Carb synch with single vacuum gauge....

It is possible, kinda, but not easy! I just did it on my 1977 1000 LTD. A friend of mine has a single vacuum gauge he bought from O'reilly. He has been trying to get me to use it knowing that I needed to synch my carbs for awhile now. Finally I couldn't resist at least trying. I had them bench synched but I knew they were off because of the rough spot coming off idle while riding.

First thing I had to buy a longer piece of tubing, 6 ft, and an aquarium air valve to dampen needle flutter.

When I checked them before starting adjusting the vacuum readings were = carbs 1-4 left to right......26, 31, 36, 28 cm Hg......a 10 cm Hg spread when the book says 2 cm Hg max between cylinders! I knew it had to be off, but wow, its way off!

At first I was chasing it all over the place, when you adjust one it changes the idle so much you have to reset idle, which changes the other three! I went back and forth back and forth and was running out of adjustment on my idle adjustment screw. I was about to get to where the idle would be too high with the screw backed all the way off. So I turned the set screws back in on all 4 carbs equal turns to get some adjustment back and rethought how I could do this. Not being able to see the readings on all 4 carbs at the same time is a definite disadvantage.

What ended up working was to set carbs #1 and #2 together and #3 and #4 together. The way I did this was to take a reading on carb #1 and then #2, then leave the vacuum gauge hooked up to #2 and adjust #1 to split the difference. Yes, adjust the carb without the gauge hooked to it. Switch the gauge back to #1 to confirm the same reading as on #2, after a few tiny adjustments I had those two synched to each other.

Now did the same for #3 and #4. So now I had both side pairs synched to each other but WAY off from one side to the other! So I hooked the gauge up to #3 and adjusted #1 and #2 equal amounts at the same time to split the difference between the pairs. (1,2 and 3,4) I just had to be sure and make tiny adjustments to the screws, turning each equal amounts on both #1 and #2 so I wouldn't loose the synch between them! Now checked all 4 again and had to make one more final tweak to #1 and #2 and then they were all 4 the same!

Well.....very close. For some reason #2 ended up being 2-3 cm Hg off from the other three. Close enough for now so I closed it up. I know It's much closer because before my fuel screws were way off from each other. They were (turns from lightly seated) #1 = 1/8th turn, #2 = 1.5 turns, #3 = 3 turns and #4 = 1.5 turns. Now after the synch they are happy at 1.5, 1.5, 1.5 and 1.75 turns.

Went for a test ride and the difference is very noticeable at idle and take off from stops. Much smoother than before.

I'm going to be riding the bike over to a fellow forum members house this week and he has a 4 gauge carb synch tool from Z1E so I'll be able to see how close I got it and make final adjustments then if needed.

Just wanted to let people know that they can at least get them much closer with a single gauge using the method I did. That should hold them over until they can get their hands on a real 4 gauge set and do it right.

Also, be sure to have a BIG @$$ fan blowing over the motor while doing this! Not just a cheap little box fan. I even killed the bike a few times to give it a rest and let it cool down a bit.

I borrowed a 4 gauge set from McDroid (thanks man!) and got it hooked up to double check my single gauge sync. Looks right on to me...



Remember when taking the photo the needles are fluttering just a bit, so #4 looks to be a touch off, but while fluttering they all look the same.

I put the tape over the gauges because each one was reading different when hooked one at a time to the same cylinder. I was going to write +1, -2, whatever, but decided to just calibrate them with the little screw on the face. Don't know why they don't come calibrated? Once I got each gauge reading the same on cylinder #1 at steady idle, I hooked them up to all 4 and sync is correct. So I got them right using a single gauge! Maybe a better set would allow an even closer setting, but this is about as close as I think you can get with this set.

Look what I missed on my ride yesterday.....123456....didn't notice it until I got home....just missed it....



So after confirming sync was right it was time for a cheap watery beer.....not much flavor but goes down like water.....

74ullc wrote: I borrowed a 4 gauge set from McDroid (thanks man!) and got it hooked up to double check my single gauge sync. Looks right on to me...

Remember when taking the photo the needles are fluttering just a bit, so #4 looks to be a touch off, but while fluttering they all look the same.

I put the tape over the gauges because each one was reading different when hooked one at a time to the same cylinder. I was going to write +1, -2, whatever, but decided to just calibrate them with the little screw on the face. Don't know why they don't come calibrated? Once I got each gauge reading the same on cylinder #1 at steady idle, I hooked them up to all 4 and sync is correct. So I got them right using a single gauge! Maybe a better set would allow an even closer setting, but this is about as close as I think you can get with this set.

Look what I missed on my ride yesterday.....123456....didn't notice it until I got home....just missed it....

So after confirming sync was right it was time for a cheap watery beer.....not much flavor but goes down like water.....



Nice job!

...and it's on sale right now...$4.98 a six-pack...