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Syncing ye olde Carbs


HoughMade
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My '79 XS 400F seemed to be idling very lean. After conversing with other ancient XS 400 owners, they confirmed what I should have known- the factory idle mixture setting are very, very lean. This led to occasional popping at idle and as revs increased. Also, it did not return to idle speed as quickly as it should. I richened the idle mixture and those problems went away. the factory setting was 2.25 turns out. At 3.5, it runs like a charm.

Also, I suspected that the carbs were out of sync. I made a simple manometer- clear tubing and auto transmission fluid. Get 20 feet of tubing (whatever fits the vaccum nipple on your bike) and find the exact center. Place the center at the bottom of a 4 foot piece of wood, then attach the tubing up the length of the board (exact length is not important, but longer give you more time to shou down if you have to). I used zip ties, but I had the thicker wall tubing- whatever you do, make sure the tubing is not collapsed. The most time consuming part of this was getting the fluid to settle in the center. To get the fluid in, I put one end in the ATF bottle and sucked the other end like a straw. I drew up about 2 feet worth of fluid, the exact amount is not important. I then hung the apparatus with the tubes in the air and waited for the fluid to settle- took 15-20 minutes. I hung mine from a ladder to use it, but I was also using that to hold the gas tank (has to come off to get to the sync screw).

After warming the bike up, hooked the tubing to the vacuum nipples on the carb holders, I started it up.....and the ATF nearly got sucked into the engine because the left cylinder was drawing much harder- way out of sync. Word to the wise, keep your thumb on the kill switch until you get it close to synced. Shut it down if one side starts to rise too fast. Longer tubing would also give more time to react...thicker fluid too. I adjusted it a couple times after having to shut it down, then it was close enough that the movement was very slow. I adjusted back and forth and got both sides of the fluid as even as possible. Perfect is near impossible, but I got it within about 1/2-3/4 inch. According to what I read, that's pretty good.

Do not use a thin fluid as it will foam and be hard to read. I used ATF because it is easy to see and a bit (if only a bit) sucked into the engine will not hurt it.

Anyhoo, here's a pic of my setup. This works great for twins with separate carbs. I have even seen these built for 4 cyls., so I imagine a 3 could be done as well.

Bike runs smoother that I thought it was supposed to. It will idles easily where it is supposed to (1200rpm) and will even hold a good idle below 1000 rpm.

manosmall.jpg

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Are we bordering on eccentricity?

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  • Moderator

i just use "simple" dial vacuum guages ;)

this is his next version using Blackpool tower

blackpooltowervacuum-tower.jpg

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you can balance the carbs with fuse wire or a 1.5 thou feeler blade. You'll be surprised how accurrate that method is.

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you can balance the carbs with fuse wire or a 1.5 thou feeler blade. You'll be surprised how accurrate that method is.

Doesn't the ATF run off the wire ?

:lol::lol:

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i just use "simple" dial vacuum guages ;)

this is his next version using Blackpool tower

blackpooltowervacuum-tower.jpg

:lol:

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i just use "simple" dial vacuum guages ;)

this is his next version using Blackpool tower

blackpooltowervacuum-tower.jpg

Very nice...you spent more time making that picture than I spent building the manometer. ;)

...and had I vacuum guages, I would use them too. However, for this particular task, $4 spent on tubing rather than $50 on vacuum guages seemed like the more fiscally prudent decision for me.

- and I am curious about carb balancing technique with a feeler guage. If you mean just setting the throttle butterflies, that, in my mind at least, is more "pre-synchronizing." It will get it close, not doubt, but the more used an engine, the less accurate it will be. Good place to start, though. If something else, please share. :)

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My '79 XS 400F seemed to be idling very lean. After conversing with other ancient XS 400 owners, they confirmed what I should have known- the factory idle mixture setting are very, very lean. This led to occasional popping at idle and as revs increased. Also, it did not return to idle speed as quickly as it should. I richened the idle mixture and those problems went away. the factory setting was 2.25 turns out. At 3.5, it runs like a charm.

Also, I suspected that the carbs were out of sync. I made a simple manometer- clear tubing and auto transmission fluid. Get 20 feet of tubing (whatever fits the vaccum nipple on your bike) and find the exact center. Place the center at the bottom of a 4 foot piece of wood, then attach the tubing up the length of the board (exact length is not important, but longer give you more time to shou down if you have to). I used zip ties, but I had the thicker wall tubing- whatever you do, make sure the tubing is not collapsed. The most time consuming part of this was getting the fluid to settle in the center. To get the fluid in, I put one end in the ATF bottle and sucked the other end like a straw. I drew up about 2 feet worth of fluid, the exact amount is not important. I then hung the apparatus with the tubes in the air and waited for the fluid to settle- took 15-20 minutes. I hung mine from a ladder to use it, but I was also using that to hold the gas tank (has to come off to get to the sync screw).

After warming the bike up, hooked the tubing to the vacuum nipples on the carb holders, I started it up.....and the ATF nearly got sucked into the engine because the left cylinder was drawing much harder- way out of sync. Word to the wise, keep your thumb on the kill switch until you get it close to synced. Shut it down if one side starts to rise too fast. Longer tubing would also give more time to react...thicker fluid too. I adjusted it a couple times after having to shut it down, then it was close enough that the movement was very slow. I adjusted back and forth and got both sides of the fluid as even as possible. Perfect is near impossible, but I got it within about 1/2-3/4 inch. According to what I read, that's pretty good.

Do not use a thin fluid as it will foam and be hard to read. I used ATF because it is easy to see and a bit (if only a bit) sucked into the engine will not hurt it.

Anyhoo, here's a pic of my setup. This works great for twins with separate carbs. I have even seen these built for 4 cyls., so I imagine a 3 could be done as well.

Bike runs smoother that I thought it was supposed to. It will idles easily where it is supposed to (1200rpm) and will even hold a good idle below 1000 rpm.

manosmall.jpg

I kinda recognise the wood in this picture - does it come from the vaulting horse that they they used in "The Great Escape" ?

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Very nice...you spent more time making that picture than I spent building the manometer. ;)

no 5 mins tops, i got photoshop :P

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