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Discussion Starter · #1 ·
Hi to all the group,
I would like to ask for some advice with regard to my sync gauge readings.
just a quick background, the bike is a 1980 cm400t american import (im in the u.k.) So the carbs have been cleaned re jetted for pods and pipes as per the sticky 95/130's. new needle in float bowl as the left one was poor and checked to 15.5 these are non adjustable type. new plugs,caps and coil. mixture screws set at 2 turns out to start with!
The problem I'm trying to solve is the imbalance of the left cylinder burning rich and sooty whilst the right is burning good to slightly lean. The left down pipe and inlet area from carb burn hotter than the right side. Also checked for intake leaks but non around all good seals.
The carbs have been bench sync'd so gauges connected and they read...Left side running at 5 pounds ( in the red zone) whilst the right was at 13 pounds and in the black zone where it should be...I think? This made it imossible to synk the right to the left as its reading was way off!! :confused:
What could these be indicating? so left is sooty plug, hot and only 5 pounds whilst the right is 'good. less hot and 12 pounds. could these be indicative of a deeper problem valves timing? any views on the readings and help and advice much appreciated. thanks.
 

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Sounds like you need to adjust the link between the the butterfly shafts to open the right side carb a little farther to equalise the vacuum draw. Adjust the mixture screws to get the highest reading on each side and re-adjust the cross shaft again as necessary to equalise the readings. You need to put a fan in front of the machine while you do this to keep the engine in a more normal temperature range.
 
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You need to check if they are 2 or 3 jet carbs. If 3 jet, you will never be 'happy' with the way it runs on pods. It also depends where the bike came from, Califonia models run a lot leaner than other parts of USA (had that problem back in 80's when I worked in dealers in Britain) The actual vacuum readings are pretty irrelevant although usually around 15cm/Hg to 25cm/Hg. I've never seen a vacuum gauge marked in pounds (even way back in late 1970's Honda dealer)
 

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Discussion Starter · #4 ·
You need to check if they are 2 or 3 jet carbs. If 3 jet, you will never be 'happy' with the way it runs on pods. It also depends where the bike came from, Califonia models run a lot leaner than other parts of USA (had that problem back in 80's when I worked in dealers in Britain) The actual vacuum readings are pretty irrelevant although usually around 15cm/Hg to 25cm/Hg. I've never seen a vacuum gauge marked in pounds (even way back in late 1970's Honda dealer)
Hi, Yes they are the 3 jet VB22's, I would hope to get the balance better than it is at the moment if possible. The bike actually came from Idaho with 7K on the clock. These are the gauges, I'm prob not reading them correctly, I have marked the readings in Yellow for info!! gauges small2.jpg
 

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Discussion Starter · #5 ·
Sounds like you need to adjust the link between the the butterfly shafts to open the right side carb a little farther to equalise the vacuum draw. Adjust the mixture screws to get the highest reading on each side and re-adjust the cross shaft again as necessary to equalise the readings. You need to put a fan in front of the machine while you do this to keep the engine in a more normal temperature range.
Hi, Forgive my ignorance but Is the link you refer to different from the screw used to sync the carbs!! the bike comes from Idaho by the way!!:D
 

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He means the linkage between them, or the screw that you use to adjust said linkage. Bench syncs are only good enough to get you an even starting place to get the bike running. Dynamic sync is always better than that because you can tune for differences between the 2 cylinders. Looks like you have all the tools to do so.

IF one cylinder is doing all the work the other will just be sooting up plugs. I would ignore the actual readings on the gauges for now and just try to get the settings as even as possible. Then do your idle mixture screw settings. Then check the sync again. Would also be a good idea to check timing and valve clearances before doing all of that as those can have an impact on things as well.
 

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Same screw. The gauges in your picture measure both pressure and vacuum. The scale to the right of the needle is marked in pounds of pressure, and to the left is marked in inches(or cm) of mercury.
 

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As Mike said in post #2.

You need to balance the carbs with the sync screw, adjust the mixtures, balance again, adjust the mixtures again (your balance is quite a ways off). The two gauges need to read the same but it doesn't matter what that reading is.
 

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Discussion Starter · #9 ·
He means the linkage between them, or the screw that you use to adjust said linkage. Bench syncs are only good enough to get you an even starting place to get the bike running. Dynamic sync is always better than that because you can tune for differences between the 2 cylinders. Looks like you have all the tools to do so.

IF one cylinder is doing all the work the other will just be sooting up plugs. I would ignore the actual readings on the gauges for now and just try to get the settings as even as possible. Then do your idle mixture screw settings. Then check the sync again. Would also be a good idea to check timing and valve clearances before doing all of that as those can have an impact on things as well.
Will do thank you, I did wonder if the timing or valves may need adjustmant, will take a look! I think I may need to jet down and with thorough carb clean to see if that changes anything! and brings the left carb and plug colour somewhere near to a reasonable start point and then work from there.
 

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Discussion Starter · #10 ·
Thanks J-T and Mike, At the moment the range between the carbs is too great and I end up with balance screw screwed in all the way so I am thinking that I need to get the left carb running better and then take it from there!,,,see reply to Frogman79. many thanks to all you experienced guys for the advice and any other thoughts welcome!
 

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Discussion Starter · #11 ·
Thanks Mike, now I understand what I am looking at!!!....always helps I find!!:confused::D lots of tinkering to do and re jetting, cleaning and checking and re checking..that seems to be the way forward judging from the many great posts on the forum!...the adventure continues, thanks once again.
 

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Same screw. The gauges in your picture measure both pressure and vacuum. The scale to the right of the needle is marked in pounds of pressure, and to the left is marked in inches(or cm) of mercury.
I had to download picture to see it properly. I think gauges may be marked as inches of water rather than mercury as the 'Normal' section is 20"~30"?
Anyway, your yellow line readings of 150 and 320 need to be moved to around 200~250 by using the single adjuster screw (with 8mm lock-nut) The 3 jet carbs are just about impossible to get running right without the stock airbox, it will run real good to around 3500~4500 rpm but when slides start to lift you will get an extreme rich mixture. I haven't had enough time on CB/CM400/450 carbs to figure out a solution. Leon Moss (LEDAR Race Developments) did a lot of work on these carbs for Honda UK throughout the 1980's. He ended up modifying them to a 2 jet conventional type or fitting CR carbs to get max power. (It was allowed for endurance racing, Le Mans, Bol'-Dor, etc) Changing carbs wasn't allowed in the American Superbike series so stock carbs had to be used although internals could be modified in any manner.. LEDAR also did a lot of work for Kawasaki when they came out with the 'ram air' GPZ600 (he developed it as 'F.A.R.T.S. but that's another story) If you can't get it running properly over 4K, your simplest solution is re-fit stock airbox, I've never seen a 3 valve twin run properly without airbox (or the 4 valve inline 4's) Good luck with carbs
 
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