Topic: Question on 77/78 CB750 Carbs

[dlhoulton]

Had some down time and was thinking. If a person had a set of 77/78 carbs from a K or F CB750 and had another set that was only good for parts. Could you put the #2 slide and arm from the parts set into the #3 carb body of the good set? This would give you two non adjusting carbs and only have to adjust carbs 1 and 4. I'm thinking sync #1 to #2 and #4 to #3 (#2 and #3 are non adjusting).
I know that no two carbs sets are alike in tuning, but the whole idea is to sync all carbs to #2. Is there that much difference in the sync vac reading from one bike and another of the same year and model if they were set up the same?
In theory you still connect all four sync hoses, but only have to adjust #1 and #4. That would make things a whole lot simpler.

[toytuff]

Everything is adjusted from number 2. I would think that it would not be a good idea but no expert. If the reading for number 2 is one thing the reading for number 3 would be different due to the cylinder it's self.

Interesting to see what others think. I'm always learning something new on this site.

tt

[flybox1]

assuming #2 and #3 pulled exactly the same vacuum...yeah, it'd probably work.  but what are the odds?
...its not like RnR one more coffin top and adjusting one more carb is that much more difficult.

[dlhoulton]

flybox1, don't get me wrong. Synching carbs isn't difficult. It's been a blast learning to work on these bikes. I was just doing something dangerous. I was thinking!!!
You brought up a good question though about vacuum. When I got my bike all tuned up and adjusted I looked at the slides and to my eye they were very close if not all the same height. I know that if you were to put a "micro measuring" device to them there would be a difference. But with using the vacuum sync gauges they look all equal. At least on my carbs. Is there that much vacuum difference that would not let you use a non adjusting carb on say #3. In my simple mind #2 doesn't have a choice and can only pull what #2 gives it. All others have to be adjusted to it. So if you treat #3 the same as #2 what would be the difference? Is it that critical that all carbs, not just #2 and #3 pull the exact same vacuum?  Or is getting close going to hurt the motor? When you sync the carbs you are only getting as close as possible with the various sync gauges you have. I know there is probably way more to this and my ignorance is shining. Hope not to offend anyone with the questions.  Only looking to learn.

[flybox1]

My bike quiets down, and runs REALLY well when my carbs are sync'd well. i can hear it.
my 350F was LOUD if it was slightly off.
if you're ok with 'close' go for it, but 'close' can mean the difference between rough and smooth running.

i'm no carb expert either....but the variables keeping 2 & 3 from being 'close' causing vacuum differences could be manufacturing defects/variances IN the carb bodies, slides, slide cutouts(if there are slight differences between 'runs') and because they are not adjustable, the slide heights themselves.
i'm sure there are many more.

[lucky]

Had some down time and was thinking. If a person had a set of 77/78 carbs from a K or F CB750 and had another set that was only good for parts. Could you put the #2 slide and arm from the parts set into the #3 carb body of the good set? This would give you two non adjusting carbs and only have to adjust carbs 1 and 4. I'm thinking sync #1 to #2 and #4 to #3 (#2 and #3 are non adjusting).
I know that no two carbs sets are alike in tuning, but the whole idea is to sync all carbs to #2. Is there that much difference in the sync vac reading from one bike and another of the same year and model if they were set up the same?
In theory you still connect all four sync hoses, but only have to adjust #1 and #4. That would make things a whole lot simpler.

No it will not work AND you will not be able to get the carbs synced!
Here is why.
You will notice there is a split in the connections of the carbs and the play in this slip joint connection always makes the 1&2 carbs OFF from the #3 and 4 carbs.
That is ONE reason you have to sync the carbs because of that slip joint having play in it.

This connection between the 1&2 and 3&4 carbs is the main reason the
sync is off between carbs.
You will notice when syncing carbs that the 3&4 carbs are always off from #1&2 and 3&4. See red circle on photo.

Click on this photo for a larger image.

[lucky]

flybox1, don't get me wrong. Synching carbs isn't difficult. It's been a blast learning to work on these bikes. I was just doing something dangerous. I was thinking!!!
You brought up a good question though about vacuum. When I got my bike all tuned up and adjusted I looked at the slides and to my eye they were very close if not all the same height. I know that if you were to put a "micro measuring" device to them there would be a difference. But with using the vacuum sync gauges they look all equal. At least on my carbs. Is there that much vacuum difference that would not let you use a non adjusting carb on say #3. In my simple mind #2 doesn't have a choice and can only pull what #2 gives it. All others have to be adjusted to it. So if you treat #3 the same as #2 what would be the difference? Is it that critical that all carbs, not just #2 and #3 pull the exact same vacuum?  Or is getting close going to hurt the motor? When you sync the carbs you are only getting as close as possible with the various sync gauges you have. I know there is probably way more to this and my ignorance is shining. Hope not to offend anyone with the questions.  Only looking to learn.

A .007 thousandths difference will make a difference! Look at .007 thousandths on a set of calipers.

[lucky]

Everything is adjusted from number 2. I would think that it would not be a good idea but no expert. If the reading for number 2 is one thing the reading for number 3 would be different due to the cylinder it's self.

Interesting to see what others think. I'm always learning something new on this site.

tt

This idea might work if it was used on the #1 and the #2 carb.
OR on the #3 and #4 carb. But it would still not be as good as having some adjustment.

[TwoTired]

Had some down time and was thinking. If a person had a set of 77/78 carbs from a K or F CB750 and had another set that was only good for parts. Could you put the #2 slide and arm from the parts set into the #3 carb body of the good set? This would give you two non adjusting carbs and only have to adjust carbs 1 and 4. I'm thinking sync #1 to #2 and #4 to #3 (#2 and #3 are non adjusting).
I know that no two carbs sets are alike in tuning, but the whole idea is to sync all carbs to #2. Is there that much difference in the sync vac reading from one bike and another of the same year and model if they were set up the same?
In theory you still connect all four sync hoses, but only have to adjust #1 and #4. That would make things a whole lot simpler.

Why stop there?  Just think. If you welded in the spark plugs, you'd never have to replace them again!  Brilliant!

You could also get rid of that troublesome moving grip on the right bar, too.  Who needs to go any different than fast!

And that messy drain plug!  Weld that up, too.  No more dumping that messy used oil!

Oh, Oh, and those pesky tappet adjusters.   Weld those up too, AND NO MORE VALVE ADJUSTMENTS ARE NEEDED!  What bliss!

The whole point about having carb slide height adjusters is so you can match the vacuum drawn by each cylinder to make them even across the bank.  Taking out an adjustment feature, prevents fine tuning.

However, if you'd rather adjust each cylinders piston ring seal, valve closure seal, cc and grind the head chamber so they have exactly the same volume, and then bring the cam lobes to exactly the same dimensions for each cylinder position, that would be MUCH easier than twisting a slide adjuster one eighth of a turn to draw equal vacuum as the rest.

[edwardmorris]

Had some down time and was thinking. If a person had a set of 77/78 carbs from a K or F CB750 and had another set that was only good for parts. Could you put the #2 slide and arm from the parts set into the #3 carb body of the good set? This would give you two non adjusting carbs and only have to adjust carbs 1 and 4. I'm thinking sync #1 to #2 and #4 to #3 (#2 and #3 are non adjusting).
I know that no two carbs sets are alike in tuning, but the whole idea is to sync all carbs to #2. Is there that much difference in the sync vac reading from one bike and another of the same year and model if they were set up the same?
In theory you still connect all four sync hoses, but only have to adjust #1 and #4. That would make things a whole lot simpler.

Why stop there?  Just think. If you welded in the spark plugs, you'd never have to replace them again!  Brilliant!

You could also get rid of that troublesome moving grip on the right bar, too.  Who needs to go any different than fast!

And that messy drain plug!  Weld that up, too.  No more dumping that messy used oil!

Oh, Oh, and those pesky tappet adjusters.   Weld those up too, AND NO MORE VALVE ADJUSTMENTS ARE NEEDED!  What bliss!

The whole point about having carb slide height adjusters is so you can match the vacuum drawn by each cylinder to make them even across the bank.  Taking out an adjustment feature, prevents fine tuning.

However, if you'd rather adjust each cylinders piston ring seal, valve closure seal, cc and grind the head chamber so they have exactly the same volume, and then bring the cam lobes to exactly the same dimensions for each cylinder position, that would be MUCH easier than twisting a slide adjuster one eighth of a turn to draw equal vacuum as the rest.

I feel terrible for the OP, but this is so laugh out loud hilarious, it might have cured my depression over making snap purchases.

[dlhoulton]

Hey, I new when I asked the question there was the possibility of getting flamed!! I've got a tough hide. It's good to know that the experts can pass on there knowledge to those of us that are still new at building and tuning these bikes. Honestly though, sometimes I think that the more experienced members forget that not everyone is on there level or have there skills. Sometimes even though you use the "SEARCH" function first, using every conceivable combination of words, you just cant find the answer so you just ask. It's the risk you take!
I've got a good bike and with the help from everyone here and tons of reading, my bike is well tuned and running great.

flybox1 and Lucky, great info. Those are the kind of responses and info that help us learn.

It was just an inquisitive question! I enjoyed the responses and valuable information that was given.

[Stev-o]

Had some down time and was thinking.
In theory you still connect all four sync hoses, but only have to adjust #1 and #4. That would make things a whole lot simpler.

Why stop there?  Just think. If you welded in the spark plugs, you'd never have to replace them again!  Brilliant!

You could also get rid of that troublesome moving grip on the right bar, too.  Who needs to go any different than fast!

And that messy drain plug!  Weld that up, too.  No more dumping that messy used oil!

Oh, Oh, and those pesky tappet adjusters.   Weld those up too, AND NO MORE VALVE ADJUSTMENTS ARE NEEDED!  What bliss!

Great response TT!

[edwardmorris]

Hey, I new when I asked the question there was the possibility of getting flamed!! I've got a tough hide. It's good to know that the experts can pass on there knowledge to those of us that are still new at building and tuning these bikes. Honestly though, sometimes I think that the more experienced members forget that not everyone is on there level or have there skills. Sometimes even though you use the "SEARCH" function first, using every conceivable combination of words, you just cant find the answer so you just ask. It's the risk you take!
I've got a good bike and with the help from everyone here and tons of reading, my bike is well tuned and running great.

flybox1 and Lucky, great info. Those are the kind of responses and info that help us learn.

It was just an inquisitive question! I enjoyed the responses and valuable information that was given.

Thick hide = good thing to have. I can assure you, these guys are help you will not find anywhere else. Don't take their candid remarks as flaming, its all in good spirit . I agree about the search tool here is hopeless but I've  had some luck using the google custom search which works a lot better.