Carburetor Synchronizing

jaybob · Post by **jaybob** » Fri Jun 12, 2009 3:07 pm

Hello all.

For my 1975 M GT550 the manual recommends disconnecting one cylinder at a time and adjusting the idle on the other two to synchronize the carbs.

What other options do I have?

The manometer method involves tapping a few new holes in the carbs and then equalizing the vacuum.

I believe much the same modification is required to use a bank of dial vacuum guages.

Has anyone ever used the Edelbrock UNI-SYN? Seems like that would be easy as pie, just put the thing over the carb bell and note the ball position. Rinse and repeat.

What methods have you fellows who have already been there been successful with?

Also I am running an electronic ignition. Will it hurt it to pull a plug wire off of a plug while the engine is running? I have heard that may not be good for the ignition.

Thanks

Jason

Post by **Suzsmokeyallan** » Fri Jun 12, 2009 5:25 pm

You could manually adjust the linkages on the butterflies so they all close and open at the same time.
To do this, remove the carbs from the bike and hold up the slides, then set the main one the idle speed screw is onto so that butterfly just seals off the bore completely.
Look in through the back while shining a light source from the front of the bore to check for when it just seals.
Then adjust the other two with their linkage setting screws so they also just seal off any light.
Lock the nuts and snap the throttle a few times, then recheck if you need to fine tune the adjustments.
This will get you as close to total sync closure across all three carbs from a visual perspective.
Its no where as good as a mercury sync balancer, but its better then guessing or using the idle drop method.

Coyote · Post by **Coyote** » Fri Jun 12, 2009 7:53 pm

Ahhhh Allan, there are no butterflies on the 550 VM carbs. Just slides.
Jason, place the carb rack on the bench. Adjust till the right slide is bottomed out and set the throttle stop / idle screw to that carb. Then adjust the other 2 so they rise at exactly the same time -- all together. It's a mind f*ck at first till you figure out what adjusts what. Drove me nuts the first couple of times I did it. When you have all slides rising at exactly the same time, mount the rack back in the bike. Fire it up and set the master throttle stop screw to a decent idle. Your carbs are as synched as the will ever get.
From here it's messing with air screws, the mains, the pilots and (oh joy) the needle positions.
Forget all that fancy vacuum crap. Works well on 4 strokes but is pretty much worthless on a 2 stroke.

jaybob · Post by **jaybob** » Fri Jun 12, 2009 8:38 pm

Sounds good guys. I did align the slides with the top of the bore as per the manual. I was just wondering what else I could do to limit my human error.

That is a good idea to use a backlight on the opening aperture. That is something I did not think to do.

Chris did you try out the pods?

Post by **Suzsmokeyallan** » Fri Jun 12, 2009 8:58 pm

OK you got me there, the 550 rack of carbs doesnt have butterfly valves, my mistake and a big one too.
Why did i even go that route, maybe my mind was on the Buffalos CVs and i was too lazy to go look at the 550 parts book,,,

jaybob · Post by **jaybob** » Fri Jun 12, 2009 9:22 pm

It's OK Allan. You still gave me good info that will help me get the job done a little better than I have it now.

If I had only half as many bikes and parts that you have rolling around in your head I wouldn't know which way to turn.

advant63 · Post by **advant63** » Fri Jun 12, 2009 10:24 pm

jaybob, you can use your manometer but you have to make a couple of pipes to insert into the ports (phillips head scew after the slides) on two of the three carbs. the third carb can be read from the diaphragm vacuum hose.
use a 12" 3/16 brake pipe, cut in half and thread the unflared end to match the screw. 4mmx1.0 i think.verify this yourself.
thread them in with a bit of sealer and cap them when not use. you can take it for a shakedown run with them installed and refit the screws when your satisfied.

Coyote · Post by **Coyote** » Sat Jun 13, 2009 7:32 am

Jason. On page 27 of the manual I sent you on CD, it tells you how to bench sync the carbs. To do it right you will need a short piece of 1/32" music wire for a gage. Being a machinist I was able to select a .031 as a go and a .040 as a no go from my thread measuring wires. Made it a little easier.
As for the new pods, I really can't tell any difference in the way it runs. It is still suffering from the slow return to idle that was there with the old pods. Been on jury duty but plan on trying to figure that out today.

jaybob · Post by **jaybob** » Sat Jun 13, 2009 7:59 am

Avant and Chris. Thank YOu!!

I was trying to figure out how to use those vacuum ports as I didn't see any other purpose for them.

I did set up my rack using page 27 and the slides are as close as I can get them by eyeball.....I think. I may have to pull the rack again to change main jets and I will for sure go through the adjustments again to verify.

Chris. Your slow to return to idle interests me. On the first running of my machine with nearly the same carb settings as you I did not notice the condition. One thing I did and am glad I did was I replaced the intake tubes. The old ones looked pretty good but if I applied some bending stress to them they had some scary cracks in the corners of the grooves that were pretty deep. Don't know if that is the case with yours but It does seem like an air leak. BTW I seem to have decent Idle and low end throttle response with the 27.5 pilot.

H2RICK · Post by **H2RICK** » Mon Jun 15, 2009 8:40 am

Jaybob:
I have a UniSyn and have used it on all my stroker carbs (and other guys' stroker carbs) as well as sled engines back in the day. It works as well as can be expected for such a crude tool....but is far better than trying to synchronize "by ear". A manometer hooked up to the vacuum ports on the carbs is BY FAR the most accurate BUT......keep in mind that sooooo many other things will affect your readings. Strokers are VERY sensitive to differences in exhaust pressure and this, in turn, will affect your manometer readings. You can have the manometer readings EXACT and still not have a properly idling/accelerating bike.

For initial synchonization, you CAN go through the process that Coyote posted but I've used the Kawi Triples & Old Brit Bike "spoons-under-the-slides" trick. Very slick, easy to do and no removal of carbs required.....just the airbox and boot. This works very well to get your initial slide synchronization quite close.....and on a Kawi Triple you have no other choice because, unlike our sophisticated Suzi Triples, they have NO vacuum port hookup points.

bikegeezer · Post by **bikegeezer** » Thu Jun 18, 2009 6:21 am

H2RICK wrote:.....and on a Kawi Triple you have no other choice because, unlike our sophisticated Suzi Triples, they have NO vacuum port hookup points.

The "factory tool" for those triples was a Uni-Syn with the vacuum cylinder separated from the disc with a long plastic tube. It worked, but it was a PIA because you had to remove the air box rubbers. Tapping the intakes for synch ports is a better solution.

Stu

jaybob · Post by **jaybob** » Thu Jun 18, 2009 8:39 am

Thanks guys.

Rick can you expound firther on the spoons? I have never heard of it but it sounds like I should know about it.

Sounds like you just put three spoons under the slides and open slowly and adjust to get them to drop evenly?

H2RICK · Post by **H2RICK** » Fri Jun 19, 2009 7:29 am

Sounds like you just put three spoons under the slides and open slowly and adjust to get them to drop evenly?

Exactly so, Jaybob. I'll also risk stating the obvious that this procedure is done with the engine cold and NOT/NOT running.

Two things to watch for are:
AA) your spoons have to be IDENTICAL. Best to go to a houswares place that sells cutlery and buy 3 NEW spoons from the same lot. Check them for "identicalness" before buying....but you get the idea. Teaspoons usually work best and fit in the space allowed more easily than larger spoons.
BB) inserting the spoon handles to identical depths under the slides. This can be helped by using one spoon to do a trial to get a rough idea of where it/they should sit and then marking that rough distance with a fine tip felt marker with the spoon still in place. Remove the spoon and use a vernier to measure your marked off distance from the end of the spoon handle. You can then mark all 3 spoons with a scriber or some such permanent marking tool using the vernier setting as your distance marking gauge.

There will still be minor variances but this method will get you VERY close for your initial slide setting. You can then do the fine tuning with your manometer....or Uni-Syn.

Tapping the intakes for synch ports is a better solution.

Agreed, Stu, but a lot of guys don't have the tools or skills or inclination to attempt such work. To you and I such an operation is a no-brainer but to many others it's a difficult/impossible job.
I haven't drilled/tapped my H2 carbs because I'm somewhat of a purist regarding stock stuff (as many on this board know

) so I've used the spoons and Uni-Syn to set any Kawi stroker I've had to work on. The spoons, especially, make the initial adjustment soooo much easier because, as you know, those old Mikuni VM's on the Kawi strokers are not the most exact instruments ever created. The first time I used the spoons on my H2 I was amazed at the difference that procedure alone made to the idle AND acceleration. The Uni-syn put the finishing touches on the job but the additional difference, IMO, would only have shown up on a dyno chart.