucked into the hose.)

So, I worked on decreasing the idle speed. I used the manual/mechanical screws (the ones you're supposed to use to adjust the carbs) to lower the idle throttle so that the idle screw would now respond the way it was supposed to. Set the idle to what reads at 1500 rpm, but I'm not sure my tach is 100% right, as there's not much range between 500-1500 on my tack. Below 1500 the bike *sounds* like it's about to stall, and much below that it actually will.

So, this time I decided to try to get the manometer gauges at the low end of the scale. And, since cylinder 3 is unadjustable, I decided to use that to balance the manometer. Started the bike, and *all 4* of the cylinders rose to the top of the sight glass.

Thinking I had something else wrong, I checked my fuel needle screws (? the ones on top of the carbs, that were under a small plate that I had to drill out when cleaning the carbs) by re-seating them and then turning then back out 2 turns. (I originally had them at 2.5 turns out.) Still, the manometer max'd out upon starting the bike.

I turned down the idle to as low as it would go without stalling the bike, and still the manometer went off the scale. I did a "by ear" adjustment of the screws (turning them up until the idle started going up, then backing them off a quarter turn) just to try to get everything in rough synch. Still, no matter what I do, cylinder 3 still goes off the scale.

If cylinder 3 is doing this, how can I get the other 3 cylinders in line?

After doing this, I do get slightly better off-the-line performance, but less top-end pick-up, and possibly less top-end speed. (I used to be able to hit the throttle at 70 mph and get a bit of a kick. Now I get just slow, gradual acceleration, with a top-end of about 90. I usually don't travel at those speeds, but I was pretty sure I had it up to about 100 previously.) Maybe this has something to do with turning in the fuel jet screws half a turn? Or was something wrong previously and this is the performance I should expect?

My assumptions: The pistons, while sitting on the bike, going left to right, are 1-2-3-4, right? Cylinder 3 is the one that can't be adjusted, correct?

Anyone have any suggestions as to how to lower the vacuum enough to allow a synch of the carbs? Am I missing something?

Side note: The manometer had 'bubbles' in the columns as soon as I hooked it up to calibrate it. Is there anything that can be done about this? I'll have a column of liquid, 1/4" (or more) of bubble, then 1/2" of liquid. Is the manometer bad? Should I send it back? Assuming that there's nothing can be done, where do you take the balance reading? at the top of the liquid above the bubble? or the top of the liquid below the bubble?

Thank you for your suggestions, both in regards to this problem and all the help from my previous posts when I was getting my bike running this past spring!
 
I had problems with my Motionpro as well. The center two tubes had fluid pulled out of them when hooked up. I called Motionpro customer service and they were VERY responsive. The tech indicated that they had some problems with a few units and said I should mail it back. They sent me a new one and a complimentary refill (I asked for it and he said "sure"). I had a very positive experience with Motionpro and the best part was that the new unit worked great. Google them on the web and you'll find their contact info.
 
Yes, you are correct in your cylinder numbering.

The reason you have such a high idle is because your carbs are out of sync. I know this sounds like you are chasing in circles, but if you had done a bench sync when you had the carbs off for cleaning, this would have been minimized. Here is what is happening. #3 is, indeed, your master carb. On either side of the throttle shaft you will find the adjusters for carbs 2 and 4. Adjust these, then try again. You will have to experiment to see which way the other throttle shaft is turning, but evidently, #3 is virtually closed and the others are still a bit open. Because the others are open, the engine is running off of them, but #3 is closed, which creates a high vacuum. What you need to do is to open #3, but since it is the master, you actually need to close the others. By the way, the Suzuki-recommended sequence is to adjust #2 to match #3, then set #1 about 1/2 cm higher, then finally set #4 about 1/2 cm higher, to match #1.

It will take you a while to get them right, this first time, but after that they should not wander much at all. Next time you take the carbs off (you may even want to do it this time), do a bench sync before putting the carbs back on the bike. All that involves is adjusting the master idle screw so that you barely see a sliver of light when looking through the carb, then adjusting the others so you see the same size sliver of light. When you put the carbs on, you will be very close, but you will still have to use your gauges.

.
 
Your on the right track. Had you bench synced the carbs you would have saved some time messin with the high idle issue but as far as maxing out the manometer you need to calibrate it at a lower level. When you calibrate it the vacuum from one cylinder is divided amongst all four chambers in the meter. When you hook it up to all four cylinders the vacuum on each individual cylinder is applied to one chamber so it pulls harder. I forget what the directions say but I calibrate the manometer to be 1/3 the way up the scale. Having air bubbles doesn't help matters either and needs to be fixed to get an accurate reading, I fixed mine by applying a little vacuum and releasing it--basically playing around with the thing until it didn't have bubbles.
 
Carbs synched...finally

Carbs synched...finally

Thanks to everyone for the great information.

I had one major initial assumption that was incorrect. I'd read in several places that you don't want to rev the engine for fear of drawing the manometer liquid into the cylinder. So, I equated "more gas" to "more vacuum". I was actually adjusting cylinders 1,2 & 4 in the *wrong* direction.

So, I backed off 1, 2 & 4 and up'd the idle. I was then able to get a decent mid-scale reading on #3, and balanced the others pretty closely to it. (Side note...I find it very interesting how adjusting 1 carb has so much effect on the other 3...rather tricky to get everything aligned. Took a bit of patience to get them all close to each other.)

Also, I stored the manometer right-side-up for a week before trying to use it again. This seemed to fix my bubble problem. I'm guessing that the unit was upside-down and/or horizontal for some portion of shipping, and that caused the gaps in the liquid that I was seeing.

I just finished this up late last night, so I just had time to take it around the block and not much more, so I can't really say how much performance (or gas mileage) difference I'm getting. It almost seemed *more* sluggish right when I hit the throttle hard, but then after the initial 'bog', it seemed to have better acceleration. The engine does seem to be running a bit smoother, which is good. I'm hoping I'm wrong about the bogging issue. Hard to tell when you're in a residential neighborhood with stop signs every 2 blocks.

I also checked/adjusted the valve clearances, and all but 2 were already in spec., and I replaced those shims. (Does it matter if you do this before or after the carb balancing? Just curious.) A dry compression test gave 175/180/170/185. Acceptible? Good?

So, I'll have to do some riding (what a chore) to see how much difference all of this made. After having this bike for over a year (and riding all spring/summer), hopefully I finally have everything cleaned, replaced, and properly adjusted and I can stop worrying about maintenance and just concentrate on the riding!

Thanks to everyone for all the help! Any other tech tidbits/advice on the info above is appreciated.
 
Did you do the compression test cold or warm? Those numbers look good, certainly well within spec. If they were cold readings, they are EXCELLENT, but even if warm, are quite good.
 
Compression test - specs

Compression test - specs

Those were cold. I balanced the carbs, then removed the valve cover to check the valve clearances. After putting the cover back on, I tested the compression while the spark plugs were still out. While I don't think it had cooled to ambient yet, it had been sitting for probably 2 hours at that point.

So, hopefully I did something right with the cylinder honing, ring replacement, and valve lapping, yes? = ) While I've done lots of auto work in the past, this is my first motorcycle, and first time taking apart an engine. Quite a learning experience, and all along the way I was worried that I was going to foul something up beyond repair...
 
sultan said:
So, I backed off 1, 2 & 4 and up'd the idle. I was then able to get a decent mid-scale reading on #3, and balanced the others pretty closely to it. (Side note...I find it very interesting how adjusting 1 carb has so much effect on the other 3...rather tricky to get everything aligned. Took a bit of patience to get them all close to each other.)
Click to expand...
Can you imagine the frustration of trying to do that with a single gauge and a manifold to read each cylinder. :eek:
That's what some on this forum have done to try to save a few bucks while trying to maintain their rides. :-k

sultan said:
I also checked/adjusted the valve clearances, and all but 2 were already in spec., and I replaced those shims. (Does it matter if you do this before or after the carb balancing? Just curious.)
Click to expand...
Yes, it makes a BIG difference. With the numbers you posted, it's probably not much of a problem, but changing valve shims can and will affect how much a cylinder can breathe. How much the cylinder can breathe will affect how hard it sucks on the closed throttle butterfly. That will affect the vacuum reading. When you sync the carbs to equalize the vacuum, you are balancing how much each cylinder is contributing to the overall power.

In any of the manuals that I have read (Factory, Clymer, Haynes), the suggested order is valves, igniton, carbs.

.
 
Not trying to post jack, but what if you have a twin? the 4 port manometer is 2x what I need, should I still get it?
 
donimo said:
Not trying to post jack, but what if you have a twin? the 4 port manometer is 2x what I need, should I still get it?
Click to expand...
If you can afford it, go for it. You will just use two of the four for your bike. If you get a four cylinder in the future, then you are set.
On my Yamaha twin, I was able to use a single automotive vacuum gauge to get it pretty close, just swapped it back and forth between cyls. It would have been a lot easier if I had two:rolleyes:
 
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