Blather and History
Benching while on the Bike. An interesting concept.
Reasoning for doing this? After I rebuilt my carbs I made all my adjustments and then put them on the engine. Only problem was I forgot to bench sync! It was one of those hectic times when I was attempting to do a 1000 things at the same time. Had I bench synced I would have noticed that the adjustment between carbs 1 & 3 was way out. In fact there was not enough adjustment left in the screw and some modification were in order. I don't know why I did not have enough adjustment - Things were pretty screwed up on this bike when I got it. Perhaps the PO had dropped one of the carbs on the linkage adjustment and bent it - stranger things have happened.
So, I have my bike running and took it to Randakk's Rally where everyone had a good laugh with my "1942 3-cylinder Farmall Tractor" sounding Gold Wing. I actually kind of like the Chug, Chug, Chug, shhh, Chug, Chug, Chug, shhh sound it was making. Bottom line was I really needed to sync the carbs.
Back at home I finally got the opportunity to put the manometer on the engine. Carbs 2 & 4 were in fairly good shape, but 1 & 3 were way off. This is when I discovered that I didn't have enough adjustment left. In order to confirm that the throttle plates were in fact coming into sync I decided to "Bench Sync" then while still on the bike. Here's what I did:
This writeup is for doing just one side. However, you could do this to both sides. Remember that it's the manifolds that hold up the carbs. You don't want to remove more than two of them at any one time. You need the other two to keep everything held onto the engine. If you remove more than two you might as well just remove the whole carb setup and throw it on the bench.
First, you must remove the two bolts that hold the manifold to the head. Also, unscrew the bracket that holds the manifold to the carb. You need to do this for both carbs on the same side. Without removing the manifold to head bolts you will not have enough flex to easily get the manifolds off.
After you removed the manifold to head bolts you can slip the o-rings out. I figured it's best to remove them so I didn't crush them or screw them up in any way.
To remove the manifolds you really need to man-handle them as the rubber seals on the carbs can be pretty tight! I held firmly to the carb body as I wiggled the manifolds out.
So, with the manifolds out you can clearly see the throttle plates. I have some thin wires I used for my syncing. Do not use copper wire - it's too soft and will dent and scratch on the throttle plate. That'll throw off your sync. I used control wire for model airplanes. However, single strand guitar wire is perfect. Make sure there are no burrs on the wire ends and that the wire is straight.
I inserted the wires and ensured they were both inserted at the same location. That's imortant! You want the wires as close to the bottom as possible as that's where the largest gap is. And you want both wires at the same location or else your sync will be off.
On carbs 1 & 3, carb 1 follows carb 3. So, turning the adjustment screw rotates carb 1's throttle plate. You need to attempt to get the resistance when you pull on the wires equal between the two throttle plates. Once you get them as close as possible lock it down with the lock nut and check once again to verify you didn't change it much.
If you are going to be using a manometer afterwards you don't have to lock it down as you'll need to fine tune it before locking down.
There was some questions as to why I didn't have enough adjustment in my linkage. So, to show how things were put togher here's a picture of my assembly of the parts. Note that there is an additional lock nut that goes on the right side. I'm just not showing it in these photos. Be sure to include your lock nut when you assemble!
I wanted to see if the thickness of the spring and washer was greater than the available space so I took a reading.
Measuring the thickness of just the spring and one washer. This thickness needs to be less than the available space between the two carb sync parts. If it's greater then you will not get the throttle plate to rotate enough to sync properly.
I took a reading of the space between the two linkage points where the adjustment screw goes. It was nearly the same as the thickness of the spring and washer. In order to give me a little more adjustment I trimmed off one loop of the spring:
Here's the reassembly:
This shows the minimum adjustment available to me. Originally, this was not small enough to get the throttle plates properly synced. After removing one loop in the spring I had enough adjustment. Remember, there's an additional lock nut that goes on the right side of the adjustment screw. I have it removed in these photos. Be sure to include the lock nut when you assemble.
Putting the manifolds back on you slip them on the carbs first. Then, insert the o-rings. It could be tricky if you don't pay attention. I used my carb sync pins to help slip the o-rings into their proper location.
Using the Manometer
An important note on using these "Cheap" vacuum gauge manometers. Verify all your gauges are in sync!!! Do this by testing each one on the same carb. Verify they all read the same. If they do not you need to adjust them so they do read the same. There is a small silver screw on the face plate of the gauge that allows for adjustment. Unscrew the plastic cover and adjust the gauges so they all read the same when attached to the same carb.
Now, with my "Bench Syncing" completed I threw on the manometer for the final adjustment. It was just a small tweak at that point before I got them this close:
After getting the carbs in sync I swapped the hose cables on my manometer to verify both gauges read the same. For me, this is a sanity check that my gauges are working correctly.
Finally after syncing both banks I went back and re-checked carbs 3 & 4 and here's there result:
Again, after I synced carbs 3 & 4 I swapped the hoses on the manometer to verify both gauges gave the same results.
Keep re-adjusting the idle speed to keep it within specs as you sync your carbs.
Note that when you sync the carbs you need to sync carbs 1 & 3 first. Follow with carbs 2 & 4. Recheck 1 & 3. After you get 1 & 3 synced and 2 & 4 synced you move on to syncing the right side with the left side. You do this by syncing 3 & 4. Once you get 3 & 4 run back through everything again to verify 1 & 3 are still in sync and 2 & 4 are still in sync. If you have to make any ajustments along the way, go back again and verify everything! A great man once said, "Everything affects everything!"
Once you get everything locked down be sure to throw on some yellow lock-tite to keep them from moving.
So, there you have it. Bench Syncing while the carbs are still on the bike followed by use of the manometer. After this my bike is running a lot smoother. No more chug, chug sound.
Do you know what it's like to fall in the mud and get kicked, in the head, by an iron boot? Of course you don't--no one does--that never happens. - Capt. Rex Kramer
'78 Honda GL1000 GoldWing
'86 Honda CMX450C Rebel