Carb Sync Help

[cp___32]

Hello,

In case my signature has disappeared, I have been working on my 1980 GS550L. I recently took the carbs off, cleaned them using an ultrasonic cleaner and plenty of spray carb cleaner. I picked up 4 rebuild kits that came with new mix screws, float needles and such. I did a float adjust on the carbs and a bench synch before reinstalling.

I got the bike running on one of the fairly warm days we've had. It was about 15C out (59 for my neighbours to the South). Let the bike warm up for about 10 minutes and then broke out my carbtune to give that a try.

I installed the carb tune, plugged off the vacuum port and started my adjustment before discovering two issues.

Firstly, I found that Cylinder 3 tended to read much lower than the other cylinders. When the bike revved, it did respond equally to the other cylinders, but it was always lower than the other three.

Secondly, when I adjusted the mixture screws in and out, I found little to no response on the gauge and in the bike's performance. Only when the screws were tightened down all the way did the bike begin to stumble and cut out. I could back off the screw and it would come back, but I was expecting to see much more 'action' on the gauge when adjusting.

I took a can of Quick Start and sprayed around the carb boots on both the engine side and the airbox side to see if there were any leaks but didn't find anything.

Anyone have any ideas what I should check next? I attached a pic of the gauge so hopefully you guys can get a better idea of what I'm reading. From what I understand the key with balancing the carbs is not necessarily that the readings are equal, just that they are stable at idle and move in unison but still, I was surprised I couldn't really dial things in more.

I did pull the plugs and they all looked about the same. If the pic of the gauge works I'll post a pic of one of the plugs which is representative of what all of them looked like.

Thanks

 

[Nessism]

Your understanding on how to sync the carbs is incorrect. You need to get all the levels equal.

You don't adjust the levels by messing with the pilot screws. Set them all to 2.5 turns from lightly seated and then start syncing.

First step is to tweak the center adjuster and get 2-3 level. Then start on the other two adjusters and bring 1 and 4 in line with the others.

Place a box fan in front of the engine and set the idle speed up to 2000 or 2500 rpm first. That's what I do anyway.

Oh, and needless to say you should have the airbox and air filter installed and sealed. The valve clearances need to be verified too.

Good luck

 

[Guest]

Yep, I will agree, ... you need to change your technique.

The manual recommends setting the idle to 1500-2000 RPM, mainly to minimize the bounce in the gauges.

As Nessism suggests, set your mixture screws to 2.5-3 turns out from lightly seated. This will richen the mixture a bit, making it easier to start and get off "choke" sooner. When the bike is running well without choke, start your balancing. Start with the center adjuster, make 2 and 3 the same. The manual suggests doing #1 next, but it doesn't really matter. Note that when you make any adjustment, there is a good chance it will affect cylinders that you are not adjusting, so it will take a few times back and forth to get them all the same.

After you get the carbs synchronized, adjust the mixture screws. Drop the idle to about 12-1300 RPM, make sure it is smooth and steady. Slowly turn the mixture screw on #1 in. Eventually, you will see the vacuum level on ALL cylinders drop a bit. Back the screw up about 1/4 turn. Repeat on each of the other carbs. When you are done, verify that a 1/4 turn in on each screw still makes the speed drop a bit, then back it out.

Verify that you have quick throttle response when you blip the throttle, and note the way it returns to idle. It should return quickly, without dropping below the set speed.

 

[Katarat]

You might get more responses in the carb section

 

[cp___32]

Thanks for all the advice. It's supposed to warm up quite a bit here over the weekend so getting it to run choke free will be much easier than it would be currently. On a few of the videos I've watched on carb balancing with homemade tools I've heard some people say it isn't necessarily the level that matters, so long as the difference between the readings remains constant and isn't increasing or decreasing constantly, but that's likely more to do with the fact they don't have a proper tool. Theory makes sense that all 4 should be working equally.

It was a toss up between posting in the carb or engine section. The adjustment deals with the carbs but the goal is getting the entire engine running correctly. I will follow the above instructions on balancing the carbs and report back on the weekend or next week.

 

[cp___32]

Hey everyone.

Thanks again for the help. It turns out (and is a little embarrassing) that I wasn't tuning the carbs correctly. I thought the bench sync was how you adjusted the butterfly valves and then used the mixture screws to balance from there. I didn't realize you still needed to fine tune the butterflies first. I loosened the jam nuts up and did the sync as described above and then moved to the mix screws after that. It was tricky because it was quite vague on when the bike would start to die out and when the vac gauges started to drop, but I think I'll see how it runs for now and one it's road worthy I'll do the plug chop method to fine tune from there.

Last thing I hope I have to do is fix the kill switch wiring. I installed a SSPB and did the kill switch mod. Now the switch latches on so once the bike has started the only way I can shut it off is using the key. I also lost the kick stand safety switch function. I'll put it all back to stock and see where I'm at. Likely crossed something up translating the instructions to work on my 550. Anyway, thanks again for the help!